OCULUS Pentacam Interpretation Manual page 128

17 Holladay Report & Holladay EKR65 Detail Report
The upper left map [axial/sagittal curvature (front)] in
power map uses a sph as the reference and the keratometric formula for power (337.5/axial radius
in mm). A steel ball with a 7.5 mm radius of curvature would have a uniform power of 45.0 D
(337.5/7.5 mm = 45.0 D) at all points. It therefore does not show the refractive power (using Snell's
Law) of the cornea except in the central 3 mm, where the difference is negligible. Since the normal
shape of the front surface of the cornea is a prolate ellipsoid with the radius of curvature steepest
in the center and flattening as we move peripherally, the colors do become cooler (toward blue) by
1 to 2 D , but are less than the actual refractive power using Snell's Law. The mean and SD for
central keratometry (axial power in 3.2 mm ring) are shown in
Central keratometry (front)*
lower limit
central K central K
(D)
mean 43.82
± SD 1.51
±2 SD 3.02
±3 SD 4.53
Tables 1-8 (N = 1243 normal eyes, internal, unpublished data from J. T. Holladay's refractive surgical
practice) give the mean, SD, 2 SD and 3 SD upper and/or lower limits for each variable assuming
a normal distribution. For 1 SD, 68% would be between the lower and upper limit, 16% would be
above 45.33 D and 16% below 42.31 D. Similarly, for 2 SD 2.3% would be below 40.80 D and 2.3%
would be above 46.8 D (1 in 44 cases) and for 3 SD 0.1% would be below 39.29 D and 0.1% would
be above 48.35 (1 in 769 cases). In general, values beyond 2 SD are suspicious of pathology and
those beyond 3 SD are considered abnormal statistically.
The steep (red) and flat (blue) semi-meridians (principal) are shown in the 3 mm, the 3 – 5 mm and
5 – 7 mm diameter zones. If these semi-meridians do not form single meridians (one line) and are
not orthogonal, then irregular astigmatism is present and the axis and magnitude are less accurate.
The lower left map [tangential curvature (front)] in
(or instantaneous curvature) map does not depend on a reference axis (axis through vertex normal),
and the radius is not the distance from the surface to the axis, as with axial radius. It is the radius
of curvature (reciprocal the curvature) relative to the surface at that point. For example, if the
earth has an average radius of 4000 miles and if there was a semicircular mountain of 6 miles,
the axial radius would be 4006 miles from the center of the earth to the top of the mountain but
the tangential radius would be only 6 miles ... a significant difference. The tangential curvature or
power is much more sensitive and shows geometrical changes much more sensitively. Because of
this increased sensitivity the default scale is in 1 D steps, whereas the axial map is in 0.50 D steps
[34] (Table
2).
maximum tangential K (front)*
lower limit
central K central K
(D)
mean 45.55
± SD 1.84
±2 SD 3.69
±3 SD 5.53
126
upper limit
central K
min max
(D) (D)
-- --
42.31 45.33
40.80 46.84
39.29 48.35
upper limit
central K
min max
(D) (D)
-- --
43.71 47.40
41.87 49.24
40.02 51.08
Figure 157, shows the axial curvature or
Table 1.
Table 1: Central keratometry (front)
(*N = 1243 normal eyes,
internal, unpublished data
by J. T. Holladay)
Figure 157 shows the front tangential curvature
Table 2: Maximum tangential K (front)
(*N = 1243 normal eyes,
internal, unpublished data
by J. T. Holladay)