The clinical and mechanical factors predisposing to a fatigue fracture
of an interlocking nail were studied in seven patients who were treated for
a fracture of the distal part of the femur. In all patients, the fracture
of the femur was five centimeters or less from the more proximal of the two
distal screw-holes. Finite-element analysis revealed that the stress on the
nail exceeded its fatigue endurance limit and that the femur had to regain
50 per cent of its original stiffness through healing to accommodate
weight-bearing without the risk of fatigue failure of the nail. The risk of
fatigue failure may be minimized by using nails that have a larger diameter
and by avoiding early weight-bearing.