By Desmond Fonn
Introduction
Patients with refractive astigmatism who would rather wear contact lenses than spectacles should be fitted with toric soft lenses. This is a sweeping statement as some would argue that for visual reasons spherical rigid gas permeable (RGP) lenses will correct astigmatism resulting in visual acuity that would closely match their vision with spectacle lenses.
The argument is flawed if the refractive astigmatism is not all corneal, resulting in residual astigmatism induced by the RGP lens. Plus, there is always approximately 10% residual of the corneal astigmatism with a spherical RGP lens, but it is dependent on the refractive index of the RGP lens material. The range of astigmatism that I am referring to is 0.75 – 3.00D, and if vertexed to the corneal plane might be closer to 2.75D.
The reason for my bold statement is that the current inventory of toric soft contact lens designs would cater for that power range and manufacturing capability is so good that practitioners and patients can be assured of a predictable correction. There is little doubt as well about the vastly superior comfort of soft lenses [1]. In the low astigmatic range, masking of astigmatism with spherical soft lenses does not provide the level of vision of toric soft lenses [2-4], and the visual performance with aspheric lenses is also worse than soft toric lenses. [5]
Prevalence of Astigmatism
In an Australian study of 179 patients in the 1970s, Holden reported that 45% of the prospective contact lens patients exhibited 0.75 D or more of astigmatism and if the astigmatism of 1.00D or greater was corrected, 35% of the patients would require toric lenses but only 25% of the cohort exhibited 1.25 D or more.[6]
Young et al’s paper on the prevalence of astigmatism in relation to soft lens fitting included a summary table of 8 papers and showed that in a large age range, the prevalence ranged from 16 - 45% with astigmatism of 0.75 D or greater. [7] Young et al worked from a database of 11,624 patients (age range 8 to 70 years) and the prevalence of astigmatism of 0.75D group was 24.1% and 1.00 D or greater in both eyes was 15.0%.
The prevalence of astigmatism of 0.75 D or greater in myopes was almost twice that of hyperopes and the prevalence of with-the-rule (WTR) astigmatism was higher than against-the-rule (32.9% vs. 29.1%). Their summary showed approximately 33% of potential contact lens wearers require astigmatic correction based on the spectacle prescription from the database.
Cho et al’s paper on spherical and toric daily disposable lenses for low astigmatic corrections included a prevalence of astigmatism summary of 4 papers [9-12]. They wrote 46 to 63% of adults had astigmatism, but the percentage was dependent on the magnitude used as the threshold.
Percentage of the correctable population fitted with toric lenses
Quite appropriately the number and percentage of patients wearing toric lenses has grown quite substantially.
Here are some examples: In 1991, Pearson estimated that only 9% of all soft lens fits in the UK were soft torics [8]. Morgan et al reported that the proportion of toric soft lenses prescribed has grown from approximately 12% to 30% [13,14], and the trends in North America are very similar, perhaps as high as 38% [15,16]. Efron has stated that toric soft lenses now represent over 35% of all soft lenses prescribed, but this survey was restricted to seven countries with the exception of Japan where the percentage there is considerably lower.
Lens designs that stabilize the axis/orientation on the eye
The diameter and posterior surface shape or base curve of toric soft lenses are very similar to spherical lens dimensions, but the thickness profile will be substantially different in order to position the correcting cylinder in the correct meridian and to maintain it in that position. The methods include prism “ballast”, periballast, eccentric lenticulation, back-surface toricity, thin/thick zones, and various combinations of these designs.
Most of the recent design developments have incorporated the latter thickness differential designs, where the top and bottom lids govern the orientation of the lens and therefore the astigmatic correction and they seem to be working very well. In a study recently conducted at the CCLR (data on file), more than 60% of the silicone hydrogel toric lenses dispensed (three currently marketed toric silicone hydrogel lenses) had the same axis as the manifest spectacle correction and the axis of the remainder were within 10 degrees of the spectacle correction.
Reorientation or recovery refers to the toric lens’ ability to speedily relocate to the proper axis after the axis is misaligned, for example when physically misaligning the lens.
A number of terms to have been used to describe the lens recovery performance e.g., “reorientation speed”, or “rotational recovery”, Young et al. used “lens reorientation” to determine whether gravity affects the lens orientation or axis mis-location.
They found that the three prism-ballast design lenses rotated away from their baseline position “significantly more” than the accelerated stabilization lens design [18] and concluded that gravity does affect prism ballasted or peri-ballasted lenses rather than the thin/thick zone lenses when patients adopted a recumbent position. The four lens designs showed similar re-orientation speeds.
Fitting toric lenses
On the basis that inventory toric lenses will be used, the majority of practitioners’ inventory will be less than the 3000 lenses that Young has suggested will be needed for 90% of patients but this method of fitting and prescribing is the best. Even if the exact prescription isn’t within stock, a close approximation will with a high degree of certainty, confirm the lens that is required. The three elements that are necessary to fulfill the correct prescription, assuming that the lens fits symmetrically and centrally over the cornea without excessive movement, are:
- 1. The refractive correction at the corneal plane
- 2. The degree of rotation (if any) of the lens
- 3. The rotational stability of the lens
If the refractive correction is known, the only function of the trial lens should be to ascertain if it fits correctly and to measure the amount of rotation of the lens and to compensate for the rotation when determining the final prescription. There are numerous texts and descriptive papers on when and how an over-refraction should be conducted, but that should not be necessary with the method that I have described.
Summary
As the average thickness of toric lenses is influenced by the by prism ballast or periballast and therefore increased thickness in parts of the lens, the oxygen transmissibility may be compromised and this will almost always be negated by the use of silicone hydrogel materials.
As silicone hydrogels,as a category, represent at least 60% of prescribed soft lenses there is every reason to believe that the same statistic applies to toric soft lenses. Fitting and prescribing should no longer be considered as specialty lenses except for those prescriptions that require custom designs.
As toric silicone hydrogel lenses are more expensive than spherical lenses practitioners should guard against non-compliance of exceeding the manufacturers recommended replacement frequency as this habit has been demonstrated in two studies [19,20].
References
1. Fonn D, Gauthier CA, Pritchard N.Patient preferences and comparative ocular responses to rigid and soft contactlenses. Optom Vis Sci 1995; 72:857-863
2. Snyder C, Talley DK: Masking of astigmatism withselected spherical soft contact lenses. J Am Optom Assoc 1989; 60:728-31.
3. Richdale K,Bersten D, Mack K et al. Visual Acuity with Spherical and Toric Soft Contact Lenses in Low- to Moderate-Astigmatic Eyes. OptomVis Sci 2007; 84: 969-75.
4. Cho P, Cheung SW, Charm J. Visualoutcome of Soflens Daily Disposable and Soflens Daily Disposable forAstigmatism in subjects with low astigmatism. Clin Exp Optom. 2012; 95: 43–47.
5. Morgan PB,Efron SE, Efron N, Hill EA: Inefficacy of aspheric soft contact lenses for thecorrection of low levels of astigmatism. Optom Vis Sci 2005; 82: 823-8.
6. Holden BA. The principles and practiceof correcting astigmatism with soft contact lenses.Aust J Optom 1975;58: 279–99.
7. Young G, Sully A, Hunt C. Prevalenceof astigmatism in relation to soft contact lens fitting. Eye Contact Lens2011;37: 20 -25
8. Pearson R. Contact lens trends in the UnitedKingdom in 1991. J Brit Contact Lens Assoc 1992; 15:17 - 23.9. Anstice J. Astigmatism–its componentsand their changes with age. Am J Optom Arch Am Acad Optom 1971; 48: 1001–1006.
10. Fledelius HC, Stubgaard M. Changes inrefraction and corneal curvature during growth and adult life. Across-sectional study. Acta Ophthalmol Copenh 1986; 64: 487–491.
11. Satterfield DS. Prevalence andvariation of astigmatism in a military population. J Am Optom Assoc 1989;60: 14–18
12. Read SA, Collins MJ, Carney LG. Areview of astigmatism and its possible genesis. Clin Exp Optom 2007; 90:5–19.
13. Morgan PB, Efron N. Prescribing softcontact lenses for astigmatism. Cont Lens Ant Eye 2009;32: 97–98.
14. Morgan P. Trends in UK contact lensprescribing 2010. Optician 2010; 239: 34–35.
15. Barr JT. Contact lenses 2001. ContLens Spectrum 2002; 17:22–28.
16. Nichols JJ. Contact lenses 2009. ContLens Spectrum 2010; 25:20–27.
17. Efron N, Morgan P, Helland M et al.Soft toric contact lens prescribing in different countries. Contact Lens & Anterior Eye. 2011; 34: 36–38
18. Young G, McIlraith R, Hunt C. Clinical Evaluation of Factors Affecting SoftToric Lens Orientation Optom Vis Sci 2009;86: E1259- E1266
19. Richter D, Dumbleton K, Guthrie S, Woods C, JonesL, Fonn D: Patient and practitioner compliance with silicone hydrogel and dailydisposable lens replacement in Canada. Can J Optom 2010;72: 10-19
20. Dumbleton K, Woods C, Jones L, Fonn D: Comfort andvision with silicone hydrogel lenses: Effects of compliance. Optom VisSci 2010;