Effect of intramedullary nail length on the biomechanical performance of internal fixation for subtrochanteric femoral fractures

OBJECTIVE: To compare the biomechanical performance of different lengths of intramedullary nails combined with reconstruction plates for fixation of Seinsheimer type IV subtrochanteric femoral fractures using finite element analysis, with additional evaluation of osteoporotic bone conditions. METHODS: A three-dimensional finite element model of Seinsheimer type IV subtrochanteric femoral fracture was constructed from CT data of a 51-year-old male volunteer. Three PFNA intramedullary nail lengths combined with a reconstruction plate were modelled: long-nail (320 mm), medium-nail (240 mm), and short-nail (160 mm) combinations. Each construct was analysed under axial (2,100 N), bending (175 N), and torsional (15 N·m) loading conditions in both healthy and osteoporotic bone subgroups (elastic modulus reduced to 60% of normal). Overall displacement, femoral stress, implant stress, and load-sharing ratio were evaluated. RESULTS: Under axial loading, the medium-nail combination exhibited the smallest maximum femoral displacement (10.7 mm healthy; 13.3 mm osteoporosis), while the short-nail combination showed the largest (13.2 mm; 19.7 mm) and the greatest sensitivity to bone quality. The short-nail combination generated the highest femoral stress under axial loading (325.4 MPa, 32.1% above the long-nail combination) and the highest nail stress under bending (199.4 MPa); the medium-nail combination produced notably high nail stress under torsion (300.7 MPa). The short-nail combination also exhibited substantially higher reconstruction plate stress under axial loading (804.5 MPa) relative to the long-nail (654.9 MPa) and medium-nail (578.8 MPa) combinations. Load-sharing analysis showed that the intramedullary nail bore the largest load fraction across all constructs (~ 47%-63%), and the long-nail combination demonstrated the most substantial load redistribution under osteoporotic conditions. CONCLUSION: The medium-nail combination may offer superior displacement control under axial loading, while the short-nail combination was associated with higher stress levels across multiple loading modes and greater biomechanical vulnerability in osteoporotic bone. Intramedullary nail length should be selected individually, accounting for fracture type, bone quality, and anticipated loading demands.