Soft Toric Lens Design and Fitting

Introduction

The use of soft toric lenses (in preference to spherical soft lenses) is indicated when there is ocular astigmatism present, be it corneal or noncorneal, that warrants correction. Unlike rigid lenses, soft lenses do not mask corneal astigmatism but rather conform to the shape of the cornea. Consequently, correcting ocular astigmatism with soft lenses requires that cylinder be incorporated into the back vertex power (BVP) of the lens.

Numerous manufacturers of soft contact lenses have made extremely optimistic and unrealistic claims of their spherical lenses being able to correct satisfactorily astigmatism of between 1.00 and 2.00 D. Only rarely is this achieved. showed that there was no statistically significant masking of corneal cylinder with standard thickness soft spherical lenses. Indeed, the most helpful indication of the likely residual astigmatism found while wearing a spherical soft contact lens is the ocular astigmatism determined from an accurate subjective spectacle refraction.

For many years it was held that prospective contact lens wearers with clinically significant astigmatism could not be successfully fitted with soft lenses. Since the early 1980s, however, notable advances in soft toric lens technology have been made such that the correction of astigmatism with soft lenses is now a viable option for the majority of these patients. Indeed, annual contact lens fitting surveys over the last 15 years have consistently demonstrated a commensurate increase in toric lens fitting as a proportion of all soft lenses fitted. This is evident from Fig. 9.1 , which shows the extent of soft toric lens fitting as a percentage of all soft spherical and toric lenses prescribed in 17 nations between 2000 and 2020 ( ).

Fig. 9.1, Extent of soft toric contact lens fitting as a percentage of all soft spherical and toric lenses prescribed in 17 nations between 2000 and 2019.

Based on the distribution of astigmatism in prospective contact lens wearers, it has been determined that, if all astigmatism of 0.75 D or more were corrected, 45% of lens wearers would need toric lenses ( ). This threshold is shown as a dotted line in Fig. 9.1 . It is evident from Fig. 9.1 that soft toric lens fitting has evolved to the point whereby, since about 2015, nearly all those with clinically significant astigmatism are being corrected with toric lenses in most countries.

Criteria for Use

When deciding whether or not to prescribe a soft toric lens, practitioners should avoid using criteria such as ‘all patients with cylinders greater than a certain amount should be fitted with soft toric lenses’. Instead, each patient should be assessed separately, taking into account the following factors.

Degree of Astigmatism

As a generalization, 1.00 D or more of astigmatism should be corrected, although there will be significant variability between patients. , in discussing the criteria for the prescribing of toric lenses, showed that 45% of the population required a cylindrical correction of up to 0.75 D and 25% of the population required a correction of 1.00 D or more. A more recent study by revealed that the prevalence of patients showing astigmatism of 0.75 and 1.00 D or greater in at least one eye was 47% and 32% respectively. As mentioned previously, the current soft toric lens prescribing rate suggests that almost all cases of astigmatism 0.75 D or more are now being fitted with soft toric lenses.

Note that soft toric lens misalignment – which is discussed in more detail later in the chapter – becomes more significant as the degree of cylinder is increased. For example, a patient with a soft toric lens incorporating a 1.25-D cylinder may be able to tolerate a 5-degree rotation from the expected lens location, whereas a toric lens patient with a 3.50-D cylinder will probably notice a significant drop in vision for the same degree of rotation off axis. Consequently – and not surprisingly – the success rate with soft toric lenses does decrease as the degree of astigmatism requiring correction is increased.

Degree of Spherical Ametropia

The amount of myopia or hyperopia associated with the astigmatism is an important consideration. Patients who are fitted with soft toric lenses that contain a low spherical component, for example +0.25/−1.50 × 180, will generally need their astigmatism to be corrected – and are often very critical of axis alignment – because the astigmatism is the most significant component of their refractive error. Conversely, there is often less need to prescribe soft toric lenses for patients with a high degree of spherical ametropia in addition to their astigmatism. For example, a contact lens patient with an ocular refraction of −6.00/−1.00 × 170 may be content with just wearing a soft spherical lens and not having the astigmatism corrected; on the other hand, a patient with an ocular refraction of −1.50/−1.00 × 170 is more likely to want their astigmatism to be corrected and so require a soft toric lens.

Cylinder Axis

The axis of the ocular cylinder is a critical factor. For example, an uncorrected cylinder with an oblique axis will cause greater degradation of visual image compared with an equivalent amount of uncorrected with-the-rule or against-the-rule astigmatism ( ). As will be discussed later in this chapter, soft toric lenses incorporating oblique cylinders may also show poorer stability due to complex lid lens interactions.

Ocular Dominance

Uncorrected astigmatism is far more likely to be accepted by the patient if it is in the nondominant eye. For example, patients may tolerate uncorrected cylinder of up to 2.00 D in their nondominant eye, while at the same time requiring that cylinder as small as 0.50 D be corrected in their dominant eye. Related to this is the situation where a patient has unequal visual acuities. In this case, higher degrees of uncorrected astigmatism will usually be tolerated in the eye with the poorer acuity.

Viability of Other Alternatives

The practitioner also needs to consider whether soft toric lenses are the best option or if the patient would be better off with spectacles or rigid lenses. For example, a patient with high degrees (>5.00 D) of both corneal and spectacle astigmatism would most likely achieve better acuities with a rigid toric lens. In addition, no form of soft toric lens can correct irregular astigmatism. Patients with astigmatic errors of this nature, for example people who have keratoconus, are usually corrected with some form of rigid contact lens.

Visual Needs of the Patient

Usually, the less critical the visual task, the greater the amount of astigmatism that can be left uncorrected (and vice versa). For example, a musician may require that a cylinder as small as 0.50 D be corrected to enable music to be read. On the other hand, a person with no specific critical visual tasks may be happy with a cylinder as high as 2.00 D left uncorrected so long as the spherical component of their refractive error is corrected.

Design Considerations

Lens Optics

The optical considerations for soft toric lenses are different from those encountered when using rigid lenses. This is primarily because a soft toric lens will align with the anterior surface of the cornea such that a negligible tear lens forms between the back surface of the lens and the front surface of the cornea. Consequently, the optical principles of rigid toric lenses do not apply. There are no tear lens calculations to perform and all the ocular astigmatism will usually be corrected by incorporation of cylinder into the BVP of the soft toric lens.

Soft toric lenses can be manufactured with a toroidal back surface and a spherical front surface or, conversely, a spherical back surface with a toroidal front surface. Regardless of which of these optical configurations is prescribed, the end result on the eye will always be a bitoric lens form due to the wrapping of the front and back surface of the lens onto the cornea. The choice of design (i.e. toric back surface vs toric front surface) is generally based more on considerations relating to manufacture, lens stability and physiological performance.

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