Purchase this article with an account.
T. Buehren, M.J. Collins, D.R. Iskander, B. Davis; Interactions Between Higher–Order Aberrations and Sphero–Cylinder Refraction . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1171.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To investigate how the presence of higher–order wavefront errors such as combinations of trefoil (along 30°), vertical coma, and spherical aberration influence the optimal sphero–cylindrical correction of the eye.
A total of 25 artificial test wavefronts, subdivided in to 5 different combinations of trefoil, coma, and spherical aberration and 5 levels of total higher–order root mean square errors (HO–RMS), were created for a 5 mm pupil zone. Four of the wavefront combinations contained coma, trefoil and spherical aberration of various signs with total HO–RMS levels between 0.1 and 0.5 microns. The other wavefront type contained only spherical aberration with HO–RMS levels also between 0.1 and 0.5 microns. Refractive power maps were created from the wavefronts and fitted with 2,025 different sphero–cylindrical combinations. For each sphero–cylinder the visual Strehl ratio based on the modulation transfer function (VSMTF) was calculated (i.e. ratio of the volumes under the MTF and the diffraction limited MTF). Retinal images and refractive power histograms were calculated for the refractive power maps corresponding to the peak of the VSMTF. Wavefronts from 3 real keratoconic eyes were also used for the analysis.
The retinal image quality (VSMTF) for wavefronts consisting of vertical coma, trefoil (along 30°) and spherical aberration, could be improved with sphero–cylinders of various magnitudes and directions (i.e. positive or negative sphere and with–the–rule or against–the rule cylinder). For example the combination of positive trefoil (Z(3,–3) = +0.325 microns), negative vertical coma (Z(3,–1) = –0.325 microns) and negative spherical aberration (Z(4,0) = –0.2 microns, i.e. total HO–RMS = 0.5 microns) was optimized with positive sphere and against–the–rule cylinder (+0.875/–0.625x90°). As expected, spherical aberration alone changed the best focal plane thereby affecting the best sphere. A clinically significant level of sphero–cylinder corrections (i.e. ≥ 0.25 D) was reached when HO–RMS levels were between 0.2 and 0.3 microns. For the keratoconic eyes, higher–order aberrations also influenced the optimal sphero–cylinder required to produce the best retinal image.
The combination of the three higher–order aberrations trefoil, coma, and spherical aberration, at levels that are often encountered in aberrated eyes, influence sphero–cylindrical refraction. These findings have implications for the understanding of optimal subjective and objective sphero–cylinder refraction results.
This PDF is available to Subscribers Only