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 (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 × 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.