degeneration, contact lens warpage, dry eye, and epithelial basement membrane dystrophy (Fig 6-7). All these conditions should be identified before surgery. Common intraoperative causes include decentered ablations and central islands, and, less commonly, poor laser optics, nonuniform stromal bed hydration, and LASIK flap complications (a thin, torn, irregular, incomplete, or buttonhole flap; folds or striae of the flap; and epithelial defects). Postoperative causes of irregular astigmatism include flap displacement, diffuse lamellar keratitis and its sequelae, flap striae, posterior corneal ectasia, irregular wound healing, dry eye, and flap edema.

Conclusion

Incorporating optical considerations into the treatment of patients undergoing keratorefractive surgery is important to enhance the visual results. Patient dissatisfaction after surgery, albeit rare, often stems from the subjective loss of visual quality, the source of which can usually be identified through a sound understanding of how refractive surgery alters the optics of the eye. A good understanding of key parameters such as corneal shape, pupil size, the ocular surface, spherical and astigmatic errors, higher-order aberrations, laser centration and the angle kappa, and irregular corneal astigmatism can help optimize visual outcomes after keratorefractive surgery.

Chapter Exercises

Questions

6.1. 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 D 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?

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6.2. For the situation described in Question 6.1, 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.

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6.3. 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

× 0.60 associated with postsurgical irregular astigmatism. What are the important signs that will aid in reaching a diagnosis?

a.difficulty in determining axis location during manifest refraction

b.discrepancy between automated refraction and manifest refraction

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

d.all of the above

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

a.better visual acuity than a prolate cornea

b.improved optics if keratorefractive surgery results in postoperative Q = –0.3

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

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

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6.5. A patient undergoing an evaluation for refractive surgery has K readings of 43.0 D/42.0 D and a manifest refraction of –9.50 D. If LASIK were performed, you would expect the postoperative average keratometry reading to be approximately

a.34.9 D

b.36.3 D

c.37.3 D

d.34.0 D

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