2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 7: Optical Considerations in Keratorefractive Surgery
7.1. Pachymetry value = t + LASIK flap thickness + RSB thickness = t + 120 μm + 300 μm. This implies that t = 100 μm. Then, t = 100 = S2 × D/3, where D = 12, the spherical component necessary to correct, which implies that S2 = 25. Therefore, S = 5 mm; that is, the largest diameter that can be used for LASIK treatment in this situation by this surgeon is 5 mm.
7.2. Pachymetry value = t + epithelium thickness + RSB thickness. This implies that t = 520 μm − 58 μm − 300 μm = 162 μm. Because t = S2 × 12/3, this implies that S2 = 162/4 = 40.5; thus, S = 6.4 mm, the largest diameter that can be used for PRK treatment in this situation by this surgeon.
7.3. d. all of the above.
7.4. b. improved optics if keratorefractive surgery results in postoperative Q = −0.3
7.5. a. +8.75 D, −11.25 D. The formula for keratometry change is approximately = 0.8 × refractive change. Here, the refractive change = (50 − 46)/0.8 = +5.00 D and (33 − 42)/0.8 = −11.25 D based on maximum steepening of the steep axis and maximum flattening of the flat axis.
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.