Role of Percentage of Tissue Altered (PTA) as a risk factor in eyes with Normal preoperative topography that developed Ectasia after LASIK

Abstract

 

Purpose

 

The purpose of this study was to investigate in detail the theoretical role of the percentage of tissue altered (PTA) on ectasia after LASIK in eyes with normal preoperative topography compared to risk factors already know.

 

Methods

 

This retrospective comparative study included eyes that developed ectasia with normal preoperative topography. Consecutive LASIK cases with preoperative normal topography that have not developed ectasia served as control. Cases were identified after a detailed analyzes of cases that were referred to the authors’ institution. The PTA is calculated through the sum of flap thickness (FT) plus the ablation depth (AD) divided by the preoperative central corneal thickness (CCT). [PTA = (FT + AD)/CCT]. Patient age, spherical equivalent refraction, CCT, and topographic patterns; perioperative characteristics, including flap thickness, ablation depth, residual stromal bed and PTA were investigated.

 

Results

 

Thirty eyes met inclusion criteria for this study. PTA was significantly higher in patients with ectasia and normal topography compared to controls. Analyzes of the eyes that developed ectasia with normal topography revealed the PTA as the most prevalent risk factor (p < 0.0001) and the stepwise logistic regression revealed the PTA with the higher influence on ectasia and a higher odds ratio followed by RSB, Age and MRSE. CCT was the risk factor with less influence

 

Conclusions

 

The PTA seems to be a significant risk factor for ectasia in eyes with pre operative normal topography compared to controls. The PTA presented a higher influence on ectasia compared to other risk factors in eyes with normal preoperative topography.

 

Keywords: 680 refractive surgery: complications • 683 refractive surgery: LASIK