Changes in the Biomechanical Properties of the Cornea Following LASIK: An Exploratory Study Using the Ocular Response Analyser

Abstract

Purpose:: To assess the changes in corneal biomechanical properties of patients undergoing laser in situ keratomileusis (LASIK) for myopia. To determine how intraocular pressure (IOP) measurements change following LASIK and which measurements are least affected by surgery.

Methods:: In this prospective pilot study, 36 eyes of 18 patients had IOP, refractive error and corneal biomechanical measurements performed prior to and 1 month following myopic LASIK. Central corneal thickness (CCT; µm) was measured with ultrasound pachymetry, and corneal hysteresis (CH; mmHg) was measured with the Reichert Ocular Response Analyser (ORA). Two further measurements of corneal biomechanical properties, the corneal constant factor (CCF; mmHg) and corneal resistance factor (CRF; mmHg), were derived from ORA readings. IOP was measured using the Tonopen (IOP_TON), and two ORA-derived measures of IOP: IOP_CC and IOP_CCF. One randomly selected eye from each patient was used for statistical analyses. Changes in corneal biomechanical properties and IOP were assessed with the paired Student t-test.

Results:: The pre- and post-LASIK CCT (mean [standard deviation]) was 540 [32] µm and 489 [44] µm, respectively. All corneal biomechanical properties significantly reduced (mean reduction [95% confidence interval, CI]: CH -1.7 mmHg [-2.3, -1.1]; CRF -3.0 mmHg [-3.7, -2.3]; CCF -3.0 mmHg [-3.8, -2.2]). These changes were not significantly associated with reduction in CCT. A significant reduction in measured IOP was found with all methods (mean reduction [95% CI]: IOP_TON -3.7 mmHg [-6.1, -1.4]; IOP_CC -3.3 mmHg [-4.6, -1.9]; IOP_CCF -1.5 mmHg [-2.7, -0.3]), but the reduction in IOP_CCF was significantly less than both the reduction in IOP_TON (P=0.046) and IOP_CC (P<0.001). A post-hoc analysis revealed a significant correlation between preoperative CH and refractive outcome (spherical equivalent): R²=0.22, P=0.048.

Conclusions:: All corneal biomechanical properties were significantly reduced following myopic LASIK. The results suggest that the IOP_CCF measure is the least affected. Corneal hysteresis may be a predictor of refractive outcome, but this needs to be confirmed in further studies.

Keywords: refractive surgery: LASIK • intraocular pressure • cornea: basic science