Finite Element Analysis Of Knee Replacement In Single Condyle

Part 1:Development of three-dimensional finite element models of normal knee joint and neutrally aligned unicompartmental knee arthroplastyObjective:The incidence of knee osteoarthritis is increasing year by year in China. Unicompartmental knee arthroplasty shows more advantages in treating antero-medial knee osteoarthritis, while related biomechanical studies, especially the finite element studies were rare in domestic. This study is aimed to develop the three-dimensional finite element models of normal knee and neutrally-aligned mobile-bearing unicompartmental knee arthroplasty and to validate them as the basis for postoperative biomechanical studies.Methods:The left knee of a healthy adult male volunteer was scanned with Computed Tomography and Magnetic Resonance Imaging. The image data processing and three-dimensional calculating were done in Mimic 17.0,and STL triangle mesh models of the normal knee structures were imported and transformed to solid models in Rapidform XOR. The assembling was done in ABAQUS 6.13,and a three-dimensional finite element model of normal knee joint was developed. Then the meshes were divided, the materials were assigned and boundary conditions were set. With a vertical compression force of 1000N applied to the femur, the load distribution,contact stresses and areas were calculated and compared to former articles for validation. Based on the validated normal knee model, bone resections and protheses assembling were simulated with the Boolean operation of ABAQUS 6.13 according to the standard surgical techniques of Oxford Phase-3 prostheses, then the three-dimensional solid model of mobile-bearing unicompartmental knee arthroplasty was developed. The meshes were divided, the materials were assigned and boundary conditions were set. With a vertical compression force of 1000N applied to the femur, the load distribution,contact stresses and bone stresses were calculated and compared with former studies for validation.Results:A three-dimensional finite element model of normal knee joint was developed successfully, which included 212128 elements and 219443 nodes. Main structures like bone,cartilage,meniscus and ligaments were included. The calculated results of load distribution,contact stresses and areas were similar to former studies, so the model was validated. Based on the validated model of normal knee joint, a three-dimensional finite element model of mobile-bearing unicompartmental knee arthroplasty was well developed, which included 218043 elements and 229524 nodes. With a vertical compression force of 1000N applied to the femur, the load distribution and stress of the tibia bone was calculated, and the results were similar to former articles.Conclusion:The three-dimensional finite element model of normal knee joint developed in this study was validated and reliable. The three-dimensonal finite element model of neutrally-aligned mobile-bearing unicompartmental knee arthroplasty contains complete structures and has strong innovation and reliability, which can be used for further biomechanical analysis.Part 2:Finite element analysis about tibial component alignments in unicompartmental knee arthroplastyObjective:Accurate positioning of tibial component is a requirement for the long-term surviorship of unicompartmental knee arthroplasty, while there are still controversies about the rational positioning. Former finite element studies all have their shortcomings and their conclusions varied. This study is aimed to develop the finite element models of unicompartmental knee arthroplasty with different varus-valgus inclinations on the coronal plane and different slopes on the sagittal plane, based on the neutrally-aligned model, to study how tibial component alignment will influence the static biomechanics of knee and to find the rational implantion angles.Methods:Based on the finite element model of neutrally-aligned unicompartmental knee arthroplasty developed in Part 1, the tibial component and meniscus bearing were rotated in different angles on the coronal plane in ABAQUS 6.13, and bone resections were simulated using Boolean operation. A set of ten solid models of unicompartmental knee arthroplasty was developed with the coronal inclination angles of tibial tray ranging from 10° valgus to 10° varus. The meshes were divided and materials were assigned. Five regions of interest were defined on the tibial surface of the models and unified boundary conditions were set. With a vertical compression force of 1000N applied to the femur, the peak Von Mises stresses,compressive strains and tensile strains of the five regions,the load percentage and contact stress in bilateral compartments were calculated. Then a set of five solid models was developed with the slope of tibial tray ranging from 0° inferior tilt to 11 ° inferior tilt using the same method. The meshes were divided and materials were assigned. Six regions were defined on the medial tibial surface of the models and unified boundary conditions were set. With a vertical compression force of 1000N applied to the femur, the peak Von Mises stresses of the six regions and stress distribution of cancellous bone were calculated, so were the load percentage and contact stress in bilateral compartments.Results:For the set of unicompartmental knee arthroplasty models with varied coronal inclination angles of tibial tray, Von Mises stresses and compressive strains at proximal medial cortical bone increased significantly as the tibial tray was in valgus inclination>4°, which may increase the risk of residual pain. Compressive strains at tibial keel slot were above the high threshold with varus inclination>4°,which may result in greater risk of component migration. Tibial bone resection corner acted as a strain-raiser regardless of the inclination angles. Compressive strains at the resected surface slightly changed with the varying inclinations and were not supposed to induce bone resorption and component loosening. Contact pressures and load percentage in lateral compartment increased with the more varus inclination, which may lead to osteoarthritis progression. For the set of unicompartmental knee arthroplasty models with varied slope of tibial tray, no significant differences in the peak Von Mises stresses of anterial medial tibial surface were found(P>0.05), while 9° and 11° slope produced significantly higher Von Mises stresses of posterior medial tibial surface than other slopes(P<0.05). Cancellous bone stresses of posterior tibia increased gradually as the slope increasing. Contact stresses in bilateral compartments were not statistic significant(P>0.05).Conclusion:Static knee biomechanics after mobile-bearing unicompartmental knee arthroplasty can be affected by tibial component alignment. Coronal alignment of 4° valgus to 4° varus and posterior slope of 0° to 7° can be considered as appropriate for placing the tibial component in mobile-bearing unicompartmental knee arthroplasty, for reducing the risk of postoperative complications and improving the long-term survival rate of prostheses.