A Method to Evaluate Motion at the Bone Fracture Interface on Intramedullary Nailing

Two methods were used to evaluate motion at the fracture site in cadaver femurs stabilized with intramedullary (IM) nails. Both bending stability at the fracture site and torsional stability of slotted and unslotted IM nails were evaluated. The first method was to manually measure the rotational stability of the device itself. The second method characterized the fracture displacement in a femur under anatomical load. Both slotted and nonslotted stainless-steel IM nails were fixed statically (proximal and distal screw fixation) in identical cadaver femurs. The distal end of each femur was fixed in epoxy and a circumferential saw cut carefully made at the mid-shaft location. The load was physiologically applied at a 12-deg angle anteriorly in the anterior-posterior plane. Displacements at the fracture site were recorded as a function of the applied load.

Results showed the bending displacement for both types of IM nails to be essentially zero. This was expected because both ends of the nails were fixed and because the opposing fracture surfaces on the medial side of the femur loaded against each other. However, the rotational displacement for the slotted nail was significantly greater than that of the unslotted nail. This was consistent with bench tests, which showed a similar trend in the rotational stiffness.