Influence of Refractive Index of the Crystalline Lens on Optical Biometric Measurements for Cataract Patients

Purpose: Measurement of the axial length (AL) of the eye and the crystalline lens thickness (LT) relies on information of the refractive index of the crystalline lens for converting optical path-length (OPL) to geometric size. The purpose of this study was to measure the lens refractive index (NLens) in vivo for patients with cataract and evaluate the influence of the NLens on AL and LT measurements. Methods: Cataract eyes of 7 patients, aged 56 to 77 years old (mean = 64.7 years), were imaged with a combined whole-eye optical coherence tomography (OCT) and Scheimpflug Imaging (SI) system before surgery. The images were processed using a custom MatLab program to trace the central light-ray from the cornea to the retina and to solve the average lens refractive index (NLens). The OPLs of other ocular segments, except the lens, were converted to geometric lengths by using the refractive indices from a model eye. The AL and LT were calculated with the measured NLens and also a constant NLens (1.41) for comparison. Results: For the patients with cataract, mean AL and LT were 23.897 ± 0.854 and 4.309 ± 0.438 mm, respectively, when the measured NLens was used for OPL conversion, and changed to 23.918 ± 0.875 mm and 4.330 ± 0.461 mm, respectively, when the NLens was set at 1.41. The lens NLens from individual eyes varied from 1.400 to 1.434 among the patients, and caused a substantial individual variation in the AL and LT measurements, as compared with those with a constant NLens in a range from -0.070 to 0.027 mm. Conclusions: The Scheimpflug imaging-equipped whole-eye OCT system provides a useful technique to measure AL and LT with the NLens determined for the cataract eye in vivo. The NLens varies substantially from individual eye to individual eye among the patients with cataract, and plays a critical role for deriving accurate AL and LT measurements. During cataract surgery the accurate AL and LT measurements should be used for IOL power calculation.