Differences in the stress distribution in the distal femur between patellofemoral joint replacement and total knee replacement: a finite element study

Computational biomechanical study on hybrid implant materials for the femoral component of total knee replacements

Multifunctional materials have been described to meet the diverse requirements of implant materials for femoral components of uncemented total knee replacements. These materials aim to combine the high wear and corrosion resistance of oxide ceramics at the joint surfaces with the osteogenic potential of titanium alloys at the bone-implant interface. Our objective was to evaluate the biomechanical performance of hybrid material-based femoral components regarding mechanical stress within the implant during cementless implantation and stress shielding (evaluated by strain energy density) of the periprosthetic bone during two-legged squat motion using finite element modeling. The hybrid materials consisted of alumina-toughened zirconia (ATZ) ceramic joined with additively manufactured Ti-6Al-4V or Ti-35Nb-6Ta alloys. The titanium component was modeled with or without an open porous surface structure. Monolithic femoral components of ATZ ceramic or Co-28Cr-6Mo alloy were used as reference. The elasticity of the open porous surface structure was determined within experimental compression tests and was significantly higher for Ti-35Nb-6Ta compared to Ti-6Al-4V (5.2 ± 0.2 GPa vs. 8.8 ± 0.8 GPa, p < 0.001). During implantation, the maximum stress within the ATZ femoral component decreased from 1568.9 MPa (monolithic ATZ) to 367.6 MPa (Ti-6Al-4V/ATZ), 560.9 MPa (Ti-6Al-4V/ATZ with an open porous surface), 474.9 MPa (Ti-35Nb-6Ta/ATZ), and 648.4 MPa (Ti-35Nb-6Ta/ATZ with an open porous surface). The strain energy density increased at higher flexion angles for all models during the squat movement. At ∼90° knee flexion, the strain energy density in the anterior region of the distal femur increased by 25.7 % (Ti-6Al-4V/ATZ), 70.3 % (Ti-6Al-4V/ATZ with an open porous surface), 43.7 % (Ti-35Nb-6Ta/ATZ), and 82.5% (Ti-35Nb-6Ta/ATZ with an open porous surface) compared to monolithic ATZ. Thus, the hybrid material-based femoral component decreases the intraoperative fracture risk of the ATZ part and considerably reduces the risk of stress shielding of the periprosthetic bone.

Influence of posterior cruciate ligament tension on tibiofemoral and patellofemoral joint contact mechanics in cruciate-retaining total knee replacement: a combined musculoskeletal multibody and finite-element simulation

The influence of posterior cruciate ligament (PCL) tension on the clinical outcome of cruciate-retaining total knee replacement (CR-TKR) remains controversial. Various numerical approaches have been used to study this influence systematically, but the models used are limited by certain assumptions and simplifications. Therefore, the objective of this computational study was to develop a combined musculoskeletal multibody and finite-element simulation during a squat motion to 90° knee flexion with a CR-TKR design to overcome previous limitations regarding model inputs. In addition, different PCL tensions (tight, lax, resected) were modeled and the influence on tibiofemoral and resurfaced patellofemoral joint dynamics and contact stresses was evaluated. The effect of the PCL on knee joint dynamics and contact stresses was more pronounced at higher flexion angles. Tibiofemoral joint dynamics were influenced and a tight PCL induced increased posterior femoral translation during flexion. The maximum contact stress in the tibial insert increased from 20.6 MPa to 22.5 MPa for the resected and tightest PCL at 90° knee flexion. Patellofemoral joint dynamics were only slightly affected by PCL tension. However, the maximum contact stress in the patellar component decreased from 58.0 MPa to 53.7 MPa for the resected and tightest PCL at 90° knee flexion. The combination of musculoskeletal multibody and finite-element simulation is a sufficient method to comprehensively investigate knee joint dynamics and contact stresses in CR-TKR. The PCL tension after CR-TKR affects joint dynamics and contact stresses at the articulating implant surfaces.

Load transfer in bone after partial, multi-compartmental, and total knee arthroplasty

Introduction: Arthroplasty-associated bone loss remains a clinical problem: stiff metallic implants disrupt load transfer to bone and, hence, its remodeling stimulus. The aim of this research was to analyze how load transfer to bone is affected by different forms of knee arthroplasty: isolated partial knee arthroplasty (PKA), compartmental arthroplasty [combined partial knee arthroplasty (CPKA), two or more PKAs in the same knee], and total knee arthroplasty (TKA). Methods: An experimentally validated subject-specific finite element model was analyzed native and with medial unicondylar, lateral unicondylar, patellofemoral, bi-unicondylar, medial bicompartmental, lateral bicompartmental, tricompartmental, and total knee arthroplasty. Three load cases were simulated for each: gait, stair ascent, and sit-to-stand. Strain shielding and overstraining were calculated from the differences between the native and implanted states. Results: For gait, the TKA femoral component led to mean strain shielding (30%) more than three times higher than that of PKA (4%-7%) and CPKA (5%-8%). Overstraining was predicted in the proximal tibia (TKA 21%; PKA/CPKA 0%-6%). The variance in the distribution for TKA was an order of magnitude greater than for PKA/CPKA, indicating less physiological load transfer. Only the TKA-implanted femur was sensitive to the load case: for stair ascent and gait, almost the entire distal femur was strain-shielded, whereas during sit-to-stand, the posterior femoral condyles were overstrained. Discussion: TKA requires more bone resection than PKA and CPKA. These finite element analyses suggest that a longer-term benefit for bone is probable as partial and multi-compartmental knee procedures lead to more natural load transfer compared to TKA. High-flexion activity following TKA may be protective of posterior condyle bone resorption, which may help explain why bone loss affects some patients more than others. The male and female bone models used for this research are provided open access to facilitate future research elsewhere.

Finite element analysis of a healthy knee joint at deep squatting for the study of tibiofemoral and patellofemoral contact

Background

In non-western countries, deep squatting is a daily activity, and prolonged deep squatting is common among occupational squatters. Household tasks, taking a bath, socializing, using toilets, and performing religious acts are among the activities frequently carried out while squatting by the Asian population. High knee loading is responsible for a knee injury and osteoarthritis. Finite element analysis is an effective tool to determine stresses on the knee joint.

Methods

Magnetic Resonance Imaging (MRI) and Computed Tomographic (CT) images were acquired of one adult without knee injuries. The CT images were acquired at the fully extended knee and one more set of images was acquired with the knee at a deeply flexed knee position. The MRI was acquired with the fully extended knee. 3-Dimensional models of bones were created using CT and soft tissue using MRI with the help of 3D Slicer software. Kinematics and finite element analysis of the knee was performed for standing and deep squatting posture using Ansys Workbench 2022.

Results

High peak stresses were observed at deep squatting compared to standing along with the reduction in the contact area. Peak von Mises stresses on femoral cartilage, tibial cartilage, patellar cartilage, and meniscus were increased from 3.3 MPa to 19.9 MPa, 2.9 MPa to 12.4 MPa, 1.5 MPa to 16.7 MPa and 15.8 MPa to 32.8 MPa respectively during deep squatting. Posterior translation of 7.01 mm, and 12.58 mm was observed for medial and lateral femoral condyle respectively from full extension to 153° knee flexion.

Conclusions

Increased stresses in the knee joint at deep squat posture may cause cartilage damage. A sustained deep squat posture should be avoided for healthy knee joints. More posterior translations of the medial femoral condyle at higher knee flexion angle warrant further investigation.

DETERMINATION OF THE STRESS OF ANTERIOR CRUCIATE LIGAMENT IN VARIOUS DEGREES OF KNEE FLEXION, COMPARISON OF NORMAL AND RECONSTRUCTED LIGAMENT

Background: Knee joint stability is enhanced by ligamentus structures such as anterior cruciate (ACL), posterior cruciate (PCL), medial collateral (MCL) and lateral collateral ligaments (LCL). Rupture of ACL is the most common knee injuries, especially in sport related activities. The aim of this study is to evaluate the stress developed in knee joint structures in various degrees of knee flexion in ACL ruptured compared to normal condition.

Method: CT scan images of knee joint were used to create 3d model of knee joint by use of Mimics software. Abaqus software was used to evaluate the stress developed in knee joint in normal and in ACL reconstructed conditions in various degrees of knee flexion.

Results: The stress developed in ACL and other knee joint structures increased significantly by increase in knee joint flexion. The stress of knee joint structures (especially in ACL) in ACL reconstructed condition was more than that of normal condition.

Conclusion: It is recommended to immobilize the knee joint in extension up to [Formula: see text] of knee flexion in those with ACL injuries. The stress of ACL increased due to an increase in tibia translation associated with knee flexion.

A numerical investigation of the infrapatellar fat pad

The infrapatellar fat pad is an adipose tissue in the knee that facilitates the distribution of synovial fluid and absorbs impulsive actions generated through the joint. The correlation between morphological configuration and mechanical properties is analyzed by a computational approach. The microscopic anatomy of the infrapatellar fat pad is studied aiming to measure the dimension of adipose lobules and the thickness of connective septa. Results from histomorphometric investigations show that the infrapatellar fat pad is an inhomogeneous tissue, constituted by large lobules in the superficial part and smaller lobules in the deepest one. Finite element models of the infrapatellar fat pad are developed. The first model considers the inhomogeneous conformation of the infrapatellar fat pad, composed of micro- and macro-chambers, while the second model considers a homogeneous distribution of adipose lobules with similar dimensions. Computational analyses are performed considering the static standing configuration and the passive flexion-extension movement. The computational results allow us to identify the different stress and strain fields within the tissue and to appreciate the variation of the mechanical performance of the overall system considering the distribution of adipose lobules. Results show that the distribution of adipose lobules in macro- and micro-chambers allows major deformation of the infrapatellar fat pad, decreasing the stress inside the tissues.

Knee kinetics and kinematics: What are the effects of TKA malconfigurations?

PURPOSE

Total knee arthroplasty (TKA) is a very successful surgical procedure. However, implant failures and patient dissatisfaction still persist. Sometimes surgeons are not able to understand and explain these negative performances because the patient's medical images "look good", but the patient "feels bad". Apart from radiograph imaging and clinical outcome scores, conventionally used follow-up methods are mainly based on the analysis of knee kinematics. However, even if kinematics remains close to the "normal" range of motion, the patient may still complain about pain and functional limitations. To provide more insight into this paradox, a better quantitative understanding of TKA mechanics must be developed. For this purpose, improved techniques for clinical follow-up, combining kinetics and kinematics analysis, should be introduced to help surgeons to assess and understand TKA performance.

METHODS

An analysis on four TKA designs was performed, and the changes in kinematics and in kinetics induced by several implant configurations (simulating implant malalignment and different knee anatomy) were compared. More specifically, analysed tibio-femoral and patello-femoral contact forces and tibio-femoral kinematics were analysed during a squat task up to 120°.

RESULTS

The results from this study show that contact forces (with changes up to 67 %) are more heavily affected by malconfigurations than kinematics, for which maximum deviations are of the order of 5 mm or 5°, similar to the simulated surgical errors. The results present a similar trend for the different designs.

CONCLUSIONS

The results confirm the hypothesis that kinematics is not the only and also not the most relevant parameter to predict or explain knee function after TKA. In the future, techniques to analyse knee kinetics should be integrated in the clinical follow-up.

All-polyethylene tibial components generate higher stress and micromotions than metal-backed tibial components in total knee arthroplasty

PURPOSE

Most total knee arthroplasty tibial components are metal-backed, but an alternative tibial component made entirely of polyethylene (all-polyethylene design) exists. While several clinical studies have shown that all-poly design performs similarly to the metal-backed, the objective of this study is to perform a biomechanical comparison.

METHODS

Loads, constraints and geometries during a squat activity at 120° of flexion were obtained from a validated musculoskeletal model and applied to a finite element model. Stresses in the tibia and micromotions at the bone-implant interface were evaluated for several implant configurations: (1) three different thicknesses of the cement penetration under the baseplate (2, 3 and 4 mm), (2) the presence or absence of a cement layer around the stem of the tibial tray and (3) three different bone conditions (physiological, osteopenic and osteoporotic bone).

RESULTS

All-polyethylene tibial components resulted in significantly higher (p < 0.001) and more uneven stress distributions in the cancellous bone under the baseplate (peak difference: +128.4 %) and fivefold increased micromotions (p < 0.001). Performance of both implant designs worsened with poorer bone quality with peaks in stress and micromotion variations of +40.8 and +54.0 %, respectively (p < 0.001). Performance improvements when the stem was cemented were not statistically significant (n.s.).

CONCLUSION

The metal-backed design showed better biomechanical performance during a squat activity at 120° of flexion compared to the all-polyethylene design. These results should be considered when selecting the appropriate tibial component for a patient, especially in the presence of osteoporotic bone or if intense physical activity is foreseen.

Deviations From Optimal Alignment in TKA: Is There a Biomechanical Difference Between Femoral or Tibial Component Alignment?

Restoration of neutral mechanical alignment is one of the prerequisites for long-term TKA survival. This study aimed to investigate the effect of deviations from neutral alignment on bone and implant stress and on ligament strain. Using a previously validated finite element model, a neutrally aligned TKA model was compared to 3 different varus and valgus configurations induced by tibial or by femoral component only and by both component simultaneously. Each model underwent a 2500 N vertical load simulating the peak walking force. Varus and valgus alignment increased polyethylene and bone stress, and altered ligament strains, as compared to the neutral aligned model. Changes in alignment of the tibial component were always associated with more detrimental effects compared to the one of the femoral component.

Modulation and predictors of periprosthetic bone mineral density following total knee arthroplasty

Total knee arthroplasty (TKA) leads to a loss of periprosthetic bone mineral density (BMD). Great importance is attached to the prevention of periprosthetic bone loss with a view to ensuring a long service life of the prosthesis. In order to provide appropriate recommendations for preventive movement therapy measures to combat peri-implant bone loss, it is necessary to know the predictors of periprosthetic BMD. The aim of this study was (1) to determine the change of periprosthetic BMD of the femur and tibia and (2) to analyse the effects of different predictors on periprosthetic BMD. Twenty-three patients with primary TKA were evaluated 10 days and 3 months postoperatively. The data analysis comprised (1) the change in periprosthetic BMD from pretest to posttest and (2) the correlations between BMD and the variables isometric maximum voluntary force, lean mass, physical activity (step count), and BMI using multiple linear regression and structural equation modelling (SEM). BMD of the distal femur was significantly reduced by 19.7% (P = 0.008) 3 months after surgery, while no changes were found in BMD of the tibia. The results of SEM demonstrate that 55% of the BMD variance was explained by the model (χ² = 0.002; df = 1; P = 0.96; χ²/df = 0.002; RMSEA < 0.01; TLI = 1.5; CFI = 1.0). A significant direct effect was only evidenced by the variable lean mass (β = 0.38; b = 0.15; SE = 0.07; C.R. = 2.0; P = 0.046). It can be assumed that a large muscle mass with accompanying distribution of high mechanical load in the bones can contribute to local changes of periprosthetic BMD. Concrete recommendations for preventing peri-implant bone loss therefore include exercises which have the aim of maintaining or building up muscle mass.

Load sharing and ligament strains in balanced, overstuffed and understuffed UKA. A validated finite element analysis

The aim of this study was to quantify the effects of understuffing and overstuffing UKA on bone stresses, load distribution and ligament strains. For that purpose, a numerical knee model of a cadaveric knee was developed and was validated against experimental measurements on that same knee. Good agreement was found among the numerical and experimental results. This study showed that, even if a medial UKA is well-aligned with normal soft tissue tension and with correct thickness of the tibia component, it induces a stiffness modification in the joint that alters the load distribution between the medial and lateral compartments, the bone stress and the ligament strain potentially leading to an osteoarthritic progression.

Stemmed TKA in a femur with a total hip arthroplasty: is there a safe distance between the stem tips?

When a stemmed TKA is needed in a femur in which a THA is already present, choosing an appropriate length for the TKA stem is crucial. Many surgeons intuitively fear that the distance between the stem tips correlates with the femur risk for fracture (RF). However, to date, no biomechanical data to support this intuition are available. Therefore, in this study, the RF in such a configuration was determined and compared for several activities, using a finite element modeling technique. During gait and sideways falling no difference in RF among different stem lengths was shown. However, a clear threshold appears during four-point bending. Stem tip distances shorter than 110 mm dramatically increased RF and, in osteoporotic bone, will certainly lead to fracture (RF>1) and thus should be avoided.