Comparison of Corneal Thickness Measurements Between Orbscan Topography System and Ultrasonic Pachymetry before and after LASIK

PURPOSE: To report the difference of the values of the corneal thickness measurement measured by Orbscan topography system and ultrasonic pachymetry is different between pre-LASIK and post-LASIK. METHODS: The corneal thickness measured by these two methods was compared in 32 pre-LASIK eyes of 18 subjects and 31 post-LASIK eyes of 20 subjects passed 3 months after laser in situ keratomileusis (LASIK). RESULTS: In the pre-LASIK eyes, the cornea measured thicker by Orbscan topography system than by ultrasonic pachymeter (16.94+/-13.1 microgram, average +/- standard deviation, p=0.001). In the post-LASIK eyes, there was no statistically significant difference in the values of corneal thickness measurement between two methods (-9.58+/-27.88 microgram, p=0.0653). CONCLUSION: We must give attention to the interpretation of values of corneal thickness measurement measured by Orbscan topography system and ultrasonic pachymetry because the correlations between the two methods in pre and post-LASIK eyes are different. We assume that differences of the values of measurement between the two methods may result from either technical errors of the measurers , interference of corneal interface, or change of tear film.

Accuracy of Orbscan Pachymetry Measurements and Ultrasonic Pachymetry before and after LASIK with Orbscan II(R) Topography

PURPOSE: To assess the the accuracy and variability in pachymetry measurements obtained by Orbscan and ultrasonic pachymeter in laser in situ keratomileusis (LASIK). METHODS: Thirty two eyes of 16 patients before LASIK were measured by ultrasonic pachymeter and Orbscan. Thirty eyes of 16 patients that had LASIK were measured by ultrasonic pachymeter and Orbscan postoperatively. The theoretical residual corneal thickness was compared to measurements by both instruments in eyes that had LASIK procedures. RESULTS: The ultrasound pachymeter measurements (543.8+/-24.2 micrometer) were thicker than the Orbscan pachymetry measurements, (541.2+/-24.9 micrometer) by mean of 2.7+/-7.4 micrometer (SD), preoperatively. But this is not statistically significant (p>0.05). Postoperatively, the ultrsound measurements were consistent with theoretical residual corneal thickness, 461.8+/-37.4 micrometer versus 467.9+/-30.71 micrometer (p=0.064), while Orbscan measurements were statistically less than the theoretical residual corneal thickness, 433.2+/-48.75 micrometer versus 464.32+/-33.4 micrometer (p<0.001). CONCLUSIONS: In LASIK eyes, the ultrasonic pachymetry was a more accurate measurement of corneal pachymetry than Orbscan topography system.

Factors Associated with the Posterior Corneal Surface Elevation in Mmyopic Eye

PURPOSE: To know the factors associated with the posterior corneal surface elevation in myopic eye. METHODS: The posterior corneal surface elevation (PCSE), keratometry, central corneal thickness (CCT), peripheral corneal thickness (PCT), thinnest corneal thickness (TCT) were measured in 64 right myopic eyes with the Orbscan topography system. The correlation between PCSE and age, refractive error, intraocular pressure (IOP), keratometry, CCT, TCT, PCT, PCT-CCT, PCT/CCT, PCT-TCT, and PCT/TCT was evaluated. RESULTS: The posterior corneal surface elevation has correlation with CCT (r:.0.3306, p:0.0071), TCT ( r :.0.4094, p:0.0007), PCT-CCT(r:0.3326, p:0.0068), PCT/CCT (r:0.3759, p:0.0020), PCT-TCT (r: 0.4750, p:0.0001), and PCT/TCT (r:0.5145, p:0.0001) but does not have correlation with age, keratometry, refractive error, IOP (r:.0.0627, p:0.6225), and PCT. CONCLUSIONS: In myopic eyes, the posterior corneal surface elevation correlates with central corneal thickness and specifically the ratio PCT to TCT.