In growing eyes, some decrease in optical aberrations can be expected as a consequence of geometric scaling, quite apart from any active emmetropization process (Kisilak M, et al.
IOVS 2005;46:ARVO E-Abstract 1971).
33 The growth (scaling)–related contributions to observed decreases in the optical aberrations were estimated for the right eyes of our control group using the following equation
\[\frac{Z_{\mathrm{t}}}{Z_{0}}{=}\ \frac{1}{k_{\mathrm{t}}^{n}}\]
where
k t is a scaling factor derived from the optical axial length data (modified from Howland
33 ; in the cited paper, cornea diameters were used). Values were normalized to the optical axial length recorded on the first measurement day; thus, the minimum
k t is 1 (i.e., day 14; note there is always a 1-day discrepancy between the axial length and aberrational data; axial length data corresponding to days 13, 16, 20, 23, and 34 are used to scale aberrations measured on days 14, 17, 21, 24, and 35). The exponent
n has an empirical value between 2 and 2.9; we used the middle value of 2.5. Estimates for primary astigmatism, coma, trefoil, and HOA, expressed as ratios of
Z t/
Z 0, are plotted against age in
Figure 3(solid line), along with normalized raw aberration data (lines with markers). Geometric growth accounts for most (92.4%) of the observed decrease in coma but only 69.8% of the decrease in HOAs, and its contributions to the changes in primary astigmatism and trefoil are even smaller: 42.7% and 59.3%, respectively.