Mathematical models of picosecond laser keratomileusis for high myopia

PURPOSE: Picosecond laser keratomileusis (PLK) is a refractive surgical pro cedure in which an intrastromal lenticle is created using the Nd:YLF picose cond laser and removed by lifting the anterior flap in the cornea, The purp ose of this study was to assess the predictability of picosecond laser kera tomileusis for high myopia. METHODS: The results from a recent clinical series of non-sighted patients who had undergone picosecond laser keratomileusis with the Nd:YLF picosecon d laser were retrospectively simulated with two mathematical models. In one model, the change in corneal curvature resulting from the collapse of a le nticular intrastromal cavity was predicted from a geometric analysis, A sec ond model was developed using the finite element method. The results of bot h predictive models were compared to the clinical results at 1 and 6 months following surgery. RESULTS: An average -13.30 D of flattening (range, -6.80 to -20.50 D) was m easured at 6 months. The geometric model predicted an average correction of -23.10 D (range, -18.00 to -34.00 D), whereas the finite element model pre dicted an average curvature change of -17.40 D (range, -14.50 to -22.50 D). An average decrease in central pachymetry of 91 mu m was measured at 6 mon ths, as compared to a predicted decrease in thickness of 112 mu m. CONCLUSIONS: The outcome of picosecond laser keratomileusis for high myopia predicted using a finite element based model shows greater change in the c orneal curvature and corneal thickness compared to clinical results. The re latively thick lenticules that are removed for high myopia cause considerab le deformational changes in the cornea, which preclude the use of a purely geometric/optical approach for predicting the resultant corneal curvature c hanges.