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[Precision and reliability of Orbscan and ultrasonic pachymetry].

PURPOSE
To compare the accuracy and reproducibility of the orbscan pachymetry and ultrasonic pachymetry in the normal eye and in the penetrating keratoplasty eye.


METHODS
Pachymetric measurements were assessed in 50 eyes of 25 normal patients and 50 eyes of 48 patients who had undergone penetrating keratoplasty using both Orbscan II and ultrasonic pachymetry (Tomey SP-2000). For each eye, 2 successive measures were recorded with both instruments. For both devices, the default setting was used. Orbscan pachymetry maps were divided into 5 groups using a modification of Liu's classification.


RESULTS
Orbscan pachymetry strongly correlated with ultrasonic pachymetry (rs = 0.91; p < 0.001). Ultrasonic pachymetry values and Orbscan pachymetry values showed no significant differences in the normal group (respectively, 557 microns +/- 36 and 555 microns +/- 34; p > 0.05). Ultrasonic pachymetry values and orbscan pachymetry values were significantly different in the penetrating keratoplasty group (respectively, 571 microns +/- 52 and 550 microns +/- 54; p < 0.001). The best value for the acoustic factor for Orbscan pachymetry in the penetrating keratoplasty group was 0.89. Ultrasonic pachymetry reproducibility and Orbscan pachymetry reproducibility were not significantly different (0.86% +/- 0.61 v. 0.67% +/- 0.63; p = 0.13 in the normal group; 1.22% +/- 0.81 v. 1.23% +/- 1.13; p = 0.92, in the penetrating keratoplasty group). Both pachymetry methods showed less reproducibility in the penetrating keratoplasty group than in the normal group (p < 0.02). Thinnest point localization was significantly different in both groups (p < 0.001). In 66% of the normal group, the thinnest point of the cornea was located in inferotemporal quadrant. This point was located at an average of 0.63 +/- 0.25 mm from the visual axis in the normal group and 1.60 +/- 0.81 mm in the penetrating keratoplasty group (p < 0.001). Whereas "Centered round" (40%) and "centered oval" (34%) were the most common patterns in the normal group, "decentered oval" (40%) and "irregular" (30%) were more frequent in the penetrating keratoplasty group (p < 0.001).


CONCLUSION
Orbscan pachymetry strongly correlated with ultrasonic pachymetry. Reproducibility of both methods is excellent and not significantly different. Thinnest point localization and pachymetric map classification are significantly different in normal eyes and in penetrating keratoplasty eyes.