Rigid Scleral and Corneoscleral Lens Design and Fitting

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 ( ).

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