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Explicit finite element simulation of eccentric loading in total knee replacement
No full textIn vivo kinematic data indicate that unicondylar or edge loading occurs during normal activities in well-aligned and malpositioned knee replacements. Using a validated explicit finite element model of a knee replacement, the effects of eccentric loading of a total knee replacement are simulated. Only minor variations were observed in the kinematics with a medial offset of the vertical load of as much as 15 mm (representing a medial:lateral loading ratio of 86:14), although the polyethylene stresses did increase by approximately 3 MPa throughout the stance phase of gait. There was a significant change in the kinematics and stresses when unicondylar loading occurred (95:5 medial:lateral loading ratio). Even for the unicondylar load case, contact always was maintained within the lateral compartment. This raises the question whether lift-off often observed in fluoroscopy studies really occurs. The model predicted regions of plastic deformation that closely resemble those observed in retrieved specimens of catastrophic wear. The explicit finite element model offers considerable insight into the kinematics and stresses generated by total knee replacement during different and varied loading conditions that occur during normal usage
Effects of eccentric loading in TKR: an explicit FE study
No full textThe stress distribution within the polyethylene component of a
TKR is ultimately dependent on the kinematics of the replaced knee. In turn,
the kinematics are dependent on the design of the implant, the relative
alignment of the components and tensions of the surrounding soft tissues.
Clinical fluoroscopy studies have shown that unicondylar loading occurs in up
to 90% of replaced knees [1]. However, the impact on the polyethylene
stresses is unknown. The aim of this study was to examine the influence of
unicondylar loading during a gait cycle on the predicted kinematics and
stresses generated by a commercially available TKR
Influence of uni-condylar loading on the stresses and kinematics on a total knee joint replacement
No full textIntroduction: The stress distribution within the polyethylene component of a TKR is ultimately dependent on the kinematics of the replaced knee. In turn, the kinematics are dependent on the design of the implant, the relative alignment of the components and tensions of the surrounding soft tissues. Clinical fluoroscopy studies have shown that unicondylar loading occurs in a high proportion of replaced knees. However, the impact on the polyethylene stresses is unknown. The aim of this study was to examine the influence of unicondylar loading during a gait cycle on the predicted kinematics and stresses generated by a commercially available TKR.
Methods: A 3D finite element model was developed using explicit FE. The femoral component the TKR was modelled as a rigid surface while the tibial insert was meshed using 4-noded solid elements with non-linear polyethylene properties. The flexion angle, axial force, A-P force and I-E torque were applied to the model as a function of the gait cycle and were similar to those used by a Stanmore knee simulator. The axial force is normally applied through the centre of the femoral component (load case 1). Unicondylar loading was simulated by displacing the action of the axial force medially, by 10 (load case 2)and 20mm (load case 3). The resulting kinematics (A-P translation and I-E rotation) and the internal stresses within the polyethylene liner are reported for each of the three load cases. .
Results: When subjected to uniform bi-condylar loading (lc1), the peak anterior translation of the polyethylene component was approx. 3.5mm and the peak internal rotation was 6 degrees. The peak von Mises stresses was approx. 16 MPa. Shifting the axial load medially by 10mm only had a minor effect on the predicted kinematics, increasing the anterior translation by 1mm and the internal rotation by 1 degree. The peak polyethylene stresses increased to 18 MPa. Shifting the axial load medially by 20mm had a significant effect on both the kinematics and stresses. The maximum anterior translation of the polyethylene increased to 8mm and there was an associated increase in the peak internal rotation, from 6 degrees to 20 degrees. The peak polyethylene stresses increased to approx. 20 MPa and significant amounts of plastic deformation were induced during the stance phase of the gait cycle.
Discussion and conclusions: Explicit finite element analysis has for the first time enabled us to simulate the abnormal kinematics caused by unicondylar loading and the associated changes in the polyethylene stresses. For the particular design examined, uneven bicondylar loading (lc2) had a minor affect on the kinematics but did increases the polyethylene stresses. Unicondylar loading (lc3) significantly affected both the kinematics and the polyethylene stresses
Some factors affecting load transfer in cemented tibial components in total knee replacement
No full text
Joint loading asymmetries in knee replacement patients observed both pre- and six months post-operation
No full textBackgroundStudies have highlighted asymmetries in knee joint moments during activities of daily living in individuals with osteoarthritis and joint replacements. However, there is a need to investigate the forces at the knee joints in order to establish the extent of loading asymmetry.MethodsTwenty healthy (mean, 62; range, 55-79 years of age) and 34 pre- to post-knee arthroplasty (mean, 64; range, 39-79 years of age) participants performed gait and sit-stand activities in a motion capture laboratory. Testing was conducted 4 weeks pre- and 6 months post- knee arthroplasty. Knee joint forces and moments were predicted using inverse dynamics and used to calculate peak loading and impulse data which were normalized to body weight. Comparisons were made in loading between affected and contralateral limbs, and changes from pre- to post-knee arthroplasty.FindingsPre-knee arthroplasty mean peak vertical knee forces were greater in the contralateral limb compared to the affected limb during both gait 3.5*body weight vs. 3.2*body weight and sit-stand 1.8*body weight vs. 1.5*body weight. During gait, peak knee adduction moment asymmetries significantly changed from pre- to post-knee arthroplasty (-0.3 to 0.8*% Body weight*m*Height), although differences in vertical knee forces remained. The sit-stand activity showed vertical ground reaction asymmetries slightly increased post- knee arthroplasty (from 0.06*body weight pre- to 0.08*body weight post). The healthy participants showed no noteworthy asymmetries.InterpretationThis study showed loading asymmetry of the ground reaction and tibio-femoral forces between affected and contralateral limbs both pre- post-knee arthroplasty. Continued over reliance of the contralateral limb could lead to pathology.<br/
The importance of tibial alignment
No full textThe influence of the tibial plateau orientation on cancellous bone stress was examined by finite element analysis for a cemented device. The objectives of the study were i) to examine the effect of the plateau-ankle angle on the cancellous bone stress, ii) to analyze the significance of the anteroposterior angles of the tibial component on these stresses, and iii) to compare the finite element predictions with clinical data. In general, positioning the tibial plateau in valgus resulted in lower cancellous bone stresses. These results support previous clinical studies, which suggest that overall alignment in valgus results in lower migration rates and lower incidence of loosening
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