Corneal tissue ablation depth and the Munnerlyn formula

Purpose: To determine whether the error in ablation depth produced by approximations inherent in the Munnerlyn formula are clinically significant when estimating residual corneal stromal depth for the evaluation before refractive surgery. Setting: Department of Ophthalmology, University of Texas Medical Branch, Galveston, Texas, USA. Methods: Using identical geometric assumptions, the exact ablation depth was calculated and compared to the approximate ablation depth predicted by the Munnerlyn formula. An adjustment factor was then derived for large optical zones and corrections. Results: The exact ablation depth is always larger than the ablation depth predicted by Munnerlyn's formula. Analysis found the error in ablation depth varied as the fourth power of the optical zone and linearly with correction. The initial corneal radius had little effect on the difference. The ablation depth could be reasonably approximated by adding an adjustment factor for large optical zones and refractive corrections. Conclusions: In patients with large optical zones, it may be preferable to calculate tissue ablation depth using the exact formula. Alternately, the Munnerlyn formula can be used to calculate ablation depth and then an adjustment factor can be added.