The effect of polyethylene thickness in fixed- and mobile-bearing total knee replacements

Finite element analysis in the optimization of posterior-stabilized total knee arthroplasty

Posterior-stabilized total knee arthroplasty (PS-TKA) is associated with high rates of satisfaction and functional recovery. This is notably attributed to implant optimization in terms of design, choice of materials, positioning and understanding of biomechanics. Finite elements analysis (FEA) is an assessment technique that contributed to this optimization by ensuring mechanical results based on numerical simulation. By close teamwork between surgeons, researchers and engineers, FEA enabled testing of certain clinical impressions. However, the methodological features of the technique led to wide variations in the presentation and interpretation of results, requiring a certain understanding of numerical and biomechanical fields by the orthopedic community. The present study provides an up-to-date review, aiming to address the following questions: what are the principles of FEA? What is the role of FEA in studying PS design in TKA? What are the key elements in the literature for understanding the role of FEA in PS-TKA? What is the contribution of FEA for understanding of tibiofemoral and patellofemoral biomechanical behavior? What are the limitations and perspectives of digital simulation and FEA in routine practice, with a particular emphasis on the "digital twin" concept? LEVEL OF EVIDENCE: V, expert opinion.

Computer-based analysis of different component positions and insert thicknesses on tibio-femoral and patello-femoral joint dynamics after cruciate-retaining total knee replacement

BACKGROUND

Positioning of the implant components and tibial insert thickness constitute critical aspects of total knee replacement (TKR) that influence the postoperative knee joint dynamics. This study aimed to investigate the impact of implant component positioning (anterior-posterior and medio-lateral shift) and varying tibial insert thickness on the tibio-femoral (TF) and patello-femoral (PF) joint kinematics and contact forces after cruciate-retaining (CR)-TKR.

METHOD

A validated musculoskeletal multibody simulation (MMBS) model with a fixed-bearing CR-TKR during a squat motion up to 90° knee flexion was deployed to calculate PF and TF joint dynamics for varied implant component positions and tibial insert thicknesses. Evaluation was performed consecutively by comparing the respective knee joint parameters (e.g. contact force, quadriceps muscle force, joint kinematics) to a reference implant position.

RESULTS

The PF contact forces were mostly affected by the anterior-posterior as well as medio-lateral positioning of the femoral component (by 3 mm anterior up to 31 % and by 6 mm lateral up to 14 %). TF contact forces were considerably altered by tibial insert thickness (24 % in case of + 4 mm increase) and by the anterior-posterior position of the femoral component (by 3 mm posterior up to 16 %). Concerning PF kinematics, a medialised femoral component by 6 mm increased the lateral patellar tilt by more than 5°.

CONCLUSIONS

Our results indicate that regarding PF kinematics and contact forces the positioning of the femoral component was more critical than the tibial component. The positioning of the femoral component in anterior-posterior direction on and PF contact force was evident. Orthopaedic surgeons should strictly monitor the anterior-posterior as well as the medio-lateral position of the femoral component and the insert thickness.

A Numerical Investigation into the Effects of Overweight and Obesity on Total Knee Arthroplasty

Overweight and obesity increase risks of knee osteoarthritis, which is a major cause of disability. Severe knee osteoarthritis can be treated by knee arthroplasty. Total knee arthroplasty has been used in overweight and obese patients; however, clinical reports showed that the outcome of this group of patients was not good as normal-weight patients. Two computer models were created in this paper to simulate the effect of excess loads on the distal femoral bone and contact pressures in total knee arthroplasty during a gait cycle. The numerical results showed increased stress in periprosthetic distal femoral bones and higher contact pressure on tibial polyethylene insert during the stance phase. Based on the computer simulation results and published research work, cementless total knee arthroplasty with thicker tibial polyethylene insert may be a better option for overweight patients.

The influence of polyethylene bearing thickness on the tibiofemoral kinematics of a bicruciate retaining total knee arthroplasty

BACKGROUND

The recently reintroduced bicruciate retaining Total Knee Arthroplasty (BCR TKA) is an effort to reproduce kinematics closer to the native knee. However, there is no data on appropriate balancing with this implant. Balancing is crucial and challenging as medial and lateral polyethylene (PE) inlays are modular, which allows for placement of different thicknesses in the medial and lateral compartments. This study aimed at providing a detailed kinematic view on balancing BCR TKA.

METHODS

Seven fresh frozen cadaver legs were mounted in a kinematic rig that applied squatting under application of physiologic quadriceps and hamstring forces. Additionally, specimen laxity was assessed using Lachman tests and varus/valgus stress tests. Following testing on the native knee, a BCR TKA was implanted in each specimen and all trials were repeated. Using one millimeter increments, five inlay thicknesses were tested to simulate optimal balancing, symmetric under-, and overstuffing, valgus constellation, and varus constellation.

RESULTS

Overall, knee kinematics following BCR TKA seem to be very close to the native knee. The changes as introduced to tibiofemoral kinematics through over- or understuffing the polyethylene inserts are affecting the system only to a minor degree and generally lack statistical significance. Reproduction of the tibial varus via PE-Inlays did not lead to kinematics much closer to the native knee.

CONCLUSIONS

The changes introduced to tibiofemoral kinematics by removal of the conforming meniscus and cartilage and replacement with a flat PE insert and femoral component are of more impact than different inlay sizes and their combinations for a BCR TKA.

Comparison of Tibial Insert Polyethylene Damage in Rotating Hinge and Highly Constrained Total Knee Arthroplasty: A Retrieval Analysis

This study compared the damage scores and damage patterns in 19 tibial inserts from rotating hinge (RH) implants with 19 inserts from highly constrained (HC) implants. Each insert was divided into 16 damage zones and each zone was subjectively graded from a scale of 0-3 for seven different damage modes. The overall damage scores were comparable for the two groups (RH: 64.1 ± 15.4; HC: 66.1 ± 29.0; P = 0.59). The HC group, however, had greater post damage (compared to the post-hole of RH) while the RH group had greater backside damage. The pattern of damage was also different, with burnishing and cold flow being more common in HC group while pitting, scratching and embedded debris were more common in the RH group.

Biomechanical comparison of fixed- and mobile-bearing for unicomparmental knee arthroplasty using finite element analysis

Unicomparmental knee arthroplasty (UKA) is a popular alternative to total knee arthroplasty (TKA) and high tibial osteotomy for unicompartmental knee conditions, especially in young patients. However, failure of UKA occurs due to either progressive osteoarthritis (OA) in the other compartment or wear on the polyethylene (PE) insert. This study used finite element (FE) analysis to investigate the effects of PE insert contact pressure and stress in opposite compartments for fixed- and mobile-bearing UKA. Analysis was performed using high kinematics displacement and rotation inputs, which were based on the kinematics of the natural knee. ISO standards were used for axial load and flexion. The mobile-bearing PE insert had lower contact pressure than the fixed-bearing PE insert. With the mobile-bearing UKA, lower stress on the opposite compartment reduces the overall risk of progressive OA in the knee. The fixed-bearing UKA increases the overall risk of progressive OA in the knee due to higher stress on the opposite compartment. However, the PE insert of mobile-bearing showed pronounced backside stress at the inferior surface.

All-polyethylene tibial components in modern total knee arthroplasty

Most total knee arthroplasty prostheses have modular tibial components with metal-backed tibial baseplates. Biomechanical studies have demonstrated mechanical advantages to a metal-backed tibial component in terms of tibial load transfer. In addition, tibial component modularity provides intraoperative flexibility and may provide an advantage in the setting of subsequent revision knee surgery. However, clinical evidence does not support the preferential use of metal-backed tibial components. Modularity introduces the potential for backside wear and associated osteolysis. Also, several recent studies have shown no significant differences in clinical and radiographic outcomes between metal-backed and all-polyethylene tibial components. In addition, all-polyethylene tibial components are less expensive than metal-backed components; increased usage of all-polyethylene components could help decrease the cost of health care.

Wear-pattern analysis in retrieved tibial inserts of mobile-bearing and fixed-bearing total knee prostheses

Components from 73 failed knee replacements (TKRs) consisting of rotating-platform, mobile-bearing and fixed-bearing implants were examined to assess the patterns of wear. The patterns were divided into low-grade (burnishing, abrasion and cold flow) and high-grade (scratching, pitting/metal embedding and delamination) to assess the severity of the wear of polyethylene. The rotating-platform group had a higher incidence of low-grade wear on the upper surface compared with the fixed-bearing group. By contrast, high-grade wear comprising scratching, pitting and third-body embedding was seen on the lower surface. Linear regression analysis showed a significant correlation of the wear scores between the upper and lower surfaces of the tibial insert (R(2) = 0.29, p = 0.04) for the rotating-platform group, but no significant correlation was found for the fixed-bearing counterpart. This suggests that high-grade wear patterns on the upper surface are reduced with the rotating-platform design. However, the incidence of burnishing, pitting/third-body embedding and scratching wear patterns on the lower surface was higher compared with that in the fixed-bearing knee.

Wear of contemporary total knee replacements--a knee simulator study of six current designs

BACKGROUND

Compared to conventional ultra-high molecular weight polyethylene (UHMWPE), crosslinked polyethylene showed significantly reduced wear rate in hip simulation and early prospective-randomized clinical studies. The crosslinking process can reduce the mechanical properties of UHMWPE, particularly the fatigue strength. UHMWPE fatigue occurs more frequently in the knee joint than in the hip joint due to its higher contact stresses and there is therefore an increased concern of mechanical failure. Therefore, the purpose of this study was to determine the wear behaviour and the wear rates of different current knee designs and bearing materials.

METHODS

In a knee-joint-simulator four kinds of crosslinked polyethylene (one produced by sequential irradiation and annealing process, three by different remelting processes, including fixed- and mobile-bearing-types) and two UHMWPE- (fixed- and mobile-bearing) inserts were tested with the appropriate femoral and tibial component recommended from the manufacturer.

FINDINGS

All types of crosslinked polyethylene produced statistically significant (P<0.05) lower wear rates than the conventional UHMWPEs without any traces of fatigue. There were no differences in the wear rates between fixed-and mobile-bearings (crosslinked polyethylenes and UHMWPEs; P>0.05). The crosslinked polyethylene tibial inserts manufactured by sequential irradiation and annealing (X3) combined with the Scorpio-knee-design had the lowest wear rates (P<0.05) overall.

INTERPRETATION

Fixed- as well as mobile-bearing crosslinked polyethylene tibial inserts are suitable for total knee arthroplasty and showed reduced wear rates compared with conventional UHMWPE. The combination of the fixed-bearing Scorpio-knee-design with a sequential irradiated and annealed crosslinked polyethylene tibial insert (X3) seems to have an advantage in wear generation compared with other fixed- and mobile-bearing knee designs.