Role of Percentage of Tissue Altered as Risk Factor for Ectasia After LASIK in Eyes With Normal Preoperative Topography
Narrative Responses:
Purpose
To investigate the role of the Percentage of Tissue Altered (PTA) on ectasia after LASIK in eyes with normal preoperative corneal topography.
Methods
This retrospective comparative case-control study included thirty eyes that developed ectasia after LASIK for myopia with bilateral normal preoperative corneal topography (ectasia group) and 174 eyes that underwent uncomplicated LASIK for myopia without developing ectasia with at least 3 years of postoperative follow-up (control group). Prevalence of risk factors and odds ratio were determined and variables were further submitted to logistic stepwise regression. The percentage of anterior tissue depth altered (PTA) during LASIK was obtained from the equation PTA = (FT + AD)/CCT Where FT = flap thickness, AD = ablation depth (AD), and CCT = preoperative central corneal thickness (CCT). Residual stromal bed (RSB), manifest refractive spherical equivalent (MRSE), age and Ectasia Risk Score System values (ERSS) were also investigated.
Results
In the ectasia group, PTA ≥ 40% was the most prevalent risk factor (97%), followed by age ≤ 30, RSB ≤ 300, and ERSS ≥ 3. The PTA ≥ 40% presented the higher odds ratio values followed by RSB ≤ 300 μm, CCT ≤ 500 μm, ERSS ≥ 4, ERSS ≥ 3, MRSE ≥ 8D and Age ≤ 25 (years). Stepwise logistic regression revealed the PTA ≥ 40% as the most important independent variable (P < 0.0001).
Conclusion
The study provides scientific evidence of the role of PTA on ectasia after LASIK in eyes with normal corneal topography and therefore should be incorporated in the preoperative screening.