Functional Anatomy and Biomechanics of the Canine Hip Joint

Abstract

Introduction: In many breeds osteoarthrosis of the hip joint is the most typical orthopaedic disease. Numerous studies concerning etiology, pathogenesis and therapy of this disease still consider an exact congruity to be the basis of a physiological joint function. No data, however, are available as far as the distribution of load in the articular surfaces is concerned.Aims: It was the aim of this investigation to describe the biomechanical features of the canine hip joint as exactly as possible with respect to long‐term and momentary loading.Material and Methods: Hip joints from 43 dogs (bodyweight >20 kg) with an average age of 6 years were investigated. The subchondral bone density was determined with the method of CT‐osteoabsorptiometry. With help of cartilaginous and osseus split lines the preferential orientation of the collagen fibres could be determined. In an in vitro experimental setup the contact areas and the contact pressure distribution within the joint have been measured.Results: In the acetabulum, maxima of subchondral bone density were distributed tricentrically: cranially, dorsally and caudally. In the femoral head, only the area close to the attachment of the ligament was highly mineralized. Cartilagineous and osseous split lines within the acetabulum gave evidence of a long‐term tensile stress. In contrast to these findings, there was no clear preferential orientation in the caput ossis femoris. With load increasing initially isolated contact areas extended and became confluent at a loading force of 75% body weight. Values of maximal joint pressure (maximal joint reaction force applied: 400% bodyweight) reached 8–10 MPa.Discussion: The findings clearly show that the articular surfaces of the hip joint are loaded inhomogeneously. For the first time, it was possible to demonstrate that the healthy canine hip joint is physiologically incongruent. Furthermore, areas of cartilage degeneration within the joint surface coincide with regions that are loaded to a higher extent a priori. From the biomechanical point of view it is recommended that all kinds of acetabular fractures – even those affecting the caudal third of the acetabulum – should be surgically repaired in an adequate way.