DEFORMATION OF THE ACETABULUM DURING WEIG HT-BEARING - INFLUENCE OF THE PUBIC RAMUS AND THE QUALITY OF TRABECULAR BONE

Introduction Several authors have reported evidence of loosening of th e acetabular component in 14 per cent of cases at 10 years follow-up. To understand this phenomena, parametric studies of the acetabulum hav e involved finite element analysis, photo elastic models or strain gau ges. The differing and sometimes contradictory results were due to the way the model was solicited. If the classic << Resultant >> is accura te for the study of the forces on the hip, Pauwels described other for ces on the pelvis during the gait. The effect of the application of th ese other forces on the acetabulum are the object of this study.Materi als and method Four pelvises were loaded with different forces. Nine s train gauges were pasted around the acetabulum. The pelvis was loaded up to 140 Kg and a compressive force was applied to the pubis by 10 Kg increments up to 40 Kg. The stress data with and without the second f orce were recorded. A cortical window was bored on the anterior inferi or iliac spine. Through this patch, the trabecular bone of the acetabu lar roof was cut up to disturb its mechanical properties. The load was once again applied to the pelvis and the stress data recorded. Result s The compressive force applied to the symphyseal surface decreases th e deformation of the posterior acetabular rim and increases deformatio n in the proximity of the ischio-iliac and iliopubic junctions. The di sruption of trabecular bone generates an increase in deformation mainl y perpendicularly to the acetabular rim. Discussion The decrease of th e stress on the posterior acetabular rim is interpreted as a reduction of the solicitations on a fragile zone and its increase on the iliopu bic and ischio iliac junctions expresses the application of the acetab ular horn on the femoral head permitting better settling of the hip du ring the gait. The constatation of a stress increase around the acetab ulum after disruption of the tracebular bone is interesting. The bone behavior with different acetabular cup models on different pelvis may be studied by strain gauges pasted on the cortical bone. The variable quality of the trabecular bone may introduce an error factor in the me asurement. Conclusion The deformation of the acetabulum during gait ha s to be studied following the different forces described by Pauwels. T he compressive force on the pubic symphysis during one leg stance perm its a decrease of the stress on the acetabular rim and an increase in the setting of the femoral head. Different qualities of trabecular bon e change the deformation of the cortical bone for an equivalent load. This incites us, in comparative studies, to be careful in the deductio n of the acetabular roof stresses from the recorded cortical bone defo rmations.