Pentacam and orbscan II measurements of posterior corneal elevation before and after photorefractive keratectomy

Purpose: To compare the measurements of posterior corneal elevation above the best-fit sphere before and after photorefractive keratectomy (PRK) using two different technologies-Scheimpflug imaging using Pentacam (Oculus) and scanning slit combined with Placido imaging using Orbscan II (Bausch & Lomb). Methods: In a retrospective observational case series study, 60 eyes measured using Orbscan II and Pentacam before or after PRK were evaluated. The data were analyzed as three groups: preoperative, early postoperative (from 1 to 3 months after surgery), and late postoprative (>3 months postoperatively but not exceeding I year). Anterior chamber depth, posterior best-fit float sphere, posterior central elevation (PCE) above the best-fit sphere, and posterior maximum elevation (PME) above the best-fit sphere in the 6.0-mm zone were analyzed. RESULTS: Posterior central elevation and PME values measured using Orbscan II were greater than those measured using Pentacam (P>.001). Anterior hamber depth values measured using Orbscan II were smaller than those measured using Pentacam in all three groups (P>.001). Using Orbscan II, PCE and PME values for the two postoperative groups were greater than for the preoperative group. Using Pentacam, the PCE values for the three groups were similar, and the PME values for the preoperative and late postoperative groups were similar. Conclusions: These findings indicate that Orbscan II measurements of posterior corneal elevation before and after excimer laser refractive corneal surgery may not be valid. Pentacam measurement showed no significant change in posterior corneal elevation from before to after excimer laser corneal surgery.