Corneal Thickness After SMILE Affects Scheimpflug-based Dynamic Tonometry

PURPOSE: To evaluate the corneal biomechanical changes due to small incision lenticule extraction (SMILE) measured by Scheimpflug-based dynamic tonometry and to assess the impact of the corneal thickness. METHODS: Sixty-eight patients measured with the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) preoperatively and 1 month after SMILE were included in this retrospective observational study. Patients were divided into three groups depending on the preoperative spherical equivalent: low from −1.00 to −3.00 diopters (D), medium from −3.25 to −5.00 D, and high from −5.25 to −7.25 D. Changes in Corvis ST parameters due to the surgery were analyzed and new indexes for correcting the impact of corneal thickness were proposed. RESULTS: First and second applanation times changed after SMILE ( P < .0001) but no differences were found in the comparison between these relative changes ( P = .31). First applanation time was correlated with central corneal thickness ( r = 0.368, P = .002) but not second applanation time ( r = −0.149, P = .23). The change in first applanation time due to SMILE was different among myopic groups ( P = .007) but equal when a new index that considers the removed central corneal thickness was used for comparison ( P = .31). Deformation amplitude was also increased after SMILE ( P < .0001), but after subtracting the removed corneal thickness from the postoperative deformation amplitude the result was equal to the preoperative deformation amplitude ( P = .26). CONCLUSIONS: SMILE produces significant changes in the Corvis ST parameters of time and deformation amplitude, but these changes are mainly explained by the confounding variable of corneal thickness. [ J Refract Surg. 2016;32(12):821–828.]