RESUMEN
There is no "gold standard" treatment for femoral mid-shaft fractures near the tip of a hip implant. Moreover, no study has quantified the changes in a femur's mechanical properties from injury through to healing. The present aim was to predict overall stiffness and peak bone stress in the same femur after injury, repair, and healing with respect to its intact condition. Stage 1 was an intact femur. Stage 2 mimicked a femur with a hip stem. Stage 3 had a 5-mm fracture gap repaired with a plate and screws. Stage 4 represented complete fracture union. Experiments were done on a synthetic femur with strain gages and subjected to 1500 N of axial force. Finite element (FE) models were validated against experiments and then re-analyzed using a clinical-level force of 3000 N. At 1500 N, FE vs. experimental strains had excellent linear agreement (R=0.94; slope=0.97). At 3000 N, FE stiffnesses were 2167 N/mm (Stage 1), 2359 N/mm (Stage 2), 973 N/mm (Stage 3), and 3348 N/mm (Stage 4), showing that Stage 3 was the least stable compared to Stage 1. At 3000 N, FE bone stresses yielded peaks of 75.7 MPa at the load application point (Stage 1), 29.0 MPa near the hip implant tip (Stage 2), 126.3 MPa at the distal portion of the plate (Stage 3), and 69.3 MPa at the proximal portion of the plate (Stage 4), showing that Stage 3 was most vulnerable to re-injury compared to Stage 1. Stress shielding and high stresses were present not only after hip implantation and plating, but also after healing.