Increasing nail-cortical contact to increase fixation stability and decrease implant strain in antegrade locked nailing of distal femoral fractures: a biomechanical study.

Background: Proximity of the distal locking screw to the fracture site potentially can cause implant failure in locked nailing of distal femoral fractures. In this biomechanical study, we investigated the effects of nail-cortical contact and nail purchase in the distal subchondral bone on fixation stability and implant strain.

Methods: Using fiberglass artificial femurs, we fixed five different distal femoral osteotomies with specially manufactured locked nails representing different conditions of nail-cortical contact. In each condition, six femur-nail constructs were loaded on the femoral head axially with 700 N; then we measured the fixation stability and strain at the superiormost distal locking screw and nail hole. The tests were conducted both with and without nail purchase in distal subchondral bone.

Results: When there was no nail-cortical contact, the load transmitted to the distal femoral fragment was completely borne by the nail and the distal locking screws. Low fixation stability and high strain on the distal screw and nail hole resulted. Greater nail-cortical contact increased the fixation stability and decreased strains on the distal screw and nail hole. Purchase of the nail tip in distal subchondral bone significantly increased the fixation stability and decreased nail hole strain except in the situation of long nail-cortical contact. The screw strain was significantly reduced in all conditions, and the reduction in strain was significantly greater than that associated with the nail hole, 32% +/- 8% versus 15.7% +/- 2.5% (p = 0.002).

Conclusions: In locked nailing of distal femoral fractures, the prime factor determining fixation stability and implant strain is nail-cortical contact. In situations without nail-cortical contact, a longer nail with purchase in the subchondral bone of the distal femur is recommended.