Introduction
Assessment of soft contact lens fit is probably the most commonly undertaken task in contact lens practice but is also one of the least discussed, possibly because it is regarded as a relatively straightforward exercise. However, soft lens fitting is not just a process of finding a soft lens that fits but also one of determining the soft lens and wearing regimen that will provide the patient with the most comfortable, convenient and safe contact lens wear.
There is a traditional view that fitting soft lenses is a less technically challenging option than fitting rigid lenses; however, the increased choices of materials, wearing regimens, care systems and lenses themselves make the decision-making process as complex for soft lenses as rigid lenses. Since these decisions rely on clinical judgement rather than measurement, soft lens fitting, when done well, is a skilled activity.
Ocular Measurement
Contrary to popular belief, keratometry is of little help in the fitting of soft lenses because the curvature of the central cornea is only one of a number of relevant ocular parameters governing soft lens fit. Normal variations in corneal asphericity and diameter have as much effect on corneal geometry (e.g. sagittal height) as the normal variation in corneal curvature ( ). Thus keratometry alone is a poor predictor of the optimum soft contact lens base curve (BC) radius. As there is a positive correlation between corneal diameter and corneal curvature (i.e. a tendency for flatter corneas to be larger in diameter), any change in sagittal height due to varying corneal radius is nullified by the corresponding variation in corneal diameter ( ). This would suggest that corneal asphericity is the most important determinant of soft lens fit.
One caveat applies: with atypical combinations of corneal diameter and curvature, may indicate extremes of sagittal height. For instance, a large cornea showing a relatively steep keratometry measurement is likely to have a relatively large sagittal height and will probably require a soft contact lens with a correspondingly large sagittal depth (i.e. steep). Conversely, a small flat cornea is likely to have a small corneal sagittal height and may require a relatively flat BC. When viewed from the side, these corneas often seem abnormally deep or shallow, even to the naked eye.
Measurement of the horizontal visible iris diameter (HVID) provides a useful guide to whether a large or small lens is required. However, this is only a rough indicator because the true corneal diameter is significantly larger than the iris diameter. The horizontal corneal diameter has been shown to be, on average, 1.5 mm larger than the HVID ( ). This can be measured to the nearest half-millimetre with a pupillary distance rule or, more accurately, using a slit-lamp graticule.
Palpebral aperture does not have the same relevance as in the fitting of rigid lenses, but extreme cases are worth noting. A narrow palpebral aperture may increase difficulties of insertion and so, given the choice, a smaller lens may be appropriate. Larger palpebral apertures are often associated with incomplete blinking. This might influence the choice of lens material, particularly when noted in combination with signs of corneal desiccation staining.
Ethnic Variations
A number of studies of ocular topography have found ethnic differences, particularly between East Asian and Caucasian populations. Several reports have confirmed a tendency for smaller corneal diameters and palpebral apertures with Asian eyes ( ). The literature on corneal curvature is less consistent; some studies noted steeper corneas in East Asian eyes compared to Caucasians, while others found the opposite. Either way, most of the reported differences are too small to affect soft contact lens fit. Studies relating to ethnic variations in soft lens fit are also contradictory. One study of toric soft lenses noted a need for a steeper lens design for Hong Kong Chinese ( ) while another found that a flatter fitting soft lens design was more successful in Chinese than Caucasian patients ( ).
Regrettably, there is less information on Afro-Caribbean populations. One study of decorative soft lenses suggested that South African patients tend to require looser fitting lenses ( ). This is consistent with the finding of smaller corneal volume in Africans compared with Caucasians ( ).
Basic Principles
Forces Acting on a Soft Lens
A range of forces act on a soft lens, keeping the lens in place on the eye but allowing it to move a small amount between blinks ( Fig. 8.1 ).
Soft lenses are required to flex in two directions to align to the shape of the cornea and sclera. As soft lenses are usually flatter than the central corneal curvature, they steepen to align with the cornea but flatten and stretch at the periphery so as to align with the sclera. A useful model is to envisage the lens periphery as a series of concentric elastic bands that stretch to align with the peripheral ocular shape ( ). When a large amount of peripheral stretching is required, this results in a tight fit. Conversely, when the lens is relatively large and there is no stretching, the lens is relatively loose and may even show edge stand-off. Typical edge strain (or stretching) for a well-fitting lens has been estimated at 3% ( ).
The stresses formed in the lens are proportional to the mechanical properties of the material as well as the dimensions of the lens. Due to the viscous nature of the tear fluid, this deformation of the lens to match the shape of the eye results in pressures being developed in the postlens tear film that have termed ‘squeeze pressure’. This squeeze pressure is related to the amount of force required to move the lens across the eye and therefore lens fit ( ). The amount of force required to move the lens is also related to the viscosity of the postlens tear film ( ) and this helps to explain why the soft lens movement can vary markedly during a given wearing period.
Soft lens retaining forces are relatively large compared with those of rigid lenses and therefore gravitational force has less of an effect.
Ideal Soft Lens Fit
The appropriate soft lens, as far as possible, should be indiscernible to the patient during wear. In other words, there should be no discomfort or disturbance of vision throughout the wearing period. Any effect on ocular physiology should be minimal and within acceptable limits. A well-fitting soft contact lens should fulfil the criteria listed in Table 8.1 .
| Requirement | Significance |
|---|---|
| Good comfort |
|
| Constant corneal coverage |
|
| Good centration |
|
| Movement on blink or version |
|
| Optimum tightness on push-up |
|
| Good peripheral fit (i.e. alignment) |
|
| Good and stable vision | Patient satisfaction |
Soft Lens Design
Contact lens practitioners have little direct control over lens design. Custom-made soft lenses can be ordered from specialist laboratories; however, these are relatively expensive. Nevertheless, they can be useful for extreme prescriptions or ocular topographies that prove unsusceptible to stock designs.
Most lenses are offered in a limited range of specifications (e.g. two BCs in a single diameter). Despite the relatively wide range of corneal diameters in the population, most spherical soft lenses are specified within a relatively narrow range of diameters: 14.0–14.5 mm. The practitioner is therefore largely dependent on the judgement of the manufacturer.
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. The design of proprietary soft lenses is governed by a number of factors in addition to geometry, including material, method of manufacture and lens power.
Lens Material and Water Content
The selection of contact lens material is relevant to several aspects of lens performance ( ):
- •
oxygen transmission
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deposit resistance
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surface wetting
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rigidity (and therefore fitting characteristics)
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lubricity
Nowadays, there is little justification for using low- Dk lenses such as poly(hydroxyethyl methacrylate) (polyHEMA) lenses. The effects of hypoxia with conventional low-water-content (i.e. <50%) lenses are well documented. Based on estimates of corneal swelling, complications such as corneal striae can be expected to affect a high proportion of wearers. An exception to this rule is in those rare cases where it proves impossible to fit patients with higher- Dk products.
The prime decision, therefore, is whether to use a silicone hydrogel or conventional hydrogel material in mid- (50%–60%) or high-water-content (>60%) lenses. All silicone hydrogel lenses provide superior oxygen transmission compared with conventional hydrogel lenses and, as a result, have become the most commonly used lens type in most countries ( ). However, some silicone hydrogel materials incorporate shortcomings in relation to mechanical properties and surface wetting, particularly the earlier versions (see Chapter 4 ).
Comparing conventional hydrogels, a higher-water-content material does not necessarily guarantee that the lens will produce better oxygen transmissibility ( ). Different material categories (e.g. ionic and nonionic) have differing strengths and weaknesses, which can be taken into account when selecting a lens type for a given patient.
Higher-water-content lenses tend to be thicker for two reasons. First, higher-water materials have a lower modulus and therefore tend to be less easy to handle in equivalent thicknesses. Second, there is a minimum thickness threshold for any material below, which lenses tend to induce corneal desiccation staining. There is some variation between patients and precise material types; however, for a 70% water content material, the critical thickness is approximately 0.12 mm, whereas with mid-water lenses it would be approximately 0.06 mm.
Method of Manufacture
The method of lens manufacture can influence the edge design. Cast moulding is the predominant method of soft lens manufacture owing to the relatively low manufacturing costs, high reproducibility and the fact that it allows thin edges to be formed. Lathing allows a greater range of parameters, but lathe-cut lenses tend to incorporate a thicker edge design than moulded lenses. Some toric designs employ a combination of moulding and lathing (hybrid), for instance, a moulded toric back surface with a lathed front surface. This allows a much wider range of prescriptions than is practicable with moulding technology alone.
Base Curve Radius (Back Optic Zone Radius)
Soft lenses are specified by BC radius, total diameter, back vertex power and material. If the lens is available in only a single specification (i.e. a one-fit lens), the lens brand name and power may be enough to specify the lens prescription. However, this apparent simplicity belies the complexity and importance of soft lens design. Lenses with apparently similar specifications can show widely differing fitting characteristics, for instance, due to variations in back surface design ( ).
Traditionally, BC is the main parameter to be modified when attempting to optimize lens fit; a steepening of BC is required to tighten lens fit and vice versa. However, even with relatively thick low water lenses, large changes in BC are required to have a significant effect on lens movement ( ). With thinner low modulus lenses, changes in BC have even less effect. The labelled BC is therefore of little help in soft lens fitting.
BC is also not helpful when comparing different brands of lens. Lenses of similar BC can show widely differing sagittal depths because of differences in back surface design ( ). This and differences in materials’ mechanical properties means that widely differing lens fits may be observed from seemingly similar lenses. With silicone hydrogel lenses of high modulus, even small (0.2 mm) changes in BC can have a significant impact on lens fit and comfort ( ).
Total Diameter
Unfortunately, the labelled total diameter is often as unhelpful as the labelled BC. The fact that the water content, and therefore the dimensions, of some lens materials varies with temperature makes it difficult to compare the labelled diameter of one lens with another. Most nonionic lenses, particularly those containing N -vinyl pyrrolidone, shrink by approximately 0.5 mm when raised from room to eye temperature. Ionic lenses are also temperature sensitive, although they shrink much less than nonionic lenses ( ). Some lenses are made larger in diameter to compensate for this.
Two further complicating factors affect the on-eye diameter. First, lenses of steeper BC have greater surface area and, therefore, larger on-eye diameter. A change in BC of 0.4 mm changes the on-eye diameter by approximately 0.2 mm ( ). Second, on-eye diameter is affected by the sagittal depth of the lens. Lenses of similar nominal diameter can vary in sagittal depth by as much as 1 mm ( ) and, as the periphery of a soft lens flattens to align with the scleral profile, the sagittal depth can have a significant effect on effective diameter.
Back Vertex Power
Some modifications are often made to the lens design at the extremes of the power range. At the lower end of the minus power range (<−1.50 D), the centre thickness is usually increased to improve lens handling. At the higher end of the power range, the centre thickness is often reduced and the optic zone diameter kept to a minimum to maximize oxygen transmission.
The thickness of high-minus lenses can be further reduced and the optical performance improved by incorporating aspheric optics to overcome lens spherical aberration ( ; ). Correcting spherical aberration can improve optical clarity for prescriptions greater than +3 D and −6 D but only for larger pupils (≥6 mm). Aspheric optics can also be incorporated to reduce the spherical aberration of the eye ( ). A number of designs are available that claim to use aspheric optics to improve visual performance; however, the published data have given conflicting results ( ).
With plus-power lenses, the optic zone diameter is again minimized to minimize centre thickness. There tends to be little difference in lens fit between low-minus and higher-minus lenses of similar design. However, plus-power lenses tend to show significantly more postblink movement than do minus lenses, which is probably due to greater interaction with the upper lid ( ).
Centre Thickness
Lens centre thickness is relevant to ease of lens handling and susceptibility to dehydration. Mid-water-content hydrogel lenses (50%–59%) are generally manufactured with centre thicknesses in the range 0.06–0.10 mm, whereas high-water-content lenses (60%) generally have centre thicknesses in the range 0.10–0.18 mm. Silicone hydrogel lenses vary widely in water content but tend to have centre thicknesses in the range 0.07–0.09 mm.
Edge Thickness and Design
Due to poor measurement repeatability, soft lens peripheral thickness is not the subject of an ISO standard and is not always routinely verified during lens manufacture. Nevertheless, variations in peripheral thickness can have a significant effect on lens fit; contrary to expectations, thicker-edged lenses often show a looser fit than do thinner lenses of similar basic design ( ).
Modern designs generally taper to a thinner edge than many older designs. Several edge shapes have been identified, including ‘rounded’, ‘knife’ and ‘chisel’ edges. The thinner knife- and chisel-edge designs appear to give better comfort, sit closer to the bulbar conjunctiva, show less movement and have less interaction with the lids than do the rounded design ( ; ).
Soft Lens Fitting Options
Many soft lenses show an acceptable fit on a wide range of eyes, but an acceptable fit is not necessarily the optimal or most comfortable fit. As well as finding the most appropriate lens design, selecting the optimal lens involves finding the lens material, lens replacement schedule and wearing regimen that best suit the individual patient. To achieve this, it is necessary to use a wide range of soft lens types and brands.
Trial Lens Fitting
Initial Lens Selection
As discussed earlier, without understanding the material and design characteristics of a particular lens, it is not possible to predict its clinical fitting performance. Even with this information, it is difficult to predict from keratometry and HVID measurements which lens is likely to be the most suitable.
The selection of a first trial lens can take into account HVID, particularly if the cornea appears to be unusually large or small. The selection of BC is a process of trial and error unless there is useful information from the patient’s previous lenses. If, for instance, the patient previously required a steeper-fitting lens to achieve a successful fit, this would suggest the need for a lens with relatively tight-fitting characteristics.
Lens material and wearing regimen are also key factors in the selection of the initial trial lens. Although compromises occasionally have to be made, these should be selected based on an assessment of the patient’s requirements rather than on prescribing habits or practice policy.
Soft Lens Insertion
- 1.
Before inserting a soft lens, place it between the index and middle fingers and rinse with saline to remove any debris.
- 2.
Allow the excess saline to drain before placing the lens on a dry forefinger.
- 3.
With the forefinger of the other hand, hold open the patient’s top lid while holding open the bottom lid with the middle finger of the other hand ( Fig. 8.2 ).
