OTA 1996 Posters - Hip Fractures & Femur Fractures

*A Mechanical Comparison of Subtrochanteric Femur Fracture Fixation

Kevin J. Pugh, MD, John T. Gorczyca, MD, Robert A. Morgan, MD, David Pienkowski, PhD

University of Kentucky Medical Center, Lexington, KY

Purpose: To determine whether a first generation interlocking femoral nail or a reconstruction nail provides superior mechanical fixation of a subtrochanteric fracture in an adult femur model.

Conclusions: Reconstruction nails are significantly stiffer than interlocking first generation nails in compression and torsion for unstable subtrochanteric femur fractures.

Methods: Twenty-four composite adult left femurs (Pacific Research, Vashon, WA) were equally divided and randomly assigned to either the first generation or reconstruction nail groups. The distal end of each femur was potted, and the specimen was tested elastically 5 times each in compression/bending and torsion to establish normal baseline data. Femurs were instrumented with either a statically locked first generation femoral nail or a reconstruction nail (Zimmer, Warsaw, IN) after reaming to a uniform canal size of 16mm. All nails were 15mm x 40cm in size. Stable fractures (fracture) were then created at 1cm, 3cm, and 5cm distal to the lesser trochanter by cutting a circumferential osteotomy around the femur to simulate bone to bone contact. Eight femurs were allocated to each fracture level, four to each nail type. After elastic testing, stable fractures were convened to unstable fractures (gap) by removing a 3cm segment distal to the original osteotomy. Mechanical testing was performed in an Instron 8521 Biaxial materials testing system. Each specimen was compressed elastically to 1.5kN at a rate of 10 N/s and compressive stiffness was calculated. Each specimen was then elastically torqued to 10 N-m at a rate of 0.1 N-m/s and torsional stiffness was calculated. All gapped specimens were then compressed to failure at a rate of 10 N/s. Compressive testing was halted when the nail bent to 90 degrees.

Results: The mechanical properties of the uninstrumented "normal" femurs were uniform, and no differences were observed between the femurs used for the reconstruction nails and those used for the first generation nails. Correlation analysis revealed that compressive stiffness was a good predictor of torsional stiffness in this model (r2 = 0.80). There were no catastrophic failures in compression or torsional testing. Reconstruction nails were consistently stiffer in compression than first generation nails in both fractured and gapped specimens at all levels (Tables 1 & 2). Reconstruction nails were also consistently stiffer in torsion, and these differences were significant in the: 1cm gap (p=.015), the 5cm fracture (p=.0002) and the 5cm gap (p=.030) groups. The reconstruction nail restored 52-61% of the compressive stiffness of the intact bone and 22-35% of the torsional stiffness in the fracture model. The interlocking nail restored 39-52% and 15-36% respectively. For the gap specimens, the reconstruction nail restored 41-58% of the compressive stiffness and 17-22% of the torsional stiffness. The interlocking nail restored 36-49% of the compressive stiffness and 11-14% of the torsional stiffness.

FRACTURE GROUP

N = 4 All groups		FRACTURE LOCATION
		1 cm	3 cm	5 cm
COMPRESSIVE STIFFNESS (N/mm)	1st Gen   Recon	513.3 (NS) 746.9	578.8 (NS) 947.2	768.4 (NS) 885.3
TORSIONAL STIFFNESS (N-mm/deg)	1st Gen   Recon	1.45 (NS) 2.53	2.06 (NS) 2.64	1.13 (p = .0002) 1.59

GAP GROUP

N = 4 All groups		FRACTURE LOCATION
		1 cm	3 cm	5 cm
COMPRESSIVE STIFFNESS (N/mm)	1st Gen   Recon	513.3 (p = .0039) 596.1	661 (p = .0026) 872.1	752.5 (p = .0057) 905.2
TORSIONAL STIFFNESS (N-mm/deg)	1st Gen   Recon	0.84 (p = .0015) 1.24	0.85 (p = .0065) 1.35	1.08 (p = .0029) 1.52

Discussion: This is the first study that quantitatively demonstrates the compressive and torsional superiority of tile reconstruction nail to the first generation interlocking nail for the treatment of subtrochanteric femur fractures. These fractures are difficult to treat and the optimal treatment is unclear. Currently both first generation and reconstruction nails are used for subtrochanteric fractures, but little mechanical data exists to support either method. Tencer et al used human femurs to compare flexible nails, Zickel nails, compression hip screws, angled blade plates and several interlocking nails for subtrochanteric fractures. Their study suffered from the large intrinsic variability in the size and material properties of varying human specimens, a problem circumvented by the standardized composite femurs used in the present study. They also were unable to compare the performance of the reconstruction nail. Although this study showed that reconstruction nails were significantly stiffer in torsion and compression throughout the subtrochanteric region, and especially with a simulated unstable fracture, additional clinical studies may be required to confirm these findings.