Abstract: BACKGROUND: Osteoporosis is a common comorbidity in patients with knee osteoarthritis. The impact of osteoporosis on the prognosis of unicompartmental knee arthroplasty is a trending topic of current research.
OBJECTIVE: To investigate the effect of different bone densities on the stress value and stress distribution of each structure in the joint after unicompartmental knee arthroplasty using finite element analysis, and to evaluate the correlation between osteoporosis and complications. 
METHODS: CT and MRI were adopted to obtain the lower limb image data of a volunteer. Geomagic Studio, Ansys workbench, and Mimics were used to establish a finite element model of the knee joint with normal sclerotin condition (T-value ≥ -1.0). The finite element model of the knee joint with osteopenia (-2.5 < T-value < -1.0) and osteoporosis (T-value ≤ -2.5) was established by changing the elastic modulus, and the validity was verified. The Boolean operation was used to simulate unicompartmental knee arthroplasty, and the unicompartmental knee arthroplasty models of three groups of sclerotin conditions were established. A 1 000 N vertical downward load was applied to the center of the femoral condyle of the three models. The stress data of the meniscus, the tibial cartilage, the polyethylene liner surface, and the surfaces of the cortical bone and the cancellous bone under the tibial prostheses, as well as the strain data of the polyethylene liner and the cancellous bone under tibial prostheses, were also observed and compared.
RESULTS AND CONCLUSION: (1) Compared with the normal sclerotin group and the osteopenia group, the peak stress of the meniscus and the polyethylene liner surface was larger, and the peak stress of the tibial cartilage was smaller in the osteoporosis group. (2) In the three models, the high-stress value of the cortical bone under the tibial prostheses increased with osteopenia, and the stress was concentrated on the internal edge of the cortical bone. The high-stress value of the cancellous bone under the tibial prostheses decreased with osteopenia, and the stress was concentrated on the posteromedial side of the cancellous bone and the curved bottom of the keel groove. (3) The strain data on the polyethylene liner and cancellous bone under tibial prostheses in the osteoporosis group were greater than those in the normal sclerotin and osteopenia groups. (4) Osteoporosis can cause biomechanical abnormalities of the internal structure of the knee joint after the treatment of unicompartmental knee arthroplasty, which may increase the incidence of complications and the risk of re-surgery.

Key words: knee osteoarthritis, arthroplasty, osteoporosis, biomechanics, finite element analysis