Rigid Corneal Lens Design and Fitting

Introduction

Fitting rigid corneal lenses is an improbable achievement. Essentially, it involves constructing a complex three-dimensional structure to sit, without the aid of any supplementary adhesive, on a vertically inclined surface while being repeatedly dislodged by a covering structure. Fortunately, the task is made easier by a number of mitigating factors:

The lids help to prevent the lens falling from the eye.
The change in curvature at the limbal junction helps to prevent sideways drift.
The capillary forces in the prelens tear film are usually strong enough to hold the lens in place but weak enough to avoid adherence.

On the other hand, a number of factors can make rigid corneal lens fitting difficult:

No two eyes have precisely the same dimensions.
The worse the refractive error, the less predictable is the fit.
Reflex tearing during adaptation can alter the lens fit.
Rigid lenses can be very uncomfortable, no matter how good the fit.

Trends in Rigid Corneal Lens Prescribing

The fit of a rigid lens is affected by a range of factors, many of which are not easily quantifiable, such as corneal and limbal anatomy, lid geometry and mechanics and extraneous factors such as pterygia. As a result, many practitioners consider rigid lens fitting to be more of an art than a science. However, the bibliometric analysis of does not support the notion that the entire field of rigid lenses (as distinct from the narrow field of clinical rigid lens fitting) can be considered as a being ‘more of an art form than a science’ Fig. 15.1 is a modified plot from , which shows 3-year moving-average trends in prescribing ‘standard’ rigid corneal lenses as new fits (i.e. excluding orthokeratology, scleral, and myopia control fits, which are considered elsewhere in this book) in 15 nations between 2000 and 2020. The mean trend on this graph (unconnected black dots) indicates an overall decline in rigid corneal lens fitting from 11% of all contact lens fits in 2000 to 6% in 2014; this level of rigid lens fitting has remained at that level (about 6%) since then.

Fig. 15.1, Three year moving-average trends in prescribing ‘standard’ rigid corneal lenses as new fits (i.e. excluding orthokeratology, scleral, and myopia control fits), in 15 nations between 2000 and 2020.

Even though rigid corneal contact lenses are now rarely fitted for routine use, there is still a need for practitioners to be familiar with the practicalities of rigid lens fitting, for several reasons: (1) they may be required to assess an existing rigid lens wearer; (2) a number of corneal conditions are best managed with rigid lenses; and (3) as orthokeratology treatment of young myopes becomes increasingly common, familiarity with rigid lens fitting will be essential.

Ocular Topography

A number of ocular dimensions are relevant to the selection of rigid corneal lenses. These are summarized in Table 15.1 , along with typical population variations. Precise measurement of each of these dimensions is not essential to successful rigid lens fitting; however, for a given patient, it is helpful to assess whether the eye is close to average or atypical, for instance, whether the corneal diameter is larger or smaller than average.

Table 15.1

Ocular Dimensions and Typical Population Variations

Data from Hall, L., Hunt, C., Young, G. & Wolffsohn, J. (2013). Factors affecting corneoscleral topography. Invest. Ophthalmol. Vis. Sci ., 54 , 3691–3701.

	Mean (mm)	±2 SD (mm)
Flattest corneal curvature – keratometry	7.85	7.25–8.45
Corneal shape factor	0.67	0.40–0.95
Corneal diameter	13.3	11.7–14.1
Horizontal visible iris diameter	11.7	10.7–12.7
Palpebral aperture	10.2	7.4–13.0

SD , Standard deviation.

Cornea

The cornea is generally aspheric in shape and is frequently described as ellipsoid ( ). In most cases, a corneal section can be approximated to a prolate ellipse – that is one that gradually flattens towards the periphery. Conventional keratometers give readings corresponding to points approximately 1.5 mm either side of the corneal apex. Given that corneas with similar keratometer readings (K - readings) can show widely differing peripheral curvature (asphericity), keratometry alone does not always indicate the optimum rigid lens fit.

The degree of corneal asphericity is often expressed as either corneal eccentricity or corneal shape factor (SF). With eccentricity ( e ) a spherical surface equals zero, while a flattening ellipse equates to a positive value. SF is a function of eccen-tricity, whereby:SF = 1−e2

Using SF, a spherical surface equals 1.0, a flattening ellipse is <1.0, while a steepening ellipse is >1.0.

With-the-rule astigmatic corneas are steeper in the vertical meridian and are generally easier to fit than against-the-rule astigmatic corneas, on which corneal lenses tend to show sideways decentration. With-the-rule astigmatic corneas often show greater corneal astigmatism in the inferior cornea compared with the superior cornea. As a rule of thumb, 1.00 D of corneal astigmatism corresponds to a difference in K-readings of approximately 0.2 mm. The peripheral cornea tends to show less toricity than does the central cornea ( ).

There is no correlation in the population between corneal asphericity and central corneal curvature ( ). However, there is a correlation between K-readings and corneal diameter such that larger corneas tend to be flatter in curvature whereas smaller corneas tend to be steeper ( ). Also, there is a correlation between asphericity and the degree of myopia; higher myopes show less peripheral corneal flattening, which is consistent with the longer axial length of the myopic eye ( ).

The cornea is invariably larger in the horizontal meridian than in the vertical. It is not easy to measure the true diameter of the cornea owing to the gradual change in transparency at the limbus. In practice, the width of the visible iris (horizontal visible iris diameter, HVID) is used as a gauge of corneal diameter. The HVID is approximately 1.5 mm smaller than the actual corneal diameter ( ). Corneal diameter influences the choice of lens diameter. The typical variation in corneal diameter of 2.4 mm (see Table 15.1 ) is mirrored by a similar variation in typical corneal lens diameter, that is 8.30–9.80 mm.

Lids

The lower lid usually aligns with the lower limbus while the upper lid tends to cover the limbus and overlap by approximately 1 mm. Both of the eyelids can help to position the lens high enough to cover the pupil and, in many cases, the lens is effectively passed from one lid to the other between blinks.

The maximum vertical distance between the lids (palpebral aperture) is generally measured prior to fitting. Smaller palpebral apertures tend to require smaller-diameter lenses; however, the relationship of the lids to the superior and inferior limbus is also relevant, and therefore there is no simple relationship between palpebral aperture and optimum lens diameter. Rigid corneal lens wear itself can cause a reduction in palpebral aperture. noted that the palpebral aperture in rigid lens wearers is, on average, 0.5 mm smaller than that in soft lens wearers.

When the upper lid is relatively high, the lens tends to decentre low and a large-diameter lens is often required to encourage the upper lid to grip the lens and hold it in place; this is known as lid attachment. A relatively low upper lid and narrow palpebral aperture tends to result in upwards decentration ( ).

In cases where the lower lid is positioned higher than the limbus, a smaller lens may be required. On the other hand, when the lower lid is low or ineffective owing to ectropion, a large-diameter lens is often required, again to encourage lid attachment.

There is a correlation between palpebral aperture and corneal diameter. Larger corneas, as well as being relatively flat, tend to be accompanied by a wider palpebral aperture.

Ethnic Variations in Ocular Dimensions

Some ethnic variations in ocular topography have been noted in the literature. In a UK-resident Chinese population, corneas have been noted to be steeper and smaller and to show less corneal flattening than Caucasian eyes ( ). This suggests a requirement for smaller rigid lenses showing less peripheral flattening for such patients. In a more recent study, however, Chinese and Japanese corneas tended to be slightly flatter but to show similar asphericity in the flatter meridian ( ). It is likely that environmental factors, such as nutrition, as well as ethnic differences influence corneal topography and probably account for the trend of corneal flattening noted in the Japanese population over a 20-year period ( ). As expected, oriental eyes tend to show a narrower palpebral aperture: on average, about 1.0 mm smaller than Caucasian eyes ( ). A comparison of Caucasian and British Asian eyes noted smaller corneas in the British Asian population but no difference in curvature or asphericity ( ).

There is less information on ethnic variations in Afro-Caribbean populations, although there is some evidence that these corneas tend to be larger and flatter than corneas of Chinese or Japanese populations ( ). One study noted that African-American corneas tended to be more prolate than in Caucasians ( ).

Forces Acting on the Rigid Corneal Lens

A number of forces act on the rigid corneal lens and have to be suitably balanced in order to achieve a satisfactory fit ( Fig. 15.2 ). The gravitational force of the lens and prelens tear film cause the lens to drop. The effect will be greater, and the lens less stable, the further forwards the centre of gravity lies. The centre of gravity is further forward in plus lenses compared with minus lenses ( Fig. 15.3 ). It is shifted posteriorly by increasing the diameter, steepening the back optic zone radius (BOZR), or decreasing the thickness of the lens. With both plus and minus lenses, the greatest shift and most effective stabilization are achieved through changing the lens diameter ( ).

Fig. 15.2, Forces acting on a lid-attached rigid lens ( Guillon, & Sammons, 1994 ). ELF , Eyelid force; ESTF , edge surface tension force; G , gravity; TFP , tear fluid pressure; VF , viscous forces.

Fig. 15.3, Rigid lens centre of gravity in a plus (left) and minus (right) power lens.

The lens is held in place by the capillary forces in the postlens tear film and the surface tensional force in the tear meniscus at the lens edge. The capillary force increases with increasingly closer alignment of the lens and the cornea. The force is therefore greater with spherical corneas compared with astigmatic corneas. Fig. 15.4 shows the change in apparent alignment with increasing astigmatism.

Fig. 15.4, Apparent percentage corneal alignment and average tear layer thickness with increasing corneal astigmatism.

Surface tension forces act at the lens edge where the edge meniscus is not covered by the lid. There will be no surface tension where a meniscus is absent owing to excessive edge clearance. This force can be increased by reducing edge clearance and edge thickness ( ).

Eyelid forces (primarily the upper lid) act to move the lens in a vertical direction during the blink. Between blinks, these forces help to stabilize the lens in the case of a lid attachment fit but have no effect in the case of an interpalpebral fit.

Rigid Corneal Lens Design

Appendix A details conventions for specifying the essential design features of contact lenses (soft or rigid) as well as the terms, symbols and abbreviations used to describe these features.

Back Optic Zone Diameter

The back optic zone diameter (BOZD) is usually fixed for a given design in a given total diameter (TD) and is generally 1–1.5 mm smaller than the TD. The BOZD should be large enough to cover the pupil in most conditions, including low illumination.

With toroidal corneas, using a smaller BOZD can increase the area of alignment and therefore improve the fit. However, if the BOZD is reduced while maintaining the same TD, this results in a wider periphery and flatter peripheral curves are required in order to maintain edge clearance.

If the BOZD is changed, it is usually necessary to change the BOZR in order to maintain a clinically equivalent fit. Reducing the BOZD without reducing the BOZR results in a sagittal depth that is shallower and therefore a flatter fit. As a rule of thumb, an increase in BOZD of 0.5 mm requires an increase in BOZR of 0.05 mm.

Front Optic Zone Diameter

The front optic zone diameter (FOZD) should be at least 0.5 mm larger than the BOZD. Except in low powers, most corneal lenses are lenticulated so as to reduce thickness and weight.

Lenses occasionally incorporate a negative carrier in order to encourage lid attachment and to centre the lens. A negative carrier is a peripheral zone that is thinner at the optic zone junction than at the lens periphery. A positive carrier or tapered-edge design – where the peripheral zone is thicker at the optic zone junction than the lens periphery – is occasionally used to discourage lid attachment in a high-riding lens.

Centre Thickness

If lenses are made too thin, not only is there a greater risk of breakage, but also the lenses tend to flex on astigmatic corneas, leaving residual astigmatism. As flexure is a function of lens thickness, it is worse with low-minus-powered lenses. For a given family of materials, flexure tends to increase with increasing Dk . It is therefore necessary to increase lens centre thickness with higher- Dk materials. Table 15.2 gives suggested centre thickness values for lenses of varying power.

Table 15.2

Suggested Centre Thicknesses

Lens power (D)	Centre thickness (mm)
−1.00	0.18
−2.00	0.17
−3.00	0.16
−4.00	0.15
≥−5.00	0.14

Add 0.02 mm for corneal astigmatism ≥2.00 D.

Edge Lift and Edge Clearance

Without a peripheral gap between the edge of the lens and the cornea, mechanical pressure leads to superficial corneal damage. Such a gap is also important for tear exchange and to enable lens removal using the lids. This gap is termed the edge clearance and can be specified axially or radially. A minimum axial edge clearance of 60–80 μm when the lens is centred is considered to be the optimal value ( ).

Whereas edge clearance relates to the lens and cornea, edge lift relates to the lens only ( Fig. 15.5 ).

Fig. 15.5, Axial edge lift and edge clearance.

Some rigid corneal lens designs follow a concept of constant edge lift throughout the range of BOZRs; however, this tends to result in greater edge clearance at the flatter end of the range. A superior design is one that gives constant edge clearance for corneas of similar asphericity. Constant edge clearance rigid corneal lens designs based on those proposed by are given in Appendix H .

Edge Form

The shape of the lens edge is one of the most important factors in minimizing any discomfort. Poor edge rounding, in particular, can result in greater edge awareness by the upper eyelid. Good rounding of the front-surface edge has been shown to be more important than rounding of the posterior edge ( ). This suggests that the interaction of the edge of the lens with the eyelid is more important in relation to comfort than the interaction with the cornea. Fig. 15.6 shows examples of edge shapes.

Spherical Versus Aspheric Designs

A wide variety of surface shapes have been used in rigid corneal lenses. These can be broadly categorized as: spherical, aspheric or a combination of the two (e.g. spherical centre with aspheric periphery). An aspheric back-surface design is theoretically capable of providing better alignment to the cornea (which itself is aspheric), although there are a number of advantages and disadvantages to both types ( Table 15.3 ).

Table 15.3

Advantages and Disadvantages of Aspheric Versus Spherical Rigid Corneal Lenses

	Spherical	Aspheric
Manufacture	Relatively easy	Easy with computer-controlled lathes Not easy with conventional lathes
Verification	Easy	Difficult
Induced astigmatism	None	Small amount with decentration
Presbyopic correction	None	Small amount
Corneal alignment	Adequate	Slightly greater
Edge clearance	Usually 80–120 μm	Usually 60–90 μm
Back-surface junctions	Can be a problem if not blended	Rarely a problem
Thickness	Can be minimized by lens design	Thinner periphery possible
Comfort	Slight to moderate discomfort	Sometimes better owing to reduced thickness and edge clearance

Spherical Designs

Spherical designs incorporating a spherical back optic zone with a number of flatter spherical peripheral zones are the most widely used and readily understood form of rigid lens. The peripheral zone is generally 1–2 mm in width and composed of one to four peripheral curves. Tricurve designs (i.e. a central curve plus two peripheral zones) are probably the most commonly used lens form. Bicurve designs are occasionally used with small lenses (e.g. <8.5 mm). Tetracurve and other multicurve designs are used with larger lenses or where a smoother transition is required between the peripheral zones.

The front surfaces of most spherical designs are bicurve, incorporating an optic zone that is slightly larger than the BOZD and a front-surface peripheral zone. The curvature of the optic zone is governed by the required lens power and that of the peripheral zone governed by the edge thickness, the power and FOZD of the lens; these parameters are invariably calculated by the manufacturing laboratory. Monocurve front-surface designs (single-cut) are occasionally used in small, low-power lenses, but most lenses are lenticulated (i.e. made with a thinner peripheral zone) in order to reduce mass and overall thickness. Multicurve front-surface designs are occasionally used with higher-power lenses in order to reduce peripheral thickness.

Aspheric Designs

From a mathematical point of view, the choice of spherical geometry is almost arbitrary, as the sphere is merely one of an infinite number of conic sections. As noted earlier, few if any corneas are spherical in shape, and therefore a spherical back surface is not the obvious choice. However, aspheric rigid lenses have two important disadvantages compared with spherical lenses: (1) they are more difficult to manufacture, particularly using conventional lathes, and (2) they cannot easily be checked using a radiuscope or keratometer. Nevertheless, they offer a number of advantages that arguably outweigh their disadvantages (see Table 15.3 ).

The main advantages of aspheric designs relate to comfort. Aspheric designs tend to show less edge clearance and therefore induce less edge sensation from contact with the palpebral conjunctiva. Poor blending of back-surface junctions in spherical lenses can cause irritation when the lens moves off-centre and the peripheral zones come into contact with the cornea. This is generally avoided with aspheric lenses unless the periphery is poorly blended. The gradual flattening of aspheric lens surfaces results in a thinner periphery, which may also help reduce edge sensation.

Optically, aspheric designs can both improve and degrade image quality. When not aligned with the visual axis, aspheric lenses will induce astigmatism. On the other hand, with higher-power lenses, aspheric optics can reduce spherical aberration. In myopic early presbyopes, the reduced minus power in the periphery of aspheric lenses can help with near vision and delay the need for a presbyopic correction.

Aspheric designs take different forms but these differences are usually too subtle to be evident other than from the product literature of the manufacturer. The simplest aspheric design is an elliptical shape selected to be close to, or slightly flatter than, the average cornea. More complex aspheric designs change their degree of flattening (or eccentricity) from centre to edge. Some designs are spherical in the centre, changing to aspheric geometry towards the periphery. Most aspheric designs incorporate a much flatter, often spherical, peripheral zone about 0.2 mm wide. This peripheral zone serves to avoid mechanical irritation when the lens decentres to the peripheral cornea.

Polymethyl Methacrylate Versus Rigid Gas-Permeable Lens Design

Given the risk of corneal exhaustion syndrome and other chronic hypoxic effects of the cornea, there is little or no justification for using polymethyl methacrylate (PMMA) lenses ( ). However, it is useful to have an appreciation of differences in design between nongas-permeable PMMA lenses and rigid gas-permeable (RGP) lenses so that, when PMMA lenses are being refitted, these differences can be taken into account. Because of the reliance on transfer of oxygen through the tears to the postlens cornea, PMMA lens designs generally incorporated a wider and flatter periphery. This results in a narrower optic zone and greater edge clearance ( ). In comparison, RGP lens designs usually incorporate the following features:

larger TD
larger BOZD
narrower periphery
steeper peripheral curves (resulting in less edge clearance)

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