A New Method to Combine Zernike Terms Into a Single Metric

Abstract

Abstract: : Purpose: To develop a new Zernike metric. Methods: A retrospective study of 60 eyes of 30 patients who received Zywave wavefront measurements (Bausch&Lomb, Rochester, New York), cycloplegic and manifest refractions, high and low contrast cycloplegic refractions, and cylcoplegic autorefractions before and six months after LASIK using one of two lasers was conducted. Wavefront measurements were calculated over a 6 mm diameter pupil. The set of coefficients of the resulting polynomial were treated as a set of error vectors. Spherical terms were treated as magnitude with no direction. All other corresponding pairs of terms were treated as components of a vector with magnitude, cylinder, and direction. The resulting sets of vectors were then summed. The 3^rd order terms , 4^th order terms, and total 3^rd through 5^th order tersm were analyzed. Regression analyses were performed comparing the average error vector, the magnitude of the error vector, and the cylinder of the error vector to the logMAR visual acuity and defocus equivalent data. Higher–order RMS (total 3^rd through 5^th) were calculated and compared to the logMAR and equivalent defocus data. Results: Pre–operatively, the cylindrical component of the 4^th order error vector correlated significantly with all defocus equivalents (p < 0.0004). There were no significant correlations for total higher–order RMS wavefront error pre–operatively. After LASIK, the magnitude of the 4^th order error vector, the average of 4^th order error vector, and RMS error correlated significantly with logMAR acuity and defocus equivalent (p < 0.008). Conclusions: This vector–based metric derived from Zernike coefficients is superior to total RMS error for prediction of vision and refractive error before LASIK. After LASIK, both metrics were similarly influenced by changes in spherical aberrations.

Keywords: refractive surgery: optical quality