Errors of Residual Stromal Thickness Estimation in LASIK

Abstract

To investigate inaccuracy and variability in residual stromal thickness estimation in LASIK by pachymetry and measurements of corneal thickness, flap thickness, and ablation depth. In 73 eyes of 37 patients, preoperative and postoperative corneal thicknesses were obtained with slit-scanning elevation topography and the ultrasound pachymeter. LASIK was performed and corneal flaps were created with a microkeratome. Flap thickness and ablation depth (expected and achieved) were calculated. Residual stromal thickness estimation error was analyzed. The mean preoperative corneal thicknesses were 559.58 +/- 23.47 and 554.92 +/- 29.95 microm for the ultrasound pachymeter and slit-scanning elevation topography, respectively. Measurement differences ranged from -36 to 30 microm. With the pachymeter, calculated mean flap thickness was 139.58 +/- 17.59 microm. With this device, predicted ablation depth differed from achieved depth by 20% or more in approximately one-third (30.14%) of treated patients; ablation differences ranged from 10.0% to 19.99% in 37% of patients and 1.00% to 9.99% in 31.5% of patients. Imprecision of microkeratome cuts, preoperative corneal pachymetry, and laser ablation depth have a significant impact on the inaccuracy of residual stromal thickness prediction. Especially in patients with borderline corneal thickness, intraoperative pachymetry measurements and a residual stromal thickness higher than the safety margin of 250 microm are recommended to minimize iatrogenic ectasia.