2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 7: Optical Considerations in Keratorefractive Surgery
Chapter Exercises
Answers
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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.
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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.
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7.3. d. all of the above.
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7.4. b. improved optics if keratorefractive surgery results in postoperative Q = −0.3
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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.