Abstract: BACKGROUND: The number of patients with femoral intertrochanteric fractures treated with proximal femoral nail anti-rotation is increasing. For patients who require removal of internal fixation after fracture healing, how to evaluate the standard of removal of internal fixation? Is bone grafting necessary after removal of internal fixation? When and how to do postoperative rehabilitation? There is little research on it in and outside China.
OBJECTIVE: Based on fracture mechanics, the feasibility of removal of proximal femoral anti-rotation intramedullary nailing after intertrochanteric fracture was analyzed, so as to provide certain reference basis and risk prediction for clinicians to evaluate the feasibility of fixation removal after intertrochanteric fracture in elderly patients.
METHODS: CT data of one volunteer were imported into Mimics 19.0 and Geomagic studio 2017 software to extract and optimize the three-dimensional model of the right femur. SolidWorks 2017 software was used to draw the internal fixation model and assemble it with femur model according to the standard operation technology. After deleting the internal fixation, the legacy of femoral bone tunnel model was imported into Hypermesh software 14.0. After meshing, defining different cortical bone, cancellous bone, and trabecular bone stress, models A-F was obtained based on Singh1-6. After setting material properties parameters, boundary conditions and loading, data were submitted into LS-DYNA software for solution, respectively. Finally, the operation result was checked in HYperview 14.0 software. The experiment was approved by the Ethics Committee of the First Affiliated Hospital of Guangzhou University of Traditional Chinese Medicine on March 14, 2019 (hospital ethics approval No. Y [2019] 164).
RESULTS AND CONCLUSION: (1) The initial fracture stress of models A-C and models D-F was similar, but the overall stress of models A-F was significantly smaller than that of models D-F. The overall fracture stress of models A-F presented an increasing trend, while the initial fracture time presented a decreasing trend. (2) From the loading stress to the complete fracture of the femoral neck, the overall number of cancellous bone with fracture disappearance in the six models was relatively small, and the medial femoral moment was the main supporting structure. (3) In models A-F, femoral neck fractures were formed, and Pauwels angle and varus angle of the femoral head showed a decreasing trend. (4) The more severe the osteoporosis in patients, the more likely the secondary fracture would occur after the removal of proximal antirotation intramedullary nail in the femoral trochanteric fracture. (5) For patients with severe osteoporosis, the proximal femoral anti-rotation intramedullary nail can be removed by crushing bone grafting to fill in bone tunnel and to increase stability.

Key words: fracture mechanics, osteoporosis, intertrochanteric fracture, internal fixation removal, feasibility, finite element analysis