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Correlation of topographic decentration indices

Poster Details

First Author: A.Abodarahim SAUDI ARABIA

Co Author(s):    M. Roshdy   S. Wahba                 

Abstract Details


To investigate the correlation between different topographic decentration indices; Inferior minus Superior value (I-S) astride horizontal meridian (h), I-S astride flattest meridian(f), and Index of Height Decentration (IHD).


Al-Watany Eye Hospital, Cairo, Egypt.


In a retrospective study; history taking, clinical examination, and rotating Scheimpflug camera scanning (by Oculyzer II equivalent to Pentacam HR) were done to 128 eyes; 82 normal, 24 forme frust keratoconus FFKC (apparently normal cornea with the fellow eye with evident keratoconus) and 22 keratoconus (KC). All cases of corneal scars or previous corneal surgeries were excluded. I-S value was calculated manually from 10 points astride the horizontal and flat meridians. The IHD is calculated automatically by the device software 1.17r119.


The mean I-Sh in normal, FFKC, and KC eyes were 0.30±0.93, 0.11±2.03, and 4.62±3.89, respectively. Those of I-Sf were 0.12±0.63, 0.38±0.62, and 3.42±3.04, respectively. Those of IHD were 0.008±0.004, 0.011±0.005, 0.066±0.067, respectively. The three indices were highly correlated (P<.0001) with higher correlation coefficient between IHD and I-Sh (r 2 =.874) than between IHD and I-Sf (r2 =.821) and between I-Sh and I-Sf (r2=.817). The deduced regression formula linking the two most correlated indices was “IHD= 0.007334 +0.005676 I-Sh +0.0007021 I-Sh2” (F-ratio = 808.4, P<.001). Hence, with lower decentration, the I-Sh of 0.5, 1, and 1.4 correspond to IHD of 0.01, 0.014, and 0.017.


All the three commonly used topographic decentration indices are correlated, and can be deduced from each other.

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