Effect of Centration Technique on the Monochromatic Wavefront Aberration Map

Abstract

Purpose:: To determine the effect of centration technique on the monochromatic wavefront aberration map.

Methods:: Wavefront aberrations up to the 6^th order were measured with a clinical aberrometer (Alcon LADARVision 4000) on 82 eyes scheduled to undergo either conventional photorefractive keratectomy (STD PRK) or wavefront guided photorefractive keratectomy (WFG PRK). Two different methods were used to center the wavefront aberration map: (1) the center of a circle formed by the limbus which is approximately equal to the corneal apex (2) the center of the circle formed by the non-dilated pupil margin. For each patient and the two methods of centration, a Zernike polynomial series was calculated using a 6mm pupil. This was done pre-operatively and at either 6 or 12 months post-operatively. To determine if wavefront aberration map is dependent on the method of centration, our analysis consisted of paired samples t-Tests comparing total root mean square (RMS) wavefront error, higher order aberrations (HOA) RMS wavefront error, and individual Zernike modes. Objective optical quality was quantified for each method by the area underneath a 2-D radially averaged modulation transfer function normalized to the diffraction limited case.

Results:: Across centration methods, the difference between Total RMS wavefront error and HOA RMS were not significantly different from zero. This was true for both the pre-operative and post-operative eyes. In pre-operative eyes, 5 individual Zernike modes were dependent on centration method: regular astigmatism, vertical and horizontal coma, secondary astigmatism, spherical aberration, and 6^th order spherical aberration. Optical quality as defined by the area underneath the rMTF was 7% greater (p=4.7e-12) when derived from the wavefront centered in the middle of a nondilated pupil when compared to the center of the limbal circle. In post-operative eyes receiving STD-PRK, a total of 8 individual Zernike modes were significantly different as a function of centration method. In post-operative eyes receiving WFG-PRK, 4 individual Zernike modes were different as a function of centration method: horizontal coma, 5^th order horizontal coma, and 6^th order spherical aberration. In post-operative eyes the area of a 2-D MTF did not significantly differ from zero as a function of centration method for either STD-PRK or WFG-PRK.

Conclusions:: The wavefront can vary as a function of centration method. Therefore, wavefront measurements derived from very large pupils may not accurately reflect the optical quality of the human eye.

Keywords: refractive surgery: optical quality • optical properties