2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 5: Contact Lenses
5.1. b. The near effectivity of the glasses decreases accommodative demand, making it easier to read than when the myopia is corrected at the cornea by the contact lenses. The glasses have their optical centers placed for distance viewing. Converging to read induces base-in prism, so that less convergence is required to look at near objects, when wearing the glasses. If the contacts are rigid, and form a plus-powered tear lens, they would blur distance vision and aid reading vision, unless the power of the lenses is properly adjusted by over-refracting with the lenses in place. The contact lens has slightly less power than the spectacle lens, because of the difference in vertex distance (see Table 5-4).
5.2. d. With contact lens involvement, we are more concerned than otherwise about infection. Use no lens or patch, consider lubrication and topical antibiotic, and see the patient soon for follow-up.
5.3. d. Increasing the sagittal depth of the contact lens tightens the fit and decreases lens movement, which may be achieved through steepening (decreasing) the base curve or increasing the diameter of the contact lens. Flattening (increasing) the base curve or decreasing the diameter of the lens decreases sagittal depth and increases the movement of the lens on the cornea. The power of the contact lens should not affect the fitting relationships.
5.4. c. The tear lens is formed by the posterior surface of the contact lens and the anterior surface of the cornea. If these 2 curvatures are the same, as with a soft lens, the tear lens is plano. If they are different (as is typical of RGP lenses), a plus or minus tear lens forms. In this case, the contact lens is flatter than K, so the tear lens is negative, or concave, in shape.
5.5. d. For every 0.05-mm radius-of-curvature difference between the base curve and K, the induced power of the tear film is 0.25 D. The power of the concave tear lens in this case is −1.00 D. The power of the RGP contact lens you should order is −3.50 D − (−1.00 D) = −2.50 D. An easy way to remember this formula is to use the following rule: SAM = steeper add minus and FAP = flatter add plus.
5.6. b. The amount and direction of rotation should be observed. In this case, they are, respectively, 1 clock-hour and rotation to the right. Each clock-hour represents 30° (360°/12 = 30°), so the adjustment should be 30°. Because the rotation is to the right, you should order a contact lens with axis 145° instead of 175°—that is, −2.50 D −1.50 × 145. An easy rule to remember is LARS = left add, right subtract.
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.