Biomechanical analysis of anatomical intramedullary nails and proximal femoral anti-rotation-II intramedullary nails for the treatment of unstable intertrochanteric fractures in older patients

Purpose To explore the biomechanical properties of Zimmer natural nail (ZNN) and proximal femoral nail antirotation (PFNA)-II nails in treating older patients with unstable intertrochanteric fractures, thereby guiding the selection of internal fixation devices in clinical practice. Method ZNN and PFNA fixation models for unstable intertrochanteric fractures (AO type 31-A2.2) were established using the three-dimensional finite element method and in vitro mechanical experiments, respectively. We compared the Von Mises stress distribution, peak stress, implants and femur locations, displacement distribution, maximum displacement, location of the models, and maximum crack distance at the fracture ends between the two groups. Additionally, we compared the axial stiffness, femoral head axial displacement, and failure load for both groups. Result Finite element analysis for AO type 31-A2.2 unstable intertrochanteric fractures showed that stresses in the ZNN and PFNA-II models concentrated at the distal locking screws, with maximum Von Mises stresses of 373.2 MPa and 811.5 MPa, respectively. The femoral stress concentrated at the distal locking screw perforation hole, with maximum Von Mises stresses of 186.5 MPa for ZNN and 1602.0 MPa for PFNA-II. According to the displacement distribution cloud map, the maximum displacements of the ZNN and PFNA-II models were 3.768 mm and 3.757 mm, respectively, both at the top of the femoral head. The maximum crack distances in the ZNN and PFNA-II models were 0.16 mm and 0.14 mm, respectively. The maximum displacements and crack distances at the fracture end were similar for both models. Biomechanical tests revealed that the axial stiffness in the ZNN group was greater than that in the PFNA-II group, with averages of 299.67 ± 27.00 N/mm and 269. 6 ± 15. 24 N/mm, respectively; the difference between the two groups was significant (T = 2. 375, P = 0. 039). In the cyclic loading test, the average axial displacement of the femoral head was 0.62 ± 0.22 mm in the ZNN group and 0.59 ± 0.23 mm in the PFNA-II group, with no significant differences (t = 0. 155, P = 0. 880). The failure load of the ZNN group significantly exceeded that of the PFNA-II group, with average failure loads of 3.95 ± 0.07 kN and 3.54 ± 0. 15 kN, respectively (t = 6.207, P < 0.001). Conclusion In AO type 31-A2.2 unstable intertrochanteric fractures, the biomechanical properties of the ZNN group were superior to those of the PFNA-II group in terms of axial stiffness and failure load. In the cyclic loading test, the axial displacement of the femoral head was similar in both groups. The finite element analysis showed a more favorable mechanical distribution for the ZNN group.