Magnitude and orientation of Zernike terms in patients with keratoconus

To measure the magnitude and orientation of the Zernike terms in keratoconic eyes, with and without rigid gas-permeable (RGP) contact lenses.

A total of 76 eyes with keratoconus, 58 eyes with keratoconus suspect, and 105 normal eyes were studied. To determine the effect of RGP lenses, 19 eyes with keratoconus, 9 eyes with keratoconus suspect, and 17 normal eyes, with and without an RGP lenses, were compared. Ocular higher-order aberrations (HOAs) were measured with a wavefront sensor for a 4-mm-diameter pupil, and the magnitudes, axes of trefoil, and coma were calculated by vector analysis.

Zernike vector analysis showed prominent vertical coma with an inferior slow pattern, with mean axes of 82.5 degrees or 91.0 degrees in the patients with keratoconus or keratoconus suspect, respectively. The mean axes of trefoil in patients with keratoconus (93.8 degrees ) and keratoconus suspect (100.6 degrees ) differed from that in normal subjects (35.4 degrees ), indicating that keratoconus has a reverse trefoil pattern from that of normal eyes. Although the total HOAs were significantly (keratoconus and keratoconus suspect, P < 0.001 and P = 0.012, respectively) reduced with an RGP lens, the patterns of the axes of coma and trefoil were reversed with the lens.

In addition to the larger amount of trefoil, coma, tetrafoil, and secondary astigmatism, keratoconic eyes tend to have a reverse coma pattern and reverse trefoil aberrations compared with normal eyes. Although RGP lenses correct the irregular astigmatism, smaller comet-like retinal images in the opposite direction remain due to residual vertical coma.