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Measurement of residual stromal thickness essential for LASIK retreatment
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NUREMBERG, Germany — Calculating residual stromal thickness is an essential component of an examination for LASIK enhancement surgery, according to a physician speaking here. The current best means of estimating residual stromal thickness is to measure the original flap thickness and to use the appropriate calculation method, said Perry Binder, MD, here at the annual international symposium of the German Ophthalmic Surgeons.
“What is the safe residual stromal thickness we must have?” Dr. Binder asked. “If you look in the literature some say it has to be more than 250 µm, others say 200 µm, some say you can’t go below 50% of the corneal thickness, others say you can’t remove more than 18% of the corneal depth, but nobody really knows. This is all theoretical material.”
According to Dr. Binder, the importance of calculating residual corneal thickness is to determine whether sufficient tissue remains after ablation to minimize the risk of ectasia and avoid structural weakening.
Dr. Binder and colleagues conducted a retrospective, comparative, interventional case study of 6,235 eyes with ultrasonic measurements performed during LASIK surgery. Ultrasonic corneal pachymetry was performed immediately before and after flap creation and immediately after laser ablation during primary procedures and after enhancements. The study included 647 enhancements.
Dr. Binder said that, using the residual stromal thickness measured at enhancement as the actual residual stromal thickness, measurements of residual stromal thickness performed immediately following laser ablation overestimated residual thickness due to laser-induced stromal dehydration and microkeratome effects. He said estimates of residual stromal thickness using a standard flap thickness or estimated flap thickness were less accurate predictors of residual stromal thickness compared to use of the theoretical laser resection with a measured flap thickness or a modified flap thickness subtracted from the postoperative corneal thickness, which provided the best estimates of residual stromal thickness.
“We want to understand the potential causes of ectasia,” Dr. Binder said. “One needs to define the parameters that are most likely responsible: flap thickness, the resection of the laser and residual stromal thickness.”