Biomechanical and optical behavior of human corneas before and after photorefractive keratectomy

Purpose To evaluate numerically the biomechanical and optical behavior of human corneas and quantitatively estimate the changes in refractive power and stress caused by photorefractive keratectomy (PRK). Setting Athineum Refractive Center, Athens, Greece, and Politecnico di Milano, Milan, Italy. Design Retrospective comparative interventional cohort study. Methods Corneal topographies of 10 human eyes were taken with a scanning-slit corneal topographer (Orbscan II) before and after PRK. Ten patient-specific finite element models were created to estimate the strain and stress fields in the cornea in preoperative and postoperative configurations. The biomechanical response in postoperative eyes was computed by directly modeling the postoperative geometry from the topographer and by reproducing the corneal ablation planned for the PRK with a numerical reprofiling procedure. Results Postoperative corneas were more compliant than preoperative corneas. In the optical zone, corneal thinning decreased the mechanical stiffness, causing local resteepening and making the central refractive power more sensitive to variations in intraocular pressure (IOP). At physiologic IOP, the postoperative corneas had a mean 7% forward increase in apical displacement and a mean 20% increase in the stress components at the center of the anterior surface over the preoperative condition. Conclusion Patient-specific numerical models of the cornea can provide quantitative information on the changes in refractive power and in the stress field caused by refractive surgery. Financial Disclosures No author has a financial or proprietary interest in any material or method mentioned. © 2014 ASCRS and ESCRS

Optical coherence tomography, Scheimpflug imaging, and slit-lamp biomicroscopy in the early detection of graft detachment after Descemet membrane endothelial keratoplasty

Purpose: To evaluate the efficacy of anterior segment optical coherence tomography (OCT), Scheimpflug imaging, and slit-lamp biomicroscopy in the early detection of a (partial) graft detachment after Descemet membrane endothelial keratoplasty (DMEK). Methods: Anterior segment OCT, Scheimpflug imaging, and slit-lamp biomicroscopy were performed in 120 eyes of 110 patients after DMEK. Results: Seventy-eight eyes showed a normal corneal clearance, and the attached Descemet grafts could not be identified with any of the imaging techniques. Forty-two eyes showed persistent stromal edema in the first postoperative month. In transplanted corneas that (partially) did not clear in the early postoperative period, OCT had an added diagnostic value in 36% of cases (15 of 42 eyes) in visualizing whether the graft was detached and, in particular, to discriminate between a "flat" graft detachment and delayed corneal clearance. In contrast, in the presence of corneal edema, Scheimpflug imaging did not provide more information than slit-lamp biomicroscopy in the detection of a graft detachment. Conclusions: Anterior segment OCT may be an effective tool in the detection of an early graft detachment after DMEK, to determine if secondary surgical intervention is indicated or is to be avoided.7 page(s

Refractive change and stability after Descemet membrane endothelial keratoplasty : effect of corneal dehydration-induced hyperopic shift on intraocular lens power calculation

Purpose: To determine the refractive change and stability of the transplanted cornea after Descemet membrane endothelial keratoplasty (DMEK) through a 3.0 mm clear corneal incision. Setting: Tertiary referral center. Design: Cohort study. Methods: Subjective and objective refractive data from pseudophakic eyes were obtained before and 3 and 6 months after DMEK. Results: The study comprised 50 eyes, 7 were phakic and 43 pseudophakic. Six months postoperatively, the corrected distance visual acuity was 20/25 (0.8) or better in 38 eyes (74%). The mean increase in spherical equivalent at 6 months (N = 50) was +0.32 diopter (D) ± 1.01 D (SD) (P=.0304) and in refractive cylinder, -0.48 ± 1.02 D (P=.001). Although Scheimpflug imaging showed a stable anterior corneal curvature, the posterior curvature increased from 5.50 ± 0.5 D preoperatively to 6.40 ± 0.4 D at 6 months and pachymetry decreased from 672 ± 82 μm to 540 ± 59 μm, respectively (both N = 32) (both P=.000). Conclusions: After DMEK, a slight preoperative to postoperative refractive change and stabilization at 3 months occurred that may induce a hyperopic shift that was not the result of the negative lenticule effect of DSEK/DSAEK. Thus, in DMEK, the hyperopic shift may result from a reversal of a preceding myopic shift induced by stromal swelling in endothelial disease. If so, normal intraocular power nomograms apply for cataract surgery before or during DMEK.10 page(s