Postoperative changes in intraocular pressure and corneal biomechanical metrics Laser in situ keratomileusis versus laser-assisted subepithelial keratectomy Mujtaba A. Qazi, MD, Jeff P. Sanderson, MD, Ashraf M. Mahmoud, Eric Y. Yoon, MD, Cynthia J. Roberts, PhD, Jay S. Pepose, MD, PhD PURPOSE: To compare intraocular pressure (IOP) and corneal biomechanical metric changes after myopic laser in situ keratomileusis and laser-assisted subepithelial keratectomy (LASEK). SETTING: Private practice, St. Louis, Missouri, USA. METHODS: The IOP, corneal biomechanical markers, and Ocular Response Analyzer (ORA) wave- form parameters were prospectively measured preoperatively and after 6 months in ablation- matched myopic LASIK eyes (mLASIK group) and LASEK eyes (mLASEK group). A retrospectively identified cohort of low myopia LASIK eyes (lmLASIK group) and fellow unoperated eyes (control) were tested at a single postoperative visit. Statistical analysis compared the percentage change in parameters between groups. RESULTS: The mean postoperative Goldmann tonometry and Goldmann-correlated IOPs were sta- tistically significant reduced in the mLASIK and mLASEK groups (P<.03). Corneal-compensated IOP, but not Pascal dynamic contour tonometry, was significantly reduced in the mLASIK group. The percentage change in corneal hysteresis (CH) and the corneal resistance factor (CRF) was greater in the mLASIK and mLASEK groups than in the lmLASIK group. The greatest percentage change in ORA signal parameters was in the mLASIK group and the smallest change, in the mLASEK group. On multivariate linear regression, the residual stromal bed was predictive of the percentage change in CH and CRF (P<.001). CONCLUSIONS: Microkeratome flap creation combined with deeper stromal ablation had the great- est effect on the ORA applanation signal, indicating corneas that are more readily deformable. The smallest change in the signal was in the group without a stromal flap (LASEK). There was a complex interaction between ablation location and depth that affected corneal biomechanical properties. J Cataract Refract Surg 2009; 35:1774–1788 Q 2009 ASCRS and ESCRS After different forms of keratorefractive surgery, pro- found changes in corneal structure and biomechanics occur secondary to iatrogenic corneal thinning, shape changes, and disruption of collagen lamellar layers. The accuracy of Goldmann applanation tonometry (GAT) is limited by the influence of inherent corneal characteristics (eg, thickness, curvature, and elastic modulus) on this measurement. 1,2 Significant alter- ation in corneal biomechanical properties and mea- sured intraocular pressure (IOP) after corneal refractive surgery has been reported. 3–6 As a result, newer methods for quantifying IOP that are report- edly less sensitive to corneal properties have been developed. 7–10 Two newer devices are the Pascal dynamic contour tonometer (DCT) (Ziemer) and the Ocular Response Analyzer (ORA) (Reichert, Inc.). In addition to generating dynamic measures of IOP (rather than the static IOP measurements with GAT), the devices provide metrics of corneal and/or ocular biomechanics. The ORA 7 uses an air pulse to produce 2 (inward and outward) applanation events (Figure 1). The change in the shape of the cornea is detected using an infrared light that reflects from the corneal surface to an aligned sensor. Maximum infrared signal ampli- tude occurs during each applanation event when the Q 2009 ASCRS and ESCRS 0886-3350/09/$dsee front matter Published by Elsevier Inc. doi:10.1016/j.jcrs.2009.05.041 1774 ARTICLE