OCULUS Pentacam Interpretation Manual page 67

11 Corneal Thickness
Percentage of Increase in Thickness (PIT)
PIT can then be calculated for each position using the simple formula:
(mean corneal thickness in the ring – thinnest corneal thickness)
PIT =
Î
thinnest corneal thickness
Clinical results
In a published study involving 46 eyes of 23 patients (13 females) diagnosed with mild to
moderate keratoconus and 364 normal eyes from 196 patients (97 females), statistically significant
differences were observed between the two groups (P < 0.01) for all positions of CTSP and PIT
[5]. Keratoconic eyes had much lower (thinner) values than normals, with an estimated average
difference of 27.3 μm. In keratoconic eyes mean TP was 428 μm, (standard deviation (SD) 72 μm,
95% confidence interval (CI95) 391-474 μm, range 245-563 μm), while in normal eyes the mean
value was 537 μm (SD 36.7 μm, CI95 513-562 μm, range 439-630 μm). For example, in keratoconic
eyes mean corneal thickness on the 4.8 mm diameter circle was 536.5 μm, (SD 48.3 μm, 95CI 516-
566 μm, range 377-623 μm), while in normal eyes, mean thickness was 589 μm (SD 36.9, 95CI
564-614.8 μm, range 467-693 μm).
The statistical significance of differences in PIT between normal and keratoconic eyes over all
locations considered was very high (p < 0.0001). Keratoconic corneas had a much higher thickness
percentage increase than normal eyes on each of the 22 diameters.
In keratoconic eyes mean PIT on the 0.4 mm diameter was 0.27% (SD 0.29, CI95 0.19-0.26, range
0.0-1.6 μm), while in normal eyes, the mean value was 0.07% (SD 0.09%, CI95 0.0-0.18%, range
0.0-0.23%). On the 4.8 mm diameter circle mean PIT in keratoconic corneas 28.2% (SD 21.4%, 95CI
13.8-34.8%, range 6.1-129%), while in normal eyes the mean value was 9.9% (SD 1.9% (95CI 8.7-
11.1%, range 3.3-17.9%).
This study demonstrated that modern corneal tomography provides us in CTSP and PIT with two
powerful discriminators of keratoconus. We also found that keratoconic eyes have thinner corneas
with less volume and a faster increase in thickness from the TP towards the periphery than do
normal corneas. The Scheimpflug images below, one of a normal thin cornea and the other of a
moderately keratoconic eye, clearly illustrate the differences in thickness profile between normal
and ectatic eyes (Figure 76).
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