Consequences of avascular necrosis of the femoral head in rabbits. A histological and radiological study.

Avascular necrosis of the head of the femur is found in many different conditions. Most often it occurs after injury or during growth. It has been generally accepted that vascular occlusion is its immediate cause (Trueta and Harrison 1953, Trueta 1957). This occlusion leads primarily to ischaemia of the tissues, which results in necrosis if the circulation is not restored in time. Morphological signs of tissue decay become perceptible only after some time (Rosingh and James 1969). The subsequent series ofchanges commonly result in deformation of the affected femoral head. There is no agreement as to the nature of the tissue changes which ultimately lead to deformation. One long-established concept holds that deformation is a consequence of the penetration of vessels into the necrotic head. This would lead to resorption of dead bone which would only gradually be compensated for by apposition of new living bone tissue (Sherman and Phemister 1947). According to other investigators, deformation appears in avascular parts of the femoral head (Axhausen and Bergmann 1937, Jons#{228}ter 1953, Brown and Abrami 1964, Catto 1965b). It is supposed that the bone tissue in the necrotic femoral head is no longer able to adapt to the functional demands. Under the influence of weight-bearing this would lead to a definite pattern ofstress fractures in the dead trabeculae, resulting in a typically deformed femoral head as seen radiographically. “ Late segmental collapse “ is the term which has been proposed for this sequence of events (Brown and Abrami 1964, Catto 1965b). Two facts are in disagreement with these theories. In the first place generalised sclerosis of the affected femoral head, as revealed in radiographs, may actually precede deformation (Waldenstr#{246}m 1934, Jons#{228}ter 1953, Hulth 1961). This observation suggests that the quantity of bone salt in the femoral head may increase before deformation occurs. In the second place, apposition of abundant quantities of new bone after revascularisation has been observed by various authors. This quantitative increase in bone mineral might be the cause of the radiographic sclerosis (Bobechko and Harris 1960, Hulth 1961, Bohr and Larsen 1965, Catto 1965b). A direct relationship between an increased quantity of bone and deformation of the femoral head is not readily apparent, however. In an attempt to obtain more information on these problems, experiments were performed in rabbits. These studies will now be reported and discussed.