Problem: Currently, non-steroidal anti-inflammatory drugs (NSAID) and radia
tion have become established as methods of choice in the prevention of ecto
pic ossifications after total hip replacement. The most effective dosis is
still not known exact for both. Conventional classification systems only pe
rmit a rough distinction of ossifications, so they cannot be used for an ex
act quantitative measuring. Further, only a limited number of categories ca
n be distinguished. We wanted to find out whether a quantitative measuremen
t of ossifications can be realized, and if small differences in prophylacti
c effect can be detected in this way. Method: By computerized digitized pla
nimetry, we measured ossifications of patients after total hip replacement
on plain X-rays of the pelvis. We followed 57 patients for up to 2 years af
ter operation. After marking the ossifications they were measured by drawin
g the outline with a magnifying glass-mouse. We checked all aspects of prec
ision and reproducibility of the measurements and the comparability with an
established classification method (Brooker). Results: The digitized planim
etry could be performed on all patients Xrays. It gave a much more differen
tiated picture compared to the method of Brooker. The method is simple and
reproducible, but time consuming. Conclusions: Digitized planimetry allows
a very exact measurement of ectopic bone formation. Compared to conventiona
l classification systems, it differentiates more subtly. In limited, well c
ontrolled studies slight differences of prophylactic methods on ossificatio
ns can be evaluated. New developments in X-ray technology will make the han
dling of this method even easier.