Posters

Are unexpected changes of astigmatism following LASIK purely random?

Poster Details

First Author: S.Patel UK

Co Author(s):    M. Anticic   M. Bohac   A. Bisevic   M. Koncarevic   N. Gabric        

Abstract Details

Purpose:

To determine [A] if the surgically induced ocular astigmatism revealed by subjective refraction was determined solely by the surgically induced change within the central optical zone of the anterior corneal surface, [B] if the vector describing the difference between the change in ocular astigmatism and change in corneal astigmatism was associated with the attempted astigmatic correction (target induced astigmatism, TIA).

Setting:

University Eye Clinic �â�€�˜Svjetlost�â�€�™ Zagreb & �â�€�˜Svjetlost�â�€�™ Split. Croatia.

Methods:

A retrospective analysis of pre-and postoperative refractive and corneal topographic(SimK) outcomes in two groups of patients. I- myopes & myopic astigmats, mean(�Â�±sd,range) spherical and cylindrical corrections were �â�ˆ�’3.16DS(�Â�±1.90DS,upto�â�ˆ�’11.00DS) and -1.09DC(�Â�±1.16DC, upto-6.00DC) II- hyperopes & hyperopic astigmats, mean(�Â�±sd,range) spherical and cylindrical corrections were �.00(�Â�±1.37DS, upto�.00DS) and -1.37DC(�Â�±1.39DC, upto-6.00DC). All cases i) had been treated for LASIK using Wavelight Allegretto Eye Q (200Hz), treatments were centered on corneal vertex and flaps were cut with Moria M2 mechanical microkeratome (90�Â�µm head) ii) were unremarkable free of any complications. SimK values were obtained using Pentacam HR.

Results:

Reporting the significant findings. Differences between the ocular and corneal surgically induced astigmatism (SIA) occurred when the preoperative ocular astigmatism was predominantly against the rule(ATR). In myopic ATR the ocular SIA exceeded corneal SIA (p=0.007) the opposite occurred in hyperopic ATR (p=0.020). In group I, multilinear regression revealed the magnitude (y3) and sine of the axis (y4) of the vector describing the mathematical difference between the ocular and corneal surface SIA was directly related to the magnitude (x3) and sine of the axis (x4) of the target induced astigmatism where, y3 =0.1783x3�â�€�“0.0804x4-0.493 (F=9.726,r=0.3138,p<0.001,n=182) y4 =0.0166x3�.1704x4�.628 (F=3.44,r=0.193,p=0.034,n=182)

Conclusions:

There was a lag between ocular and corneal surface SIA that depended on the axis of the preop ocular astigmatism. Vector analysis suggests the forces accounting for the differences between ocular and corneal surface SIA values are strongly linked to the attempted target astigmatic correction in myopic corrections. The greater the attempted correction, the greater the vector accounting for the difference between ocular and corneal SIA. This was not the case in the hyperopic corrections. Other factors are responsible for the divergence between ocular and corneal surface SIA in hyperopic cases.

Financial Disclosure:

NONE