Assessing the accuracy of Orbscan II post-LASIK: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures

Purpose: To establish a relationship between the forward protrusion of the cornea (keratectasia), following successful LASIK, and anterior chamber depth. Methods: One hundred and fifteen eyes of 59 patients who had undergone Zyoptix LASIK and 39 eyes of 21 patients who had undergone PlanoScan LASIK at the Eye Institute, Centre for Refractive Surgery, Remuera, Auckland, New Zealand, were included in the study. The results of Orbscan II acquisitions, taken before and 8.3 weeks ± 4.0 (mean ± standard deviation) following the procedure, were analysed to determine the amount of forward protrusion of the central 2 mm diameter of the posterior surface. The apparent keratectasia was then related to the Orbscan II anterior chamber depth (ACD) measurement in a linear mixed model analysis that included ‘right or left eye’, ‘sex’, ‘type of LASIK procedure’, ‘change in pachymetry’, ‘postoperative pachymetry’, ‘change in anterior corneal curvature’, ‘postoperative anterior corneal curvature’, ‘planned ablation depth’ and ‘time until follow‐up’ as factors and covariates. Results: The mean ± SD amount of keratectasia determined by Orbscan II was 22.8 ± 26.8 µm for the Zyoptix group and 16.9 ± 24.6 µm for the PlanoScan group. The reduction in depth of the anterior chamber was 14.7 ± 79.3 µm for the Zyoptix group and 18.2 ± 68.8 µm for the PlanoScan group. The mixed model analysis demonstrated strong relationships with postoperative pachymetry and change in anterior corneal curvature (P