Comparison of corneal pachymetry using ultrasound and Orbscan-II before LASIK

Objective: To assess the accuracy and variability in pachymetry measurements of th e thinnest corneal thickness obtained by Orbscan-Ⅱ and by DGH ultrasound in pr imary laser in myopia.Methods: The thinnest corneal thickness in 306 healthy eyes of 161 myopic subjects were measured with Orbscan-Ⅱ and DGH ultrasound pachymetry. The results were treated statistically and the differences were analysed.Results: There was no statistical difference(independent samples test,P0.05) in corneal thickness between left eye and right eye which use Orbscan-Ⅱ or DG H ultrasound pachymetry. The mean thinnest corneal thickness was(537.83±32.7 96)μm with Orbscan-Ⅱ system,and (528.79±27.689)μm with DGH ultrasound pachymetry. There was significant difference statistically(paired sa mples test,P0.05). There were significant linear correlations betw een scan ning-slit topography and ultrasound pachymetry(r=0.926,P 0.001),the differen ce of the results between these two methods was a mean value(9.24±12.692)μm,in overall the DGH ultrasound measurements were thicker than the Orbscan-Ⅱ meas urements by a mean of (9.24±12.692)μm. This sample(306 eyes) were divided in to 4 groups according to the thinnest corneal thickness by Orbscan-Ⅱ system,gr oup 1,34 eyes with less than 500 μm. Group 2,164 eyes with 500 μm to 550 μm. Gro up 3,95 eyes with 550 μm to 600 μm;and group 4,13 eyes with more than 600 μm. Amo ng these groups the difference of the results between these two methods,group 1 was( -3.29±12.692)μm,group 2 was(6.95±11.64)μm,group 3 was(16.93±10.42)μm and g roup 4 was( 15.38±12.52)μm,there was significant difference statiastically among these gro ups of the difference of the results between these two methods(ANOVA,F=33 .13,P0.05).Conclusion: When measuring the thinnest corneal thickness,the value obtained with Orbscan-Ⅱ topography was higher than that with ultrasonic pachymetry,but t he difference of the results between these two methods is not identical amomg th ese groups based on the thinnest corneal thickness by Orbscan-Ⅱ system,specia ll y the thinnest corneal thickness was less than 500 μm by Orbscan-Ⅱ. Ultrason ic pachymetry is still the most accurate method in corneal pachymetry,but Orbsca n-Ⅱ topography system is a useful instrument for the evaluation of corneal shape an d thickness which is helpful in corneal refractive surgeries.