Pachymetric Measurements Using Orbscan after Excimer Refractive Surgery

ABSTRACT

PURPOSE: To assess the accuracy of pachymetric measurement using Orbscan(Bausch and Lomb, USA) after laser in situ keratomileusis(LASIK), photorefractive keratectomy(PRK), and laser epithelial keratomileusis (LASEK). METHODS: Central corneal thickness of 159 nonoperated normal eyes, 55 eyes after LASIK, 12 after PRK, and 17 after LASEK was measured using ultrasound and Orbscan II. The acoustic factor (AF) was adjusted, based on the results obtained in the normal eye group, to minimize the difference between ultrasound and Orbscan pachymetric values. RESULTS: Using the adjusted AF (0.9607), the mean difference between Ultrasonic pachymetric and Orbscan measurement was 0.00 +/- 21.08, 20.44 +/- 24.12, 27.63 +/- 36.87, and 0.56 +/- 17.26 micro meter in the normal, LASIK, PRK, and LASEK groups, respectively. In the LASIK and PRK groups, there was a statistically significant difference between the two methods (both P<0.001). The postoperative ultrasound measurements were consistent with the theoretical residual corneal thickness in the LASIK and PRK groups (-2.96 +/- 18.00 micro meter, -1.00 +/- 13.59 micro meter), while Orbscan measurements were statistically significantly less than the theoretical residual corneal thickness in the LASIK and PRK groups (21.84 +/- 31.06 micro meter, p<0.001; 21.88 +/- 35.91 micro meter, P=0.036). CONCLUSIONS: Orbscan pachymetric values may be underestimated and less accurate after excimer refractive surgery, especially after LASIK and PRK.