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Queensland University of Technology (QUT), Centre for Vision and Eye Research, School of Optometry and Vision Science, Contact Lens and Visual Optics Laboratory, Brisbane, Australia
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.
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.
]. 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 [
], and while preformed ground glass lenses addressed this to some extent, the challenge of corneal oedema (Sattler’s veil or Fick’s phenomenon) remained [
] 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 [
], 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 [
]. 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 [
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.
]. 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 [
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) [
]. 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 [
] and the 2017 ISO standards for 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. 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.
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 [
]. 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) [
]. 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 [
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 % [
]. Further potential customisations include notches or localised vaulting to avoid conjunctival anomalies, fenestrations to reduce suction and aid lens removal [
], 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) [
] (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 [
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.
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 [
].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 [
]. For example, patients fitted with toric landing zones report improved comfort, increased wearing time and overall satisfaction, and better optical and visual outcomes [
] 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 [
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 [
], the sclera shows greater irregularity. Additionally, in keratoconus, the axis of greatest scleral asymmetry appears to align with the cone location [
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 [
]. 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 [
In vivo assessment of the anterior scleral contour assisted by automatic profilometry and changes in conjunctival shape after miniscleral contact lens fitting.
]. 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 [
], 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 [
]. 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 [
]. 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 [
], 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 [
] 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 [
]. 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 [
]. 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 [
] and to improve patient communication about their condition. Images can be sent electronically to other eye care practitioners to aid with diagnosis [
] 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 [
]. Slit lamp photography is recommended when significant corneal epitheliopathy is present since fluorescein staining should improve with scleral lens wear [
]. If epitheliopathy is worse than baseline images on subsequent examination, a lens design change or additional therapeutic intervention may be indicated [
]. 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 [
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 [
]. 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 % [
Review of corneal endothelial specular microscopy for FDA clinical trials of refractive procedures, surgical devices and new intraocular drugs and solutions.
]. 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 [
Short-term adaptation of the human corneal endothelium to continuous wear of silicone hydrogel (lotrafilcon A) contact lenses after daily hydrogel lens wear.
] 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 [
], 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 [
]. 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 [
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 [
]. 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 [
]. 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 [
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 [
]. 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 [
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) [
]. 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 [
]. 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 [
]. 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 % [
]. 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 [
] 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 [
]. Larger overall lens diameters also often require a toric or customised landing zone since scleral elevation asymmetry increases further from the limbus [
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 [
] and a 0.5 mm difference between meridians may influence lens centration and the fluid reservoir profile for a standard spherical back surface profile [
]. 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 [
]. 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 [
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 [
]. 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 [
]. 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 [
]. 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 [
]. 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 [
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 [
]. 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 [
]. 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) [
], 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 [
], 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 [
]. 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) [
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 [
] 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 [
]. 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 [
]. 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 [
]. 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 [
], 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 [
]. 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 [
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 [
]. 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 [
] 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 [
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 [
], 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 [
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 [
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 [
] 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 % [
]. 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 [
Another approach to increase rigid lens surface wettability is through the application of a polyethylene glycol polymer coating following a plasma treatment [
]. 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 [
]. 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 [
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 [
]. 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 [
]. 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. 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.
]. 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 [
] which can make assessing mid-peripheral and limbal vault challenging (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 [
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 [
]. 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) [
]. 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 [
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