Questions

• 7.1. The Munnerlyn formula approximates the depth of excimer laser tissue ablation:

  

  where t is the central ablation depth in micrometers, S is the diameter of the optical zone in millimeters, and D is the degree of refractive correction in diopters. For a LASIK patient with a refractive correction of −6.00 −6.00 × 90 and a central corneal thickness of 520 µm, and for whom the LASIK flap thickness is 120 µm, an extremely conservative surgeon would not want to have a residual stromal bed (RSB) thickness of less than 300 µm. According to the Munnerlyn formula, what is the largest optical zone diameter that can be used for this treatment?

• 7.2. For the situation described in Question 7.1, what is the largest optical zone diameter that can be used if PRK, rather than LASIK, is planned? Assume an epithelium thickness of 58 µm and an RSB thickness of 300 µm.

• 7.3. A patient with a preoperative manifest refraction of −3.50 D, normal keratometry (K) readings, and a pachymetry measurement of 550 µm undergoes keratorefractive surgery. Three months postoperatively, the patient has an uncorrected visual acuity of 20/30 with a refraction of +2.00 −3.00 × 60 associated with postsurgical irregular astigmatism. What are the important signs that will aid in reaching a diagnosis?

  1. difficulty in determining axis location during manifest refraction

  2. discrepancy between automated refraction and manifest refraction

  3. no improvement or change in visual acuity with large powers of cylinder at markedly different axes

  4. all of the above

• 7.4. Corneal asphericity is represented by Q value. A spherical cornea with asphericity of Q = 0 is associated with

  1. better visual acuity than a prolate cornea

  2. improved optics if keratorefractive surgery results in postoperative Q = −0.3

  3. improved optics if keratorefractive surgery results in postoperative Q = 0

  4. improved optics if keratorefractive surgery results in postoperative Q = +0.3

• 7.5. A patient undergoing an evaluation for refractive surgery has K readings of 46.0 D/42.0 D. If LASIK were performed, what are the largest hyperopic and myopic spherical corrections tolerable?

  1. +5.00 D, −11.25 D

  2. +3.00 D, −11.25 D

  3. +3.00 D, −6.25 D

  4. +5.00 D, −7.00 D

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.