Purpose: To compare posterior corneal features and their discriminating power for differentiating normal corneas from subclinical keratoconus using the Placido dual-Scheimpflug analyzer.
Methods: Patients were retrospectively included in the study. The preoperative normal right eyes of 79 patients imaged with a Placido dual-Scheimpflug system and with a stable postoperative LASIK follow-up of a minimum of 36 months were included in the normal group and were compared to 39 contralateral topographically normal eyes with clinically evident keratoconus in the fellow eye. The posterior surface variables measured were categorized according to the feature of the corneal shape they were characterizing (curvature, elevation, asymmetry, and eccentricity) and compared between the two groups using the Student's two-sample t test. The discriminating ability of the posterior surface variables was compared by receiver operator characteristics curves.
Results: Variables that related to asymmetry and elevation of the posterior surface were statistically significantly different between groups (P < .05), whereas eccentricity and curvature-related parameters were not. Receiver operator characteristics curves analysis showed that the maximum posterior elevation over the best-fit toric and aspheric surface reference shape had the highest discriminating ability for distinguishing normal corneas from subclinical keratoconus, with an area under the curve of 0.877, followed by the asphericity asymmetry index, with an area under the curve of 0.871, and posterior inferior-superior value, with an area under the curve of 0.851.
Conclusions: Posterior cornea measured with a dual-Scheimpflug analyzer provides useful parameters for differentiating normal corneas from subclinical keratoconus. Of the posterior surface parameters, asymmetry and elevation seem to be the most sensitive shape modifications for differentiating both populations. [J Refract Surg. 2018;34(10):664-670.].
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