Prediction of muscle and joint loads after segmental femur replacement for osteosarcoma.

The authors designed a mathematical model that simulated 28 proximal and distal femoral resections and reconstructions for tumor and then predicted muscle and joint forces during gait and stair climbing. Such models will become increasingly useful as improvements are available, as tumor patient longevity increases, and as the design and manufacture of custom devices become more common. Inertial property changes with surgery were minor and can be ignored for most purposes. For most simulations, physiologically reasonable muscle forces were predicted, suggesting that patients would be able to walk in a normal or near-normal fashion. Large joint and muscle loads were predicted after extensive muscle resections, particularly during stair climbing, suggesting that patients would limp. The quadriceps muscle group was sufficiently strong to allow normal gait with partial excisions. With complete quadriceps excision and transfers, however, high joint loads were predicted. Furthermore, if an endoprosthesis had inherent varus-valgus stability in such cases, the moments to be satisfied by the prosthesis would predispose to high interface stresses and loosening. If, conversely, the endoprosthesis did not have inherent stability, muscle forces in the remaining muscles would be unrealistically high, suggesting that such a patient would limp or require ambulation assists.