|
They ranged in age from 20 to 71 years, with a mean of 41. The study enrolled equal numbers of males and females and included both myopes and hyperopes with spherical equivalent values ranging from -11.6 D to +7.6 D. "Obviously as we seek to develop refractive surgery techniques to optimise vision by correcting higher order aberrations, it is important to understand the distribution of ocular wavefront aberrations in the general population and how they may be affected by age," Douglas D Koch MD noted. The study excluded any eyes with previous ocular surgery, a corneal scar or other corneal pathology. All patients had best corrected visual acuity better of than 20/40. Recent history of contact lens wear was an additional exclusions criterion. The wavefront evaluation was performed with a Hartmann-Shack system (WaveScan, VisX) across physiological, 6.0mm pupils. The analyses of higher order aberrations (HOA) included the determination of 22 Zernike coefficients and RMS values for third to sixth order aberrations, as well as separate evaluations for spherical aberration (fourth and sixth order) and coma (third and fifth order). For the overall cohort, algebraic means for the various Zernike terms tended to be very close to zero with the exceptions of trefoil, third order coma and spherical aberration. All values showed a wide range from negative to positive with high standard deviations. For example, the algebraic mean RMS value for third order vertical coma was -0.055 microns, but the range was -0.56 to +0.38 microns, the standard deviation 0.142 microns and the absolute mean 0.121 microns. Dr Koch performed further calculations based on the absolute values for the Zernike coefficients to examine the magnitude of HOA in the population. Mean RMS for total HOA was 0.305 microns, and the absolute mean RMS was seen to decrease progressively with increasing order from third to sixth order. The third order aberrations accounted for the majority of total HOA and fourth order aberrations contributed to a lesser but still prominent degree. Total mean RMS values for fifth and sixth order aberrations were relatively low (0.08 microns). Absolute mean RMS values for coma and spherical aberration were 0.170 and 0.128 microns respectively. Of the 22 Zernike coefficients analysed, 13 (59%) exhibited mirror symmetry with significant correlations between right and left eyes. The significant correlations were most common among third (75%) and fourth order (80%) terms. The presence of significant correlations between right and left eyes fell off with higher order aberrations and was present for only 33% of the fifth order terms and 57% of the sixth order terms. With the patients divided into age groups by 10-year brackets, mean HOA RMS was seen to increase gradually with age, rising from 0.262 microns for those in the 20 to 29-year-old group to 0.367 microns for the 60 to 71-year-olds. Both spherical aberration and coma showed age-related rises, although the change was greater for spherical aberration. The mean RMS for spherical aberration was 0.1 microns in the youngest age group and 0.187 microns for the oldest; corresponding values for coma were 0.151 microns and 0.207 microns. "Based on the findings of a separate study of changes in aberrations of the anterior corneal surface as a function of age, we believe the age-associated increase in spherical aberration is almost exclusively lenticular, whereas the increase in coma with age is largely corneal in origin," he reported. Correlation analysis showed a statistically significant association between age and total HOA, although the R2 value was only 0.1004. Spherical aberration and coma were similarly noted to correlate weakly with age. "Those results show that the wide variation in overall HOA, spherical aberration and coma in the population is only minimally explained by age. This suggests that we have much to learn about the role these aberrations play in the daily visual lives of our patients," Dr Koch said. Li Wang was the first author on this paper, which will be published in the Journal of Cataract and Refractive Surgery. Douglas D Koch MD |
