Femoral stress and strain changes post-hip, -knee and -ipsilateral hip/knee arthroplasties: a finite element analysis.

OBJECTIVE: To identify the optimal ratio of free femur for minimizing the risks of periprosthetic fracture. METHODS: Three dimensional models of the femur with hip and knee stem elongation were constructed. With the distal femoral condylar surface fixed in a three dimensional model, the femoral head loading was performed according to the methods described by Huiskes and van Rietbergen in the models of hip replacement, knee replacement with or without hip stem or knee stem elongation. The maximum principal stress (MPS) and maximum principal elastic strain (MPES) of the femur were recorded and their relationships to the free femur ratio were analyzed using Pearson's correlation analysis. RESULTS: There were no obvious changes in MPS and MPES with hip stem elongation from 100 to 180 mm. In ipsilateral hip and knee replacement, the MPS and MPES had a tendency to decrease with knee and hip stem elongation. The MPS and MPES were mainly located in the anterior medial side of the middle to distal femur post-hip replacement and distalized with stem elongation. When the knee stem had been elongated more than 120 mm, the stress and strain concentrated strongly in the middle of the femoral shaft. There was a positive correlation between MPS and MPES to the free femur ratio (P < 0.01); however, no optimal ratio of free femur that would minimize the risks of periprosthetic fracture was identified. CONCLUSION: Positive correlations were found between implant free femur and stress and strain changes in total knee arthroplasty, total hip arthroplasty and ipsilateral hip and knee replacement.