218 LENS

Refractive Lens Surgery

CLEAR LENS EXTRACTION Often used for high myopia; some studies have shown up to a 17% long-term RD risk, but is likely much less.

IOL CALCULATION AFTER REFRACTIVE SURGERY Contact lens method is probably the most accurate, as it determines a corrected K value by overrefracting with a plano RGP. If the patient has a myopic shift with the overrefraction, convert the RGP base curve into diopters, and subract the myopic shift diopters to obtain the new K.

Example: a post-LASIK patient’s current spherical equivalent ¼ þ0.25. With overrefraction using a plano RGP and with a base curve of þ35.0 D, the spherical equivalent changes to 2.00 D. Because of the myopic shift, the cornea must be weaker than the base curve of the CL by 2.25 D. Therefore, the new cornea K must be 32.75 (35.0 2.25).

Keratometer and topographers are based on a net index of refraction ¼ 4/3 (n ¼ 1.33). However, because refractive surgery changes the shape of the anterior but not the posterior cornea, the 4/3 index is no longer valid. These instruments thus overestimate the power of the cornea by up to 14%.

PHAKIC IOL Early studies showed a high incidence of pupillary block glaucoma, especially in hyperopes, and usually prevented by two patent LPIs preoperatively. About 2% incidence of focal lens opacity and <1% incidence of visually significant cataract are seen within a few years of surgery.

Goodman, Ophtho Notes © 2003 Thieme

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