Influence of muscular contractions on the stress analysis of distal femoral interlocking nailing

Background In the literature, the commonly assumed loading conditions on the proximal femur are hip compression and/or gluteus contractions. However, no study has discussed the influence of muscle forces on failure of distal nail holes and locking screws. Methods This finite-element study analyzed the influence of muscular contractions on stress analysis of distal nail holes and locking screws. Three loading conditions were used for comparison, comprised of either hip compression alone or with muscle contractions. The head displacement of intact and fractured femur, the nail and screw stresses vs. fixation depth, and the stress distribution at the distal nail–screw interfaces were chosen as the comparison indices. Findings The addition of trochanteric and diaphysial muscles showed the more physiologically reasonable displacement of the femoral head. However, all loading conditions consistently showed the hole and screw stresses increase as the nail was inserted deeper. The stress distribution at the distal nail–screw interfaces was remarkably different under the condition of with or without the muscular contractions. The exertion of muscles predicted the fatigue cracking originated at the edge of the nail holes on the medial rather than lateral side. Interpretation Only hip compression and/or gluteus contraction generated a characteristic bending stress pattern and medially deflected nail curvature. Comparatively, the trochanteric and diaphysial muscles stabilized the femoral head and resulted in the higher stress concentration at the distally medial nail–screw interfaces. However, further experimental and clinical studies, focusing on the failure sites of the distal femoral hardware, should be undertaken to validate such findings.