Single-vision soft contact lenses have a number of optically attractive features. They centre well on the cornea with only small amounts of lateral movement and hence introduce little additional asymmetric aberration into the lens–eye system. The diameter of their optic zone normally exceeds that of the entrance pupil of the eye under all lighting conditions; thus the ‘haloes’ around light sources that are observed at night following excimer refractive surgery or during wear of some rigid lenses are avoided. Their large overall diameter, greater than that of the cornea, ensures the absence of the discontinuities, flare and stray light effects that can arise with smaller-diameter rigid lenses in the peripheral visual field, due to refractive and scattering effects at the lens edges. Their refractive index (about 1.37–1.48) is quite close to that of the cornea, so that Fresnel reflection losses are comparable to those in the natural eye. They shape themselves so that their back surface conforms closely to the anterior surface of the cornea, thus minimizing fitting problems.
On the other hand, the tendency to drape to conform to the corneal surface and the consequent near-elimination of tear lens effects mean that, unlike rigid lenses, soft lenses cannot compensate for modest amounts of corneal astigmatism. Only by using a well-stabilized toric lens can the latter be corrected (see Chapter 9 ). Other possible disadvantages from the optical point of view include lens flexure and hydration variations, which may result in on-eye power changes. This chapter will primarily be concerned with such on-eye power changes and aberration effects, insofar as they apply to the optic zone of spherical corrections.
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.
On-Eye Power Changes
Before being placed on the eye, the back vertex power of a soft lens is normally checked when the lens is at room temperature in a fully hydrated state (see Chapter 7 ). As has already been noted, when the soft lens is worn, its inherent flexibility allows it to ‘drape’, so that the shape of the posterior surface approximates closely to that of the anterior cornea. While this greatly simplifies fitting, any associated changes in the curvatures of the lens surfaces and lens thickness may result in the on-eye power of the lens differing slightly from that measured off-eye.
Although draping implies that the tear lens between the contact lens and the cornea ought to have zero power, this may not always be the case. found no evidence for a significant tear lens, and several subsequent authors agreed with this result ( ). However, and suggested that a tear lens about 10 µl in volume may sometimes exist and contribute about −0.15 D of power to the combined lens–eye system. went on to propose that, although low-minus lenses may entrap only a small volume of tears (about 5.5 µL), thicker, low-plus lenses may entrap a greater volume (about 9.5 µL), giving a correspondingly greater tear lens effect (up to −2.00 D).
Changes in hydration, which are a function of the lens design and material ( ), the wearer, the visual task and the environmental conditions ( ) will affect the refractive index and geometry of any soft lens, and hence its power. Typically, hydration may fall by around 5%–10% after the first hour of lens wear ( ). Thinner lenses reach equilibrium after about 5 minutes, whereas thicker, high-power positive lenses may continue to dehydrate for 30 minutes or more after insertion. Effects appear to be material dependent ( ); in particular, high-water-content lenses dehydrate more and reach equilibrium sooner than lower-water-content lenses of comparable thickness. There is a strong suggestion that greater dehydration may occur during near work, due to reduced blinking, and when atmospheric humidity is low. Water loss can occur by several pathways, including evaporation into the atmosphere, drainage into the nasolacrimal system and, possibly, absorption into the conjunctival capillaries.
As the corneal temperature is around 32°C–35°C ( ), while the room temperature is normally about 20°C, there is a change in temperature when the lens is put on the eye: this affects all the lens parameters ( ), including hydration and refractive index. The measurements made on a variety of soft lens materials by , however, suggest that the maximum increase in ( n L −1), where n L is the refractive index, for a temperature rise of 14°C is only about 1 part in 40, so that the associated power change in a lens of this material would only be about 0.25 D for a ±10 D lens.
Many authors have attempted to develop either theoretical or empirical models to allow overall on-eye power changes to be predicted ( ). Among the more mathematically sophisticated models of flexure alone are those based on the concept that the flexed lens always retains constant volume ( ) or that the arc length of the back optic zone of the lens remains constant ( ). Experimental studies, whose results include the effects of all the factors mentioned, not just those of flexure, agree in indicating that any power changes are small for negative lenses but that larger, clinically significant changes, which increase with the lens power, occur for positive lenses.
This can be seen in Fig. 6.1 , which compares a smooth continuous curve derived from best fits to a substantial body of experimental data by , with more recent experimental data ( ).
Also shown (dashed) is a curve based on the empirical equation proposed by , that is,
have suggested that the paraxial powers of contemporary thin, disposable, soft lenses do not change when the lens is placed on the eye, and that any small apparent discrepancies in the expected subjective correcting effect might be associated with the spherical aberration of the lens and eye. This would be expected to shift the optimal focus away from the paraxial value towards the marginal focus in a way that depended upon the pupil diameter and the sign and magnitude of the aberration.
It should be mentioned that unusually thick (up to 0.6 mm) soft lenses have occasionally been suggested for the correction of keratoconus ( ). These show less draping during wear and might be expected to be accompanied by substantial tear lens effects and effective on-eye power changes.
Thus at the present time neither modelling nor experimental data provide fully convincing predictions for on-eye power changes. All the earlier models involve inadequately justified assumptions and it is probable that additional factors such as material, lens design, fitting philosophy, patient tear flow and lens–lid interactions also influence the results occurring in practice. It may be that the greater thickness of the first generation of soft lenses unduly influenced the early results. A recent theoretical study suggests that additional factors, including base curve and lens diameter, may also affect the effective power changes in both spherical and toric soft lenses ( ). Although predictions based on Fig. 6.1 should give a general guide to the magnitude of the on-eye power changes to be expected, for practical work, it still seems more sensible to follow the conventional approach of trial lens fitting, since after suitable equilibration the trial lens will display similar on-eye effects as the ordered lens of the same design and power ( ).
pointed out that, for foveal vision and well-centred, rotationally symmetric contact lenses having steeply curved surfaces, the classical lens aberration of greatest potential importance would be spherical aberration, in which the power of the contact lens varies with distance from its axis. This is in contrast to spectacle lenses where, since the eye moves with respect to the lens, oblique astigmatism, distortion and field curvature are all introduced whenever the visual axis moves away from the optical centre of the lens. Indeed, spherical aberration is of little importance in spectacle lenses whereas control of the off-axis aberrations is a major design aim. Since for primary spherical aberration, the wavefront aberration varies with the fourth power of the radius in the pupil and the change in zonal power with the square of the radius, the greatest optical impact of spherical aberration is likely to occur when the pupil diameter is large, that is, under mesopic and scotopic conditions.
With the exception of diffractive lenses for presbyopes (see Chapter 22 ), longitudinal chromatic aberration is normally of negligible importance in either spectacle or contact lens design, since any contribution from the correcting lens is much smaller than that of the eye itself (see Chapter 3 ). While transverse chromatic aberration associated with prismatic effects during oblique viewing is of great significance in relation to spectacle lens materials and their dispersion, it plays little role in relation to well-centred contact lenses.
Early work in this area concentrated on exploration of the benefits of aspheric lens surfaces in reducing the spherical aberration of the contact lens in isolation ( ). It has since been realized that what matters is the combined aberration of the lens–eye system and that a contact lens with minimal spherical aberration does not necessarily lead to the best visual performance ( ). In principle, ideally the aberration of the lens should balance that of the eye, so that the combined system has minimal aberration ( Fig. 6.2 ).
It is clear that, if any series of aspheric lenses designed to have a fixed level of spherical aberration for all lens powers is to offer useful improvements in the vision of patients by compensating for the eye’s own spherical aberration, several conditions must be fulfilled:
The spherical aberration of the eye should be consistent from patient to patient.
Spherical aberration should be the major ocular aberration, so that its neutralization produces a substantial reduction in root mean square (RMS) wavefront error.
The on-eye spherical aberration introduced by the lenses should not differ from the off-eye aberration due to flexure or other factors, or, if it does, it should vary in a systematic, predictable way.
Lens wear should not induce any change in the ocular aberrations, for example through corneal warpage.
Any decentration of the aspheric contact lens with respect to the axis of the eye should not itself introduce significant aberration.
The neural components of the visual system should be capable of appreciating any benefits in optical image quality that correction might provide.
Although various early attempts were made to assess the impact on visual performance of soft lenses designed to have different levels of spherical aberration ( ), they produced somewhat mixed results, with some patients showing improvements, some no change and others a loss in visual performance. Under photopic conditions, with relatively small pupils, mean high- and low-contrast visual acuities produced with spherical and aspherical versions of the same lens showed no significant differences ( ). were, however, able to demonstrate that if the aberration of the lens was systematically varied, mesopic contrast sensitivity at 12 c/deg passed through a peak for each patient: the optimal spherical aberration of the correcting lens tended to become more negative with age. Two groups of subjects aged between 20 and 45 years were used: one consisted of emmetropes and the other of myopes with corrections between −5.00 and −10.00 D. The range of lenses used was designed to display varying amounts of primary spherical aberration of the form