Comparison of the standard combined (bidirectional) radial keratotomy technique with the undercut technique in human donor eyes

BACKGROUND: We evaluated the efficacy of a new radial keratotomy technique, using a diamond designed to undermine the central clear zone without incising superficial stroma. METHODS: An 8-incision radial keratotomy at a 3-mm central clear zone was performed on nine pairs of human donor globes. One eye of each pair was incised using the standard combined (bidirectional) technique diamond and the contralateral eye, using the undercut bidirectional technique diamond. Paired t-tests were used to compare changes in central corneal curvature between these two groups. Microscopic analysis of incision morphology was performed on four eyes. RESULTS: Corneal topography at the 1-, 3-, and 5-mm annular zones revealed corneal flattening of 7.70 +/- 1.50 diopters (D), 6.70 +/- 1.30 D, and 5.10 +/- 1.00 D, respectively, in the undercut bidirectional technique group versus 6.20 +/- 1.70 D, 5.30 +/- 1.50 D, and 4.00 +/- 1.20 D, respectively, in the standard bidirectional technique group (P <0.01 for each annular zone). Light microscopy (serial sections) revealed an average incision depth of 80.9 +/- 3.9% in the undercut bidirectional technique group versus 72.7 +/- 4.5% in the combined group (P <0.01). The undercut bidirectional technique incisions undermined the central clear zone for a distance of approximately 350 mu m compared to about 140 mu m for the standard bidirectional incisions. CONCLUSIONS: In the human cadaver eye, the undercut technique of radial keratotomy provided greater flattening than the standard bidirectional technique. The greater amount of flattening may result from greater central extension of the undercut incisions beneath the central clear zone, from greater incision depth, or from a combination of both factors.