A mechanical study of gap motion in cadaveric femurs using short and long supracondylar nails

To determine the relative stability achieved in unstable supracondylar femur fractures treated with long (36 cm) and short (20 cm) retrograde intramedullary nails using 1 or 2 proximal locking bolts. We hypothesized that longer nails would reduce fracture site motion compared with short nails and that 2 proximal locking bolts would improve stability compared with 1 proximal locking bolt.

Nine pairs of matched human cadaveric femurs were instrumented with 20-cm and 36-cm retrograde intramedullary nails (all 12-mm diameter, Biomet, Warsaw, IN) following reaming to 13 mm. Transverse supracondylar gap (6 mm) osteotomies were created. The femurs were mounted and cyclically tested separately in coronal plane bending and sagittal plane bending on a materials testing system. Fracture site translation was measured using a digital caliper in the respective plane.

Orthopaedic biomaterials laboratory.

With 2 proximal locking bolts, average sagittal translation was 7.2 mm and 1.8 mm, respectively, for the 20-cm and 36-cm nails. Coronal translation was 6.3 mm and 4.3 mm, respectively. With a single proximal locking bolt, average sagittal translation was 7.6 mm and 2.2 mm, respectively, for the 20-cm and 36-cm nails. Coronal translation was 13.6 mm and 4.4 mm, respectively. A statistically significant difference in fracture site translation was found in each pairing by Student t test (P < 0.005), except coronal translation with 2 proximal locking bolts (P = 0.056). Free-body analysis predicts higher local stresses at the proximal interlocks of the shorter nail.

Longer nails provide improved initial fracture stability when compared with short retrograde nails for supracondylar femur fractures due to a more stable mechanical interaction between the femoral diaphysis and the nail. A second proximal locking bolt in the long nail provides no additional stability.