Why wouldn’t you fit your astigmatic patients with toric silicone hydrogel lenses?

Some 35 years ago, Holden determined that, in a soft lens wearing population, if all astigmatism of 0.75D or more was corrected, 45% of lens wearers would need toric lenses; and, if all astigmatism of 1.00D or more was corrected, that 35% of all wearers would require toric lenses. [1] However, a significant number of practitioners have historically ignored low levels of astigmatism, choosing instead to fit either spherical lenses or aspheric lenses, in an attempt to fit simpler lens designs and reduce costs for patients. Despite the fact that it has been shown that “masking of astigmatism” with spherical soft lenses is minimal [2] and that the visual performance with aspheric lenses is inferior to that of soft torics, [3] the numbers of patients fitted with toric soft lenses remains lower than it should be.

So, what percentage of soft lens wearers are fitted with soft torics? In 1991, Pearson estimated that only 9% of all soft lens fits in the UK were soft torics. [4] More recent surveys have suggested that this has gradually increased, [5-7] to the point where toric lenses now account for 28-38% of soft lens fits in Canada, the US and the UK, [7] suggesting that in these markets practitioners are routinely utilizing soft torics in soft lens patients with 1-1.25D of astigmatism, a figure confirmed by Morgan and Efron. [6] However, many countries continue to lag significantly behind these rates, with practitioners in Bulgaria, China, Hong Kong, Japan and Russia only fitting 5-13% of their soft lens patients with soft torics. [7] It is highly unlikely that these differences represent true differences in amounts of astigmatism in the population, but rather reflect the degree to which practitioners have embraced fitting soft torics to their patients.  This is of considerable concern for the contact lens industry, as poor vision is a significant factor driving contact lens dissatisfaction and dropout. [8-10]

In order to assist with toric lens stability, incorporate the more complex optics and minimize rotation, toric lenses are typically larger and thicker than their spherical counterparts. When made in traditional low oxygen permeability (Dk) hydrogel materials, this increased mass and correspondingly lower oxygen transmissibility (Dk/t) has resulted in substantial hypoxic compromise for the majority of toric lens wearers. The measurably poor Dk/t [11] has resulted in many patients exhibiting significant levels of neovascularization, [12] myopic creep and corneal distortion, even in daily wear.

When silicone hydrogels were first introduced in the late 1990s, a frequently asked question by practitioners related to when these materials would be available in toric designs, and their wishes were granted in 2005, with the launch of PureVision Toric. Since that time, four other silicone hydrogel torics have been released (see Table 1). These lenses are now available in a wide variety of parameters, provide greater amounts of oxygen to the eye than hydrogel torics, [13, 14] induce less corneal swelling [15] and are expected to be rapidly adopted by clinicians around the globe. Although no global data has yet been published on what percentage of prescribed soft torics are made in silicone hydrogel materials, a recent estimate suggested that 60% of soft torics dispensed in the US are made from silicone hydrogel materials. [16]

Table 1: Silicone Hydrogel Toric Contact Lenses

† Dk/t will vary according to lens power
*Cylinder powers not available in all spherical powers and axes, check with manufacturers.

The lenses have been shown to be highly reproducible, easy to fit and predictable in terms of their stability and rotation. [17, 18] In a study recently conducted at the CCLR (data on file), more than 60% of the silicone hydrogel toric lenses dispensed (Acuvue OASYS for Astigmatism, Air Optix for Astigmatism and PureVision Toric) had the same axis as the manifest spectacle correction and the axis of the remainder were within 10 degrees of the manifest spectacle correction.

So why would practitioners not prescribe a silicone hydrogel toric lens? One potential reason could be cost, as toric silicone hydrogel lenses are usually one third to two thirds more expensive than their spherical counterparts. Consequently patients wearing a toric silicone hydrogel lens may be more likely to wear the lens for longer than the manufacturer's recommended replacement frequency, when compared with spherical silicone hydrogel lenses. This was indeed shown to be the case in two recent studies investigating compliance with replacement frequency conducted at the CCLR. [19, 20] Another reason for not refitting patients could relate to concerns regarding the ease with which current HEMA toric wearers can be converted into silicone hydrogel torics. Studies suggest that modern silicone hydrogel toric designs are extremely good and practitioners should have no concerns about undertaking such refits. [18]

In closing, the widespread availability of fitting sets that address the majority of patients' astigmatic needs has made upgrading patients to silicone hydrogel toric lenses infinitely simpler.  Given the substantial improvements in oxygen transmission, excellent comfort and ease of fitting, it is recommended that these lenses should become the toric lenses of choice for both neophytes and existing patients needing soft lens torics.

References

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4. Pearson R: Contact lens trends in the United Kingdom in 1991. J Brit Contact Lens Assoc 1992; 15; 17 - 23.
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12. Westin E, McDavid K, Benjamin W: Inferior corneal neovasculrisation associated with extended wear of prism ballasted toric hydrogel lenses. Int Cont Lens Clin 1989; 16; 20 - 22.
13. Forister JF, Chao J, Khy K, Forister E, Weissman BA: Predicted tear layer oxygen tensions under two designs of silicone hydrogel toric lenses. Cont Lens Anterior Eye 2008; 31;5: 228-41; quiz 274-5.
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15. Dumbleton K, Moezzi A, Fonn D, Simpson T: Comparison of overnight corneal swelling induced by lotrafilcon B toric versus alphafilcon A toric lenses. Contact Lens And Anterior Eye 2006; 29;4: 193.
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18. Secor G: A clinical comparison of two toric lenses. Contact Lens Spectrum 2008; 22;5: 42 - 44.
19. Richter D, Dumbleton K, Guthrie S, Woods C, Jones L, Fonn D: Patient and practitioner compliance with silicone hydrogel and daily disposable lens replacement in Canada. Can J Optom 2009; In press.
20. Dumbleton K, Woods C, Jones L, Fonn D: Comfort and vision with silicone hydrogel lenses: Effects of compliance. Optom Vis Sci 2010; In press.