Effects of misalignment during corneal topography

Purpose

To evaluate consequences of misalignment during corneal topography.

Setting

Maisonneuve-Rosemont Hospital, Montreal, Quebec, Canada.

Methods

Twenty-nine healthy subjects were enrolled in a study of 2 types of misalignment during Orbscan topography acquisition. The first was rotation, which was defined as angular deviation of the subject's visual axis away from the central fixation target, and the second was translation, defined as off-center displacement of the joined half slits on the computer screen during alignment by the technician. The effect of vertical and horizontal misalignments, by rotation or translation, was assessed. Study parameters included simulated keratometries (SimKs), mean corneal power (mean power), and corneal apex (R) and the asphericity factor (Q) values. Inferior–superior (I–S) and temporal–nasal (T–N) indices were calculated. Differences in parameter values between aligned and misaligned positions were studied. A mathematical model was also developed to simulate rotational and translational misalignment and consolidate clinical experimentation.

Results

The mean SimK values were slightly but significantly affected by rotation. The I–S value increased significantly with downward rotation and decreased with upward rotation of the eye, while T–N values increased significantly with temporal rotation and decreased with nasal rotation. The R and Q values increased with rotation. Overall, the topography parameters were minimally affected by translation. Comparable results were obtained with the theoretical model.

Conclusions

Rotational misalignment of the eye during acquisition influenced topography more than the translational misalignment during focusing by the technician. Rotational misalignment induced corneal asymmetry on elevation and curvature maps as well as pseudokeratoconus patterns.