The relationship between peripheral and paracentral corneal astigmatism and refractive astigmatic error in pseudophakic eyes

Session Details

Session Title: Corneal Astigmatism Assessment. Surgery Equipment

Session Date/Time: Saturday 05/09/2015 | 11:00-13:00

Paper Time: 11:06

Venue: Main Auditorium

First Author: : P.Buckhurst UK

Co Author(s): :    C. Hamer   H. Buckhurst   C. Purslow   N. Habib           

Abstract Details

Purpose:

The effectiveness of a toric intraocular lens implant (IOL) is dependant on its alignment with the steepest corneal meridian as determined by keratometry, topography or tomography. These techniques are generally regarded to be interchangeable however many devices assess corneal power over different cord lengths. Given that corneal shape is not spherical it is likely that the differing cord length has an effect on the measurement of astigmatism. This study examines the relationship between corneal astigmatism at different chord lengths and refractive astigmatic error (RAE).

Setting:

Royal Eye Infirmary, Derriford Road, Plymouth, UK

Methods:

The study examined 75 subjects (74.8±9.6 years) who had undergone small incision sutureless cataract surgery with postoperative corneal astigmatism of >0.75DC. All subjects received a monofocal, non-toric, aspheric IOL. Scheimpflug tomography was used to determine corneal power at 2mm and at 5mm. An investigator masked to the tomography results conducted subjective refraction to determine RAE. The astigmatic power of the cornea and the overall manifest refraction were assessed following conversion into vector format (J0/J45). The relationship between corneal power at 2mm, 5mm and RAE was assessed through repeated measures ANOVA and a stepwise multiple linear regression.

Results:

The mean RAE and astigmatism at 2mm and 5mm were 1.32D (J0: -0.45±0.43; J45: -0.07±0.35), 1.49D (J0 -0.30±0.65; J45: -0.01±0.45) and 1.15D (J0: -0.25±0.46; J45: 0.04±0.37), respectively. The absolute magnitude of astigmatism found with a 2mm chord length was similar to the RAE (p=0.208), however, the 5mm chord length measured a significantly lower magnitude of astigmatism (p=0.04). The 2mm chord length was found to account for 30% of the variation in RAE; the stepwise regression model found that the 2mm zone and not the 5mm zone was a significant predictor of RAE.

Conclusions:

When compared with a larger peripheral chord length, astigmatism measured with a 2mm chord length showed a greater association with the overall RAE. These results would suggest that it is important to select the power and position of a toric IOL in accordance with the more paraxial central corneal astigmatic zone.

Financial Interest:

NONE