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  • 2020–2021 BCSC Basic and Clinical Science Course™

    Go to Academy Store Learn more and Purchase.

    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.

    CLINICAL EXAMPLE 5-4

    The refractive correction is −3.50 +1.75 × 90, and the K measurements along the 2 principal meridians are 7.80 mm horizontal (43.25 D at 180°) and 7.50 mm vertical (45.00 D at 90°). The contact lens base curve is 7.50 mm. What is the anticipated power of the contact lens?

    The refractive correction along the flattest corneal meridian (7.80 mm) is −1.75 D (convert the refractive error to minus cylinder form), and the lens has been fitted steeper than flat K, creating a tear lens of +1.75 D. Thus, a corresponding amount of minus power must be added (recall the SAM rule: steeper add minus), giving a corrective power of −3.50 D in that meridian.

    The refractive correction along the steepest meridian (7.50 mm) is −3.50 D. The lens is fitted “on K”; therefore, no tear lens power is created. The corrective power for this meridian is also −3.50 D.

    Accordingly, the power of the contact lens should be −3.50 D (see Fig 5-6).

    CLINICAL EXAMPLE 5-5

    A patient with a refraction of −2.00 −2.00 × 180 desires contact lens correction. The keratometry measurement is 44.00 sphere. What is the residual refractive error if this eye is fitted with a spherical RGP contact lens? A spherical soft contact lens? A toric soft contact lens?

    This eye has lenticular but not corneal astigmatism. Correction with spherical soft or rigid contact lenses will result in residual astigmatism. A soft toric contact lens may be used to correct the lenticular astigmatism.

    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.

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