Biomechanical study of bone defects and unicompartmental prosthesis implantation position in spontaneous osteonecrosis of the knee

Finite element analysis was used to study the effects of the anterior load-bearing column of the femoral component and bone defects at different locations on stress changes in the internal structures of the knee joint in fixed-bearing unicompartmental knee arthroplasty (FB-UKA). On this basis, the critical value of bone defect depth in UKA for treating SONK was further analyzed, providing a theoretical basis for clinicians to choose between UKA and TKA. A normal knee joint model was established and validated. An elliptical bone defect with a major axis of 6 mm and a minor axis of 4 mm was created on the weight-bearing surface of the medial femoral condyle. The following two groups of finite element models were established: The first group was used to evaluate the effect of bone defect depth: while maintaining the alignment relationship unchanged, three models were sequentially established with bone defect depths corresponding to 1/3, 2/3, and 3/3 of the total length of the anterior load-bearing pillar of the femoral prosthesis. The second group was used to evaluate the effect of prosthesis medial translation: the bone defect depth was fixed at the total length of the anterior load-bearing pillar of the femoral prosthesis, and starting from the center of the bone defect, the femoral prosthesis was translated medially by 1 mm, 3 mm, and 5 mm, respectively, to construct three models. In total, seven different UKA models with bone defects were established. The stress variations on the surfaces of the femoral prosthesis, polyethylene insert, tibial prosthesis, and the femoral-side and tibial-side cement mantles were analyzed. As defect depth increased, peak stresses on all components and cement mantles exhibited an upward trend. However, when the defect depth was fixed at full pillar length and the component was translated medially by 3 mm, the peak stress on the femoral-side cement increased markedly, exceeding the typical compressive strength of polymethyl methacrylate (PMMA) bone cement (approximately 110 MPa). This biomechanical observation suggests that under such extreme defect and offset conditions, the risk of cement mechanical failure may increase; however, the potential impact on long-term prosthesis survival and clinical loosening rates requires further validation through clinical studies. Under the finite element model conditions established in this study, when FB-UKA is used for the treatment of SONK, the model analysis demonstrated that when the bone defect depth did not exceed the length of the intact pillar and medial translation was ≤3 mm, more favorable stress distribution and reduced stress concentration were observed in the bone cement mantle, which we hypothesize may potentially reduce the risk of bone cement mantle fatigue, postoperative loosening, and femoral component dislocation in clinical settings. It should be noted that this finding is derived from specific finite element model parameters and boundary conditions, and therefore possesses certain theoretical reference value. However, the potential clinical implications we proposed require further validation through subsequent clinical cohort studies or randomized controlled trials, as these clinical outcomes were not directly evaluated in the present finite element analysis. Nevertheless, these biomechanical findings may provide a preliminary reference for clinical practice, helping to guide intraoperative decisions regarding the extent of bone grafting, cement volume, and component positioning, thereby offering a biomechanical basis for optimizing surgical strategies and potentially reducing the risk of postoperative complications, which requires further clinical confirmation.