Mathematical Model to Compare the Relative Tensile Strength of the Cornea After PRK, LASIK, and Small Incision Lenticule Extraction

PURPOSE: To develop a mathematical model to estimate the relative differences in postoperative stromal tensile strength following photorefractive keratectomy (PRK), LASIK, and small incision lenticule extraction (SMILE). METHODS: Using previously published data where in vitro corneal stromal tensile strength was determined as a function of depth, a mathematical model was built to calculate the relative remaining tensile strength by fitting the data with a fourth order polynomial function yielding a high correlation coefficient (R 2 = 0.930). Calculating the area under this function provided a measure of total stromal tensile strength (TTS), based only on the residual stromal layer for PRK or LASIK and the residual stromal layers above and below the lenticule interface for SMILE. RESULTS: Postoperative TTS was greatest after SMILE, followed by PRK, then LASIK; for example, in a 550- μ m cornea after 100- μ m tissue removal, postoperative TTS was 75% for SMILE (130- μ m cap), 68% for PRK, and 54% for LASIK (110- μ m flap). The postoperative TTS decreased for thinner corneal pachymetry for all treatment types. In LASIK, the postoperative TTS decreased with increasing flap thickness by 0.22%/ μ m, but increased by 0.08%/ μ m for greater cap thickness in SMILE. The model predicted that SMILE lenticule thickness could be approximately 100 μ m greater than the LASIK ablation depth and still have equivalent corneal strength (equivalent to approximately 7.75 diopters). CONCLUSIONS: This mathematical model predicts that the postoperative TTS is considerably higher after SMILE than both PRK and LASIK, as expected given that the strongest anterior lamellae remain intact. Consequently, SMILE should be able to correct higher levels of myopia. [ J Refract Surg. 2013;29(7):454–460.]