(W.Haigis)
The K-index
problem has been addressed in a separate paper (http://www.augenklinik.uni-wuerzburg.de/eulib/texte/kprobl/kprobl.htm
. To illustrate the influence of the keratometer index on the optimization of
lens constants, we performed the respective calculations for two intraocular
lenses, for which ULIB had received data sets with pre- and postoperative
results as described on http://www.augenklinik.uni-wuerzburg.de/eulib/dload.htm
.
The results
in terms of optimized constants are shown in the following tables.
Assuming
that the reported Ks stem from a keratometer with an index of 1.3375,
e.g. a Javal-type instrument or a topography system, the results are:
|
|
nominal |
Haigis |
HofferQ |
Holl.1 |
SRK/T |
SRK II |
|
IOL#1 |
117.8 |
a0=0.03;
a1=0.4; a2=0.1 |
pACD=4.21 |
sf=0.47 |
A=116.9 |
A=117.0 |
|
IOL#2 |
118.0 |
a0=0.88;
a1=0.4; a2=0.1 |
pACD=5.04 |
sf=1.35 |
A=118.4 |
A=118.9 |
Assuming
that the reported Ks stem from a keratometer with an index of 1.332, e.g.
a Zeiss or a Topcon instrument, the results are:
|
|
nominal |
Haigis |
HofferQ |
Holl.1 |
SRK/T |
SRK II |
|
IOL#1 |
117.8 |
a0=0.50;
a1=0.4; a2=0.1 |
pACD=4.66 |
sf=0.88 |
A=117.5 |
A=117.7 |
|
IOL#2 |
118.0 |
a0=1.32;
a1=0.4; a2=0.1 |
pACD=5.44 |
sf=1.72 |
A=118.9 |
A=119.5 |
It can be
seen that the resulting constants differ by some 0.5 - 0.7 D. If e.g. it was erroneously assumed that the
keratometer used was a standard 1.3375 instrument, while actually it applied
1.332, the optimized constants would be too weak. Accordingly, IOL powers
calculated with these constants will also be too weak thus making patients
hyperopic.
This again
demonstrates the necessity of being aware of the effective keratometer
calibration.
W.Haigis, June 02, 2003