Corneal Astigmatism Measurement by Ray Tracing Versus Anterior Surface–Based Keratometry in Candidates for Toric Intraocular Lens Implantation

Abstract PURPOSE: To compare corneal astigmatism derived from total corneal refractive power (CATCRP), a ray-tracing method, with that derived from simulated keratometry (CASimK), an anterior surface-based method, in candidates for toric intraocular lens (IOL) implantation. DESIGN: Reliability analysis. METHODS: In one eye of 200 consecutive patients (mean age 71 ±9 years) with cataract, a Pentacam HR (Oculus, Wetzlar, Germany) was used to measure CATCRP and CASimK. Eyes with CATCRP >1 diopter (D) were defined as candidates for toric IOL implantation. The estimation difference between CATCRP andCASimK was analyzed by calculating the arithmetic, absolute, and vector differences between the two methods. RESULTS: Seventy-seven candidates were identified. In the candidates, CASimK was <1D in 22% of cases and underestimated the astigmatism magnitude, on average, by -0.15 ±0.34 D. The mean absolute and vector difference between CATCRP and CASimK was 0.31 ±0.29 D and 0.30±0.29 D X 180°, respectively. The absolute and vector differences between the two methods were >0.50 D in 21/77 (27.2%) and 25/77 (32.5%) eyes, respectively; the proportions were significantly (P < 0.001) higher than in the eyes with CATCRP ≤1D (0.8% and 9.8%, respectively). The difference in steep meridian alignment between CATCRP and CASimK was >5 degrees in 22.1% and >10 degrees in 2.6% of cases. CONCLUSION: The difference between CATCRP and CASimK, as calculated by Pentacam HR, is greater in candidates for toric IOL implantation than in the general population. That difference considerably influences the candidate and toric IOL power selection in a large proportion of cases.