Accuracy of Orbscan pachymetry measurements and DHG ultrasound pachymetry in primary laser in situ keratomileusis and LASIK enhancement procedures

PURPOSE: To assess the accuracy and variability in pachymetry measurements obtained by Orbscan and by DGH ultrasound in primary laser in situ keratomileusis (LASIK) and LASIK enhancement procedures.

SETTING: Gimbel Eye Centre, Calgary, Alberta, Canada.

METHODS: A retrospective analysis of 906 consecutive primary LASIK and 183 consecutive LASIK enhancement procedures for which preoperative DGH ultrasound and Orbscan pachymetry measurements were obtained. The theoretical residual corneal thickness was compared to measurements by both instruments in 60 eyes that had primary procedures and enhancements.

RESULTS: In primary LASIK eyes, the DGH ultrasound measurements were thicker than the Orbscan measurements by a mean of 18.4 microm +/- 17.4 (SD). The DGH ultrasound measurements were thicker than the Orbscan measurements by a mean of 50.1 +/- 40.7 microm in preenhancement pachymetry measurements. The DGH ultrasound measurements were consistent with theoretical residual corneal thickness, 493.0 +/- 42.0 microm versus 487.0 +/- 31.0 microm (P =.65), while Orbscan measurements were statistically less than the theoretical residual corneal thickness, 431.0 +/- 42.0 microm versus 468.0 +/- 39.0 microm (P =.0001).

CONCLUSION: DGH ultrasound was a more accurate measurement of corneal pachymetry than Orbscan. The discrepancies between DGH ultrasound and Orbscan pachymetry measurements were more prominent in eyes that had had LASIK.