http://www.journalofrefractivesurgery.c ... hing=13972
Original Articles:
Probability Model of the Inaccuracy of Residual Stromal Thickness Prediction to Reduce the Risk of Ectasia After LASIK Part I: Quantifying Individual Risk
Journal of Refractive Surgery Vol. 22 No. 9 November 2006
Dan Z. Reinstein, MD, MA(Cantab), FRCSC; Sabong Srivannaboon, MD; Timothy J. Archer, BA(Oxon), DipCompSci(Cantab); Ronald H. Silverman, PhD; Hugo Sutton, MD; D. Jackson Coleman, MD
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 µm 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 than a specified “cut-off.” The model was applied using 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 µm for VHF digital ultrasound measurement of pachymetry,
30.3 µm for Moria LSK-One flap thickness, and 11.2 µm for NIDEK EC-5000 ablation depth. Assuming negligible laser ablation depth bias, the model found the
probability that the actual RST will be <200 µm given a target RST of 250 µm is 7.56% with the Moria LSK-One. The model applied to published flap statistics revealed a range of probabilities of leaving <200 µm given a target RST of 250 µm 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. [J Refract Surg. 2006;22:851-860.]
AUTHORS
From London Vision Clinic, London, United Kingdom (Reinstein, Archer); the Department of Ophthalmology, University of British Columbia, Vancouver, Canada (Reinstein, Srivannaboon, Sutton); the Department of Ophthalmology, Weill Medical College of Cornell University, NY (Reinstein, Silverman, Coleman); the Department of Ophthalmology, St Thomas’ Hospital - Kings College, London, United Kingdom (Reinstein); Centre Hospitalier National d’Ophtalmologie, Paris, France (Reinstein, Srivannaboon); and Siriraj Hospital, Mahidol University, Bangkok, Thailand (Srivannaboon).
Drs Reinstein, Silverman, Sutton, and Coleman have a proprietary interest in the Artemis technology (Ultralink LLC, St Petersburg, Fla) through patents administered by the Cornell Research Foundation, Ithaca, NY. The remaining authors have no proprietary or financial interest in the materials presented.
Some of the aspects of this study were presented at the Association for Research in Vision and Ophthalmology Annual Meeting; May 9-14, 1999; Fort Lauderdale, Fla.
Preparation in partial fulfillment of the requirements for the doctoral thesis, University of Cambridge, for Dr Reinstein.
Correspondence: Dan Z. Reinstein, MD, MA(Cantab), FRCSC, London Vision Clinic, 8 Devonshire Place, London W1G 6HP, United Kingdom. Tel: 44 207 224 1005; Fax: 44 207 224 1055; E-mail:
dzr@londonvisionclinic.com
Received: April 14, 2005
Accepted: January 1, 2006
