ABSTRACT
OBJECTIVES: To measure quantitatively the strains in the concave and convex sides of a femur to substantiate the Arbeitsgemeinschaft für Osteosynthesefragen (AO) tension band principle. METHODS: Eight cadaveric fresh-frozen femurs were cleaned of soft tissue and prepared for mounting the strain gages (Vishay, Raleigh, NC). The student strain gages were mounted at the anterior, lateral and medial surfaces of a femur. They were loaded at 100, 250, 500, 1,000 and 1,500 N axial forces in a mechanical testing machine (Instron Model 4202, Norwood, MA) and the micro strains were measured for intact and after fixation of a simulated fracture by an eight hole broad 4.5 mm stainless steel compression plate (Synthes LCP). In order to simulate the fracture conditions, where all eight screw holes might not be filled, three different arrangements were tested: 8 screws, 4 screws and 2 screws. The microstrains under different loads were analyzed by Analysis of Variance (ANOVA) with Holm-Sidak multiple comparison method. RESULTS: At the same gage location micro strains were not significantly different between the intact, and the femur fixated by 8, 4 and 2 screws. For intact bones at 1,500 N the strain ratio between the tension to compression sides was -0.21 proximally and -0.04 distally. The comparison of these strain ratios at different loads did not show any significant differences at p = 0.05 and power of 0.8. CONCLUSIONS: The data showed a trend validating the tension band principle as tensile strains lowered and compressive strains increased after placement of the plate.
