If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
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
Centre for Ocular Research & Education (CORE), School of Optometry and Vision Science, University of Waterloo, Waterloo, Ontario, CanadaCentre for Eye and Vision Research (CEVR), Hong Kong Special Administrative Region
Orthokeratology (ortho-k) is the process of deliberately reshaping the anterior cornea by utilising specialty contact lenses to temporarily and reversibly reduce refractive error after lens removal. Modern ortho-k utilises reverse geometry lens designs, made with highly oxygen permeable rigid materials, worn overnight to reshape the anterior cornea and provide temporary correction of refractive error. More recently, ortho-k has been extensively used to slow the progression of myopia in children.
This report reviews the practice of ortho-k, including its history, mechanisms of refractive and ocular changes, current use in the correction of myopia, astigmatism, hyperopia, and presbyopia, and standard of care. Suitable candidates for ortho-k are described, along with the fitting process, factors impacting success, and the potential options for using newer lens designs. Ocular changes associated with ortho-k, such as alterations in corneal thickness, development of microcysts, pigmented arcs, and fibrillary lines are reviewed. The safety of ortho-k is extensively reviewed, along with an overview of non-compliant behaviours and appropriate disinfection regimens. Finally, the role of ortho-k in myopia management for children is discussed in terms of efficacy, safety, and potential mechanisms of myopia control, including the impact of factors such as initial fitting age, baseline refractive error, the role of peripheral defocus, higher order aberrations, pupil size, and treatment zone size.
Orthokeratology (ortho-k) is the process of deliberately reshaping the anterior cornea by utilising specialty contact lenses to temporarily and reversibly reduce refractive error after lens removal [
]. Ortho-k originated in the 1950’s when eyecare practitioners (ECPs) observed changes in corneal curvature and refractive error in some patients wearing flat-fitting rigid corneal contact lenses [
]. Today, modern ortho-k utilises reverse geometry lens designs, made with highly oxygen permeable rigid materials, worn overnight to reshape the anterior cornea and provide temporary correction of refractive error.
This report reviews the practice of ortho-k, including its history, mechanisms of refractive and ocular changes, current use in the correction of myopia, astigmatism, hyperopia, and presbyopia, and standard of care. The role of ortho-k in myopia management for children is discussed in terms of efficacy, safety, and potential mechanisms of myopia control. Ultimately, the goal is to provide ECPs evidence-based guidance on ortho-k, to offer the best care for children with progressing myopia. To conclude, the potential future of ortho-k is considered.
2. Historical overview
At the International Society of Contact Lens Specialists meeting in 1962, Jessen described his technique of “ortho-focus” as a rigid polymethyl methacrylate (PMMA) contact lens moulding the cornea over months of daily wear so that the patient would not rely on corrective lenses to see clearly [
]. Over the next two decades, academics researched the validity of daily wear ortho-k. Kerns, at the University of Houston, formalised a clinical study in 1976 [
]. Their conclusions were in agreement: daily PMMA ortho-k lens wear was safe, but was of limited clinical benefit due to the slow onset and relatively small and temporary reduction of myopia [
]. Following these four publications, minimal research was conducted over the next decade, possibly reflecting a waning interest in ortho-k and due to commonly encountered fitting challenges, such as lens decentration and induced astigmatism [
In the late 1980's, Wlodyga and Stoyan developed the first true reverse geometry rigid corneal contact lenses, with a back optic zone radius flatter than the adjacent peripheral curve [
]. With the advent of reverse geometry lens designs, approval for overnight wear in high oxygen permeability (Dk) rigid materials (for the correction of myopia), computerised numerical controlled lathes, and sophisticated topographers which accurately quantify corneal changes, ortho-k lenses are now worn overnight to provide clear, unaided vision during the day. Since the effect is temporary, the lenses need to be worn as a retainer, usually every night.
For over a century, ECPs have considered various methods to slow the progression of myopia [
] reported no refractive progression in seven to 19-year-old children with previously steadily progressing myopia who wore flat-fitting rigid corneal lenses (∼2.00 D flatter than the flattest corneal meridian) for at least 18 h a day. Since the late 1990's, ortho-k has been perceived by ECPs to be one of the most effective methods of myopia control [
], a prospective single-arm intervention study suggested that ortho-k had the capacity to modulate myopia progression in children, relative to a historical control group of single-vision distance spectacle lens wearers [
]. This was followed by numerous peer-reviewed studies, including randomised and controlled clinical trials (Section 8.3 and Table 1), describing reductions in myopia progression in children and the myopia control effect of ortho-k for up to two years follow-up [
HM: significant difference in axial elongation between groups in year 1 only. OK: Euclid lenses with correction up to -6.00D. Residual myopia in HM group corrected with SVS.
A – refractive astigmatism; AL – axial length; BL – baseline; D – dioptres; LM – low myopia; MM – moderate myopia; HM – high myopia; MC – myopia control (difference in mean change in axial length between groups in mm and as percentage of change in control group); m – months; n – sample size; OK – orthokeratology; RC – reverse curve; SD – standard deviation; SCL – soft contact lenses; SER – spherical equivalent refraction; SVS – single-vision spectacles.
Statistical comparison between the ortho-k and control groups at the final study time point (unless otherwise specified) for axial elongation over the entire study period (* p ≤ 0.05, # p ≤ 0.01, ^ p ≤ 0.001, X p > 0.05, NR not reported).
Modern ortho-k lenses are typically approved by regulatory bodies for overnight wear for the temporary correction of refractive error of up to ∼6.00 D of myopia, and 1.75 D of astigmatism. Currently, no lenses have been approved for the correction of hyperopia or presbyopia, and only two ortho-k lens series (Bloom Night [Menicon] (up to 4.00 D myopia and ≤1.50 D with-the-rule astigmatism and ≤0.75 D against-the-rule astigmatism) and Paragon CRT [CooperVision] (up to 6.00 D myopia and 1.75 D astigmatism) have been granted marketing authorisation (CE approval) for myopia control in Europe. Therefore, the use of ortho-k for myopia control is considered “off-label” (prescribing a licensed product outside of the approved scope of treatment) for the vast majority of ortho-k lens designs in most countries. Eye care practitioners are encouraged to prescribe on-label products if available and appropriate and consider off-label alternatives if on-label approaches are not effective [
]; myopia up to approximately 4.50 D, corneal or refractive astigmatism up to ∼3.00 D, and a pupil diameter less than ∼6.00 mm in dim illumination to minimise symptoms associated with post-treatment elevated higher order aberrations (HOAs). Soft lens wearers who suffer from lens related discomfort or dryness [
] may also benefit from ortho-k, and the typical requirements and contraindications for commencing contact lens wear apply (e.g. a healthy ocular surface, eyelids, tear film) (see CLEAR Evidence-based Practice Report) [
]. Refractive correction of hyperopia and presbyopia is possible with ortho-k, but the outcomes are less predictable and highly variable compared to the correction of myopia (see Section 4.1.1.1).
3.2 Spherical lens designs
Ortho-k lens designs consist of four or five zones including [
]; a central back optic zone radius, a reverse curve adjacent to and steeper than the back optic zone radius, one or two alignment curves that have the greatest influence on lens centration and movement, and a final peripheral curve that provides axial edge lift (Fig. 1) (for toric lens designs see Section 3.4).
Fig. 1Spherical orthokeratology lens in situ. (A) optic zone, (B) reverse curve, (C) alignment curve/s, (D) peripheral curve. (Courtesy PolyU, HK and Jason Lau).
For the correction of myopia, the back optic zone radius is fitted flatter than the flattest corneal meridian by the desired amount of refractive correction (and an additional refractive buffer of ∼0.75 D [the Jessen or compression factor] to account for daily regression in corneal flattening). Ortho-k lenses are fitted with apical clearance ∼≤10 μm for myopic corrections and do not bear on the central cornea. Fig. 2 demonstrates the post-lens tear layer thickness profile for ortho-k lens designs with different target refractive corrections. Conversely, for the correction of hyperopia and presbyopia, the central back optic zone radius is fitted steeper than the flat K.
Fig. 2Top: Post-lens tear layer thickness profile for a four zone orthokeratology lens design for myopia. (A) optic zone, (B) reverse curve, (C) alignment curve, (D) peripheral curve. Bottom: Variation in post-lens tear layer thickness profile for orthokeratology lens designs targeting different refractive errors. Note: the width of the reverse and alignment curves vary between these lens designs for myopic and hyperopic corrections.
The development of computer systems has allowed topographers to analyse thousands of points on the corneal surface, reconstruct the corneal shape, and chart them as color-coded maps to aid ECP interpretation [
]. Advances in corneal topography have contributed to the resurgence of modern ortho-k. Eyecare practitioners are not only able to visualise the initial corneal shape, but also assess patient suitability for ortho-k, and design lenses. Topographers can accurately quantify changes in corneal shape, which allows the ECP to monitor lens centration, assess and troubleshoot the lens fit, as well as manage patient treatment [
An ortho-k lens can be fitted using a suite of diagnostic lenses and assessing the lens in situ. For an optimum fitting lens, the optic zone is centred on the pupil with sodium fluorescein pooling within the reverse curve, and approximately 1 mm of movement upon blinking [
], and for myopic ortho-k lens designs the central post-lens tear layer is less than this. The fit of the lens in an open eye condition may also not necessarily inform how the lens fits when worn in closed eye conditions. Consequently, in modern ortho-k practice, corneal topography (corneal height, curvature, and eccentricity data) guides initial lens selection and informs lens modifications. Therefore, obtaining reliable pre-treatment corneal topography maps that are not affected by misalignment, poor tear film quality, or eyelid artefacts are essential.
The initially selected or ordered lens is worn for an overnight trial, and the patient is reviewed the following morning, within two hours of waking, as signs of corneal oedema will likely resolve after two hours of eyelid opening. Following lens removal, ocular health is assessed (with particular attention to signs of corneal hypoxic stress and central corneal staining due to lens bearing), along with visual acuity, refractive error, and corneal topography. If this lens is considered satisfactory, it is dispensed to the patient. Corneal topography difference maps (post-lens wear minus pre-lens wear) are used to assess the location of the treatment zone (the central region of desired corneal flattening for the correction of myopia) and the magnitude of change in corneal power. Common corneal topography difference maps are displayed in Fig. 3.
Fig. 3Common tangential power difference maps (post-orthokeratology lens wear minus pre-lens wear). (A) early bullseye, (B) central island, (C) frowny face, (D) smiley face with false central island. (Courtesy PolyU, HK).
For the correction of myopia, the desired outcome is a ‘bulls-eye’ pattern over the pupil, which indicates the lens was well centred overnight and resulted in central corneal flattening and mid-peripheral corneal steepening [
]. A laterally decentred bulls-eye pattern (nasal or temporal lens decentration) may be due to an overall lens diameter that is too small, an alignment curve that is too flat, or corneal astigmatism (which may require a toric or quadrant specific back surface lens design) (see Section 3.4). A ‘smiley face’ pattern arises due to a high-riding lens, when the sagittal height of the lens is less than that of the cornea (alignment curve too flat or total diameter too small), and the sagittal height of the lens needs to be increased to ensure there is central apical clearance during overnight wear. Conversely, a ‘frowny face’ pattern is caused by a lens that decentres inferiorly due to the alignment curve being too steep, or the overall diameter too small, requiring a decrease in the sagittal height of the lens. Any lens, and therefore treatment zone, decentration can induce astigmatism and coma which adversely affect visual performance [
A ‘central island’ pattern (the opposite of a bulls-eye pattern) indicates a region of central corneal steepening surrounded by an annulus of corneal flattening (this is the intended outcome in hyperopic/presbyopic ortho-k). In ortho-k for myopia, this arises due to reverse and alignment curves that are too steep or a total diameter that is too large, and requires refitting with a lens with less apical clearance, readily diagnosed using corneal topography [
]. An incomplete bulls-eye pattern (i.e. an incomplete central circular region of flattening) can be confused for a central island at the initial aftercare visit. However, the apical power of the post-lens wear topography map will not be steeper than the pre-treatment map, and the bulls-eye pattern will become complete following continued lens wear without modification. Based on the corneal topography difference maps at aftercare visits, overnight lens wear can either continue or the lens may need to be modified to improve the lens fit with reassessment after another overnight trial. A recommended review schedule is outlined in Section 6 and Table 2.
Table 2Elements of good clinical practice in orthokeratology.
Details
Practice standard
Office equipment
•
Corneal topographer
•
Slit lamp with 40x magnification
•
Non-contact optical biometer for axial length measurement (for myopia control therapy) [
Eye care practitioners should have undergone proper ortho-k training and have a thorough understanding of ortho-k
•
Support staff should be well trained on handling emergency calls, lens handling procedures and be able to provide accurate ortho-k information
•
All staff should behave professionally when communicating with patients/adult caregivers and ensure they fully understand the information listed on the information and consent sheet
Before commencing ortho-k
Information sheet
•
Explain ortho-k benefits, potential risks, limitations, on- and off-label treatments with respect to myopia control
•
Fee schedule and refund policy
•
Importance of compliance of lens use and aftercare
•
Role and responsibilities of adult caregivers to help monitor lens use and care
1st aftercare (after delivery of every lens): early the next morning after an overnight wear, within 2 h after waking, to assess binding, and ocular health, including corneal oedema
•
Subsequent aftercare (7 days, 14 days (optional), 1 month, 3 months, and 3 or 6 monthly after commencement of lens wear thereafter): any time of the day
•
Early morning aftercare every 6 months recommended (to assess ocular health, including corneal oedema)
•
Unscheduled aftercare: in case of unexpected symptoms
Ortho-k lenses with a spherical back optic zone radius applied to a toric cornea (typically with greater than 1.50 D corneal toricity) can result in lens decentration, often infero-temporally) [
] until the development of back surface toric reverse geometry lenses in the mid to late 2000’s. Currently, the majority of toric ortho-k lenses available cater for with-the-rule astigmatism up to ∼1.75 D (with FDA approval up to 1.50 D for some lenses), and only a few lenses that correct for greater magnitudes of astigmatism (e.g. DRL [Precilens] 4.00 D at any axis, and Z-Night [Menicon] 2.50 D with-the-rule).
Toric back surface ortho-k lens designs may have a spherical [
], along with different sagittal heights and tangent angles along orthogonal meridians of the reverse, alignment, or peripheral lens curves to improve lens stabilisation and treatment zone centration. Back surface toric designs produce an elliptical shaped treatment zone which decreases in size with increasing corneal toricity [
]. For an 8 mm chord diameter, the relationship between the anterior corneal height profile and corneal toricity are linear and highly correlated (approximately 1.00 D of corneal astigmatism per 25 μm difference in corneal height) (Fig. 4). Thresholds for corneal toricity or corneal height difference to consider a toric lens design vary between manufacturers and lens designs. However, corneal toricity greater than ∼1.50 D or a corneal height difference along the principal meridians of ∼30 μm over an 8 mm chord may benefit from a back surface toric design with respect to centration [
Fig. 4Relationship between anterior corneal height difference (along principal meridians) and anterior corneal toricity for an 8 mm chord. Lines of best fit extracted from data of 305 healthy participants using Pentacam (red) and Sirius instruments (green) [
] between the anterior corneal height difference along principal meridians and back surface toricity incorporated into the lens design (also for an 8 mm chord).
] included 43 children aged six to 12 years with myopia up to 5.00 D and with-the-rule refractive astigmatism between 1.25 and 3.50 D. A fenestrated peripheral toric back surface (toric alignment curve with spherical back optic zone radius) ortho-k lens design was used and after one month of lens wear, on average refractive astigmatism had reduced by 79% (1.91 ± 0.64 to 0.40 ± 0.39 D) and corneal toricity had reduced by 44% (2.30 ± 0.51 to 1.28 ± 0.53 D). Myopia also reduced by 81% after one month (2.53 ± 1.31 to 0.41 ± 0.43 D), and since many myopic children are also astigmatic, the myopia control efficacy of this toric ortho-k lens design has also been investigated (see Section 8.3.2).
4. Ocular changes associated with orthokeratology
4.1 Corneal changes
During ortho-k, the cornea undergoes changes at the cellular level that are clinically observed as changes in thickness and topography. Unless otherwise specified, the changes reported in this section are associated with ortho-k for myopia.
4.1.1 Changes in corneal topography
Corneal topographical changes observed in modern ortho-k are believed to be due to hydraulic forces in the post-lens tear film that cause tangential stresses across the corneal epithelial surface, resulting in changes to anterior corneal shape and thickness [
4.1.1.1 Orthokeratology for hyperopia or presbyopia
Compared to ortho-k for the correction of myopia, research regarding ortho-k for hyperopia is limited. Ortho-k for hyperopia or presbyopia induces central corneal steepening surrounded by an annulus of mid-peripheral corneal flattening [
] (Fig. 5). The goal of steepening the central cornea is to increase central corneal refractive power to correct hyperopia or presbyopia (typically using a monovision approach). The time course of refractive change for mild hyperopia (+1.50 D) is similar to myopic ortho-k, with the largest refractive change taking place after the first night of lens wear and reaching the full target correction by the seventh day [
]. However, the region of central steepening is typically smaller than the region of central flattening for an equivalent myopic correction and duration of lens wear [
]. Variable results have been reported for higher refractive errors (e.g. for a +3.50 D target correction, after seven nights of lens wear participants were on average 1.50 D to 2.00 D under-corrected) [
]. Further, refractive regression of the hyperopic correction has been observed throughout the day, with greater retention after seven days of lens wear [
Fig. 5The mean change in best vision sphere (BVS) on day one and seven of overnight orthokeratology lens wear for; (A) the treatment of presbyopia in emmetropes (dashed line denotes target spherical equivalent refraction change of 2 D) [
It is hypothesised that the changes in corneal topography induced by hyperopic ortho-k are a result of para-central corneal compressive forces and a central suction force within the post-lens tear film. However, since lens fenestrations do not appear to affect the corneal response in hyperopic ortho-k, the observed corneal changes may be due to a moulding effect, where the cornea conforms to the posterior lens surface, rather than hydraulic forces [
]. Further investigation is required to understand the exact mechanism by which hyperopic ortho-k induces corneal changes.
The correction of presbyopia using ortho-k is possible through a monovision or multifocal correction. In monovision, the dominant eye is corrected for distance viewing and the non-dominant eye is corrected for near viewing. After fitting a hyperopic ortho-k lens with a target correction of +2.00 D in one eye of 13 emmetropic presbyopes, monovision provided functional near vision that did not compromise distance vision (although only a modest 1.00 D refractive change after one week of lens wear was achieved) [
Ortho-k lenses can also provide both distance and near refractive correction by reducing the treatment zone size and creating a multifocal effect. In hyperopic ortho-k, the area of central corneal steepening is surrounded by an annulus of corneal flattening creating a centre-near optical effect that increases negative spherical aberration and the depth of field [
]. For myopic ortho-k, the opposite occurs where central corneal flattening and mid-peripheral corneal steepening creates a centre-distance optical effect [
]. Further research to better understand and evaluate the clinical performance of ortho-k for the correction of presbyopia is required.
4.1.1.2 Orthokeratology for myopia
In contrast, ortho-k for the correction of myopia induces central corneal flattening surrounded by an annulus of mid-peripheral corneal steepening. This mid-peripheral steepening creates significant changes in relative peripheral refraction in the horizontal, vertical, and oblique meridians [
]. While the exact mechanism by which ortho-k lenses work to slow myopia progression is not understood, it has been hypothesised that the myopic relative peripheral refraction shift contributes to this effect [
]. Smaller magnitude corneal changes have also been reported in older (19 - 57 years) compared to younger (six to 12 years) participants, suggesting a more rapid corneal response in younger eyes [
Fig. 6(A) The mean change in apical corneal power immediately after ortho-k lens removal (AM) and 8.5 h later (PM) during the first 90 days of lens wear, for an early three zone ortho-k lens design (Contex) [
]. (B) Change in spherical equivalent refraction (SER) for lower (mean initial SER -1.5 ± 0.3 D) and higher levels of myopia (mean initial SER -3.2 ± 0.5 D) during the first month of ortho-k lens wear, along with the change in vertical treatment zone diameter (lower and higher myopia groups combined) [
]. Greater flattening has been observed in the temporal compared to the nasal sector within the central treatment zone, along with greater steepening in the temporal mid-periphery compared to the nasal mid-periphery [
]. The treatment zone diameter and shape (circle or oval) is dependent on multiple variables, such as the back optic zone diameter, baseline corneal curvature and toricity, and target refractive error [
], adapted from corneal ablation calculations in refractive surgery to epithelial thinning in ortho-k for the correction of myopia, the treatment zone diameter (TZD) is inversely related to the target refractive change (Fig. 7). Munnerlyn’s formula (below), using an assumed corneal refractive index of 1.337, provides good predictions of refractive outcomes for myopia up to 4.00 D after three months of lens wear [
Fig. 7Relationship between target refractive change, central epithelial thinning, and treatment zone diameter in orthokeratology for myopia, based on Munnerlyn’s formula, assuming a total corneal refractive index of 1.377 [
Reducing the back optic zone diameter of an ortho-k lens can reduce the treatment zone area, resulting in an annulus of greater mid-peripheral corneal refractive power and inducing a positive shift in spherical aberration [
]. Further studies are needed to determine if treatment zone size influences the myopia control efficacy of ortho-k (see Section 9.4.2).
In general, the time taken for ortho-k induced ocular changes in corneal topography and SER to regress to pre-treatment levels following the cessation of lens wear increases with increasing treatment duration and baseline level of myopia. However, large individual variations have been observed [
], on average, corneal flattening and the change in SER had regressed by ∼80 to 100% compared to pre-treatment values one week after ceasing lens wear. It should be noted that variations from the pre-treatment SER may be related to myopia progression in studies of six months or more in duration. Following longer-term lens wear in children (mean four years), residual corneal flattening was still apparent along the flatter corneal meridian (mean 0.07 mm) two weeks after ceasing lens wear, which was greater for children with more myopia prior to treatment [
]. In adults, following 12 months of ortho-k lens wear, on average, changes in SER and contrast sensitivity were not statistically different from pre-treatment values after one [
Recovery of corneal irregular astigmatism, ocular higher-order aberrations, and contrast sensitivity after discontinuation of overnight orthokeratology.
] of ceasing lens wear. However, changes in some HOAs (e.g. primary spherical aberration) may remain elevated one month after ceasing lens wear for larger pupil diameters (5 mm or greater) [
Recovery evaluation of induced changes in higher order aberrations from the anterior surface of the cornea for different pupil sizes after cessation of corneal refractive therapy.
]. Based on recovery data obtained within the first 72 h after ceasing lens wear, the regression to baseline SER appears to take longer for higher levels of baseline myopia [
Total corneal thickness changes following overnight ortho-k for myopia and hyperopia are displayed in Fig. 8.
Fig. 8Average total thickness changes across the central cornea (0 mm denotes corneal apex) on the morning of day 4 of orthokeratology treatment for myopia (red) [
]. More recent work using optical pachymetry revealed central stomal thickening following overnight hyperopic ortho-k, which resolved over the course of the day, and mid-peripheral epithelial thinning that persisted throughout the day, which likely contributes to the refractive correction [
The epithelial and total corneal thickness across the horizontal corneal meridian have been measured using a modified optical pachymeter before and after ortho-k for the correction of myopia [
]. Central corneal thickness decreased due to epithelial thinning, while the mid-peripheral cornea became thicker due to epithelial and stromal changes. Later, these results were confirmed with optical coherence tomography [
]. Stromal oedema may be partly responsible for the mid-peripheral corneal thickening related to the oxygen transmissibility of the contact lens material [
]. These analyses provide evidence that redistribution of the anterior corneal tissue, rather than an overall bending of the cornea, is responsible for the refractive effects of ortho-k. Epithelial thinning of 15 to 17 μm has been observed for a target treatment of 2.50 to 2.75 D of myopia [
]. Small meridional variations in mid-peripheral epithelial tissue changes have also been reported for toric corneas fitted with spherical ortho-k lens designs [
To better understand the cellular mechanism underlying corneal thickness changes with ortho-k, studies have investigated changes in corneal morphology. Using confocal microscopy, an increase in endothelial cell polymegethism has been reported after one year of ortho-k lens wear in 15 young adults, which reduced after one month of cessation of ortho-k, but did not return to the baseline level [
]. Similarly, the middle and posterior stromal keratocyte densities do not change significantly after one year of ortho-k. However, a decrease in anterior stromal keratocyte density has been reported, along with an increase in activated keratocytes (those with highly reflective nuclei) [
Correlation of corneal pigmented arc with wide epithelial thickness map in orthokeratology-treated children using optical coherence tomography measurements.
]. Although the composition of the pigmentation has not been confirmed, due to similarities in clinical appearance with other corneal iron depositions, it is presumed that the pigmented arc or ring is comprised of iron [
Correlation of corneal pigmented arc with wide epithelial thickness map in orthokeratology-treated children using optical coherence tomography measurements.
]. The pigmented arc typically originates in the inferior cornea, presumably in the epithelial layer, and has been observed as early as two weeks after commencing ortho-k [
]). It surrounds the treatment zone corresponding to the location of the reverse curve of the ortho-k lens (Fig. 9), and this clinically benign observation has been shown to be reversible after ceasing lens wear for approximately two months [
], but likely varies with the duration of lens wear and target refraction. The incidence and intensity of corneal pigmented arcs or rings increase with longer duration of lens wear [
] and incidence rates of over 90% have been reported in retrospective case series of Taiwanese children wearing ortho-k lenses for an average period of almost two years [
Correlation of corneal pigmented arc with wide epithelial thickness map in orthokeratology-treated children using optical coherence tomography measurements.
Fig. 9Slit lamp image of a pigmented ring in an orthokeratology lens wearer corresponding to the location of the reverse curve after one year of lens wear. The right image provided without colour to enhance visualisation of the ring. (Courtesy PolyU, HK).
In the normal cornea, fibrillary lines appear as vertically or slightly curved greyish-white fibrils located within the corneal epithelium and sub-epithelial layer [
]. Retro-illumination can be used to differentiate fibrillary lines from superficial corneal dystrophies, since corneal dystrophies are readily visualised using this technique, while fibrillary lines are not [
] (Fig. 10). It is now believed that they represent an altered sub-basal nerve plexus, probably due to epithelial neural reorganisation, an altered epithelial migratory pattern or corneal biomechanical stress induced by ortho-k lenses [
Such changes have also been reported using corneal confocal microscopy, with a redistribution of the sub-basal nerve plexus away from the treatment zone centre (Fig. 11), and a reduction in nerve density [
]. The causal nature of this relationship cannot be confirmed from these studies, but these changes appear to have no significant visual or ocular health consequence and are reversible upon cessation of lens wear [
Fig. 11The sub-basal corneal nerve plexus (black lines) superimposed upon a tangential power corneal topography map of a short-term (one year) (A) and long-term (B) (9 years) orthokeratology lens wearer from Lum et al. [
In summary, corneal changes induced by ortho-k are often reversible after ceasing lens wear, which is compatible with the recovery of the topographic and refractive effects induced by the treatment. While corneal thickness recovery occurs within days or weeks of lens discontinuation, the reversal of other effects, such as the redistribution of the sub-basal plexus, nerve may take longer, likely dependent upon the duration of treatment and target refraction.
4.2 Corneal biomechanics
In altering the corneal shape, ortho-k may also affect corneal biomechanics, which in turn, could affect the ability of the cornea to be reshaped by an ortho-k lens. Corneal biomechanics characterise the tissue’s response to external forces. Some devices measure corneal biomechanics through corneal applanation with controlled pressure, while other devices use corneal indentation. The clinical meaning of these metrics remains relatively unknown. Corneal biomechanics may be used to predict the corneal response to the forces imposed by ortho-k lens wear and may change during the course of the treatment. Thus measuring such changes may help to better understand the corneal reshaping process.
The Ocular Response Analyzer (Reichert Inc., USA) and CorVis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) are two instruments currently available to characterise corneal biomechanical properties. The former is more commonly used and measures corneal biomechanics through applanation pressure to reflect viscoelastic properties and resistance of the cornea known as corneal hysteresis and corneal resistance factor, respectively. Earlier studies reported a mild reduction in corneal hysteresis and corneal resistance factor by approximately 1 mmHg or less on average, after ortho-k [
]. Using corneal indentation, the measured corneal tangent modulus in short-term ortho-k lens wear is not significantly different to daily rigid corneal lens wear in the contralateral eye [
]. Although there was a reduction in corneal hysteresis and corneal resistance factor, this observation may be associated with the variability of the Ocular Response Analyzer, rather than actual changes in the intrinsic properties of the cornea.
A short-term pilot study showed that the onset of changes in corneal topography might be related to corneal hysteresis [
], with a slower onset and recovery in topographical and thickness changes observed for corneas with higher baseline corneal hysteresis values. A lower corneal hysteresis and a higher tangent modulus has also been associated with greater corneal flattening along the flattest meridian in ortho-k-treated eyes [
]. In a 6-month study, individuals who dropped out of the study due to significant residual refractive error (mean residual sphere -1.90 D) or were “non-responsive” to the treatment had flatter baseline corneal curvatures and a significantly lower tangent modulus compared to those who completed the study [
]. While the geometric properties of the cornea change following ortho-k, the magnitude of change does not correlate with metrics of corneal biomechanics. A recent study [
Role of corneal biomechanical properties in predicting of speed of myopic progression in children wearing orthokeratology lenses or single-vision spectacles.
] reported a significant difference in corneal hysteresis in spectacle-wearing children showing slow and fast axial elongation and hence, it may be beneficial to recommend ortho-k as early as possible for myopic children with low corneal hysteresis.
At present, it is not clear, with the methodologies available, if corneal biomechanical properties are able to predict the corneal response to ortho-k treatment, if long term ortho-k wear leads to modification of natural corneal biomechanics, or if there are diurnal changes in these properties.
4.3 Intraocular pressure
Most tonometers apply a force to the corneal tissue to derive the internal pressure of the eye, either with direct contact or using non-contact methods. This measurement is influenced by the properties of the tissue, particularly corneal thickness. Eyes with thinner corneas tend to underestimate the actual intraocular pressure while those with thicker corneas tend to overestimate the actual intraocular pressure [
]. Besides thickness, the rigidity of corneal tissue may also affect applanation tonometry, such that stiffer corneas result in a relative overestimation of intraocular pressure, while more flexible corneas result in an underestimation [
Studies that have attempted to investigate the influence of ortho-k on intraocular pressure measurements have shown small and clinically insignificant variations (under 1 mmHg, on average) using different methods of tonometry [
]. Data from these reports also suggest that ortho-k does not affect the reliability of the tonometric measurements. Since there is no evidence of any physiological changes associated with aqueous humour production or outflow during ortho-k treatment which might explain the changes in intraocular pressure reported, it is likely that such variations in intraocular pressure are artefacts related to measurements obtained from the reshaped cornea, rather than actual changes in the true intraocular pressure.
5. Safety of orthokeratology
The safety of ortho-k has been a topic of interest due to its popular use as a myopia control intervention for children, involving overnight wear and hence, potential increased risk of serious corneal infection. In the early 2000′s, several independent case reports of microbial keratitis (MK) in ortho-k lens wearers were published [
] (see Section 5.1.4). Inappropriate lens care and non-compliance, along with continuing lens wear despite significant discomfort, were identified as risk factors for MK, with another report [
] specifying that topping up care solutions and use of tap water were examples of non-compliant behaviours increasing risk. These reports demonstrate that ortho-k can lead to sight-threatening MK, however, the less common use of ortho-k, compared to other contact lens modalities (for vision correction), renders it hard to identify cases even in larger meta-analyses and studies [
]. The estimated incidence of MK related to ortho-k is 13.9 per 10 000 patient years in children (95% confidence interval [CI] 1.7 - 50.4) and 0 per 10 000 patient years in adults (95% CI 0 - 31.7), based on at least three months of lens wear [
Importantly, the vast majority of complications that arise during ortho-k lens wear are not serious adverse events. Over a 10-year follow-up period, 11.1% of Japanese children aged eight to 16 years wearing ortho-k lenses experienced an adverse event (95% CI 8.2 - 14.6%), the most common being conjunctivitis and superficial corneal staining [
]. The incidence of adverse events was comparable to a soft lens control group. Another study of European children aged six to 12 years wearing ortho-k lenses for one year [
] estimated the incidence of all adverse events as 13.3% of eyes (95% CI 8.4 - 20.6%) per annum, the majority being non-significant adverse events (i.e. an adverse event of no immediate clinical concern not warranting discontinuation from lens wear) (9.2% of eyes [95% CI 5.2 - 15.7%] per annum).
5.1 Complications
5.1.1 Corneal staining
Superficial trace corneal staining is the most common complication observed in ortho-k lens wearers [
] which may be related to the greater corneal flattening required for the correction of higher degrees of myopia. Close observation is required during overnight lens wear due to the risk of potential complications associated with corneal staining. Persistent central corneal staining is associated with lens binding [
]. Corneal epithelial trauma induced by the adherent lens edge causes localised corneal distortion and the indentation ring typically displays fluorescein pooling rather than staining [
]. The negative ‘suction’ pressure under the centre of the lens and positive pressure at the edge of the lens was originally hypothesised to cause conventional (not reverse geometry) lens binding, however steeper fitting lenses result in less frequent binding than flat fitting lenses [
]. Pressure exerted by the eyelids during overnight lens wear and a consequent thinning and increased viscosity of the post-lens tear film due to the expulsion of the aqueous tear layer may play a role [
]. It has been claimed that modifying the ortho-k lens fit to increase post-lens tear exchange may reduce the recurrence of ortho-k lens binding and subsequent corneal indentation ring [
] examining lens adherence following overnight rigid corneal lens wear indicated that flat fitting lenses with apical bearing are more likely to result in lens binding upon waking, that persists for a longer duration than steep fitting lenses. Therefore, slightly increasing the central post-lens tear layer thickness may potentially be of benefit, but could also alter the refractive outcome. Lens fenestrations reduce the incidence of lens binding in children wearing ortho-k lenses by ∼20%, but does result in dimple veiling during the first week of lens wear [
]. Importantly, patients must be aware of the potential for lens binding and taught how to use lubricating drops upon waking and apply gentle pressure using the eyelid adjacent to the inferior and superior limbus to mobilise a bound lens prior to attempting lens removal.
5.1.3 Microcysts
Corneal microcysts are a clinical sign of hypoxia [
]. Due to the use of high Dk rigid lens materials, microcysts have been observed only occasionally in ortho-k lens wearers and have not warranted clinical management due to the small numbers detected [
]. Anecdotal reports indicate that microcysts are more commonly seen in children who require higher myopia correction and in those who wear ortho-k lenses for longer periods overnight [
Guo B, Cho P, Efron N, A case of microcystic corneal oedema associated with over-wear of decentered orthokeratology lenses during COVID 19 lockdown. Clin Exp Optom; in press.
], which may be due to reduced oxygen availability to the cornea.
5.1.4 Microbial keratitis
MK is the most significant and potentially vision-threatening complication associated with ortho-k lens wear. No studies to date have investigated and quantified the risk factors associated with the development of MK with ortho-k, however, most initial cases of MK reported were from Asia and involved female lens wearers [
]. A review of the first 16 peer-reviewed articles between 2001 and 2005, which included the first 50 reports of MK in ortho-k, was performed to characterise microbes causing MK, and identify risk factors [
]. Most cases were from East Asia (80%) and in patients between nine to 15 years of age (61%), likely reflecting the demographic of ortho-k lens wearers and their use for myopia control. MK was associated with Pseudomonas aeruginosa as the primary infective organism in 52% of cases, with Acanthamoeba the second most common, at 30% of cases. The most common complication of MK was the development of corneal scars, which significantly affected the visual outcome after the resolution of the infection [
] of published reports of MK in ortho-k spanning 2001 to 2007, including the first 50 cases outlined above and an additional 73 cases, reiterated that the majority of patients affected were East Asian females aged between eight and 15 years. The infectious organism was Pseudomonas aeruginosa in 38% of cases and Acanthamoeba in 33% of cases.
] involving 173 eyes of 166 patients (cases published in the literature between 2002 and 2014) also confirmed that most of the patients with MK were young females (mean age at diagnosis: 15 years) using ortho-k for myopia control. Positive cultures were obtained in 70% of patients (from either the cornea, contact lens, or case) and Pseudomonas aeruginosa (36%) and Acanthamoeba (32%) were the most frequently identified microorganisms. The high prevalence of cases of Acanthamoeba keratitis associated with ortho-k in these reviews emphasises the importance of eliminating the use of tap water in care regimens for overnight ortho-k [
], the incidence of MK associated with overnight ortho-k was estimated in children and adults using de-identified data from 1317 patients, including 640 adults (49% and 1164 patient-years of lens wear) and 677 children (51%, 1435 patient-years of lens wear), provided by randomly selected ECPs, stratified by lens order volume and lens company. Eight incidents of corneal infiltrates associated with a painful red eye were identified (six in children and two in adults). Two were classified as MK (based on an expert panel consensus) and occurred in children, but neither resulted in a loss of visual acuity. The overall estimated incidence of MK was 7.7 per 10 000 patient years in all patients (95% CI 0.9 - 27.8), 13.9 per 10 000 patient years in children (95% CI 1.7 - 50.4), and 0 per 10 000 patient years in adults (95% CI 0 - 31.7), based on at least 3 months of lens wear. The estimated incidence of MK in overnight ortho-k wear is greater than estimates for daily wear of soft lenses (annualised incidence per 10 000 wearers of 1.2 [95% CI 1.1 - 1.5] for soft lenses worn on a daily wear basis, and 2.0 [95% CI 1.7 - 2.4] for daily disposable lenses) [
]. For some patients, a viable myopia correction or myopia control strategy may be available in these soft lens modalities, which should be a consideration when discussing the potential management options. Further prospective studies are still required to estimate the true incidence of MK associated with overnight ortho-k lens wear and identify potential risk factors.
5.2 Non-compliance
Non-compliance related to hygiene procedures and care procedures remains a serious concern and is a major causative factor of serious ocular complications for all contact lens modalities [
]. A later study investigating microbial contamination of lenses and accessories in a group of existing ortho-k lens wearers reported the highest contamination rate for the suction holder, followed by the lens case and lenses. The authors recommended using fingers for ortho-k lens removal and avoiding the use of a suction holder to minimise the risk of potential infection [
]. Both studies identified high contamination rates in lens accessories, due to failure to clean and/or disinfect and replace regularly, which were frequently overlooked by patients/adult caregivers and ECPs. The presence of Gram-negative rods in contact lenses and accessories also indicated poor drying of hands after washing in ortho-k lens wearers [
]. These studies emphasise two critical areas that require attention from ECPs and patients: proper hand hygiene and lens and accessories care. A list of reported common non-compliant behaviours in ortho-k lens wearers is presented in Table 3.
Table 3List of common non-compliant behaviours in orthokeratology wearers.
00017-5&id=gr1.jpg){kind=link}
00017-5&id=gr2.jpg){kind=link}
00017-5&id=gr3.jpg){kind=link}
00017-5&id=gr4.jpg){kind=link}
00017-5&id=gr5.jpg){kind=link}
00017-5&id=gr6.jpg){kind=link}
00017-5&id=gr7.jpg){kind=link}
00017-5&id=gr8.jpg){kind=link}
00017-5&id=gr9.jpg){kind=link}
00017-5&id=gr10.jpg){kind=link}
00017-5&id=gr11.jpg){kind=link}