Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK - Part I: Quantifying individual risk

PURPOSE: To measure the imprecision of microkeratome cuts, preoperative corneal pachymetry, and laser ablation depth and develop a statistical model to describe the probability of the residual stromal bed thickness (RST) after myopic LASIK being significantly thinner than predicted. METHODS: Preoperative corneal thickness, flap thickness, ablation depth, and RST were measured in 36 eyes by a prototype three-dimensional very high-frequency (VHF) 50 MHz digital ultrasound scanning device (< 1.2 turn precision), precursor to the commercially available Artemis 2. All eyes had undergone LASIK with the Moria LSK-One microkeratome and the NIDEK EC-5000 excimer laser. Based on the statistically combined uncertainty (standard deviation) and bias (accuracy to intended value) of corneal thickness measurement, flap thickness, and ablation depth, a continuous probability function was devised describing the chance of obtaining an actual RST less "cut-off". The model was applied using than a specified the data collected from the cohort of eyes. The model was also applied using published flap thickness statistics on a series of microkeratomes. RESULTS: Precision (standard deviation) was 0.74 mu m for VHF digital ultrasound measurement of pachymetry, 30.3 mu m for Moria LSK-One flap thickness, and 11.2 mu m for NIDEK EC-5000 ablation depth. Assuming negligible laser ablation depth bias, the model found the probability that the,actual RST will be < 200 mu m given a target RST of 250 mu m is 7.56% with the Moria LSK-One. The model applied to published flap statistics revealed a range of probabilities of leaving < 200 mu m given a target RST of 250 mu m from < 0.01% to 33.6%. CONCLUSIONS: The choice of microkeratome, laser, and pachymeter has a significant impact on the variation of the depth of keratectomy and thus on the risk of ectasia. This model together with high-precision microkeratomes, preoperative pachymetry and knowledge of laser ablation precision would enable surgeons to determine the specific imprecision of RST prediction for individual LASIK cases and minimize the risk of ectasia.