2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 5: Contact Lenses
Contact Lens Fitting
Toric Soft Contact Lenses
Perhaps one-third of contact lens wearers have at least 1.00 D of astigmatism. Up to 0.75 or 1.00 D of astigmatism will likely be “masked” by a spherical soft contact lens enough to offer best acuity (Table 5-8). Greater degrees of astigmatism will usually not be well-corrected by spherical soft contact lenses.
Table 5-8 Astigmatism and Lens Fitting
Soft toric contact lenses are available in several designs. To prevent lens rotation, various techniques are used:
adding prism ballast, placing extra lens material on the bottom edge of the lens, whose weight aligns the lens—commonly used
creating thin zones; that is, making lenses with a thin zone on the top and bottom, so that eyelid pressure tends to keep the lens in the appropriate position
truncating or removing the bottom of the lens to form a straight edge that aligns with the lower eyelid—not as commonly used
Fitting soft toric lenses is similar to fitting other soft lenses, except that lens rotation must also be evaluated. Toric lenses typically have a mark to note their 6-o’clock position. If a slit-lamp examination shows that the lens mark is consistently rotated to one side of the 6-o’clock axis, the amount of rotation should be noted, in degrees (1 clock-hour equals 30°). To adjust the prescription for lens rotation, follow the LARS rule (left add; right subtract). This is illustrated in Clinical Example 5-6 and Figure 5-11.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.