Biomechanics of femur fractures secondary to gunshot wounds.

Nonlethal gunshot wounds to the femur produce four classical types of fractures. The 'drill-hole,' incomplete, and 'butterfly' fractures have been described in both clinical and experimental settings. The remote spiral fracture, sometimes attributed to the fall on the femur after wounding, is unique to the weight-loaded femur. A mathematical model relying on the beam in bending and the mechanics of a cylindrical shell is offered. An experimental method utilizing an apparatus (the 'osteoclast') designed to apply torsion, bending, or combined forces to cadaveric long bones is discussed. Femur failure proximal or distal to the impact site of a projectile is a function of stress risers operating at relatively great distances. Predictable primary and secondary peak moment locations for fracture are illustrated.