Cadaveric lumbar spine specimens of "motion segments", each including two vertebrae and the linking disc and facet joints, were compressed. The pressure across the facet joints was measured using interposed pressure-recording paper. This was repeated for 12 pairs of facet joints at four angles of posture and with three different disc heights. The results were that pressure between the facets increased significantly with narrowing of the disc space and with increasing angles of extension. Exfra-articular impingement was found to be caused, or worsened, by disc space narrowing. Increased pressure or impingement may be a source of pain in patients with reduced disc spaces. At each lumbar intersegmental level the intervertebral disc and the facetjoints act together to resist the resultant force acting through the "motion segment". This force can be resolved into two components, one acting perpen-104 86 85 F 31

In previous studies on the mechanics of the human hip joint (Afoke, Byers and Hutton 1980, 1984) it was found that the inter-cartilaginous space was variable in size and location, and that it changed shape with different positions of the joint. It was concluded that this space would be of limited value as a load-distribution mecha nism and that cartilage per se should be well able to distribute the applied load. If incongruity had any function, it seemed to be in promoting bulk fluid movement for the purpose of lubrication and nutrition (Afoke, Hutton and Byers 1984).To obtain a better understanding of the mechanics of the joint, it was thought advantageous to measure the pressure distribution at the cartilage interface during load-bearing. Previous pressure measurements in the hip have been carried out either directly or indirectly. Direct measurements have mostly involved the insertion of piezo-electrictransducersin the acetabulum(Christel, Derethe and Sedel 1976; Mizrahi et al. 1981; Adams and Swanson1985);in theseexperiments, a limited number of transducers were used and it was difficult to infer the pressure distribution over the whole of the cartilage surface. On the other hand, Rushfeld, Mann and Harris (1979), using an instrumented endoprosthesis, were able to obtain the pressure contours on the cartilage when the hip was loaded. However, the use of a prosthesis has its Requests for reprints should be sent to Dr P D Byers. body weight (BW), it would be oflimited value as a load distribution mechanism and that cartilage per se is able to distribute the applied load evenly. ©1987British Editorial Societyof Bone and Joint Surgery MATERIALS AND METHODSHip joints were obtained from routine necropsies and were stored in sealed polythene bags at â€" 20°Cuntil required. Before testing, they were thawed at room temperature and all the excess soft tissue and the labrum were removed. The disarticulated specimen was then mounted by first cutting the ilium and the femoral shaft in such a way that the direction of the applied force simulated that in vivo. The cut ends of the joint components were mounted in cups using quick-setting dental cement. Wood screws were inserted in the cortical bone to improve the fixing ofthe specimen to the cement. From The Polytechnicof Central LondonThe pressuredistribution between the cartilaginous surfaces in the humanhipjoint was measured using pressure-sensitive film. Five cadaveric hip joints (aged between 58 and 87 years) were measured at three @fions and loads, representing three instants in the stance phase of the walking cycle. The pressure distribution was not uniform, indicating that the cartilage did not, to any great extent, distribute the applied load. The maximum pressures recorded were about 10 MN/m2. The anterosuperiorsurface of the cartilage was identified as an area of high pressure.

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