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The Biomechanical Effect of Proximal Tumor Defect Location on Femur Pathological Fractures

Objectives: The femur is the most common long bone affected by cancerous metastasis. Femoral tumor defects are known to induce pain and functional impairment in patients. Although prior studies exist evaluating the clinical and biomechanical effect of tumor defect size, no biomechanical studies have experimentally examined the risk of pathological fracture with respect to the anterior, posterior, medial, and lateral surfaces on which a proximal tumor defect is located on the femur. Methods: Circular tumor-like defects of 40-mm diameter were created proximally in the subtrochanteric region on the Anterior (n = 5), Posterior (n = 5), Medial (n = 5), and Lateral (n = 5) sides of 20 synthetic femurs. Intact femurs served as a control group (n = 4). Femurs were tested for lateral, “offset” torsional, and axial stiffness, as well as axial strength. Results: Lateral stiffnesses (range, 121–162 N/mm) yielded no differences between groups (P = 0.069). “Offset” torsional stiffnesses (range, 135–188 N/mm) demonstrated that the Medial group was less stiff than the Intact, Anterior, and Lateral groups (P ⩽ 0.012). Axial stiffnesses (range, 1057–1993 N/mm) showed that the Medial group was less stiff than the Intact group (P = 0.006). Axial strengths (range, 3250–6590 N) for the Medial group were lower than Anterior (P = 0.001) and Posterior (P = 0.001) specimens, whereas the Lateral group had a lower strength than Anterior specimens (P = 0.019). No other statistical differences were noted. Axial failure of Medial and Lateral specimens involved the tumor-like defect in 100% of cases, whereas 100% of Intact femurs and 80% of Anterior and Posterior femur groups failed only through the neck. Conclusions: In 2 of 3 test modes, the Medial tumor-like defect group resulted in statistically lower stiffness values compared with Intact femurs and had lower strength than Anterior and Posterior groups in axial failure.