Scleral Lenses

Introduction and History

A scleral contact lens rests solely on the sclera. It is intended to vault the cornea in its entirety and to retain a fluid reservoir between the lens and the eye ( Fig. 14.1 ). Scleral lenses were first described in 1888 by . In the early years, glass was the only material available, making lenses difficult to create, wear and reproduce. By the 1940s glass had been superseded by polymethylmethacrylate (PMMA), which could be both heat moulded and lathe-cut, increasing manufacturing versatility and reproducibility. Fenestrating the lenses to promote tear exchange and reduce the risk of hypoxic sequelae became possible, but this complicated the fitting process as bubbles were introduced into the precorneal fluid reservoir. The advent of corneal lens designs, which had a greater facility for tear exchange due to their smaller-diameter and movement on eye, meant that scleral lenses fell largely out of use. When rigid gas permeable (RGP) materials became available in the 1980s, interest in this lens type was renewed and has continued to grow ever since.

Terminology

In order for a lens to be described as a scleral contact lens, it must have a bearing surface on the bulbar conjunctiva overlying the sclera, hereafter referred to simply as the sclera, and should vault the whole cornea and limbus and retain a precorneal fluid reservoir. To meet these criteria, the minimum diameter must be just greater than the corneal diameter to allow for some limbal clearance plus the annular width of the bearing surface. Given a normal corneal diameter between 11 and 12 mm, the range of lenses meeting the requirement for a definition of a scleral lens is from approximately 15 mm to just less than the separation of the medial and lateral recti muscle insertion points, which on average is 23–24 mm.

The terms corneo-scleral, semi-scleral, mini-scleral and scleral lenses have historically been variously used for such lenses. The authors consider that the term corneo-scleral is most appropriate for lenses where the intended bearing surface is spread over the peripheral cornea and anterior sclera. Lenses with a bearing surface solely on the sclera are scleral lenses irrespectively of diameter. This is supported by the nomenclature used by the Scleral Lens Society (USA). What is most important is that the intended fitting objectives for the lens design to be used is fully understood prior to the fitting process and shown in Table 14.1 .

There is now a plethora of lens designs, the diameters of which are greater than the visible iris or cornea. Fig. 14.2 illustrates two designs with a diameter of approximately 16 mm. Figs 14.2(a) and (c) having a narrow but clearly demarked scleral bearing surface with cornea clearance and Figs 14.2 (b) and (d) with scleral and peripheral cornea bearing.

Relevant ocular topography

The sclera appears to be conical in the paralimbal region, as seen in Fig. 14.3 , but from observation of eye impression casts, it is evident that there is a large variation in the scleral topography further towards the equator of the globe. The contour appears to be more curved, often becoming increasingly toroidal further out. There is also increasing intrameridional asymmetry further towards the equator, the nasal sclera being typically flatter than the temporal. This was first described by and is shown in Fig. 14.4a and b . These topographical features affect the design of the scleral zone of a lens required to achieve optimum alignment fitting. The clinical objective of scleral lens fitting is corneal clearance, therefore there is no requirement for alignment or matching to the central corneal topography. More relevant is the relative forward projection of the cornea from the plane of the sclera which will determine the sagittal height of the lens required. It has been shown that corneas with comparable central corneal curvature can have different sagittal heights at a given diameter, and this is largely determined by the peripheral corneal angle ( ).

Lens configuration

Scleral lenses are constructed with an optic (corneal or central) zone and a scleral (landing or haptic) zone linked by a transition (limbal or mid peripheral) zone.

Optic Zone

The back surface of the optic zone may consist of single or multiple curves and may be spherical or aspheric.

Key Point

Given a constant optic zone diameter and a monocurve optic radius, steepening or flattening the radius increases or decreases the sagittal height of the lens, respectively, as illustrated in Fig. 14.5a . Alternatively, the optic zone curves can be varied to alter the mid peripheral profile whilst retaining the same sagittal height as illustrated in Fig. 14.5b .

The refractive indices for a schematic cornea and water are calculated to be 1.376 and 1.336, respectively; hence it is considered that most astigmatism arising from the anterior corneal surface is neutralised by the tear lens. However, it is not uniform across the whole thickness of the cornea. calculated the mean refractive index of the stromal anterior and posterior surfaces to be 1.380 and 1.373, respectively, but the epithelium was shown to be 1.401. Therefore it should be recognised that there are limitations of liquid lens neutralisation of the cornea if the topography is especially irregular, as is the case in advanced keratoconus with a clear cornea.

Scleral Zone

As mentioned earlier, the peripheral scleral zone bearing surface is, or simulates, a conic section for smaller diameter lenses but is spherical for larger lenses. Most standard diagnostic scleral zones are symmetrical about the axis, but toroidal peripheral designs are available from some manufacturers and have been shown to improve the comfort and stability in some cases ( ). Quadrant-specific modifications can also be requested for some smaller-diameter lenses. Localised notching or vaulting allows for bearing surface irregularities such as pterygia or pingueculae ( Fig. 14.6 ), or after glaucoma filtration surgery (see also Section 9 , Addendum, available at: https://expertconsult.inkling.com/ ).

Anterior segment topography devices may contribute to the refinement of the scleral zone design to an increasing extent in the future (see Fig. 14.28 ).

Transition Zone

A series of slopes or curves facilitate an optimum transition zone between the optic and scleral zones. Its geometry creates different limbal profiles that usually flatten from the edge of the optic zone outwards (prolate) or, less commonly, reverse designs where the first curve after the central optic radius is steeper (oblate) ( Fig. 14.7a and b).

The prolate shape is most appropriate for:

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  regular, non-pathological eyes

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  keratoconic topographies.

Oblate profiles may be useful for:

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  post-refractive surgery

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  peripheral corneal degenerations

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  corneal transplants.

Changing the specification of the transition can increase or decrease the depth and breadth of the limbal clearance ( Fig. 14.8 ) and thus the effective BOZD or the overall sagittal height of the lens.

Sagittal Height (Sag)

Most modern scleral lenses are defined by their sagittal height (sag) rather than the BOZR, as it is not the cornea per se that is being fitted but rather the relationship between the scleral bearing zone and the relative projection of the cornea.

The reference plane from which the sag of smaller-diameter scleral lenses is determined varies between designs. Some manufacturers define the optic zone sagitta from a chord at the perimeter of the lens, and others define it from the chord between the optic and transition zones. However, for most smaller diameters, overall sag is an integrated function of the scleral zone, transition and optic; therefore an appreciation of the peripheral configuration is important to allow effective modification of the lens design or to interchange between designs.

In contrast, traditional large-diameter scleral lenses are better defined in terms of optic zone projection, that is, the projection of the optic zone from the extrapolation of the scleral curve. This is because fitting depends on selecting an optimum scleral zone alignment to minimise vaulting and decentration, which is a prerequisite for proceeding to optic zone assessment.

Lens Substance (Centre Thickness)

There is an optimum thickness for a given combination of material and diameter which retains dimensional stability. Insufficient substance may lead to excessive flexure with an unpredictable change to the fitting dynamic, possibly resulting in a lens that is tighter in situ than expected, or adversely affecting the dimensional stability. Increased substance may reduce the risk of both but could be bulky and reduce oxygen available to the cornea.

Centre thicknesses up to 0.6 mm have not been shown to significantly increase central corneal swelling during wear in normal eyes ( ). However, this may not be the case if the cornea is already compromised, for example, in the event of endothelial cell loss following a corneal transplant.

Front Optic (see also Section 9 , Addendum, available at: https://expertconsult.inkling.com/ )

The front optic radius is determined by the optical power required, which is most often spherical, although a simultaneous vision multifocal is available.

Key Point

The optic zone thickness is determined by the power of the lens:

• For negative-powered lenses, there has to be a minimum central substance at the apex of the optic zone.

• For positive-powered lenses, there has to be a minimum central substance at the perimeter of the optic zone.

Advantages and Applications of Scleral Lenses

A summary of the relative advantages of smaller- and larger-diameter scleral lenses is shown in Tables 14.2 and 14.3 .

Scleral lenses have many potential advantages over other lens types:

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  As mentioned earlier, the tear reservoir retained behind the lens circumvents the need for precise alignment with the cornea; therefore virtually any corneal topography can be addressed successfully as stability and movement are not a concern.

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  The large diameter and scleral bearing eliminates:

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    lid sensation

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    issues associated with lens movement on the eye, e.g. variable visual acuity and instability.

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  Compared with a corneal lens, the absence of corneal contact and minimal movement reduces the potential for mechanical damage to the cornea.

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  There is a good chance of optically correcting:

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    primary ectatic corneal conditions, i.e. when the cornea becomes both thin and distended: early and advanced keratoconus ( Fig. 14.9 ), keratoglobus, pellucid marginal degeneration and Terriens marginal degeneration

    

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    after corneal transplants

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    high degrees of nonpathological ametropia.

The retention of a liquid reservoir can be invaluable as a therapeutic application to:

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  maintain corneal hydration in cases of epithelial desiccation subsequent to dry eye or tear film dysfunction

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  protect the cornea from exposure in cases of lid closure dysfunction

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  reduce ocular discomfort caused by these conditions.

Conditions where scleral lenses can be beneficial include:

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  the aftermath of Stevens-Johnson syndrome (SJS)

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  ocular cicatricial pemphigoid (OCP)

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  Sjögren syndrome

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  exposure keratitis

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  filamentary keratitis

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  seventh nerve palsy

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  persistent epithelial defects

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  neurotrophic cornea.

The increased substance of scleral lenses can be used to increase the palpebral aperture in cases of phthisis bulbi, ptosis or microcornea. Alternatively, some types of ptosis can be ameliorated nonsurgically by building a prop onto the front surface ( Fig. 14.10a and b).

Scleral Lens-Fitting Principles

The general fitting principles are similar for all scleral lenses; however, some of the nuances are dependent on the diameter.

Diameter

There is no universally optimum diameter, and initial selection depends mainly on practitioner preference, but there are some factors that may influence this decision:

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  Corneal profile

Small total diameters may be first choice for a topographically normal cornea or a moderately protrusive ectatic cornea with a central apex. The flatter BOZR required reduces the difference between the central curvature and the scleral bearing surface curvature or slope, with the transition zone a comparatively close match to the peripheral corneal profile. Fig. 14.11 illustrates the difference.

A larger back optic zone diameter (BOZD) is necessary to clear a more eccentric apex, reducing the bearing surface and transition zone width unless the total diameter is also increased ( Fig. 14.12 ). In addition, a larger BOZD may be beneficial where glare/flare is a known problem.

A more protrusive corneal profile needs a greater sagittal depth to clear the cornea. A larger-diameter allows a broader limbal clearance and a more gradual transition from the optic zone to the scleral zone.

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  Naturally occurring astigmatism

Moderate to high nonpathological or nonsurgical astigmatism is likely to be associated with a toroidal sclera which tends to increase towards the equator of the globe. If a large-diameter lens is preferable for other reasons, such as practitioner/patient preference or an anticipated handling problem with smaller lenses, a provisional wearing trial would be advised to verify sufficient sealing on the sclera. It may be apparent at an early stage that the scleral zone fitting is too irregular to productively proceed further than an initial trial lens. Small-diameter lenses bear on the most co-axial region of the sclera, i.e. just beyond the limbus, hence may give better scleral zone alignment.

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  Monocular or binocular requirement

The improved centration of smaller-diameter lenses reduces vertical prismatic effect; hence a smaller-diameter may be preferred if the fellow eye is sighted but not wearing a scleral lens. The lesser substance of a smaller-diameter lens also reduces forced widening of the palpebral aperture, which may be cosmetically more acceptable in unilateral cases.

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  Eyelid configuration and method of application/removal

Application and removal techniques are quite different depending on the diameter. Smaller lenses are normally placed directly onto the cornea, with the lids parted to create an aperture wider than the diameter of the lens. Larger diameters are applied by sliding the lens under the upper lid, which is then manually closed over the lens, and the lower lid everted. Small-diameter lenses are likely to dislodge onto the superior sclera if this method is tried.

Intuitively, a smaller diameter would seem to be a more logical choice if the palpebral aperture is also narrow. However, if the eyelids cannot be parted sufficiently to place the lens directly onto the cornea, a larger-diameter lens may be applied more easily. This is the same if applying the lens causes excessive eyelid squeezing or for wearers with especially tight lids.

It may be apparent at an early stage of the fitting process that the handling method is an issue affecting selection of the optimum diameter. If so, a provisional instruction on application and removal methods to confirm the patient's ability or preference may be crucial in deciding on the diameter for the initial lens. A more in-depth discussion of handling techniques is covered in Section 9 , Addendum, available at: https://expertconsult.inkling.com/ .