Abstract: BACKGROUND: The elastic modulus of β-tricalcium phosphate bioceramic rods is close to that of normal bone tissue, and it exhibits excellent biocompatibility and mechanical properties. It can be used as a supporting material inside the femoral head after core decompression. However, there are few biomechanical studies on osteonecrosis of the femoral head and the changes in stress and displacement of the femoral head after ceramic rod implantation. 
OBJECTIVE: To explore the biomechanical effects of core decompression with ceramic rod implantation in the treatment of osteonecrosis of the femoral head during the peri-collapse stage. 
METHODS: A total of 21 hips were selected from 19 patients with osteonecrosis of the femoral head implanted with ceramic rods at the peri-collapse stage. Preoperative and postoperative imaging data were obtained, and relevant CT images were loaded in Mimics 21.0 software to construct a three-dimensional model of the femoral head. A global model of the proximal femur that includes cortical and cancellous bone, as well as a model of the proximal cancellous bone of the femur were created. The preoperative MRI image data of the patients were imported, and the necrotic lesion model was made by using the graphic matching technology, which was saved in .stl format. They were transferred to Geomagic 2012 software for smooth and precise surfacing processing. The ceramic rod was designed and modeled in SolidWorks 2021 software, and the relevant models were imported for assembly and Boolean operation. After ensuring that the interference check was correct, the stress and displacement of the weight-bearing area and necrotic area of the femoral head during the one-legged standing and walking post-pedal stages were calculated and observed by the ANSYS 2021 software.
RESULTS AND CONCLUSION: (1) The area of maximum stress on the femoral head was located above the anterolateral aspect of the necrotic part, and the stress values of the postoperative weight-bearing area and necrotic area were significantly lower than those before operation when standing on one foot (P < 0.05). The risk of femoral head collapse was lower than that before operation (P < 0.05). (2) In the postoperative pedaling stage, with the increase of the load, the stress values and collapse risk of the postoperative weight-bearing area and necrosis area increased, but they were still lower than those before the operation (P < 0.05). (3) Medullary core decompression combined with ceramic rod implantation can help to reduce the load on the weight-bearing area of the femoral head surface, so that the stress in the weight-bearing area can be effectively dispersed, and the load part is transferred to the direction of the femoral moment, which can improve the local stress concentration state and effectively support the femoral head to prevent further collapse. 

Key words: osteonecrosis of the femoral head, peri-collapse stage, ceramic rod, core decompression, biomechanics, finite element analysis, orthopedic implant