Figure 6 shows preoperative (white bars) and postoperative (black bars) values
of the RMS spherical aberration, for a 6.5-mm pupil. It appears as
positive in all cases, because it is given in terms of the RMS
corresponding to
Z 4 0 and
Z 6 0 . 29 Eyes are sorted by increasing myopic spherical error, as in
Figure 5 .
There is a dramatic increase in spherical aberration after LASIK in all
but three eyes (1, 10, and 11). The spherical-aberration RMS increase
ranged from 0.002 to 0.97 μm.
Table 1 , row 5, shows the average
increase of spherical aberration (by a factor of 3.99) in this group of
eyes, which is highly statistically significant (
P <
0.0001). For a 3-mm pupil, the RMS corresponding to spherical
aberration increases by a factor of 7.48 on average, and this increase
is again significant (
P = 0.002). The increase of 4th
order aberration terms (
Table 1 , row 3) is mostly due to the increase
in spherical aberration. The fact that the asphericity of the cornea
changes with surgery (from a prolate to an oblate shape or from
negative to positive asphericity) is reflected in a change of spherical
aberration toward more negative values, i.e., more myopic at the pupil
edges (mean
Z 4 0 =−
0.13 preoperatively and −0.68 postoperatively).
Figure 7 shows that the increment of spherical aberration is directly related to
the preoperative refraction: the larger the correction, the larger
amount of spherical aberration is induced by LASIK. Symbols represent
individual data, and the solid line denotes a linear regression to the
data. The increase is highly statistically significant
(
r = 0.76;
P < 0.0001). A slightly
higher correlation coefficient (
r = 0.88) is found when
the spherical equivalent correction programmed in the laser system
(i.e., attempted correction) is used in the calculation. However, there
is a good agreement between the preoperative spherical error derived
from the Zernike polynomial expansion and the value used in the surgery
(
r = 0.98 D, average difference = 1.02 D). Two
eyes (10 and 11) do not seem to follow the mentioned trend; they have
postoperative spherical-aberration RMS below 0.2 μm while having a
preoperative myopia of >6 D.