Corneal Thickness After SMILE Affects Scheimpflug-based Dynamic Tonometry

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.

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.

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).

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.].