Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy

Objective: To assess the validity of corneal power measurement and standard intraocular lens power (IOLP) calculation after photorefractive keratectom y (PRK). Design: Nonrandomized, prospective, cross-sectional, clinical study. Participants: A total of 31 eyes of 21 females and 10 males with a mean age at the time of surgery of. 32.3 +/- 6.6 years (range, 24.4-49.5 years). Intervention: Subjective refractometry, standard keratometry, TMS-1 corneal topography analysis, and pachymetry were performed before and 15.8 +/- 10. 4 months after PRK for myopia (n = 24, -1.5 to -8.0 diopters [D], mean -5.4 +/- 1.9 D) or myopic astigmatism (n = 7, sphere -2.0 to -7.5 D, mean -4.4 +/- 1.9 D; cylinder -1.0 to -3.0 D, mean -1.9 +/- 0.7 D). The IOLP calculat ions were done using two different formulas (SRK/T and HAIGIS). Main Outcome Measures: Keratometric power (K) and topographic simulated ker atometric power (TOPO) as measured (K-meas, TOPOmeas) and as calculated acc ording to the change of power of the anterior corneal surface or according to the spherical equivalent change after PRK (K-calc, TOPOcalc), IOLP for e mmetropia, and postoperative ametropia for calculated corneal powers were a ssessed in a model. Results: After PRK, mean K-meas and TOPOmeas were significantly greater (0. 4-1.4 D, maximum 3.3 D) than mean KRcalc and TOPOcalc (P < 0.0001). On aver age, the relative flattening of the cornea after PRK was underestimated by 14% to 30% (maximum, 83%) depending on the method of calculation. The mean theoretical IOLP after PRK ranged from + 17.4 D (SRK/T, TOPOmeas) to +20.9 D (HAIGIS, K-calc) depending on the calculation method for corneal power an d IOLP calculation formula used. For both formulas, IOLP values using kerat ometric readings were significantly higher (>1 D) than IOLP values using to pographic readings (P < 0.0001). The theoretically induced mean refractive error after cataract surgery ranged from +0.4 to +1.4 (maximum, +3.1) D. Co rneal power overestimation and IOLP underestimation correlated significantl y with the spherical equivalent change after PRK (P = 0.001) and the intend ed ablation depth during PRK (P = 0.004). Conclusions: To avoid underestimation of IOLP and hyperopia after cataract surgery following PRK, measured corneal power values must be corrected. The calculation method using spherical equivalent change of refraction at the corneal plane seems to be the most appropriate method. In comparison with t his method, direct power measurements underestimate corneal flattening afte r PRK by 24% on average. Use of conventional topography analysis seems to i ncrease the risk of error. However, because this study is retrospective and theoretical, there is still a need for a large prospective investigation t o validate the authors' findings.