Comparison of Keratometric Cylinder with Aphakic Refractive Cylinder

Abstract

Purpose: : The purpose of this work is to show that refractive "aphakic" cylinder is different from keratometric cylinder based on anterior corneal curvature and the keratometric index (1.3375).

Methods: : Traditionally astigmatic correction at the time of cataract surgery (LRIs or toric IOLs) is planned based on the preoperative keratometric cylinder (keratometer or corneal topographer) because the patient’s preoperative refractive astigmatism is compromised by lenticular cylinder. With the advent of intraoperative wavefront aberrometry, the refractive cylinder can be measured after the cataractous lens has been removed. This intraoperative cylinder value, which is based on a measurement along the patient’s line of site, versus the keratometric measurement centered on the corneal apex, will be a better representation of the patients "true" astigmatism. We compare data from 3 sources to demonstrate these differences: 1) 100 post op cataract surgery cases (standard monofocal lenses) for which manifest refraction and keratometry were measured, 2) 350 pre LASIK eyes for which cycloplegic refraction and Pentacam data were collected and 3) 200 eyes with preoperative keratometric data and intraoperative aphakic refraction.

Results: : For the 100 post operative cataract surgery patients, the mean absolute deviation in cylinder magnitude between manifest refraction and keratometry was 0.45 D. For this group, the mean axis difference was 17 degrees in 65 eyes with ≥0.50 D manifest cylinder. The difference in cylinder magnitude ranged between 0.0 D to 1.9 D and the axis difference ranged between 0^o to 67^o. For the 350 pre LASIK cases, the mean cylinder difference was 0.48 D. The differences ranged between 0.0 D to 1.6 D and 0^o to 87^o for the 204 eyes with cycloplegic astigmatism ≥0.5 D. For the 200 cases with intraoperative cylinder measurement and preoperative keratometry, the mean difference was also 0.46 D with a range of 0.0 D to 1.6 D. For this group, the mean axis difference was 22 degrees with a range of 0^o to 87^o in 151 eyes with keratometric cylinder was ≥0.5 D. Vector analysis of this data will also be presented.

Conclusions: : These results show that the keratometric cylinder derived from the anterior corneal curvature and the standard keratometric index (1.3375) does not represent the patient’s true refractive cylinder. The refractive aphakic cylinder obtained via intraoperative aberrometry at the time of cataract surgery should be the value used in planning LRIs or selecting toric lens cylinder power and axis placement during cataract surgery.

Keywords: astigmatism • cataract • topography