Biomechanical evaluation of extramedullary versus intramedullary fixation for reverse obliquity intertrochanteric fractures.

OBJECTIVES: This study evaluated the 135-degree hip screw, 95-degree hip screw, and intramedullary hip screw (IMHS) for fixation of reverse obliquity intertrochanteric fractures.

METHODS: Twelve matched pairs of human femora (mean age 64 years) were obtained. Osteotomies were created in left femurs at a 33-degree angle, running inferolaterally from the lesser trochanter to mimic reverse obliquity intertrochanteric fractures. Right femora acted as controls. Three groups of left femora (n = 4 per group) had a 135-degree hip screw, 95-degree hip screw, or IMHS inserted. Strain gages were placed distal to the fracture site to monitor fragment strain. A linearly variable differential transformer measured lateral displacement of the proximal femur. An Instron tester applied vertical loads to the femoral head. Outcome measures of stiffness, strain, and lateral displacement were determined at 25-degree adduction, 25-degree abduction, 25-degree flexion, and 90-degree flexion. A 2-cm bone gap was then created at the fracture site to simulate comminution and the mechanical tests repeated. Failure load was assessed in 25-degree adduction with a bone gap.

RESULTS: There was no difference in normalized stiffness between constructs before creation of a gap. After gap creation, stiffness of all constructs was reduced (P = 0.03), and there was a significant difference in adduction (135-degree hip screw, 46.6% +/- 3%; 95-degree hip screw, 22.9% +/- 2%; and IMHS, 53.7% +/- 7.8%) (P < 0.05). Similar results were noted for abduction and flexion. There was no significant difference in lateral displacement between constructs before (P = 0.92) or after (P = 0.26) gap creation. Failure load was significantly different (135-degree hip screw, 1222 +/- 560 N; 95-degree hip screw, 2566 +/- 283 N; and IMHS, 4644 +/- 518 N) (P = 0.02).

CONCLUSIONS: With bone contact, there were no statistically significant differences in the stiffness between the constructs. With a gap, however, the IMHS bone implant construct was significantly stiffer and had a greater load to failure than the 135-degree and 95-degree constructs.