Introduction: To evaluate heterotopic ossifications (HTO) in total hip replacement several classifications have been developed. Most often the classifications according to Brooker or Arcq are used. These systems enable the description of HTO on x-rays in ap view, functional aspects like range of motion are not evaluated. In several studies we could observe that patients with HTO IV degrees according to Brooker ("ankylosis") had a free or excellent range of motion. We introduce a modified Brooker classification system which enables the evaluation of HTO in two X-ray planes and allows in addition a description of hip function.
Patients and method: On the a.-p. X-ray view the original Brooker classification is used, on the lateral view the Brooker classification ventrally and dorsally of the longitudinal axis of the stem of the total hip replacement is used. In addition functional deficits as a sequalae of HTO are documented (1: yes; 2: no). The new classification system is used in 10 cases with HTO IV degrees according to Brooker in the a.-p .view.
Results: 8 cases with "ankylosis" according to Brooker showed a good range of motion. In these cases HTO I-III degrees according to Brooker were found in the lateral view. In the ap view these HTO were evaluated as HTO IV degrees because of an overlapping effect.
Discussion: The new classification using X-rays in two planes allows to describe HTO sufficiently especially in respect to hip function. The most important advantage of this new classification is the fact that it modifies only a widly used and accepted system. This may lead to a high acceptance of the new classification.