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

PURPOSE: To compare corneal astigmatism derived from total corneal refractive power,(CA(TCRP)), a ray-tracing method, with that derived from simulated keratometry (CA(SimK)), an anterior surface-based method, in candidates for toric intraocular lens (IOL) implantation. DESIGN: Reliability analysis. METHODS: In 1 eye of 200 consecutive patients (mean age 71 +/- 9 years) with cataract, a Pentacam HR (Oculus) was used to measure CA(TCRP) and CA(SimK). Eyes with CA(TCRP) > 1 diopter (D) were defined as candidates for toric IOL implantation. The estimation difference between CA(TCRP) and CA(SimK) was analyzed by calculating the arithmetic, absolute, and vector differences between the 2 methods. RESULTS: Seventy-seven candidates were identified. In the candidates, CA(SimK) was < 1 D in 22% of cases and underestimated the astigmatism magnitude, on average, by -0.15 +/- 0.34 D. The mean absolute and vector difference between CA(TCRP) and CA(SimK) was 0.31 +/- 0.29 D and 0.30 +/- 0.29 D X 180 degrees, respectively. The absolute and vector differences between the 2 methods were > 0.50 D in 21 of 77 (27.2%) and 25 of 77 (32.5%) eyes, respectively; the proportions were significantly (P < .001) higher than in the eyes with CA(TCRP) <= 1 D (0.8% and 9.8%, respectively). The difference in steep meridian alignment between CA(TCRP) and CA(SimK) was > 5 degrees in 22.1% and > 10 degrees in 2.6% of cases. CONCLUSION: The difference between CA(TCRP) and CA(SimK), 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.