Magnitude and Orientation of Zernike Terms in Patients With Keratoconus

Abstract

Abstract: : Purpose:The existence of pairs in Zernike terms sometimes makes it difficult to analyze the characteristics of ocular higher–order aberrations. Describing the Zernike terms as vector components with a magnitude and orientation approach similar to the cylinder power and axis might be helpful. Therefore, we investigated the magnitudes and orientations of Zernike terms using Campbell’s simplified Zernike functions in keratoconic eyes. Methods:The ocular higher–order aberrations for the central 4 mm diameter were measured in 56 keratoconic (KC), 51 keratoconic suspect (KCS), and 60 normal eyes (CONT) using a wavefront sensor (KR–9000PW, Topcon). For each pair of the standard Zernike terms, a single magnitude and axis value were calculated for the third– and forth–order aberrations. The magnitudes and orientations of coma, trefoil, secondary astigmatism, and tetrafoil in addition to the spherical aberration were compared between the three groups. Results:The mean magnitudes (RMS,µm) @ axis (degrees) of the coma in KCS, KC and CONT were 0.31@108.6°, 0.77@93.8°, and 0.07@189.9°, respectively. These findings indicate a prominent vertical coma with an inferior fast wavefront in KC and KCS patients as expected. Interestingly, the axis of trefoil in KC (71.9) and KCS (76.3) were significantly different (P=0.001) from that in normal (24.8) indicating that keratoconus has an infero–superior reverse pattern in trefoil compared to normals. Also, the angle of tetrafoil tended to be located near 45 degrees for all groups. Although the RMS of the secondary astigmatism in KCS and KC were higher than in CONT (P=0.001), there were no significant trends in the axis orientation among the three groups. For spherical aberration, KC eyes have significant negative values compared to eyes of CONT (P=0.001). Conclusions:Compared to normals, keratoconic eyes tend to have higher trefoil with infero–superior reverse pattern and negative spherical aberration in addition to the prominent vertical coma.

Keywords: keratoconus • refraction • imaging/image analysis: clinical