Resumen de Orbscan evaluation of corneal power after myopic PRK: placido disc-based vs scanning slit topography

Nicola Rosa, Luigi Capasso, Emma Minutillo, Maria Borrelli, Maddalena De Bernardo, Michele Lanza

• 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 3mm (r²= 0.81, r²= 0.88, r²= 0.87, respectively) in Mean Power 5mm (r²= 0.68, r²= 0.78, r²= 0.78, respectively) in Anterior Mean Power 3mm (r²= 0.88, r²= 0.85, r²= 0.85, respectively) in Anterior Mean Power 5mm (r²= 0.75, r²= 0.7, r²= 0.75, respectively) in Total Mean Power 3mm (r²= 0.75, r²= 0.71, r²= 0.79, respectively) in Total Mean Power 5mm (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.