Orbscan Evaluation of Corneal Power after Myopic PRK: Placido Disc-Based vs Scanning Slit Topography

Abstract

Purpose: To compare Placido disc-based and scanning slit topography in the evaluation of corneal power changes after PRK. Methods: The changes in corneal power, evaluated with an Orbscan II utilizing both Placido disc-based and Scanning Slit topography, before and 1, 3 and 6 months after PRK were compared with refractive changes in 109 individuals (149 eyes) who underwent photorefractive keratectomy (PRK) with a sphero equivalent refraction, calculated at the corneal apex, ranging from -0.25 to -12 D (mean -4.58±2.54 D). Results: 1,3 and 6 months refractive changes showed a good correlation with changes in BFS (r2=0.64, r2= 0.53, r2= 0.62, respectively); in Sim K's (r2= 0.77, r2= 0.8, r2= 0.78, respectively) in Mean Power 3 mm (r2= 0.81, r2= 0.88, r2= 0.87, respectively) in Mean Power 5 mm (r2= 0.68, r2= 0.78, r2= 0.78, respectively) in Anterior Mean Power 3 mm (r2= 0.88, r2= 0.85, r2= 0.85, respectively) in Anterior Mean Power 5 mm (r2= 0.75, r2= 0.7, r2= 0.75, respectively) in Total Mean Power 3 mm (r2= 0.75, r2= 0.71, r2= 0.79, respectively) in Total Mean Power 5 mm (r2= 0.75, r2= 0.71, r2= 0.78, respectively). However, all differences were statistically significant (P<0.001). Conclusion: The data obtained with the scanning slit system seem to be more reliable, but need correcting factors to improve their precision.

Keywords

• corneal power
• myopic PRK
• placido disc
• scanning slit topography