Comparison of vector and polar methods for the calculation of surgically-induced astigmatism

Session Details

Session Title: Pseudophakic IOLs: Toric I

Session Date/Time: Saturday 10/09/2016 | 16:00-18:00

Paper Time: 16:54

Venue: Hall C2

First Author: : C.Hamer UK

Co Author(s): :    N. Habib   N. Mandour   D. Adams   H. Buckhurst   C. Purslow   P. Buckhurst     

Abstract Details

Purpose:

Quantifying the impact of surgical incisions on the corneal shape is achieved through the calculation of surgically induced astigmatism (SIA). There are various methods available to calculate this, which produce different results. The Cartesian vector result combines direction and magnitude, describing the change for the whole surface. The polar method produces a description of the change in reference to a specific meridian with an additional description of the torsional force orthogonal to this [KP(θ), KP(θ+45)]. This study aims to investigate the disparity between the two separate methods of calculating the SIA and the error (pseudo-SIA) associated with each method.

Setting:

Plymouth University, Peninsular Allied Health Centre, UK and the Royal Eye Infirmary, Plymouth, UK

Methods:

The corneal curvatures of two subject groups were assessed at two separate occasions with an auto-keratometer, placido-disc topographer and Scheimpflug tomographer. In the first group: one hundred healthy phakic patients were assessed on two occasions in the absence of surgery. In the second group: eighty-three otherwise healthy cataract patients were assessed pre- and post routine phacoemulsification surgery. The operations were performed by a single surgeon (NM) with a 2.8 clear corneal bi-planar incision placed superior temporally. Cartesian Vector analysis and the Polar method were used to determine the Pseudo-SIA (error in repeatability, Group 1) and SIA (Group 2).

Results:

There was a significant difference found between the mean Pseudo-SIA (group 1) and SIA (group 2) results calculated by the Cartesian Vector method for all three instruments (p<0.0001). However there was no significant difference found between the Pseudo-SIA and SIA groups for all readings produced by the Polar methods (p<0.05), apart from the description of torsional change in the Auto-keratometry readings (p>0.05). The readings produced by the placido disc and scheimpflug results were comparable (p>0.005) but the autokeratometer results were found to be different to both (p<0.001).

Conclusions:

There was a significant difference found between the mean Pseudo-SIA (group 1) and SIA (group 2) results calculated by the Cartesian Vector method for all three instruments (p<0.0001). However there was no significant difference found between the Pseudo-SIA and SIA groups for all readings produced by the Polar methods (p<0.05), apart from the description of torsional change in the Auto-keratometry readings (p>0.05). The readings produced by the placido disc and scheimpflug results were comparable (p>0.005) but the autokeratometer results were found to be different to both (p<0.001).

Financial Disclosure:

NONE