Introduction and Historical Overview
Scleral lenses were the first contact lenses used to protect the ocular surface, restore vision in keratoconus and correct simple ametropia ( ). Initially manufactured in glass and later polymethylmethacrylate, Sattler’s veil (corneal oedema) was common ( ) and required periodic lens removal to allow the cornea to recover. Other approaches to eliminate or delay the onset of corneal oedema included the use of different buffering agents within the scleral bowl and later fenestrating the lens to enhance tear exchange ( ) or maintain the osmotic balance between the postlens fluid reservoir and the corneal epithelium ( ). Following the introduction of corneal rigid and soft contact lenes, by the time scleral lenses were manufactured in low Dk oxygen-permeable rigid lens materials in the 1980s ( ), they were primarily used as a specialty lens reserved for clinical situations in which other lens designs could not achieve a satisfactory visual outcome, provide an acceptable fit, or the desired ocular protection.
The prescribing of larger diameter rigid lenses, including both corneoscleral and scleral lenses, was virtually nonexistent towards the end of the 20th century and the beginning of the 21st century. However, since 2003, there has been a substantial upsurge in the prescribing of such lenses ( ). This upward trend is demonstrated in Fig. 17.1 , which is an updated version of data presented by . This figure shows data from 40 countries that returned information for at least 1000 contact lens fits, surveyed between 2000 and 2020. It can be seen that the fitting of corneoscleral and scleral lenses increased from less than 1% of all contact lens fits in 2003 to more than 3% in 2020.
This significant increase in fitting corneoscleral and scleral lenses this century is most likely due to advances in lens manufacturing and ocular imaging such as anterior segment optical coherence tomography (OCT) and corneoscleral topography which facilitate scleral lens design and fitting ( ) without the need to obtain an impression of the ocular surface or rely solely on the interpretation of vascular compression patterns beneath the lens haptic. Highly permeable lens materials (up to Dk ~180) ( ), new coatings to improve rigid lens wettability ( ) and smaller overall lens diameters (average diameter prescribed ~16 mm; ) compared to earlier scleral lenses (up to 24 mm diameters) may also have contributed to the rise in modern scleral lens prescribing.
Terminology
A rigid lens that vaults the cornea and rests entirely upon the conjunctival tissue is considered a scleral lens ( ), while a corneoscleral lens bears upon corneal and conjunctival tissue ( ). Previous classification systems defined scleral lenses based on overall diameter ( ); however, since a small-diameter lens may completely vault some smaller corneas, lenses are now categorised based on where the lens contacts the anterior ocular surface. Ventilated lenses refer to scleral lenses that have been intentionally modified in an attempt to facilitate tear exchange, and therefore oxygen delivery to the cornea, through a fenestration, channel or slot ( ). Nonventilated or sealed lenses refer to scleral lenses without such modifications, which typically result in minimal to no tear exchange following lens settling ( ).
Indications
Throughout the 1930s and 1940s, approximately 65% of scleral lenses were fitted to correct ametropia with only 15% prescribed for keratoconus ( ). In contrast, the optical correction of an irregular anterior corneal surface is the primary indication for modern corneoscleral and scleral lens fitting; ~50%–60% primary corneal ectasia (keratoconus, pellucid marginal degeneration) and ~20% postgraft ( ). Therapeutic rehabilitation of the ocular surface accounts for ~5%–10% of larger-diameter rigid lens fittings ( ) including the treatment of a wide range of anterior segment conditions ( ) by providing constant lubrication over the entire cornea, and protection from external forces including the eyelids. Other specific indications for sealed scleral lenses include dusty environments that may result in foreign bodies becoming trapped behind a corneal rigid lens, requirements for very stable vision (e.g. sport) and the risk of other lens types dislodging (e.g. water sports).
Advantages and Disadvantages
The major advantage of corneoscleral and scleral lenses is the ability to obtain a stable lens fit in eyes with a highly irregular corneal shape, which may not be possible with a smaller- diameter rigid lens, and potentially delay or eliminate the need for corneal surgery ( ). Improved comfort ( ) is another benefit, due to reduced eyelid interaction with the lens edge, landing zone contact restricted to the less sensitive conjunctival tissue (for scleral lenses), and the entire cornea remains constantly hydrated due to the retained postlens fluid reservoir. A disadvantage of larger-diameter rigid lenses relates to difficulties with lens handling initially; on average, 2.4 and 1.9 attempts for application and removal, respectively, which reduces to ~1 attempt after 1 year of scleral lens wear ( ). Some patients also need to remove the lens, refill with an application fluid and reapply the lens throughout the day due to a build-up of reservoir debris. The cost of scleral lenses is greater compared to other lens designs; however, there is the potential for the lenses to provide an acceptable fit for a longer period of time if fitted with modest corneal clearance compared to a corneal rigid lens (i.e. a scleral lens may potentially accommodate progressing corneal ectasia for longer than a corneal rigid lens with minimal clearance).
Fitting Principles
Corneoscleral Lenses
The major advantages of larger-diameter lens designs compared with traditional corneal rigid lenses are improved comfort (partly due to the reduced interaction between the lens edge and eyelid) and centration with a larger optical zone diameter. They are particularly useful for inferiorly located cones ( ) or if soft, corneal rigid, piggyback or hybrid lenses have failed to provide an acceptable visual outcome. Corneoscleral designs can be customised to improve lens centration and the overall fit (e.g. multicurve and aspheric designs or toric/quadrant-specific peripheral curves as per modifications to corneal rigid lenses, see Chapter 15 ), and the location of corneal bearing varies with lens design and fitting philosophy ( ) ( Fig. 17.2 ). Importantly, limbal compression must be avoided, since insult to the stem cells can potentially trigger a neovascular response ( ). Corneoscleral lenses display movement upon blinking (up to ~0.5 mm), but less than corneal rigid lenses (1–2 mm). Consequently, oxygen delivery is greater for corneoscleral designs compared to sealed scleral lenses due to greater tear exchange and a thinner fluid reservoir which minimises corneal oedema.
Scleral Lenses
Terminology
The Scleral Lens Education Society ( ) recommends a simplified three zone lens description; the optic zone, which houses the refractive correction, the landing zone (i.e. the portion of the lens that aligns with the conjunctival tissue) and the transition zone, which connects the optic and landing zone, and may consist of multiple mid-peripheral curves.
Preformed Scleral Lens Diagnostic Fitting
Initial Lens Selection
Preformed lens fitting involves in vivo lens assessment of corneal clearance, lens centration and visual performance using a suite of diagnostic lenses to determine the required lens parameters. The initial diagnostic lens selected is typically based on the corneal diameter which informs the required lens diameter to ensure limbal clearance (~65% of lenses fitted are 15–17 mm overall diameter; ), or based on the sagittal height of the eye (from OCT imaging or corneoscleral topography) at a specified chord diameter which informs the required sagittal depth of the lens. Unlike corneal rigid lens fitting, the back optic zone radius is not critical during the initial fitting process since the lens vaults the central cornea, and the optimal back optic zone radius may not correlate with central anterior corneal curvature ( ).
Corneal Vault
Scleral lenses rest entirely on the conjunctiva and settle back into this tissue throughout the day (i.e. the postlens fluid reservoir reduces by ~100 µm or more centrally; ; ) and continue to settle back further over months of lens wear ( ) ( Fig. 17.3 ). Therefore scleral lenses must be fitted with sufficient initial vault to accommodate this settling to avoid corneal bearing after longer periods of wear.
Manufacturer recommendations for fluid reservoir thickness values vary from ~200 to 400 µm centrally after lens application and <100 µm at the limbus following settling ( ). Ideally, a thinner fluid reservoir is desirable to minimise corneal hypoxic stress ( ), midday fogging ( ) and lens decentration ( ). However, some patients can successfully wear lenses with central vault up to ~1000 µm without adverse effects ( ).
The fluid reservoir thickness should be monitored since the progression of ectasia can also result in contact between the lens and central cornea ( Fig. 17.4 ). A uniform fluid reservoir thickness may not always be achievable due to the location of the cone in keratoconus or regional variations in lens settling ( ) ( Fig. 17.5 ).
An estimation of the amount of vault can be made by creating an optic section with a slit lamp biomicroscope and comparing the fluid reservoir thickness to the centre thickness of the lens, which usually is specified by the manufacturer but in any case can be measured. Examples of this approach are presented in Appendix L .
More reliable quantification of corneal vault requires OCT or Scheimpflug imaging ( Fig. 17.6 ). Again, the extent of corneal vault can be estimated by comparing the reservoir thickness to the known central lens thickness; however, practitioners typically overestimate the magnitude of clearance using this approach ( ).
Landing Zone Alignment
Inappropriate alignment between the landing zone and the underlying conjunctiva can result in a number of unwanted physiological and optical outcomes ( ). A sub-optimal fitting relationship can arise when a spherical landing zone is fitted to a nonspherical sclera. The vast majority of scleral profiles are nonspherical (6% spherical, 29% toric, 41% asymmetric) ( ), and the degree of variation in the scleral elevation profile increases further from the limbus ( ) ( Fig. 17.7 ). Scleral asymmetry is also greater in keratoconic compared to nonkeratoconic eyes ( ), and the asymmetry of the scleral elevation profile has been linked with the location of the cone ( ).
Landing zone alignment influences comfort, lens centration, tear exchange and midday fogging. Toric, meridian-specific or impression-based landing zone designs are available to improve lens centration and minimise compression of the underlying conjunctival tissue and vasculature. Other landing zone modifications may be required to minimise tissue compression or inflammation when altering the overall lens diameter is not feasible ( ). For example, a localised notch or elevation within the landing zone can be used to avoid conjunctival lesions or anomalies such as pinguecula, pterygium, cysts, symblepharon and surgical sites (e.g. blebs or tubes following glaucoma filtration surgery) ( Fig. 17.8 ).
The landing zone is typically assessed using a slit lamp to evaluate compression of the conjunctival vasculature and edge lift. The ingress of vital dyes into the fluid reservoir can also highlight regions of landing zone misalignment that may result in fluid reservoir debris or bubbles. Following lens removal, conjunctival staining, tissue compression and hyperaemia within the landing zone may indicate inappropriate alignment. OCT imaging can also be used to assess landing zone alignment with the lens on eye but does produce a displacement imaging artefact ( ).
Empirical Fitting
Scleral curvature and elevation derived from corneoscleral topography and OCT imaging can be used to empirically design scleral lenses, reducing the need for the application of multiple diagnostic lenses. Corneoscleral topographers (e.g. Pentacam, Eye Surface Profiler, sMap3D) utilise the ocular height profile in conjunction with lens data provided by manufacturers to determine the required lens parameters to achieve a theoretical optimal lens fit. Scleral height data obtained using OCT imaging can also be used to estimate the required landing zone curvature and toricity ( ).
Impression-Based Fitting
This fitting approach utilises an impression of the anterior ocular surface to design the back surface of the scleral lens ( ) and is typically reserved for advanced ocular disease with irregular corneal or scleral shapes often when an acceptable lens fit cannot be obtained using preformed lenses or topography-based designs. Impression-based lenses ensure close alignment of the landing zone with the underlying conjunctiva and can result in improved visual outcomes compared to a preformed scleral lens with a spherical or toric landing zone design due to improved centration ( ). Patients must cease lens scleral lens wear before the impression is obtained to allow recovery of conjunctival and scleral compression. Up to three impressions and five lenses may be required to arrive at an optimal fitting impression-based lens (mean 1.3 and 2.1, respectively), based on the EyePrint-PRO system ( ).
Ventilated Scleral Lenses
Scleral lenses may be ventilated by a fenestration ( ), slot ( ) or channel. These modifications are thought to improve tear exchange, oxygen delivery and facilitate the removal of debris from the fluid reservoir; however, there has been limited research to confirm these claims. For example, reported that fenestrating a scleral lens (a 1- to 1.5-mm fenestration at the limbus) had minimal impact on tear exchange. Compared to nonventilated scleral lenses, fenestrated designs are typically fitted with minimum central clearance [e.g. ~30 µm ( ) to 100–150 µm ( )] to ensure that any air bubbles that enter the fluid reservoir remain near the peripheral cornea and do not disturb vision. Fenestrations are typically positioned over the limbus ( ) and can vary in size (0.25–1.5 mm), number and arrangement (e.g. triangular configuration) ( ). A common approach is to use a number of fenestrations to ensure at least one is not blocked by the upper and lower lids if the lens rotates. These lenses also have a greater potential for breakage at the site of the ventilation. Channels or a fenestration within the landing zone can also be used to minimise lens suction and aid lens removal.
Application Solutions
Nonventilated scleral lenses must be filled with a preservative free solution prior to application since the fluid reservoir remains in contact with the cornea throughout lens wear. Most practitioners recommend single use or multidose preservative free saline or preservative free artificial tears ( ). Preservative free aerosol salines are not recommended as they can introduce bubbles into the fluid reservoir which can affect vision and lead to corneal desiccation. The use of application solutions that mimic the ionic composition of the natural tear film may enhance subjective comfort and vision compared to other solutions ( ).
Since nonventilated scleral lenses exhibit minimal movement or tear exchange, the fluid reservoir has been used as a depot for off-label sustained drug delivery through the cornea ( ). Fluoroquinolone antibiotics have been utilised as a prophylactic measure during continuous scleral lens wear following the re-epithelialisation of significant corneal defects ( ) or to treat severe infectious keratitis ( ). Antivascular endothelial growth factor (1 drop of 1% bevacizumab within the reservoir) has also been used as a long-term treatment for significant corneal neovascularisation in a range of anterior eye conditions ( ) with minimal adverse effects.
Care and Maintenance
The care and maintenance procedures for corneoscleral and scleral lenses are the same as described in Chapter 18 for rigid lenses. However, special lens cases may be required to accommodate lenses with larger diameters and sagittal depths. Some alcohol-based, abrasive and enzymatic cleaning products are not recommended for lenses coated with a polyethylene glycol (PEG) polymer to improve deposit resistance, wettability and potentially comfort ( ), since these can remove the coating.
Optical Considerations
Optimally fitted rigid lenses of varying diameter provide equivocal optical performance and visual outcomes for mild to moderate corneal ectasia ( ). However, as corneal disease progresses, corneoscleral and scleral lens designs may provide better visual outcomes than corneal rigid lenses in some instances ( ), most likely due to improved lens centration and stability.
Decentration
Scleral lenses often decentre inferiorly due to gravitational and eyelid forces and temporally due to the typically flatter and more elevated nasal sclera ( ). The mislocation of the anterior lens surface can induce residual astigmatism, and higher-order aberrations, particularly for aspheric or wavefront-guided front surface designs ( ). A nonuniform fluid reservoir can also create a vertical prismatic effect which can prevent fusion in unilateral scleral lens wearers. Decentration can be reduced by improving landing zone alignment and reducing apical vault. For some lens designs it is also possible to offset the optic zone to ensure alignment with the pupil. The extent of lens decentration can be reliably quantified using over-topography ( ), or en-face imaging with customised software ( ).
Flexure
Scleral lenses with a spherical landing zone may flex if the underlying sclera is not spherical (toric or asymmetric) in nature. Lens flexure manifests as an astigmatic over-refraction and can be confirmed using videokeratoscopy with the lens on eye. For thinner scleral lenses, the extent of lens flexure is approximately linearly proportional to the degree of scleral toricity (0.2 D lens flexure per 100 µm of scleral toricity) ( ). Flexure can be minimised by improving the landing zone alignment. Flexure must be distinguished from lens warpage, in which a spherical lens displays persistent front surface toricity both on and off the eye.
Front Surface Optics
While the fluid reservoir significantly reduces most anterior corneal astigmatism and higher-order aberrations ( ), some eyes fitted with scleral lenses may require a toric, aspheric or wavefront-guided front surface design to correct residual internal astigmatism or higher-order aberrations. To optimise the refractive correction, these lens designs require a landing zone design that ensures alignment of the optic zone with the pupil. The principles of front surface toric lens designs for corneal rigid lenses discussed in Chapter 16 also apply for scleral lenses; however, for scleral lenses, a toric back surface optic zone is not required (a toric or customised landing zone ensures suitable alignment).
A front surface aspheric lens design can minimise residual spherical aberration arising from the combination of the scleral lens and the internal optics of the eye. The appropriate front surface correction to optimise visual outcomes varies with pupil size and can be difficult to predict without a measurement of residual higher-order aberrations during spherical front surface scleral lens wear ( ). Wavefront-guided front surface scleral lenses are available from some manufacturers, which aim to correct residual higher-order aberrations up to the fifth radial order (this includes spherical aberration, coma and trefoil). This approach requires quantification of residual or internal aberrations, excellent optic zone alignment with the pupil and a period of adaptation to achieve superior visual outcomes compared to conventional front surface designs ( ).
Back Surface Geometry
The asphericity of the back surface of the optic zone can also be altered to improve alignment with abnormal corneal shapes (e.g. keratoconus or postrefractive surgery) with minimal impact on spherical aberration. The back surface geometry can also be manipulated to alter the power of the postlens fluid reservoir, which may be desirable to minimise the correction required within the scleral lens, and therefore the lens thickness.
Lens Material and Coatings
Increasing scleral lens oxygen permeability beyond a Dk of 100 has minimal effect on corneal oxygenation in healthy eyes ( ) and may make the lens more susceptible to scratching ( ). However, increased short-term comfort has been reported for a lens material with a Dk of 180, when controlling for other lens parameters ( ). The lens material can also affect in vitro optical quality measurements ( ), but these variations would not significantly alter visual outcomes in vivo. Plasma treatments are available for scleral lenses and can enhance lens wettability by ~40% ( ) but are not permanent. A PEG polymer lens coating applied after a plasma treatment can also improve scleral lens wettability ( ), with no impact upon in vitro optical performance ( ). Compared to uncoated scleral lenses, PEG-coated scleral lenses can improve comfort in established scleral lens wearers with dry eye ( ).
Complications
While serious complications can occur with scleral lenses (e.g. microbial keratitis) ( ), it should be noted that scleral lenses are often prescribed for compromised eyes with advanced corneal or ocular surface disease. This section summarises complications or anomalies that are unique to scleral lenses.
The Fluid Reservoir
Air Bubbles
In fenestrated scleral lens wear, a mobile air bubble is intentionally introduced into the fluid reservoir. For sealed scleral lenses, large static bubbles are problematic as they can lead to corneal desiccation and if located centrally will disturb vision. If a bubble is present immediately after lens application, the lens must be removed, refilled with application solution (enough to create a convex meniscus) and reapplied, without touching the lids or lashes. Lens handling can be challenging for patients initially, but the number of attempts required to apply the lens reduces over time and stabilises after about 6–12 months ( ). If multiple small bubbles (a frothy appearance) are visible in the fluid reservoir, ensure the patient is not using a preservative free aerosol saline to fill the lens. Bubbles that arise after a period of lens wear may indicate a region of landing zone misalignment. The point of entry can be identified by applying a vital dye to the eye and observing its ingress into the fluid reservoir, and the landing zone modified to create a sealed system if desired.
Midday Fogging
Misty or cloudy vision (midday fogging) is common symptom reported by ~30%–50% of scleral lens wearers ( ) and the debris that clouds the postlens fluid reservoir ( ) can begin to accumulate immediately after lens application ( Fig. 17.9 ). Midday fogging has been linked with inflammation ( ) and tear film debris external to the lens ( ) and must be differentiated from reduced visual acuity due to corneal oedema. Midday fogging is managed with varying success by lens removal and reapplication (sometimes multiple times per day), using a more viscous gel to fill the scleral bowl, aggressively treating any eyelid disease that may contribute to tear film debris and improving landing zone alignment ( ).
