Bone-muscle interaction of the fractured femur.

The interaction forces of a fractured femur among the bone, muscle, and other soft tissues are not well understood. Only a small number of in vivo measurements have been made and with many limitations. Mathematical modeling is a useful alternative, overcoming limitations and allowing investigation of hypothetical simulated reductions. We aimed to develop a model to help understand best practices in fracture reduction and to form a base to develop new technologies and procedures. The simulation environment allows muscle forces and moments to deform a fractured femur, and the behavior of forces during reduction can be found. Visual and numerical output of forces and moments during simulated reduction procedures are provided. The output can be probed throughout the reduction procedure down to the individual muscle's contribution. This is achieved by construction of an anatomically correct three-dimensional mathematical model of the lower extremity and muscles. Parameters are fully customizable and can be used to investigate simple, oblique, and some comminuted fractures. Results were compared with published in vivo measurements and were of the same magnitude. A simple midshaft fracture had a maximum resulting force of 428 N, whereas traction from the hip reached a maximum value of 893 N at 60 mm of displacement. Monte Carlo analysis revealed that the deforming force was most sensitive to the muscles' rest lengths. The developed model provides greater understanding and detail than in vivo measurements have to date. It allows new treatment procedures to be developed and importantly to assess the outcome.