BCLA CLEAR - Scleral lenses

      Abstract

      Scleral lenses were the first type of contact lens, developed in the late nineteenth century to restore vision and protect the ocular surface. With the advent of rigid corneal lenses in the middle of the twentieth century and soft lenses in the 1970’s, the use of scleral lenses diminished; in recent times there has been a resurgence in their use driven by advances in manufacturing and ocular imaging technology. Scleral lenses are often the only viable form of contact lens wear across a range of clinical indications and can potentially delay the need for corneal surgery. This report provides a brief historical review of scleral lenses and a detailed account of contemporary scleral lens practice including common indications and recommended terminology. Recent research on ocular surface shape is presented, in addition to a comprehensive account of modern scleral lens fitting and on-eye evaluation. A range of optical and physiological challenges associated with scleral lenses are presented, including options for the clinical management of a range of ocular conditions. Future applications which take advantage of the stability of scleral lenses are also discussed. In summary, this report presents evidence-based recommendations to optimise patient outcomes in modern scleral lens practice.

      Abbreviations:

      IOP (intraocular pressure), OCT (optical coherence tomography), PMMA (poly (methyl methacrylate))

      Keywords

      1. Introduction

      For over a century, scleral lenses have been used for refractive and therapeutic rehabilitation in a wide range of ocular conditions [
      • Fick A.
      A contact lens.
      ,
      • Pearson R.M.
      August Müller’s inaugural dissertation.
      ,
      • Pearson R.M.
      The Sämisch case and the Müllers of Wiesbaden.
      ]. They often provide a visual solution when numerous other optical treatments have failed and can delay the need for corneal surgery [
      • DeLoss K.S.
      • Fatteh N.H.
      • Hood C.T.
      Prosthetic Replacement of the Ocular Surface Ecosystem (PROSE) scleral device compared to keratoplasty for the treatment of corneal ectasia.
      ,
      • Koppen C.
      • Kreps E.O.
      • Anthonissen L.
      • Van Hoey M.
      • Dhubhghaill S.N.
      • Vermeulen L.
      Scleral lenses reduce the need for corneal transplants in severe keratoconus.
      ,
      • Rosenthal P.
      • Croteau A.
      Fluid-ventilated, gas-permeable scleral contact lens is an effective option for managing severe ocular surface disease and many corneal disorders that would otherwise require penetrating keratoplasty.
      ,
      • Ling J.J.
      • Mian S.I.
      • Stein J.D.
      • Rahman M.
      • Poliskey J.
      • Woodward M.A.
      Impact of scleral contact lens use on the rate of corneal transplantation for keratoconus.
      ]. This review explores the evolution of the initial glass and poly (methyl methacrylate) PMMA haptic lenses to modern scleral lens designs including the evidence guiding current scleral lens practice and avenues for future research.

      2. Scleral lens history

      The first therapeutic (non-optical) glass blown scleral shell was manufactured in 1887 and was worn continuously to protect the ocular surface [
      • Pearson R.M.
      The Sämisch case and the Müllers of Wiesbaden.
      ,
      • Müller A.
      Brillenglässer und Hornhautlinsen: Inaugural-Dissertation zur Erlangung der Doctorwürde… der Universität Kiel. L. Handorff.
      ]. Descriptions of glass scleral lenses to restore vision in high myopia, aphakia, and keratoconus were published shortly thereafter [
      • Fick A.
      A contact lens.
      ,
      • Pearson R.M.
      August Müller’s inaugural dissertation.
      ,
      • Panas P.
      Meeting of 20th March.
      ]. Glass blown scleral lenses had poor optical reproducibility [
      • Pearson R.M.
      The Sämisch case and the Müllers of Wiesbaden.
      ,
      • Rugg-Gunn A.
      Contact glasses.
      ], and while preformed ground glass lenses addressed this to some extent, the challenge of corneal oedema (Sattler’s veil or Fick’s phenomenon) remained [
      • Sattler C.
      Erfahrungen mit haftgläsern.
      ,
      • Dallos J.
      Sattler’s veil.
      ]. The evolution of scleral lenses from glass to PMMA and gas permeable materials [
      • Ezekiel D.
      Gas permeable haptic lenses.
      ,
      • Rosenthal P.
      Clinincal performance and fitting principles of “rigid” super permeable contact lenses.
      ,
      • Lyons C.
      • Buckley R.
      • Pullum K.
      • Sapp N.
      Development of the gas‐permeable impression‐moulded scleral contact lens: a preliminary report.
      ,
      • Pullum K.W.
      Feasibility study for the production of gas permeablescleral lenses using ocular impression techniques.
      ] has minimised the adverse physiological effects induced by the original scleral lenses, and advances in manufacturing techniques and ocular imaging have led to a resurgence in scleral lens prescribing in recent years [
      • Vincent S.
      The rigid lens renaissance: a surge in sclerals.
      ,
      • Woods C.A.
      • Efron N.
      • Morgan P.
      • International Contact Lens Prescribing Survey Consortium
      Consortium ICLPS. Are eye‐care practitioners fitting scleral contact lenses?.
      ].

      3. Scleral lens indications

      There are numerous optical and therapeutic indications to prescribe scleral lenses [
      • van der Worp E.
      • Bornman D.
      • Ferreira D.L.
      • Faria-Ribeiro M.
      • Garcia-Porta N.
      • González-Meijome J.M.
      Modern scleral contact lenses: a review.
      ,
      • Schornack M.M.
      Scleral lenses: a literature review.
      ,
      • Fadel D.
      • Kramer E.
      Potential contraindications to scleral lens wear.
      ]. During the 1980’s, when scleral lenses were first manufactured in gas permeable materials [
      • Ezekiel D.
      Gas permeable haptic lenses.
      ,
      • Rosenthal P.
      Clinincal performance and fitting principles of “rigid” super permeable contact lenses.
      ,
      • Lyons C.
      • Buckley R.
      • Pullum K.
      • Sapp N.
      Development of the gas‐permeable impression‐moulded scleral contact lens: a preliminary report.
      ,
      • Pullum K.W.
      Feasibility study for the production of gas permeablescleral lenses using ocular impression techniques.
      ,
      • Muller F.A.
      • Muller A.C.
      Das Kunstliche auge.
      ], the most common conditions treated were; high ametropia (aphakia and myopia, 44 %), primary corneal ectasia (keratoconus, pellucid marginal degeneration, keratoglobus, 32 %), post-penetrating keratoplasty (12 %), and ocular surface disease (7 %) (based on a weighted analysis of Ezekiel, Pullum, and Trodd [
      • Ezekiel D.
      Gas permeable haptic lenses.
      ,
      • Pullum K.
      • Trodd T.
      The modern concept of sclerallens practice.
      ]. Since the advent of silicone hydrogel soft contact lenses used for the refractive correction of aphakia and high myopia, the most common clinical conditions treated with modern highly oxygen permeable scleral lenses are; primary corneal ectasia (53 %), ocular surface disease (18 %), and post-penetrating keratoplasty (17 %) (weighted analysis of data [
      • Rosenthal P.
      • Croteau A.
      Fluid-ventilated, gas-permeable scleral contact lens is an effective option for managing severe ocular surface disease and many corneal disorders that would otherwise require penetrating keratoplasty.
      ,
      • Visser E.-S.
      • Visser R.
      • van Lier H.J.
      • Otten H.M.
      Modern scleral lenses part I: clinical features.
      ,
      • Visser E.-S.
      • Van der Linden B.J.
      • Otten H.M.
      • Van der Lelij A.
      • Visser E.-S.
      Medical applications and outcomes of bitangential scleral lenses.
      ,
      • Otten H.M.
      • van der Linden B.J.
      • Visser E.-S.
      Clinical performance of a new bitangential mini-scleral lens.
      ,
      • Lee J.C.
      • Chiu G.B.
      • Bach D.
      • Bababeygy S.R.
      • Irvine J.
      • Heur M.
      Functional and visual improvement with prosthetic replacement of the ocular surface ecosystem scleral lenses for irregular corneas.
      ,
      • Dimit R.
      • Gire A.
      • Pflugfelder Sc
      • Bergmanson Jp.
      Patient ocular conditions and clinical outcomes using a PROSE scleral device.
      ,
      • Baran I.
      • Bradley J.A.
      • Alipour F.
      • Rosenthal P.
      • Le H.-G.
      • Jacobs D.S.
      PROSE treatment of corneal ectasia.
      ,
      • Pecego M.
      • Barnett M.
      • Mannis M.J.
      • Durbin-Johnson B.
      Jupiter scleral lenses: the UC Davis Eye Center experience.
      ,
      • Pullum K.W.
      • Whiting M.A.
      • Buckley R.J.
      Scleral contact lenses: the expanding role.
      ,
      • Kanakamedala A.D.
      • Salazar H.
      • Chamberlain P.D.
      • Vadapalli S.
      • Orengo-Nania S.
      • Khandelwal S.S.
      Outcomes of scleral contact lens use in veterans.
      ,
      • Segal O.
      • Barkana Y.
      • Hourovitz D.
      • Behrman S.
      • Kamun Y.
      • Avni I.
      • et al.
      Scleral contact lenses may help where other modalities fail.
      ,
      • Arumugam A.O.
      • Rajan R.
      • Subramanian M.
      • Mahadevan R.
      PROSE for irregular corneas at a tertiary eye care center.
      ,
      • Stason W.B.
      • Razavi M.
      • Jacobs D.S.
      • Shepard D.S.
      • Suaya J.A.
      • Johns L.
      • et al.
      Clinical benefits of the Boston ocular surface prosthesis.
      ]. Modern scleral lenses are also used for the correction of simple refractive errors, including presbyopia, particularly when other modalities fail due to vision or comfort issues [
      • Nau C.B.
      • Harthan J.
      • Shorter E.
      • Barr J.
      • Nau A.
      • Chimato N.T.
      • et al.
      Demographic characteristics and prescribing patterns of scleral lens fitters: the SCOPE study.
      ].

      4. Scleral lens terminology and anatomy

      4.1 The evolution of scleral lens terminology

      The Scleral Lens Education Society previously classified large diameter rigid lenses based on the overall lens diameter relative to the horizontal visible iris diameter (e.g. corneoscleral, mini-scleral, semi-scleral, and large scleral lenses) [
      • Ritzmann M.
      • Caroline P.J.
      • Börret R.
      • Korszen E.
      An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses.
      ]. However, any lens ‘fitted to vault over the entire cornea, including the limbus, and to land on conjunctiva overlying the sclera’ is now considered a scleral lens [
      • Michaud L.
      • Lipson M.
      • Kramer E.
      • Walker M.
      The official guide to scleral lens terminology.
      ]. Supplementary Table 1 highlights the main differences between the Scleral Lens Education Society scleral lens terminology guidelines [
      • Michaud L.
      • Lipson M.
      • Kramer E.
      • Walker M.
      The official guide to scleral lens terminology.
      ] and the 2017 ISO standards for Ophthalmic optics (Contact lenses Part 1: Vocabulary, classification system and recommendations for labelling specifications) [
      • Standardization IOf
      Ophthalmic optics - Contact Lenses. Part 1: vocabulary, classification system and recommendations for labelling specifications.
      ]. The Scleral Lens Education Society guidelines adopt a simplified three zone description (optic, transition, and landing zones) and this terminology is used throughout this paper (Fig. 1A and B).
      Fig. 1
      Fig. 1A. This image illustrates the three zones of a scleral lens on an eye. Image credit Daddi Fadel. B. This image illustrates the three zones of a scleral lens on an eye with sodium fluorescein. Image credit Daddi Fadel.

      4.2 The optic zone

      The optic zone houses the refractive correction of a scleral lens and can be customised similarly to rigid corneal lenses. For example, the optic zone can be elliptical in shape to optimise the fit [
      • Fadel D.
      The influence of limbal and scleral shape on scleral lens design.
      ], or offset from the geometric centre of the lens to improve alignment of the optics with the pupil [
      • Vincent S.J.
      • Collins M.J.
      A topographical method to quantify scleral contact lens decentration.
      ,
      • Vincent S.J.
      • Fadel D.
      Optical considerations for scleral contact lenses: a review.
      ]. In addition to including front surface toricity to correct residual astigmatism, front surface asphericity can be manipulated to minimise residual higher order aberrations (primarily spherical aberration) [
      • Gumus K.
      • Gire A.
      • Pflugfelder S.C.
      The impact of the Boston ocular surface prosthesis on wavefront higher-order aberrations.
      ,
      • Hussoin T.
      • Le H.-G.
      • Carrasquillo K.G.
      • Johns L.
      • Rosenthal P.
      • Jacobs D.S.
      The effect of optic asphericity on visual rehabilitation of corneal ectasia with a prosthetic device.
      ], or a customised wavefront guided front surface can also reduce other non-symmetrical higher order aberrations [
      • Sabesan R.
      • Johns L.
      • Tomashevskaya O.
      • Jacobs D.S.
      • Rosenthal P.
      • Yoon G.
      Wavefront-guided scleral lens prosthetic device for keratoconus.
      ,
      • Marsack J.D.
      • Ravikumar A.
      • Nguyen C.
      • Ticak A.
      • Koenig D.E.
      • Elswick J.D.
      • et al.
      Wavefront-guided scleral lens correction in keratoconus.
      ,
      • Hastings G.D.
      • Applegate R.A.
      • Nguyen L.C.
      • Kauffman M.J.
      • Hemmati R.T.
      • Marsack J.D.
      Comparison of wavefront-guided and best conventional scleral lenses after habituation in eyes with corneal ectasia.
      ] (see Section 11.1).

      4.3 The transition zone

      The transition zone connects the optic and landing zones and is often referred to as the limbal, peripheral, or intermediate zone [
      • Michaud L.
      • Lipson M.
      • Kramer E.
      • Walker M.
      The official guide to scleral lens terminology.
      ]. The transition zone may contain multiple curves and can be manipulated to adjust the fluid reservoir depth over the mid-peripheral cornea and limbus through alterations in its curvature or tangent angle.

      4.4 The landing zone

      The landing zone contacts the conjunctival tissue overlying the sclera and may be spherical, toric, quadrant, multi-meridian specific, or completely customised based on an ocular impression (e.g. EyePrintPRO™ (Advanced Vision Technology, USA)), or scleral profilometry (e.g. sMap3D (Precision Ocular Metrology, Mexico, USA, distributed by Visionary Optics, Virginia), Eye Surface Profiler (Eaglet Eye, Netherlands, and Pentacam (Oculus, Germany). The alignment of the landing zone influences lens seal off, suction, centration, and tissue compression [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ,
      • Consejo A.
      • Behaegel J.
      • Van Hoey M.
      • Iskander D.R.
      • Rozema J.J.
      Scleral asymmetry as a potential predictor for scleral lens compression.
      ,
      • Consejo A.
      • Behaegel J.
      • Van Hoey M.
      • Wolffsohn J.S.
      • Rozema J.J.
      • Iskander D.R.
      Anterior eye surface changes following miniscleral contact lens wear.
      ,
      • Macedo-de-Araújo R.J.
      • Van der Worp E.
      • González-Méijome J.M.
      In vivo assessment of the anterior scleral contour assisted by automatic profilometry and changes in conjunctival shape after miniscleral contact lens fitting.
      ]. A recent study showed that, in comparison to habitual scleral lens corrections (including spherical and toric landing zones), a quadrant-specific scleral lens design resulted in an improvement of one line or more visual acuity in 50 % of eyes and a reduction in the incidence of midday lens removal from 30 % to 7 % [
      • Barnett M.
      • Carrasquillo K.G.
      • Schornack M.M.
      Clinical outcomes of scleral lens fitting with a data-driven, quadrant-specific design: multicenter review.
      ]. Further potential customisations include notches or localised vaulting to avoid conjunctival anomalies, fenestrations to reduce suction and aid lens removal [
      • Fadel D.
      • Ezekiel D.F.
      Fenestrated scleral lenses: back to the origins? Review of their benefits and fitting techniques.
      ], venting channels to enhance fluid exchange between the landing zone and the optical zone, and modifications to aid stabilisation (e.g. ballast, slab off designs).

      4.5 Scleral lens specifics

      While the back optic zone radius is critical in rigid corneal lens fitting, the sagittal depth of a scleral lens is the key initial parameter to ensure apical vault. In particular, the sagittal depth of the lens perpendicular to the chord where the landing zone first contacts the ocular surface (i.e. the primary functional sagittal depth) [
      • Michaud L.
      • Lipson M.
      • Kramer E.
      • Walker M.
      The official guide to scleral lens terminology.
      ] (Fig. 2). Scleral lenses are also substantially thicker than rigid corneal lenses to minimise potential lens warpage and on eye lens flexure. However anterior lens flexure is often related to the interaction between the landing zone and scleral elevation [
      • Vincent S.J.
      • Kowalski L.P.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      The influence of centre thickness on miniscleral lens flexure.
      ]. This increased thickness has implications for oxygen delivery to the cornea for lower Dk lens materials [
      • Michaud L.
      • Van Der Worp E.
      • Brazeau D.
      • Warde R.
      • Giasson C.J.
      Predicting estimates of oxygen transmissibility for scleral lenses.
      ,
      • Pullum K.W.
      • Hobley A.J.
      • Davison C.
      100+ Dk: does thickness make much difference?.
      ,
      • Kim Y.H.
      • Tan B.
      • Lin M.C.
      • Radke C.J.
      Central corneal edema with scleral-lens wear.
      ]. The centre lens thickness is typically between 200 and 500 μm ([
      • Barnett M.
      • Toabe M.
      Scleral lens handling. Contemporary scleral lenses: theory and application.
      ]. Similar to soft and rigid corneal lenses, the thickness profile varies with centre thickness, lens power, and between scleral designs [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      Scleral contact lens thickness profiles: the relationship between average and centre lens thickness.
      ].
      Fig. 2
      Fig. 2This image shows the lens primary functional sagittal depth, which is perpendicular to the chord where the landing zone first contacts the ocular surface. Image credit Daddi Fadel.

      5. Ocular surface shape

      The understanding of ocular surface shape and elevation has been improved by recent advances in anterior segment imaging such as optical coherence tomography (OCT) and corneoscleral profilometry [
      • Walker M.K.
      • Schornack M.M.
      • Vincent S.J.
      Anatomical and physiological considerations in scleral lens wear: conjunctiva and sclera.
      ,
      • Bandlitz S.
      • Esper P.
      • Stein M.
      • Dautzenberg T.
      • Wolffsohn J.S.
      Corneoscleral topography measured with Fourier-based profilometry and scheimpflug imaging.
      ]. The morphology of the anterior ocular surface varies diurnally [
      • Read S.A.
      • Alonso‐Caneiro D.
      • Free K.A.
      • Labuc‐Spoors E.
      • Leigh J.K.
      • Quirk C.J.
      • et al.
      Diurnal variation of anterior scleral and conjunctival thickness.
      ], during accommodation [
      • Niyazmand H.
      • Read S.A.
      • Atchison D.A.
      • Collins M.J.
      Effects of accommodation and simulated convergence on anterior scleral shape.
      ,
      • Woodman-Pieterse E.C.
      • Read S.A.
      • Collins M.J.
      • Alonso-Caneiro D.
      Anterior scleral thickness changes with accommodation in myopes and emmetropes.
      ], with age [
      • Read S.A.
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Bremner A.
      • Fothergill A.
      • Ismail B.
      • et al.
      Anterior eye tissue morphology: scleral and conjunctival thickness in children and young adults.
      ], refractive error [
      • Niyazmand H.
      • Read S.A.
      • Atchison D.A.
      • Collins M.J.
      Anterior eye shape in emmetropes, low to moderate myopes, and high myopes.
      ,
      • Consejo A.
      • Rozema J.J.
      In vivo anterior scleral morphometry, axial length and myopia.
      ], and disease [
      • Walker M.K.
      • Schornack M.M.
      • Vincent S.J.
      Anatomical and physiological considerations in scleral lens wear: conjunctiva and sclera.
      ].While presently there are no evidence-based scleral lens fitting guidelines based on anterior segment imaging to optimise visual and physiological outcomes, toric, quadrant-specific, or customised landing zones can improve scleral alignment and have numerous advantages. Back surface landing zone customisation reduces lens decentration, lens flexure [
      • Van der Worp E.
      A guide to scleral lens fitting.
      ], excessive debris [
      • Van der Worp E.
      A guide to scleral lens fitting.
      ], the formation of air bubbles, conjunctival prolapse, localised conjunctival vessel blanching [
      • Visser E.
      • Visser R.
      Case report: bitorische scleralens bij keratitis sicca.
      ,
      • Visser E.
      • Visser R.
      • Van Lier H.
      • Otten H.
      A cross sectional survey of the medical indications for and performance of scleral contact lens wear in the Netherlands.
      ] and lens impingement [
      • Mahadevan R.
      • Jagadeesh D.
      • Rajan R.
      • Arumugam A.O.
      Unique hard scleral lens post-LASIK ectasia fitting.
      ,
      • Schornack M.
      Toric haptics in scleral lens design: a case series.
      ]. For example, patients fitted with toric landing zones report improved comfort, increased wearing time and overall satisfaction, and better optical and visual outcomes [
      • Visser E.-S.
      • Visser R.
      • van Lier H.J.
      • Otten H.M.
      Modern scleral lenses part I: clinical features.
      ,
      • Visser E.-S.
      • Van der Linden B.J.
      • Otten H.M.
      • Van der Lelij A.
      • Visser E.-S.
      Medical applications and outcomes of bitangential scleral lenses.
      ,
      • Visser E.-S.
      • Visser E.-S.
      • Van Lier H.J.
      Advantages of toric scleral lenses.
      ].

      5.1 Scleral elevation

      The scleral elevation profile is not spherical, but is typically asymmetric or toric. In a study of 140 eyes using corneoscleral topography [
      • DeNaeyer G.
      • Sanders D.
      • van der Worp E.
      • Jedlicka J.
      • Michaud L.
      • Morrison S.
      Qualitative assessment of scleral shape patterns using a new wide field ocular surface elevation topographer.
      ] only 5 % of eyes were identified as spherical and 29 % of eyes displayed regular scleral toricity. Most eyes exhibited an asymmetry in scleral elevation (41 %) or periodicity (25 %). The asymmetry in scleral elevation increases further from the limbus, which suggests that for larger diameter scleral lenses, a toric or customised landing zone is required to achieve acceptable alignment [
      • Walker M.K.
      • Schornack M.M.
      • Vincent S.J.
      Anatomical and physiological considerations in scleral lens wear: conjunctiva and sclera.
      ].
      Unlike corneal toricity, scleral toricity is typically quantified as a difference in the elevation profile, rather than curvature, between two meridians at a specified chord length. Corneal and scleral toricity are not typically correlated in healthy eyes with minimal astigmatism [
      • Ritzmann M.
      • Caroline P.J.
      • Börret R.
      • Korszen E.
      An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses.
      ,
      • Vincent S.J.
      • Kowalski L.P.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      The influence of centre thickness on miniscleral lens flexure.
      ]. In cases of high degrees of astigmatism [
      • DeNaeyer G.
      • Sanders D.R.
      • Michaud L.
      • Morrison S.
      • Walker M.
      • Jedlicka J.
      • et al.
      Correlation of corneal and scleral topography in cases with ectasias and normal corneas.
      ,
      • Consejo A.
      • Rozema J.J.
      Scleral shape and its correlations with corneal astigmatism.
      ] and irregular corneas [
      • DeNaeyer G.
      • Sanders D.R.
      • Michaud L.
      • Morrison S.
      • Walker M.
      • Jedlicka J.
      • et al.
      Correlation of corneal and scleral topography in cases with ectasias and normal corneas.
      ,
      • Van Nuffel S.
      • Consejo A.
      • Koppen C.
      • Kreps E.O.
      The corneoscleral shape in keratoconus patients with and without specialty lens wear.
      ], the sclera shows greater irregularity. Additionally, in keratoconus, the axis of greatest scleral asymmetry appears to align with the cone location [
      • DeNaeyer G.
      • Sanders D.R.
      • Michaud L.
      • Morrison S.
      • Walker M.
      • Jedlicka J.
      • et al.
      Correlation of corneal and scleral topography in cases with ectasias and normal corneas.
      ,
      • Dhaese S.E.
      • Kreps E.O.
      • Consejo A.
      Scleral shape and its correlation with corneal parameters in keratoconus.
      ].

      5.2 Scleral curvature

      The anterior sclera just beyond the limbus is typically tangential in shape (more straight than curved), but because of differences in tangent angles the sclera is flatter nasally [
      • van der Worp E.
      • Bornman D.
      • Ferreira D.L.
      • Faria-Ribeiro M.
      • Garcia-Porta N.
      • González-Meijome J.M.
      Modern scleral contact lenses: a review.
      ,
      • Hall L.A.
      • Young G.
      • Wolffsohn J.S.
      • Riley C.
      The influence of corneoscleral topography on soft contact lens fit.
      ,
      • Hall L.A.
      • Hunt C.
      • Young G.
      • Wolffsohn J.
      Factors affecting corneoscleral topography.
      ,
      • Choi H.J.
      • Lee S.-M.
      • Lee J.Y.
      • Lee S.Y.
      • Kim M.K.
      • Wee W.R.
      Measurement of anterior scleral curvature using anterior segment OCT.
      ] and steepest temporally [
      • Ritzmann M.
      • Caroline P.J.
      • Börret R.
      • Korszen E.
      An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses.
      ,
      • Choi H.J.
      • Lee S.-M.
      • Lee J.Y.
      • Lee S.Y.
      • Kim M.K.
      • Wee W.R.
      Measurement of anterior scleral curvature using anterior segment OCT.
      ,
      • Macedo-de-Araújo R.J.
      • Amorim-de-Sousa A.
      • Queirós A.
      • van der Worp E.
      • González-Méijome J.M.
      Relationship of placido corneal topography data with scleral lens fitting parameters.
      ,
      • Bandlitz S.
      • Bäumer J.
      • Conrad U.
      • Wolffsohn J.
      Scleral topography analysed by optical coherence tomography.
      ], with the greatest meridional difference found between the nasal and temporal quadrants [
      • Hall L.A.
      • Young G.
      • Wolffsohn J.S.
      • Riley C.
      The influence of corneoscleral topography on soft contact lens fit.
      ,
      • Hall L.A.
      • Hunt C.
      • Young G.
      • Wolffsohn J.
      Factors affecting corneoscleral topography.
      ]. These regional variations in scleral curvature may be due to the anatomical position of the extraocular muscle insertion points which influence scleral thickness and lens centration along the horizontal meridian. Several studies have reported changes in scleral elevation, thickness, and topography following short-term lens wear [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ,
      • Consejo A.
      • Behaegel J.
      • Van Hoey M.
      • Iskander D.R.
      • Rozema J.J.
      Scleral asymmetry as a potential predictor for scleral lens compression.
      ,
      • Consejo A.
      • Behaegel J.
      • Van Hoey M.
      • Wolffsohn J.S.
      • Rozema J.J.
      • Iskander D.R.
      Anterior eye surface changes following miniscleral contact lens wear.
      ,
      • Macedo-de-Araújo R.J.
      • Van der Worp E.
      • González-Méijome J.M.
      In vivo assessment of the anterior scleral contour assisted by automatic profilometry and changes in conjunctival shape after miniscleral contact lens fitting.
      ,
      • Courey C.
      • Courey G.
      • Michaud L.
      Conjunctival inlapse: nasal and temporal conjuctival shape variations associated with scleral lens wear.
      ]. However further research is still needed regarding the long-term effect of conjunctival and scleral tissue changes beneath and adjacent to the landing zone.

      5.3 The corneoscleral junction

      The profile of the ocular surface as the scleral lens approaches and lands on the eye can influence several aspects of the fit including centration, tissue compression, and optical outcomes. The corneoscleral junction is the angle at the junction of the cornea and the sclera, which typically flattens substantially from the peripheral cornea to the anterior sclera, greater flattening indicated by a more acute corneoscleral junction angle. Asymmetries in the corneoscleral junction can affect scleral lens movement and position [
      • DeNaeyer G.
      • Sanders D.
      • van der Worp E.
      • Jedlicka J.
      • Michaud L.
      • Morrison S.
      Qualitative assessment of scleral shape patterns using a new wide field ocular surface elevation topographer.
      ] and are more pronounced in keratoconic eyes, particularly in advanced cases [
      • Piñero D.P.
      • Martínez-Abad A.
      • Soto-Negro R.
      • Ruiz-Fortes P.
      • Pérez-Cambrodí R.J.
      • Ariza-Gracia M.A.
      • et al.
      Differences in corneo-scleral topographic profile between healthy and keratoconus corneas.
      ]. Temporal scleral lens decentration often occurs due to misalignment of the landing zone along the horizontal meridian [
      • Seguí-Crespo M.
      • Ariza-Gracia M.Á
      • Sixpene Nd L.D.
      • Piñero D.P.
      Geometrical characterization of the corneo-scleral transition in normal patients with Fourier domain optical coherence tomography.
      ], which can result in displacement of the optic zone, variation in the limbal fluid reservoir thickness, and decreased comfort due to lens movement. Inferior lens decentration can be amplified when the lens is fitted with a higher initial central reservoir clearance, although the mass of the lens does not appear to play a significant role [
      • Kowalski L.P.
      • Collins M.J.
      • Vincent S.J.
      Scleral lens centration: the influence of centre thickness, scleral topography, and apical clearance.
      ]. The corneoscleral junction is less pronounced in Asian compared to Caucasian eyes [
      • Seguí-Crespo M.
      • Ariza-Gracia M.Á
      • Sixpene Nd L.D.
      • Piñero D.P.
      Geometrical characterization of the corneo-scleral transition in normal patients with Fourier domain optical coherence tomography.
      ], and Asian eyes may have a smaller corneal diameter and a more prolate corneal shape along the vertical meridian [
      • Hickson-Curran S.
      • Brennan N.A.
      • Igarashi Y.
      • Young G.
      Comparative evaluation of Asian and white ocular topography.
      ]. Some laboratories manufacture scleral lens designs that account for known anatomical variations related to race, and this may become more common in the future as lens design becomes more customized for each individual eye.

      6. Instrumentation

      While scleral lenses have been prescribed for over a century using a diagnostic fitting approach, several instruments are now available that can assist with ocular health assessment and documentation, initial lens selection, haptic customisation, and troubleshooting.

      6.1 Corneoscleral profilometry

      Several scleral topographers are now available including the Eye Surface Profiler (Eaglet Eye, Netherlands), the sMap3D™ corneoscleral topographer (Visionary Optics, USA), and the Corneo Scleral Profile report module with the Pentacam® (Oculus Optikgeräte, Germany). The Eye Surface Profiler and sMap3D instruments measure ocular surface elevation up to a 22 mm chord diameter using fluorophotometry, which requires the instillation of sodium fluorescein. The Eye Surface Profiler acquires measurements in primary gaze while the sMap3D captures images during primary, superior, and inferior gaze directions and combines the three images to generate a single stitched map with greater surface coverage [
      • DeNaeyer G.
      • Sanders D.R.
      • Farajian T.S.
      Surface coverage with single vs. multiple gaze surface topography to fit scleral lenses.
      ,
      • Jesus D.A.
      • Kedzia R.
      • Iskander D.R.
      Precise measurement of scleral radius using anterior eye profilometry.
      ,
      • Iskander D.R.
      • Wachel P.
      • Simpson P.N.
      • Consejo A.
      • Jesus D.A.
      Principles of operation, accuracy and precision of an Eye Surface Profiler.
      ]. Fluorophotometry scleral topographers are reliable for clinical applications based on reports of repeatability, accuracy, and precision [
      • DeNaeyer G.
      • Sanders D.R.
      sMap3D corneo-scleral topographer repeatability in scleral lens patients.
      ]. The Corneo Scleral Profile software integrated in the Scheimpflug system allows the assessment of the ocular surface up to 18 mm horizontally and 17 mm vertically and displays consistent scleral elevation measurements over a 16 mm chord [
      • Sindt C.W.
      • Lay B.
      • Danno R.
      Repeatability and validation of scheimpflug scleral data.
      ].

      6.2 Optical coherence tomography

      Anterior segment OCT has a range of applications in modern scleral lens practice and complements diagnostic lens fitting [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ]. OCT imaging is often used to determine the initial diagnostic lens (based on measures of corneal sagittal height [
      • Ritzmann M.
      • Caroline P.J.
      • Börret R.
      • Korszen E.
      An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses.
      ,
      • Sorbara L.
      • Maram J.
      • Fonn D.
      • Woods C.
      • Simpson T.
      Metrics of the normal cornea: anterior segment imaging with the Visante OCT.
      ,
      • Sorbara L.
      • Maram J.
      • Mueller K.
      Use of the VisanteTM OCT to measure the sagittal depth and scleral shape of keratoconus compared to normal corneae: pilot study.
      ], or to refine the limbal curves or landing zone based on the corneoscleral or scleral angle. During the fitting process, cross-sectional images allow quantification of the fluid reservoir or scleral lens thickness [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      Scleral contact lens thickness profiles: the relationship between average and centre lens thickness.
      ] and en face imaging can be used to determine optic zone centration or rotation relative to the pupil centre or assess anterior lens surface wettability [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ]. OCT imaging has also been used to quantify the ocular response to scleral lens wear with respect to corneal oedema, tissue compression beneath the landing zone [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ,
      • Courey C.
      • Courey G.
      • Michaud L.
      Conjunctival inlapse: nasal and temporal conjuctival shape variations associated with scleral lens wear.
      ], and fluid reservoir debris [
      • Carracedo G.
      • Serramito‐Blanco M.
      • Martin‐Gil A.
      • Wang Z.
      • Rodriguez‐Pomar C.
      • Pintor J.
      Post‐lens tear turbidity and visual quality after scleral lens wear.
      ]. It should be noted when performing OCT imaging through a rigid contact lens, thickness measurements will be slightly underestimated due to the difference between the refractive index of lens material and the refractive index assumed by the instrument software [
      • Ramasubramanian V.
      • Glasser A.
      Distortion correction of Visante optical coherence tomography cornea images.
      ,
      • Lin R.C.
      • Shure M.A.
      • Rollins A.M.
      • Izatt J.A.
      • Huang D.
      Group index of the human cornea at 1.3-μm wavelength obtained in vitro by optical coherence domain reflectometry.
      ].

      6.3 Anterior segment photography

      The slit-lamp biomicroscope can be used in conjunction with various systems or accessories [
      • Kaya A.
      Ophthoselfie: detailed self-imaging of cornea and anterior segment by smartphone.
      ,
      • Jalil M.
      • Ferenczy S.R.
      • Shields C.L.
      iPhone 4s and iPhone 5s imaging of the eye.
      ,
      • McLean C.J.
      • Tossounis C.M.
      • Saleh G.M.
      Camera adapter for anterior segment slitlamp photography.
      ,
      • Nagra M.
      • Huntjens B.
      Smartphone ophthalmoscopy: patient and student practitioner perceptions.
      ,
      • Huntjens B.
      • Basi M.
      • Nagra M.
      Evaluating a new objective grading software for conjunctival hyperaemia.
      ] to capture images of anterior segment conditions for documentation, monitoring treatment progression, teaching, medico-legal purposes [
      • Barsam A.
      • Bhogal M.
      • Morris S.
      • Little B.
      Anterior segment slitlamp photography using the iPhone.
      ], or contact lens fitting. Recording and storing the images of the anterior segment offers the possibility to refer to prior visits [
      • Wolffsohn J.S.
      • Naroo S.A.
      • Christie C.
      • Morris J.
      • Conway R.
      • Maldonado-Codina C.
      Anterior eye health recording.
      ] and to improve patient communication about their condition. Images can be sent electronically to other eye care practitioners to aid with diagnosis [
      • Barsam A.
      • Bhogal M.
      • Morris S.
      • Little B.
      Anterior segment slitlamp photography using the iPhone.
      ] or to lens manufacturers to troubleshoot fitting challenges with specific lens designs. The fluid reservoir thickness can also be reliably estimated using anterior segment photography and basic image analysis software [
      • Macedo-de-Araújo R.J.
      • Amorim-de-Sousa A.
      • van der Worp E.
      • González-Méijome J.M.
      Clinical findings and ocular symptoms over 1 year in a sample of scleral Lens wearers.
      ]. Slit lamp photography is recommended when significant corneal epitheliopathy is present since fluorescein staining should improve with scleral lens wear [
      • Schornack M.M.
      • Pyle J.
      • Patel S.V.
      Scleral lenses in the management of ocular surface disease.
      ]. If epitheliopathy is worse than baseline images on subsequent examination, a lens design change or additional therapeutic intervention may be indicated [
      • Baldwin B.
      Contemporary scleral lenses: theory and application.
      ].

      6.4 Specular microscopy

      Specular microscopy is a non-invasive procedure that utilises specular reflection to evaluate corneal endothelial cell morphology [
      • Böhnke M.
      • Masters B.R.
      Confocal microscopy of the cornea.
      ]. The parameters obtained from specular microscopy include cell size, endothelial cell density, the coefficient of variation, defined as the ratio between the standard deviation of cell sizes and the average cell size, indicative of polymegethism, and the hexagonal cell index, defined as the percentage of 6-sided cells, indicative of pleomorphism [
      • McCarey B.E.
      • Edelhauser H.F.
      • Lynn M.J.
      Review of corneal endothelial specular microscopy for FDA clinical trials of refractive procedures, surgical devices and new intraocular drugs and solutions.
      ]. Coefficient of variation and hexagonal cell index are thought to provide a better indication of how the cornea responds under stress than the endothelial cell density [
      • Mannis M.J.
      Cornea.
      ]. Normative values for the corneal endothelium cell density in middle aged adults are >2700 cells/mm2, coefficient of variation of 0.27 and a hexagonal cell index >60 % [
      • McCarey B.E.
      • Edelhauser H.F.
      • Lynn M.J.
      Review of corneal endothelial specular microscopy for FDA clinical trials of refractive procedures, surgical devices and new intraocular drugs and solutions.
      ] (see also CLEAR Anatomy Report [
      • Downie L.E.
      • Bandlitz S.
      • Bergmanson J.P.G.
      • Craig J.P.
      • Dutta D.
      • Maldonado-Codina C.
      • et al.
      CLEAR - anatomy and physiology of the anterior eye.
      ]. However, endothelial cell morphology varies regionally, with a greater endothelial cell density and more regular cell shape in the mid-peripheral and peripheral cornea compared to centrally [
      • Amann J.
      • Holley G.P.
      • Lee S.-B.
      • Edelhauser H.F.
      Increased endothelial cell density in the paracentral and peripheral regionsof the human cornea.
      ,
      • Tavazzi S.
      • Cozza F.
      • Colciago S.
      • Zeri F.
      Slit-lamp based assessment of peripheral versus central regions of the human corneal endothelium.
      ]. In addition, a limitation of specular microscopy is that the procedure only images one side of an endothelial cell [
      • Bergmanson J.
      Histopathological analysis of corneal endothelial polymegethism.
      ].
      Corneal endothelial morphology is influenced by short [
      • Doughty M.J.
      • Aakre B.M.
      • Ystenaes A.E.
      • Svarverud E.
      Short-term adaptation of the human corneal endothelium to continuous wear of silicone hydrogel (lotrafilcon A) contact lenses after daily hydrogel lens wear.
      ,
      • Giasson C.J.
      • Rancourt J.
      • Robillard J.
      • Melillo M.
      • Michaud L.
      Corneal endothelial blebs induced in scleral Lens wearers.
      ,
      • Ohya S.
      • Nishimaki K.
      • Nakayasu K.
      • Kanai A.
      Non-contact specular microscopic observation for early response of corneal endothelium after contact lens wear.
      ] and long-term contact lens wear [
      • Ahmad A.
      • Mohd-Ali B.
      • Ishak B.
      Changes in the morphology of corneal endothelial cells in young myopic adults after 6 months of wearing soft contact lenses: a Malaysian perspective.
      ,
      • Bourne W.M.
      The effect of long-term contact lens wear on the cells of the cornea.
      ,
      • Chang S.-W.
      • Hu F.-R.
      • Lin L.L.-K.
      Effects of contact lenses on corneal endothelium–a morphological and functional study.
      ,
      • Dogan C.
      • Hagverdiyeva S.
      • Mergen B.
      • Iskeleli G.
      Effect of the rigid gas-permeable contact Lens use on the endothelial cells in patients with keratoconus.
      ,
      • Doughty M.J.
      An observational cross-sectional study on the corneal endothelium of medium-term rigid gas permeable contact lens wearers.
      ,
      • Erickson P.
      • Doughty M.J.
      • Comstock T.L.
      • Cullen A.P.
      Endothelial cell density and contact lens-induced corneal swelling.
      ,
      • Esgin H.
      • Erda N.
      Endothelial cell density of the cornea during rigid gas permeable contact lens wear.
      ,
      • Hiraoka T.
      • Furuya A.
      • Matsumoto Y.
      • Okamoto F.
      • Kakita T.
      • Oshika T.
      Influence of overnight orthokeratology on corneal endothelium.
      ,
      • Lee J.
      • Park W.S.
      • Lee S.H.
      • Oum B.S.
      • Cho B.M.
      A comparative study of corneal endothelial changes induced by different durations of soft contact lens wear.
      ,
      • Leem H.S.
      • Lee K.J.
      • Shin K.C.
      Central corneal thickness and corneal endothelial cell changes caused by contact lens use in diabetic patients.
      ,
      • Mohd-Ali B.
      • Chen L.Y.
      The morphology of corneal endothelial cells in long term soft contact lens wearers in Kuala Lumpur.
      ,
      • Tsubota K.
      • Hata S.
      • Toda I.
      • Yagi Y.
      • Sakata M.
      • Shimazaki J.
      Increase in corneal epithelial cell size with extended wear soft contact lenses depends on continuous wearing time.
      ,
      • Walline J.J.
      • Lorenz K.O.
      • Nichols J.J.
      Long-term contact lens wear of children and teens.
      ,
      • Wiffen S.J.
      • Hodge D.O.
      • Bourne W.M.
      The effect of contact lens wear on the central and peripheral corneal endothelium.
      ]. Endothelial blebs have also been noted after short-term scleral lens wear [
      • Giasson C.J.
      • Rancourt J.
      • Robillard J.
      • Melillo M.
      • Michaud L.
      Corneal endothelial blebs induced in scleral Lens wearers.
      ]. Age [
      • Yee R.W.
      • Matsuda M.
      • Schultz R.O.
      • Edelhauser H.F.
      Changes in the normal corneal endothelial cellular pattern as a function of age.
      ], diabetes [
      • Goldstein A.S.
      • Janson B.J.
      • Skeie J.M.
      • Ling J.J.
      • Greiner M.A.
      The effects of diabetes mellitus on the corneal endothelium: a review.
      ], penetrating keratoplasty [
      • Lass J.H.
      • Sugar A.
      • Benetz B.A.
      • Beck R.W.
      • Dontchev M.
      • Gal R.L.
      • et al.
      Endothelial cell density to predict endothelial graft failure after penetrating keratoplasty.
      ,
      • Patel S.V.
      • Hodge D.O.
      • Bourne W.M.
      Corneal endothelium and postoperative outcomes 15 years after penetrating keratoplasty.
      ], and Fuchs’ endothelial dystrophy [
      • Zhang J.
      • Patel D.V.
      The pathophysiology of Fuchs’ endothelial dystrophy–a review of molecular and cellular insights.
      ] also result in a reduction in endothelial cell density, an increase in polymegethism and pleomorphism, and the potential for increased corneal oedema due to scleral lens induced hypoxic stress.
      Evaluation of the corneal endothelium prior to scleral lens fitting, especially in post-keratoplasty eyes, where a reduction in endothelial cell density is common [
      • Lass J.H.
      • Sugar A.
      • Benetz B.A.
      • Beck R.W.
      • Dontchev M.
      • Gal R.L.
      • et al.
      Endothelial cell density to predict endothelial graft failure after penetrating keratoplasty.
      ], can provide some insight into how the cornea may respond to hypoxic stress. Periodic evaluation of the corneal endothelium is indicated in overnight contact lens wear, the use of contact lenses with low oxygen transmissibility, and in corneal endothelial abnormalities [
      • Sonsino J.
      • Eiden B.
      • Kojima R.
      Instrumentation.
      ]. Currently, there are no established thresholds for specular microscopy metrics that would contraindicate scleral lens wear, and a trial lens fitting can be used to assess the corneal response (see Section 10.2.2.1 and CLEAR Medical Uses Report [
      • Jacobs D.S.
      • Carrasquillo K.G.
      • Cottrell P.D.
      • Fernandez-Velasquez F.J.
      • Gil-Cazorla R.
      • Jalbert I.
      • et al.
      CLEAR - medical uses of contact lenses.
      ]).

      7. Prescribing scleral lenses

      7.1 Empirical lens fitting

      Scleral lens parameters are generally determined through lens assessment on eye using a diagnostic fitting set. However, the recent development of technology to reliably quantify the scleral profile allows for empirical scleral lens design and fitting. Empirical rigid corneal lens fitting using corneal topography has a number of advantages including reduced chair time with higher first fit success rate [
      • Ruston D.M.
      The challenge of fitting astigmatic eyes: rigid gas-permeable toric lenses.
      ], greater initial patient satisfaction [
      • Kwong H.
      • Gundel R.
      Empirical fitting with polycon II lenses.
      ], and optimised lens fitting [
      • van der Worp E.
      • de Brabander J.
      • Lubberman B.
      • Marin G.
      • Hendrikse F.
      Optimising RGP lens fitting in normal eyes using 3D topographic data.
      ]. Empirical fitting may also be a safer approach, since diagnostic multi-use lenses are not required, and the first lens applied as part of the fitting process is customised to the patient’s eye [
      • Sindt C.
      • Bennett E.
      • Szczotka-Flynn L.
      • Sclafani L.
      • Barnett M.
      Technical Report: Guidelines for Handling of Multipatient Contact Lenses in the Clinical Setting.
      ]. This circumvents lens handling by multiple people, the handling of contaminated materials, and the risk of potential infection.
      When fitting empirically, the required scleral lens back vertex power can be determined by applying a rigid corneal lens of known power and back optic zone radius and performing an over-refraction, estimated from previously worn rigid corneal lenses, or using the manifest refraction and central corneal curvature. Alternatively, an over refraction can be performed using the initial empirically designed lens. However, a second pair of lenses may be required.

      7.1.1 Corneoscleral profilometry

      Corneoscleral profilometry provides a variety of important quantitative anatomical data to guide initial scleral lens selection including the ocular sagittal height profile, scleral and corneal (limbal) asymmetry, and conjunctival irregularities. Based on the data collected, customised software provides the parameters of the optimal first lens to apply on the eye from a database of lens designs or allows total customisation. Currently, no studies have investigated the agreement between empirically designed scleral lenses and the final optimal fitting lens based on an in-vivo assessment. However, several case studies have highlighted the utility of scleral topography for landing zone customisation [
      • DeNaeyer G.
      • Sanders D.R.
      sMap3D corneo-scleral topographer repeatability in scleral lens patients.
      ,
      • Piñero D.P.
      • Soto-Negro R.
      Anterior eye profilometry-guided scleral contact lens fitting in keratoconus.
      ].

      7.1.2 Optical coherence tomography

      The elevation profile of the cornea and sclera obtained through OCT imaging has been used to aid initial lens selection, assess landing zone alignment, and customise scleral lens peripheral curves. Ocular surface height data obtained using an OCT over a 16 mm chord has been utilised to design customised scleral lenses for complex ocular shapes when an optimal fit could not be achieved with spherical diagnostic lenses [
      • Gemoules G.
      A novel method of fitting scleral lenses using high resolution optical coherence tomography.
      ]. Scleral height data along the horizontal and vertical meridians has also been used to estimate the required landing zone toricity, however there was only a modest association between the OCT derived scleral toricity and the toricity of the optimal fitting scleral lens [
      • Le H.-G.T.
      • Tang M.
      • Ridges R.
      • Huang D.
      • Jacobs D.S.
      Pilot study for OCT guided design and fit of a prosthetic device for treatment of corneal disease.
      ].

      7.1.3 Impression based fitting

      Impression based fitting is a valuable empirical scleral lens fitting technique, particularly for highly irregular corneal or scleral shapes (e.g. an atypical corneal profile, significant ocular surface asymmetry, or conjunctival elevations due to ocular pathology or surgery) [
      • Harthan Js.
      EyePrintPRO fitting for a patient with refractory ocular surface disease and a glaucoma drainage device.
      ,
      • Nguyen M.T.
      • Thakrar V.
      • Chan C.C.
      EyePrintPRO therapeutic scleral contact lens: indications and outcomes.
      ,
      • McMahon J.
      • Fiden B.
      Scleral lens challenges.
      ]. The process of obtaining an ocular impression has minimal impact on corneal shape [
      • Miller D.
      • Holmberg A.
      • Carroll J.
      • Exford J.
      • Boyd M.
      Effect of impression taking on the shape of the cornea in scleral lens fitting.
      ]. However, in order to obtain a reliable impression of the ocular surface scleral lens wear must be ceased for a period of time to allow recovery of the conjunctival shape. The minimum amount of time out of scleral lenses to allow full tissue recovery is unknown, but is likely dependent upon the duration of lens wear, landing zone design, and conjunctival and scleral tissue properties. Impression based technology allows close alignment of the scleral lens to the anatomy of the anterior ocular surface.
      The EyePrint-PRO™ platform utilises a scan of an ocular impression to generate an impression based back surface scleral lens design. These scans of the ocular impression are similar to corneoscleral profile measurements obtained using Scheimpflug imaging [
      • Sclafani L.
      • Slater D.
      • Lay B.
      • Sindt C.W.
      Topographic elevation data to design scleral lenses.
      ]. A link to external software is available to design a customized lens. Currently, only Pentacam® Corneo Scleral Profile and Eye Surface Profiler data can be exported into ScanFitPro™ software to generate a highly customised scleral lens.

      7.2 Diagnostic Lens fitting

      While empirical scleral lens fitting based on corneal and scleral elevation and curvature can aid lens customisation and reduce the number of lenses manufactured to arrive at an optimal fit, the lens must still be assessed in vivo. Practitioners should understand the rationale and evidence underpinning the selection of specific parameters for an initial diagnostic scleral lens and how to modify these parameters to achieve an optimal fitting lens.
      When selecting an initial diagnostic lens, it is recommended to first consider the overall lens diameter, sagittal depth, and posterior lens surface profile. Unlike rigid corneal lenses, the back optic zone radius is not critical to obtain a reasonable fit during diagnostic fitting since the anterior corneal curvature only weakly correlates with the back optic zone radius of the final optimal fitting scleral lens [
      • Schornack M.M.
      • Patel S.V.
      Relationship between corneal topographic indices and scleral lens base curve.
      ]. Many manufacturers diagnostic lens kits include a suite of lenses with a wide range of sagittal depths and back optic zone radius, 1–3 different overall diameters, and 1–2 posterior lens profiles (prolate or oblate). Once these initial parameters have been established, modifications can be made to further optimise the lens fit (limbal and peripheral curves, landing zone) and visual performance (front surface asphericity, front surface toric, multifocal).

      7.2.1 Overall lens diameter

      The selection of the overall lens diameter is influenced by the corneal diameter and sagittal depth, ocular condition, eyelid morphometry, presence of conjunctival anomalies, and patient dexterity. The spread of overall lens diameters prescribed by practitioners follows an approximate normal distribution centred around 16 mm; <15 mm: 18 %, 15−17 mm: 65 %, >17 mm: 17 % [
      • Harthan J.
      • Nau C.B.
      • Barr J.
      • Nau A.
      • Shorter E.
      • Chimato N.T.
      • et al.
      Scleral lens prescription and management practices: the SCOPE study.
      ,
      • Vincent S.J.
      • Kowalski L.P.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      The influence of centre thickness on miniscleral lens flexure.
      ]. The overall lens diameter is typically larger for the therapeutic treatment of ocular surface disease compared to visual rehabilitation for corneal ectasia. However, smaller lens designs can also aid corneal rehabilitation [
      • Porcar E.
      • Montalt J.C.
      • España-Gregori E.
      • Peris-Martínez C.
      Fitting scleral lenses less than 15 mm in diameter: a review of the literature.
      ] and as corneal ectasia advances and the required sagittal depth of the lens increases, a larger diameter lens (with a larger landing zone) may also be required to enhance lens stability and provide a larger region of bearing for support [
      • Fadel D.
      Modern scleral lenses: mini versus large.
      ]. Larger overall lens diameters also often require a toric or customised landing zone since scleral elevation asymmetry increases further from the limbus [
      • Ritzmann M.
      • Caroline P.J.
      • Börret R.
      • Korszen E.
      An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses.
      ,
      • Walker M.K.
      • Schornack M.M.
      • Vincent S.J.
      Anatomical and physiological considerations in scleral lens wear: conjunctiva and sclera.
      ,
      • Consejo A.
      • Llorens‐Quintana C.
      • Bartuzel M.M.
      • Iskander D.R.
      • Rozema J.J.
      Rotation asymmetry of the human sclera.
      ,
      • Abass A.
      • Lopes B.T.
      • Eliasy A.
      • Salomao M.
      • Wu R.
      • White L.
      • et al.
      Artefact-free topography based scleral-asymmetry.
      ].
      Ideally the posterior lens surface must not contact the limbus, so the overall diameter should exceed the corneal diameter or horizontal visible iris diameter by 1.5–2 mm [
      • Van der Worp E.
      A guide to scleral lens fitting.
      ,
      • Bergmanson J.P.
      • Martinez J.G.
      Size does matter: what is the corneo‐limbal diameter?.
      ]. On average, the adult cornea is elliptical in shape (0.2 to 0.4 mm wider horizontally than vertically [
      • Hall L.A.
      • Young G.
      • Wolffsohn J.S.
      • Riley C.
      The influence of corneoscleral topography on soft contact lens fit.
      ,
      • Hall L.A.
      • Hunt C.
      • Young G.
      • Wolffsohn J.
      Factors affecting corneoscleral topography.
      ,
      • Read S.A.
      • Collins M.J.
      • Carney L.G.
      The influence of eyelid morphology on normal corneal shape.
      ] and a 0.5 mm difference between meridians may influence lens centration and the fluid reservoir profile for a standard spherical back surface profile [
      • Fadel D.
      The influence of limbal and scleral shape on scleral lens design.
      ]. Scleral lenses with an elliptical optic zone and overall lens diameter can be produced, dependent on the manufacturer, and typically requires a toric or customised landing zone or prism ballast for stabilisation.

      7.2.2 Lens sagittal depth

      To vault the anterior corneal surface, the sagittal depth of the lens must exceed the corneal sagittal height at the same chord diameter (Fig. 3). Theoretically this relates to the location of the proximal edge of the landing zone, where the lens first contacts the conjunctiva (i.e. the primary functional lens diameter) rather than the overall lens diameter). The sagittal depth at this location has been termed the functional sagittal lens depth [
      • Michaud L.
      • Lipson M.
      • Kramer E.
      • Walker M.
      The official guide to scleral lens terminology.
      ]. Ideally, a standardised approach to quantify and report the primary functional lens diameter and sagittal depth would simplify diagnostic lens fitting and allow a more direct comparison between various lens designs [
      • Michaud L.
      • Lipson M.
      • Kramer E.
      • Walker M.
      The official guide to scleral lens terminology.
      ].
      Fig. 3
      Fig. 3This image shows a scleral lens vaulting the anterior corneal surface. The sagittal height of the lens is exceeding the ocular sagittal height calculated at the same chord diameter (15.79 mm). Image credit Daddi Fadel.
      Because the primary functional lens diameter (or landing zone width) may not be known, the corneal sagittal height measured at a chord equal to the overall lens diameter is an appropriate starting point to estimate the required depth of the lens [
      • Macedo-de-Araújo R.J.
      • Amorim-de-Sousa A.
      • Queirós A.
      • van der Worp E.
      • González-Méijome J.M.
      Relationship of placido corneal topography data with scleral lens fitting parameters.
      ]. This can be done using anterior segment OCT, scleral profilometry, or extrapolating corneal height data obtained using videokeratoscopy over a smaller chord length, allowing for addition sagittal depth to ensure adequate corneal vault. For example, an average increase in corneal sagittal depth of ∼2000 μm from a 10 mm to a 15 mm chord diameter has been reported in healthy young adults [
      • Ritzmann M.
      • Caroline P.J.
      • Börret R.
      • Korszen E.
      An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses.
      ]. Therefore, the initial lens sagittal depth required for ∼14−17 mm diameter lenses could be estimated by adding ∼2000 μm (and the desired initial fluid reservoir thickness) to the corneal height data measured at a 10 mm chord using a topographer. Manufacturers typically recommend an initial diagnostic lens within a diagnostic fitting set based on an estimate of corneal sagittal height or corneal condition.

      7.2.3 Back surface profile

      The asphericity of the posterior lens surface can be modified to improve the alignment with the anterior cornea and create a thinner and more uniform fluid reservoir. A prolate back surface describes a standard lens design with a back optic zone radius steeper than the adjacent transition curve (a gradual flattening from the centre of the optic zone), while an oblate (reverse geometry) design has a flatter back optic zone radius relative to the adjacent transition curve. Oblate designs are typically indicated in post-surgical conditions in which the central cornea is substantially flatter than the periphery [
      • Gemoules G.
      Therapeutic effects of contact lenses after refractive surgery.
      ,
      • Gemoules G.
      • Morris K.M.
      Rigid gas-permeable contact lenses and severe higher-order aberrations in postsurgical corneas.
      ]. A prolate back surface design fitted to an oblate cornea will provide central corneal vault; however mid-peripheral corneal bearing may be present. Oblate back surface designs can also be utilised in advanced prolate ectasia, to generate a more minus powered tear layer, and reduce the need for a high minus powered scleral lens, which will reduce lens mass and overall thickness.

      7.2.4 Fluid reservoir thickness

      7.2.4.1 Central or apical vault

      The apical vault refers to the fluid reservoir thickness at the location of greatest corneal elevation. In a normal cornea, this is typically located near the geometric centre, however in corneal ectasia (e.g. keratoconus and pellucid marginal degeneration) the apex is often displaced inferiorly [
      • DeNaeyer G.
      • Sanders D.R.
      • Michaud L.
      • Morrison S.
      • Walker M.
      • Jedlicka J.
      • et al.
      Correlation of corneal and scleral topography in cases with ectasias and normal corneas.
      ], whereas following corneal surgery (e.g. post-penetrating keratoplasty) the peak elevation is in the mid-periphery, near the graft/host interface [
      • Anand V.
      Postkeratoplasty contact lens fitting.
      ]. In corneas with an atypical corneal elevation profile it can be challenging to achieve a uniform post lens tear layer (and in some cases, impossible, without an impression-based lens design). The central vault can be modified by altering the sagittal depth of the lens, the back-optic zone radius, and the transition curves between the optic and landing zone.
      The optimal initial central post-lens fluid reservoir thickness is one that does not adversely affect corneal physiology or optical performance throughout long-term lens wear. That is, the lens has sufficient clearance that the posterior lens surface does not bear on the corneal epithelium towards the end of the day (or in the long-term), but not an excessive amount that results in lens tilt or decentration. The recommended target central fluid reservoir thickness varies between lens manufacturers, ranging from approximately 300–500 μm immediately after application to 100–300 μm after settling [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ,
      • Harthan J.
      • Shorter E.
      • Nau C.
      • Nau A.
      • Schornack M.M.
      • Zhuang X.
      • et al.
      Scleral lens fitting and assessment strategies.
      ].
      Another physiological consideration regarding the fluid reservoir thickness is oxygen availability to the cornea in a sealed system with no tear exchange after lens settling. A positive trend between increasing central corneal vault and inflammatory markers within the fluid reservoir has been reported [
      • Postnikoff C.K.
      • Pucker A.D.
      • Laurent J.
      • Huisingh C.
      • McGwin G.
      • Nichols J.J.
      Identification of leukocytes associated with midday fogging in the post-lens tear film of scleral contact lens wearers.
      ], and several theoretical models have estimated less oxygen delivery (greater oedema) with increasing central corneal vault [
      • Michaud L.
      • Van Der Worp E.
      • Brazeau D.
      • Warde R.
      • Giasson C.J.
      Predicting estimates of oxygen transmissibility for scleral lenses.
      ,
      • Kim Y.H.
      • Tan B.
      • Lin M.C.
      • Radke C.J.
      Central corneal edema with scleral-lens wear.
      ,
      • Jaynes J.M.
      • Edrington T.B.
      • Weissman B.A.
      Predicting scleral GP lens entrapped tear layer oxygen tensions.
      ,
      • Weissman B.A.
      • Ye P.
      Calculated tear oxygen tension under contact lenses offering resistance in series: piggyback and scleral lenses.
      ,
      • Compan V.
      • Oliveira C.
      • Aguilella-Arzo M.
      • Mollá S.
      • Peixoto-de-Matos S.C.
      • González-Méijome J.M.
      Oxygen diffusion and edema with modern scleral rigid gas permeable contact lenses.
      ,
      • Compañ V.
      • Aguilella-Arzo M.
      • Edrington T.B.
      • Weissman B.A.
      Modeling corneal oxygen with scleral gas permeable lens wear.
      ], however this appears to be overestimated for open and closed eye conditions in healthy young adults [
      • Fisher D.
      • Collins M.J.
      • Vincent S.J.
      Post-lens fluid reservoir thickness and corneal edema during open eye scleral lens wear.
      ,
      • Fisher D.
      • Collins M.J.
      • Vincent S.J.
      Fluid reservoir thickness and corneal oedema during closed eye scleral lens wear.
      ]. Relatively small differences in oxygen uptake [
      • Giasson C.J.
      • Morency J.
      • Melillo M.
      • Michaud L.
      Oxygen tension beneath scleral lenses of different clearances.
      ] and corneal oedema have been observed in healthy eyes fitted with high Dk scleral lenses of varying central vault [
      • Fisher D.
      • Collins M.J.
      • Vincent S.J.
      Post-lens fluid reservoir thickness and corneal edema during open eye scleral lens wear.
      ,
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The time course and nature of corneal oedema during sealed miniscleral contact lens wear.
      ], although these differences may become clinically relevant in eyes with compromised endothelial function.
      For modern sealed scleral lens designs, the central fluid reservoir decreases by ∼100 to 200 μm over 8 h [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ,
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The temporal dynamics of miniscleral contact lenses: central corneal clearance and centration.
      ,
      • Kauffman M.J.
      • Gilmartin C.A.
      • Bennett E.S.
      • Bassi C.J.
      A comparison of the short-term settling of three scleral lens designs.
      ,
      • Esen F.
      • Toker E.
      Influence of apical clearance on mini-scleral lens settling, clinical performance, and corneal thickness changes.
      ] as the lens settles back in to the conjunctiva [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ]. Consequently, an initial central reservoir thickness of 100 μm or less will be insufficient for some patients [
      • Harthan J.
      • Nau C.B.
      • Barr J.
      • Nau A.
      • Shorter E.
      • Chimato N.T.
      • et al.
      Scleral lens prescription and management practices: the SCOPE study.
      ]. The reduction in central vault follows an exponential decay, with ∼50 % of the total settling observed after 30 min of lens wear, which stabilises after ∼2−4 h [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The temporal dynamics of miniscleral contact lenses: central corneal clearance and centration.
      ,
      • Kauffman M.J.
      • Gilmartin C.A.
      • Bennett E.S.
      • Bassi C.J.
      A comparison of the short-term settling of three scleral lens designs.
      ,
      • Esen F.
      • Toker E.
      Influence of apical clearance on mini-scleral lens settling, clinical performance, and corneal thickness changes.
      ,
      • Courey C.
      • Michaud L.
      Variation of clearance considering viscosity of the solution used in the reservoir and following scleral lens wear over time.
      ]. The magnitude and time course of lens settling does not vary with the fluid used to fill the scleral bowl (e.g. preservative free saline or a viscous gel) [
      • Courey C.
      • Michaud L.
      Variation of clearance considering viscosity of the solution used in the reservoir and following scleral lens wear over time.
      ], which suggests that the tissue beneath the landing zone provides the majority of support to the lens. The extent of lens settling varies to some extent with different diameter lenses [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ], and the lens design and scleral profile are additional confounding factors. Some studies have reported a greater reduction in central fluid reservoir (in μm) for lenses fitted with greater initial central vault [
      • Esen F.
      • Toker E.
      Influence of apical clearance on mini-scleral lens settling, clinical performance, and corneal thickness changes.
      ,
      • Rathi V.M.
      • Mandathara P.S.
      • Dumpati S.
      • Sangwan V.S.
      Change in vault during scleral lens trials assessed with anterior segment optical coherence tomography.
      ,
      • Otchere H.
      • Jones L.W.
      • Sorbara L.
      Effect of time on scleral lens settling and change in corneal clearance.
      ], but when this is converted to a percentage reduction in vault, the extent of settling is similar [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      Regional variations in postlens tear layer thickness during scleral Lens Wear.
      ]. Compared to modern high Dk sealed scleral systems, early channelled or fenestrated lenses were intentionally fitted with a thinner fluid reservoir after settling (50−70 μm) [
      • Woodward E.G.
      Preformed scleral lens fitting techniques.
      ,
      • Ruben M.
      • Trodd T.
      Analysis of scleral lenses fitted to keratoconus patients.
      ], to mobilise air bubbles.
      Few studies have quantified the extent of longer-term scleral lens settling. Based on clinical observation without any quantification, an early report in the 1950’s Bier [
      • Bier N.
      Contact lens routine and practice.
      ] suggested that larger diameter lenses (>20 mm) took up to 8 weeks to fully settle back into the conjunctiva (with longer daily wearing time decreasing the time to achieve full settling). For smaller diameter (16.5 mm) modern scleral lens design, a mean settling of ∼150 μm after one month of wear has been reported [
      • Mountford J.
      Scleral contact lens settling rates.
      ]. In another prospective study, a further reduction in central fluid reservoir thickness of ∼20 μm (7 %) between 1 and 2 months of lens wear was observed when controlling for the duration of lens wear at each visit [
      • Macedo-de-Araújo R.J.
      • Amorim-de-Sousa A.
      • van der Worp E.
      • González-Méijome J.M.
      Clinical findings and ocular symptoms over 1 year in a sample of scleral Lens wearers.
      ]. Longer-term scleral lens settling is difficult to predict and is likely influenced by a range of factors including conjunctival and scleral tissue properties, lens diameter, and design [
      • Walker M.K.
      • Schornack M.M.
      • Vincent S.J.
      Anatomical and physiological considerations in scleral lens wear: conjunctiva and sclera.
      ].

      7.2.4.2 Limbal vault

      Similarly, the posterior lens surface must vault the limbus during lens wear to avoid mechanical insult [
      • Nixon A.D.
      • Barr J.T.
      • VanNasdale D.A.
      Corneal epithelial bullae after short-term wear of small diameter scleral lenses.
      ], oedema, peripheral neovascularisation, potential compromise of limbal stem cells, and corneal conjunctivalisation [
      • Dua H.S.
      • Azuara-Blanco A.
      Limbal stem cells of the corneal epithelium.
      ]. In some instances, appropriately fitted scleral lenses can be a therapeutic treatment for limbal stem cell deficiency [
      • Kim K.H.
      • Deloss K.S.
      • Hood C.T.
      Prosthetic replacement of the ocular surface ecosystem (PROSE) for visual rehabilitation in limbal stem cell deficiency.
      ,
      • Schornack M.M.
      Limbal stem cell disease: management with scleral lenses.
      ], and should be considered prior to surgical interventions [
      • Deng S.X.
      • Kruse F.
      • Gomes J.A.
      • Chan C.C.
      • Daya S.
      • Dana R.
      • International Limbal Stem Cell Deficiency Working Group
      Global consensus on the management of limbal stem cell deficiency.
      ]. Few studies have reported on the reduction of limbal fluid reservoir thickness throughout lens wear. Limbal settling of ∼50 to 120 μm (70–84 %) has been reported after 3 h of lens wear for a range of initial central and limbal reservoir thickness values which resulted in a settled limbal vault of 10−50 μm [
      • Fisher D.
      • Collins M.J.
      • Vincent S.J.
      Post-lens fluid reservoir thickness and corneal edema during open eye scleral lens wear.
      ]. In another study [
      • Kaya A.
      Ophthoselfie: detailed self-imaging of cornea and anterior segment by smartphone.
      ], limbal settling was similar for lenses fitted with low (∼160 μm) or high (∼200 μm) initial limbal vault after 2 h of wear (∼34 μm or a 20 % reduction on average) and greater comfort was reported for lenses fitted with greater limbal vault [
      • Yeung D.
      • Murphy P.J.
      • Sorbara L.
      Objective and subjective evaluation of clinical performance of scleral Lens with varying limbal clearance in keratoconus.
      ]. The limbal vault can be varied by modifying various lens parameters dependent upon the design and manufacturer, for example; the sagittal depth of the lens, the overall lens diameter, or the curvature or tangent of the transition zone. Manufacturer recommendations for limbal clearance following lens settling vary between 50 and 100 μm [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ].

      7.3 Other lens considerations

      When selecting a rigid lens material, important considerations include oxygen permeability, mechanical properties in relation to flexure, and scratch resistance and surface wettability, keeping in mind the wearing modality (e.g. daily or overnight wear) and refractive error [
      • Fatt I.
      • Ruben C.M.
      Oxygen permeability of contact lens materials: a 1993 update.
      ]. The ideal lens material will have a high Dk and a low contact angle to optimise wettability and oxygenation to the cornea. However, increasing the Dk of a sealed scleral lens beyond 100 has minimal effect on oedema in young healthy eyes and may improve comfort [
      • Dhallu S.
      • Trave-Huarte S.
      • Bilkhu P.
      • Boychev N.
      • Wolffsohn J.S.
      Effect of Scleral Lens Oxygen Permeability on Corneal Physiology.
      ]. A range of rigid lens coatings have been developed in recent years that may be of benefit in regards if wettability to scleral lens wearers.

      7.3.1 Oxygen permeability

      Scleral lens materials range in Dk value from approximately 88–180 with centre thickness from 250 microns to greater than 500 microns [
      • Jedlicka J.
      Initial lens selection. Contemporary scleral lenses: theory and application Sharjah.
      ]. Table 2 [
      • Fadel D.
      Scleral lens complications: their recognition, etiology, and management.
      ] shows the relationship between the material wetting angle and Dk.
      Highly oxygen permeable rigid materials (∼≥100 Dk) provide increased oxygen transmission and result in less bacterial adhesion to the corneal epithelium following overnight lens wear [
      • Ren D.H.
      • Petroll W.M.
      • Jester J.V.
      • Ho-Fan J.
      • Cavanagh H.D.
      The relationship between contact lens oxygen permeability and binding of Pseudomonas aeruginosa to human corneal epithelial cells after overnight and extended wear.
      ] compared to lower Dk materials, and may reduce the possibility of adverse hypoxic complications. They are therefore an excellent choice for hyperopic prescriptions (due to the increased lens centre thickness). However, higher Dk materials are less scratch resistant [
      • Tranoudis I.
      • Efron N.
      Scratch resistance of rigid contact lens materials.
      ] and therefore may require more frequent replacement [
      • Jones L.
      • Woods C.A.
      • Efron N.
      Life expectancy of rigid gas permeable and high water content contact lenses.
      ]. Small variations in optical quality with different rigid lens materials have been measured in-vitro [
      • Domínguez‐Vicent A.
      • Esteve‐Taboada J.J.
      • Ferrer‐Blasco T.
      • García‐Lázaro S.
      • Montés‐Micó R.
      Optical quality comparison among different Boston contact lens materials.
      ], although the magnitude of these variations in higher order aberrations would have minimal impact on visual performance compared to other factors during lens wear such as lens centration [
      • Atchison D.A.
      Aberrations associated with rigid contact lenses.
      ] or wettability.

      7.3.2 Wettability

      Rigid lens surface wettability describes how well water (an approximation of the tear film) spreads across or adheres to the anterior lens surface, and affects surface deposition, patient comfort, and vision [
      • Bourassa S.
      • Benjamin W.
      Clinical findings correlated with contact angles on rigid gas permeable contact lens surfaces in vivo.
      ]. Wettability is influenced by a range of factors including the presence of manufacturing residue [
      • Macmillan T.F.
      • Benjamin W.J.
      New RGP wettability-strategies for cleaning and wet storage.
      ], the quantity, quality, and chemistry of the tear film, blinking efficiency, conditioning solution [
      • Chowhan M.A.
      • Asgharian B.
      • Fontana F.
      In vitro comparison of soaking solutions for rigid gas-permeable contact lenses.
      ] and the lens material [
      • Shirafkan A.
      • Woodward E.G.
      • Port M.J.
      • Hull C.C.
      Surface wettability and hydrophilicity of soft contact lens materials, before and after wear.
      ].
      There has been limited research concerning rigid lens wettability published over the past two decades. Recent studies have focused on plasma treatments, a process in which the lens surface is ionised with oxygen plasma to create a hyper-clean, hydrophilic rigid surface. A plasma polymer [
      • Yasuda H.
      • Bumgarner M.
      • Marsh H.
      • Yamanashi B.
      • Devito D.
      • Wolbarsht M.
      • et al.
      Ultrathin coating by plasma polymerization applied to corneal contact lens.
      ] applied to PMMA corneal lenses was a different (although similarly named) approach to improve lens wettability in the 1970s. In-vitro studies suggest that plasma treatment can increase wettability (decrease the wetting angle) by 40 % [
      • Shin H.S.J.
      • J. K
      • Kwon Y.S.
      • Mah K.C.
      Surface modification of rigid gas permeable contact lens treated by using a low-temperature plasma in air.
      ], and decreases bacterial adhesion [
      • Wang Y.
      • Qian X.
      • Zhang X.
      • Xia W.
      • Zhong L.
      • Sun Z.
      • et al.
      Plasma surface modification of rigid contact lenses decreases bacterial adhesion.
      ]. However, the treatment is not permanent, and benefits of increased lens wettability and comfort decrease after weeks of lens wear [
      • Fonn D.
      Targeting contact lens induced dryness and discomfort: what properties will make lenses more comfortable.
      ]. The use of an ultraviolet laser system to generate a hydrophilic rough texture rigid lens surface has also been proposed, and in-vitro data shows a reduced contact angle (improved wettability) both with and without plasma treatment [
      • Tsai H.-Y.
      • Hsieh Y.-C.
      • Lin Y.-H.
      • Chang H.-C.
      • Tang Y.-H.
      • Huang K.-C.
      Fabrication of hydrophilic surface on rigid gas permeable contact lenses to enhance the wettability using ultraviolet laser system.
      ].
      Another approach to increase rigid lens surface wettability is through the application of a polyethylene glycol polymer coating following a plasma treatment [
      • Sato T.
      • Kobayashi K.
      • Tanigawa H.
      • Uno K.
      The effect of the poly (ethylene glycol) chain on surface exchange of rigid gas-permeable contact lenses.
      ]. In-vitro studies suggest this process can significantly increase rigid lens surface wettability (the contact angle decreased by 50 %) [
      • Sato T.
      • Kobayashi K.
      • Tanigawa H.
      • Uno K.
      The effect of the poly (ethylene glycol) chain on surface exchange of rigid gas-permeable contact lenses.
      ], reduce protein and lipid deposition [
      • Sato T.
      • Kobayashi K.
      • Tanigawa H.
      • Uno K.
      The effect of the poly (ethylene glycol) chain on surface exchange of rigid gas-permeable contact lenses.
      ], with no clinically significant effect upon the optical properties of scleral lenses [
      • Hastings G.D.
      • Zanayed J.Z.
      • Nguyen L.C.
      • Applegate R.A.
      • Marsack J.D.
      Do Polymer Coatings Change the Aberrations of Conventional and Wavefront-guided Scleral Lenses?.
      ]. In-vitro studies of polyethylene glycol coatings applied to PMMA have also demonstrated improved wettability and decreased bacterial adhesion [
      • Ko J.
      • Cho K.
      • Han S.W.
      • Sung H.K.
      • Baek S.W.
      • Koh W.-G.
      • et al.
      Hydrophilic surface modification of poly (methyl methacrylate)-based ocular prostheses using poly (ethylene glycol) grafting.
      ]. However, no clinical studies of performance of this type of polymer coating have been published. As polyethylene glycol coatings gradually diminish with daily lens handling and cleaning, it is important to recommend specific solutions for polyethylene glycol coated scleral lenses to prolong the life of the coating (e.g. using non-abrasive cleaners rather than alcohol-based or abrasive cleaners). While recent in-vitro studies suggest there are several different approaches to improve rigid material wettability by modifying or treating the lens surface, in-vivo studies of the performance of such lenses are still required to understand their clinical utility.
      Poor wettability typically presents as a greasy anterior lens surface and is associated with discomfort, reduced wearing time, and disturbed vision [
      • Fadel D.
      • Toabe M.
      Scleral lens issues and complications related to handling, care and compliance.
      ]. Patients with ocular surface disease (e.g. blepharitis, Sjøgren’s disease, ocular rosacea or graft versus host disease) require treatment prior to and during scleral lens wear to aid wettability. If lens wettability is compromised at delivery, the cause is likely due to laboratory related issues (substances transferred during the manufacturing process or by manipulation). If initial lens wettability was adequate and wettability issues arise following a period of lens wear, patient lens handling and hygiene should be reviewed since the use of oily soaps or makeup prior to lens application may affect wettability along with protein and lipid deposition [
      • Tonge S.
      • Jones L.
      • Goodall S.
      • Tighe B.
      The ex vivo wettability of soft contact lenses.
      ].
      If poor lens wettability persists, the lens may need to be removed and periodically and cleaned before reapplication. The lens may be conditioned whilst on the eye using a cotton swab or removal plunger, moistened with rigid lens conditioning multipurpose disinfecting solution. In some extreme cases, a daily disposable soft contact lens has been applied to the anterior scleral lens surface to improve scleral lens wettability. Overall, practitioners must consider material selection, the use of additional lens coatings, appropriate solutions, and patient lens handling and maintenance to optimise anterior lens surface wettability.

      8. Scleral lens evaluation

      The general principles of scleral lens fitting are that they rest on the conjunctiva, avoid corneal and limbal touch, with centration around the cornea and minimal movement [
      • Van der Worp E.
      A guide to scleral lens fitting.
      ,
      • Barnett M.
      • Fadel D.
      ]. During diagnostic lens fitting, initial evaluation of the fluid reservoir immediately after lens application (filling the scleral lens with preservative free saline and sodium fluorescein) is recommended to assess the central vault and check for bubbles, confirm adequate placement, and estimate corneal vault. If the initial central vault appears excessive (e.g. > 500 μm) or insufficient (e.g. < 100 μm), a diagnostic lens with a different sagittal height should be applied. If application bubbles are present, the lens must be removed and reapplied. Once a diagnostic lens with appropriate initial central clearance has been applied (e.g. 200–400 μm), the lens should be allowed to settle for 30 min prior to assessing the fit and performing an over-refraction. Approximately 50 % of the lens settling and decentration observed after ∼8 h will occur during this period [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The temporal dynamics of miniscleral contact lenses: central corneal clearance and centration.
      ]. Using biomicroscopy initially, an “in-out” approach can be used to examine the fit from the lens centre to the edge, assessing the fluid reservoir thickness from the apex to the limbus, conjunctival alignment (conjunctival bearing and vasculature) and edge profile in all quadrants, and the need for customisation (e.g. notches, localised vaulting).

      8.1 Assessing fluid reservoir thickness

      The fluid reservoir thickness can be estimated by comparing the extent of vault between the anterior cornea and the posterior lens surface with the thickness of the scleral lens using biomicroscopy. To estimate vault using a slit lamp, a white light optic section should be used with the illumination source positioned 45 degrees from the axis of observation. The thickness of the fluid reservoir (preservative free saline with sodium fluorescein) is then compared to the thickness of the scleral lens (Fig. 4).
      Fig. 4
      Fig. 4Slit lamp assessment of the liquid reservoir thickness with (A and B) and without (C and D) sodium fluorescein comparing with the lens thickness. Note how the lens thickness varies across the lens (centrally, inferiorly, and superiorly). Image credit Daddi Fadel.
      While many practitioners use this approach [
      • Harthan J.
      • Shorter E.
      • Nau C.
      • Nau A.
      • Schornack M.M.
      • Zhuang X.
      • et al.
      Scleral lens fitting and assessment strategies.
      ], there is a tendency to subjectively overestimate the extent of vault (with [
      • Yeung D.
      • Sorbara L.
      Scleral lens clearance assessment with biomicroscopy and anterior segment optical coherence tomography.
      ] and without [
      • Fuller D.G.
      • Chan N.
      • Smith B.
      Neophyte skill judging corneoscleral lens clearance.
      ] sodium fluorescein in the fluid reservoir), particularly for lower reservoir thickness values [
      • Fuller D.G.
      • Chan N.
      • Smith B.
      Neophyte skill judging corneoscleral lens clearance.
      ]. If the centre lens thickness is used as a reference, it should be noted that the actual lens centre thickness can vary by ±100 μm from the ordered thickness and still be within the manufacturing tolerance [
      • Standardization IOf
      Ophthalmic optics - contact lenses. Part 2: tolerances.
      ]. The lens thickness profile also varies substantially across the lens (Fig. 4) [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      Scleral contact lens thickness profiles: the relationship between average and centre lens thickness.
      ] which can make assessing mid-peripheral and limbal vault challenging (Fig. 5).
      Fig. 5
      Fig. 5Slit lamp assessment of a diagnostic scleral lens applied to an eye with keratoconus. Estimation of apical (A), mid-peripheral (B), and limbal vault (C) with sodium fluorescein filled fluid reservoir. Note the asymmetry in the fluid reservoir thickness in (A), common in keratoconus. Estimation of apical (D), mid-peripheral (E), and limbal vault (F) without sodium fluorescein. Landing zone assessment under low (G) and high magnification (H). Assessing the mid-peripheral and limbal vault is challenging since the lens thickness in that area is different than center lens thickness. Image credit Maria Walker.
      Estimation of limbal clearance can be difficult with sodium fluorescein since the human eye can only detect fluorescence of tear layers at least 15−20 μm thick (see CLEAR Evidence-based Practice Report [
      • Wolffsohn J.
      • Dumbleton K.
      • Huntjens B.
      • Kandel H.
      • Koh S.
      • Kunnen C.M.E.
      • et al.
      CLEAR - evidence based contact lens practice.
      ]. To reliably quantify the fluid reservoir thickness, anterior segment OCT [
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Optical coherence tomography and scleral contact lenses: clinical and research applications.
      ] or Scheimpflug imaging can be used [
      • Schornack M.M.
      • Nau C.B.
      Changes in optical density of postlens fluid reservoir during 2 hours of scleral lens wear.
      ] (Fig. 6). If automated imaging is not available, practitioners must be able to estimate vault without sodium fluorescein in the fluid reservoir.
      Fig. 6
      Fig. 6Image of a scleral lens with anterior segment OCT. Image credit Tom Arnold.

      8.2 Assessing the landing zone

      The landing zone should be assessed using a slit lamp with low illumination and a wide beam, beginning with low magnification (Fig. 5G) and increasing magnification as needed in each quadrant (Fig. 5H) to evaluate blood vessel impingement, blanching, and edge lift. It is also important to evaluate the conjunctival appearance after lens removal, looking for features such as rebound hyperaemia, compression, and staining. Diffuse white light should be used for hyperaemia assessment on biomicroscopy, and lissamine green is recommended to stain devitalised cells caused by excessive compression.
      Tissue compression during scleral lens wear is primarily superficial (conjunctival and episcleral tissue [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ]) and varies with anatomical location, wearing time, lens design, and potentially the instrument used (e.g. OCT or scleral topography) [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ,
      • Consejo A.
      • Behaegel J.
      • Van Hoey M.
      • Iskander D.R.
      • Rozema J.J.
      Scleral asymmetry as a potential predictor for scleral lens compression.
      ,
      • Courey C.
      • Michaud L.
      Variation of clearance considering viscosity of the solution used in the reservoir and following scleral lens wear over time.
      ]. Recovery of tissue compression has only been reported in one study, with 50 % recovery observed 3 h after lens removal [
      • Alonso-Caneiro D.
      • Vincent S.J.
      • Collins M.J.
      Morphological changes in the conjunctiva, episclera and sclera following short-term miniscleral contact lens wear in rigid lens neophytes.
      ]. The optimal time without scleral lens wear to allow full recovery from lens induced compression prior to obtaining scleral topography or an impression of the ocular surface is unknown.
      At an aftercare visit, once a lens has been worn for several hours, vital dyes can be helpful to evaluate the alignment of the landing zone with the underlying conjunctiva. Applied over the top of the lens, sodium fluorescein or lissamine green that enters the landing zone can highlight subtle regions of edge lift and may be used as a gross evaluation of tear exchange for a settled lens. Pingueculae, pterygia, or a glaucoma drainage device may become inflamed after several hours of lens wear, thus landing zone customisation may be required (e.g. a localised notch or vault to avoid tissue compression) [
      • Walker M.K.
      • Schornack M.M.
      • Vincent S.J.
      Anatomical and physiological considerations in scleral lens wear: conjunctiva and sclera.
      ].

      8.3 Centration and movement

      Horizontal and vertical lens decentration can be quantified using a slit lamp graticule, or imaging techniques such as videokeratoscopy [
      • Vincent S.J.
      • Collins M.J.
      A topographical method to quantify scleral contact lens decentration.
      ] and OCT [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The temporal dynamics of miniscleral contact lenses: central corneal clearance and centration.
      ]. En face imaging also allows the measurement of lens rotation [
      • Ticak A.
      • Marsack J.D.
      • Koenig D.E.
      • Ravikumar A.
      • Shi Y.
      • Nguyen L.C.
      • et al.
      A comparision of three methods to increase scleral contact Lens on-eye stability.
      ]. An optimally fitting sealed scleral lens should exhibit minimal movement on blinking or eye movements. However, fenestrated lenses are slightly more mobile due to reduced suction forces. Excessive movement may occur with excessive central vault or poor landing zone alignment. Recent studies report greater mean lens decentration inferiorly than temporally ranging from ∼0.1 to 1 mm horizontally and ∼0.2 to 1.7 mm vertically [
      • Vincent S.J.
      • Collins M.J.
      A topographical method to quantify scleral contact lens decentration.
      ,
      • Sabesan R.
      • Johns L.
      • Tomashevskaya O.
      • Jacobs D.S.
      • Rosenthal P.
      • Yoon G.
      Wavefront-guided scleral lens prosthetic device for keratoconus.
      ,
      • Marsack J.D.
      • Ravikumar A.
      • Nguyen C.
      • Ticak A.
      • Koenig D.E.
      • Elswick J.D.
      • et al.
      Wavefront-guided scleral lens correction in keratoconus.
      ,
      • Kowalski L.P.
      • Collins M.J.
      • Vincent S.J.
      Scleral lens centration: the influence of centre thickness, scleral topography, and apical clearance.
      ,
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The temporal dynamics of miniscleral contact lenses: central corneal clearance and centration.
      ,
      • Ticak A.
      • Marsack J.D.
      • Koenig D.E.
      • Ravikumar A.
      • Shi Y.
      • Nguyen L.C.
      • et al.
      A comparision of three methods to increase scleral contact Lens on-eye stability.
      ]. However, these values will vary significantly based on the landing zone design, scleral topography, apical clearance, and wearing time.

      8.4 Review schedule

      The frequency of aftercare visits will vary depending on the patient and the severity and stability of the underlying condition. The first scheduled aftercare is recommended 1–3 weeks after the initial dispense to allow for a gradual build-up of wearing time. Established wearers may require 3, 4, 6–12 month reviews based on the severity of the condition and potential for changes or progression. Aftercare visits should be scheduled after several hours of lens wear, so the extent of lens settling can be observed, and potential complications such as midday fogging, corneal oedema, and elevated IOP can be screened for, in addition to routine aftercare procedures (Fig. 7). Documentation of the lens parameters, lens fit, and ocular health after lens removal is critical in the follow-up of patients. When possible, anterior segment photography should be used for photographic documentation of ocular health.
      Fig. 7
      Fig. 7Image of midday fogging in the reservoir of a scleral lens. Image credit Tom Arnold.

      9. Scleral lens challenges

      9.1 Optical performance

      In general, scleral lenses provide similar visual outcomes to rigid corneal lenses [
      • Levit A.
      • Benwell M.
      • Evans B.J.W.
      Randomised controlled trial of corneal vs. Scleral rigid gas permeable contact lenses for keratoconus and other ectatic corneal disorders.
      ]. High-contrast visual acuity assessment alone does not adequately reflect the visual enhancement by scleral lenses on irregular corneas [
      • Hussoin T.
      • Le H.-G.
      • Carrasquillo K.G.
      • Johns L.
      • Rosenthal P.
      • Jacobs D.S.
      The effect of optic asphericity on visual rehabilitation of corneal ectasia with a prosthetic device.
      ]. For example, other metrics such as low-contrast visual acuity, night vision assessment, aberrometry and subjective visual disturbance can vary significantly between different patients with similar high contrast visual acuity [
      • Macedo-de-Araújo R.J.
      • Faria-Ribeiro M.
      • McAllinden C.
      • van der Worp E.
      • González-Méijome J.M.
      Optical quality and visual performance for one year in a sample of scleral Lens wearers.
      ].
      As corneal disease progresses, scleral lenses may be required to obtain a stable fit and, in such cases, can yield superior visual acuity [
      • Kumar P.
      • Bandela P.K.
      • Bharadwaj S.R.
      Do visual performance and optical quality vary across different contact lens correction modalities in keratoconus?.
      ]. Scleral lenses typically decentre infero-temporally [
      • Visser E.-S.
      • Van der Linden B.J.
      • Otten H.M.
      • Van der Lelij A.
      • Visser E.-S.
      Medical applications and outcomes of bitangential scleral lenses.
      ,
      • Otten H.M.
      • van der Linden B.J.
      • Visser E.-S.
      Clinical performance of a new bitangential mini-scleral lens.
      ,
      • Vincent S.J.
      • Collins M.J.
      A topographical method to quantify scleral contact lens decentration.
      ,
      • Kowalski L.P.
      • Collins M.J.
      • Vincent S.J.
      Scleral lens centration: the influence of centre thickness, scleral topography, and apical clearance.
      ,
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      The temporal dynamics of miniscleral contact lenses: central corneal clearance and centration.
      ,
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      • Beanland A.
      • Lam L.
      • Lim C.C.
      • et al.
      Hypoxic corneal changes following eight hours of scleral contact lens wear.
      ] due to the effects of gravity, eyelid morphology, and scleral topography. Inferior decentration can induce a small base down prismatic effect due to an asymmetric fluid reservoir [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      Regional variations in postlens tear layer thickness during scleral Lens Wear.
      ], which can be problematic for unilateral lens wearers [
      • Vincent S.J.
      • Fadel D.
      Optical considerations for scleral contact lenses: a review.
      ], and create residual higher order aberrations [
      • Atchison D.A.
      Aberrations associated with rigid contact lenses.
      ]. Residual astigmatism may also arise due to lens flexure, typically associated with thinner lenses with rotationally symmetric landing zone fitted on a non-spherical sclera [
      • Vincent S.J.
      • Kowalski L.P.
      • Alonso-Caneiro D.
      • Kricancic H.
      • Collins M.J.
      The influence of centre thickness on miniscleral lens flexure.
      ]. These optical effects can be minimised by reducing the extent of decentration by fitting a toric or customised landing zone [
      • Ticak A.
      • Marsack J.D.
      • Koenig D.E.
      • Ravikumar A.
      • Shi Y.
      • Nguyen L.C.
      • et al.
      A comparision of three methods to increase scleral contact Lens on-eye stability.
      ] and reducing apical clearance [
      • Kowalski L.P.
      • Collins M.J.
      • Vincent S.J.
      Scleral lens centration: the influence of centre thickness, scleral topography, and apical clearance.
      ]. Transient changes in corneal optics have also been observed following scleral lens removal [
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      • Beanland A.
      • Lam L.
      • Lim C.C.
      • et al.
      Hypoxic corneal changes following eight hours of scleral contact lens wear.
      ,
      • Kumar M.
      • Shetty R.
      • Lalgudi V.G.
      • Vincent S.J.
      Scleral lens wear following penetrating keratoplasty: changes in corneal curvature and optics.
      ,
      • Vincent S.J.
      • Alonso-Caneiro D.
      • Collins M.J.
      Corneal changes following short-term miniscleral contact lens wear.
      ,
      • Serramito-Blanco M.
      • Carpena-Torres C.
      • Carballo J.
      • Piñero D.
      • Lipson M.
      • Carracedo G.
      Anterior corneal curvature and aberration changes after scleral lens wear in keratoconus patients with and without ring segments.
      ,
      • Vincent S.J.
      • Alonso‐Caneiro D.
      • Collins M.J.
      Miniscleral lens wear influences corneal curvature and optics.
      ,
      • Soeters N.
      • Visser E.S.
      • Imhof S.M.
      • Tahzib N.G.
      Scleral lens influence on corneal curvature and pachymetry in keratoconus patients.
      ], which can have implications for monitoring corneal disease. Front surface lens wettability also influences visual performance during scleral lens wear (see section 7.3.2).

      9.2 Injury and infection (abrasion and microbial keratitis)

      A small number of case series suggest that microbial keratitis (MK) does occur with scleral lens wear [
      • Severinsky B.
      • Behrman S.
      • Frucht-Pery J.
      • Solomon A.
      Scleral contact lenses for visual rehabilitation after penetrating keratoplasty: long term outcomes.
      ,
      • Zimmerman A.B.
      • Marks A.
      Microbial keratitis secondary to unintended poor compliance with scleral gas-permeable contact lenses.
      ,