COMPENSATING FOR CHANGES IN MUSCLE LENGTH IN TOTAL HIP-ARTHROPLASTY -EFFECTS ON THE MOMENT GENERATING CAPACITY OF THE MUSCLES

Alterations in the location of the hip center may change the lengths a nd moment arms of the muscles, and thereby affect their capacity to ge nerate-force and moment about the hip; This study demonstrates some of the differences between compensating and not compensating for changes in muscle length that arise from displacement of the hip center. A co mputer model was developed to estimate the maximum isometric moment ge nerating cal pacity of the hip muscles under two conditions. In the co mpensated condition, the hip center was displaced, but the muscles wer e restored to their original lengths and orientations by altering prox imal femoral geometry, In the uncompensated condition, femoral geometr y remained constant; thus, muscle lengths and-orientations changed wit h displacement of the hip center; The computer simulations showed larg e differences between the two conditions. For example, a 2-cm superior displacement of the hip center decreased the moment generating capaci ty of the hip abductors 18% with compensation and 49% without compensa tion. Similarly, a 1-cm medial displacement of the hip center increase d the moment generating capacity of the abductors 17% with compensatio n, but decreased it 4% without compensation. In contrast, a 1-cm infer ior displacement decreased the moment generating capacity of flexors 6 % with compensation, but increased it 12% without compensation. The re sults presented here demonstrate that compensating for changes in musc le length can be important in terms of preserving the moment generatin g capacity of the muscles when the hip center is displaced superiorly and medially, but not when the hip center is displaced in the inferior direction.