Effects of A-P translation and rotation on the wear of UHMWPE in a total knee joint simulator

Computational wear of knee implant polyethylene insert surface under continuous dynamic loading and posterior tibial slope variation based on cadaver experiments with comparative verification

Background

The effect of posterior tibial slope on the maximum contact pressure and wear volume of polyethylene (PE) insert were not given special attention. The effects of flexion angle, Anterior-Posterior (AP) Translation, and Tibial slope on the max contact pressure and wear of PE insert of TKR were investigated under loadings which were obtained in cadaver experiments by using Archard’s wear law. This study uses not only loads obtained from cadaver experiments but also dynamic flexion starting from 0 to 90 degrees.

Method

Wear on knee implant PE insert was investigated using a 2.5 size 3 dimensional (3D) cruciate sacrificing total knee replacement model and Finite Element Method (FEM) under loadings and AP Translation data ranging from 0 to 90 flexion angles validated by cadaver experiments. Two types of analyses were done to measure the wear effect on knee implant PE insert. The first set of analyses included the flexion angles dynamically changing with the knee rotating from 0 to 90 angles according to the femur axis and the transient analyses for loadings changing with a certain angle and duration.

Results

It is seen that the contact pressure on the PE insert decreases as the cycle increases for both Flexion and Flexion+AP Translation. It is clear that as the cycle increases, the wear obtained for both cases increases. The loadings acting on the PE insert cannot create sufficient pressure due to the AP Translation effect at low speeds and have an effect to reduce the wear, while the effect increases with the wear as the cycle increases, and the AP Translation now contributes to the wear at high speeds. It is seen that as the posterior tibial slope angle increases, the maximum contact pressure values slightly decrease for the same cycle.

Conclusions

This study indicated that AP Translation, which changes direction during flexion, had a significant effect on both contact pressure and wear. Unlike previous similar studies, it was seen that the amount of wear continues to increase as the cycle increases. This situation strengthens the argument that loading and AP Translation values that change with flexion shape the wear effects on PE Insert.

Backside wear of tibial polyethylene components is affected by gait pattern: A knee simulator study using rare earth tracer technology

The aim of this study was to determine the effect of two in-vivo-determined gait patterns, one with low and one with high anteroposterior (AP) motion, on total and backside polyethylene insert wear in comparison with the ISO (International Organization for Standardization) standard 14243-3. In order to differentiate and accurately quantify topside and backside wear, a novel technique was employed where different lanthanide tracers were incorporated into the polyethylene during manufacture. Wear particle analysis was conducted following established protocols. For all tested liners and motion protocols, the chemically calculated wear rates correlated closely with gravimetrically determined wear. Both in vivo motion groups displayed higher wear rates than the ISO group following the order of the AP motion amplitudes. Backside wear for ISO constituted 2.76% ± 0.90% (mean ± SE) of the total wear, increasing significantly to 15.8% ± 3.2% for the low AP and further increasing to 19.3% ± 0.95% for the high AP motion protocol. The mean wear particle sizes were under 200 nm for all three motion patterns, being largest for the protocol with high AP motion. Particle release from the low and high AP gait patterns was 1.9 to 2.8 times that from the ISO protocol. Testing for the proportion of backside wear across various activities of daily living should be an important consideration in evaluating knee prostheses wear.

Influence of joint kinematics on polyethylene wear in anatomic shoulder joint arthroplasty

BACKGROUND

Despite the positive results in total shoulder arthroplasties (TSAs), a higher revision rate is documented compared with total hip and knee replacements. Wear is the possible main cause of TSA failure in the long-term. This study investigated the effect of joint kinematics and the influence of the rotator cuff on the polyethylene wear performance in an anatomic TSA.

METHODS

Lifting a load of 2 kg with an abduction/adduction of 0° to 90° was simulated for 2 × 10⁶ cycles as a primary motion using a fully kinematic joint simulator. A combined rotation in anteversion-retroversion of ±5° and ±10° was also simulated. The force in the superior-inferior direction and the axial joint compression were applied under force control based on in vivo data of the shoulder. A soft tissue restraint model was used to simulate an intact and an insufficient rotator cuff.

RESULTS

The highest wear rate in the intact rotator cuff group was 58.90 ± 1.20 mg/10⁶ cycles with a combined rotation of ±10°. When an insufficient rotator cuff was simulated, the highest polyethylene wear rate determined was 79.67 ± 4.18 mg/10⁶ cycles.

CONCLUSIONS

This study confirms a high dependency of the polyethylene wear behavior and dimension on the joint kinematics in total shoulder replacement. This can be explained by an increasing cross-shear stress on the polyethylene component. The results obtained indicate that additional combined kinematics are an indispensable part of wear tests on anatomic shoulder replacements.

Fixed Bearing Knee Congruency – Influence on Contact Mechanics, Abrasive Wear and Kinematics

The objective of our study was to evaluate the in vitro wear behavior of fixed bearing designs for total knee arthroplasty in relation to contact mechanics and resultant kinematics for different degrees of congruency.

A finite element model was created for three knee articulations with increasing degrees of tibio-femoral congruency (flat, curved, and dished design). For the three different knee design configurations, in vitro wear simulation was performed according to ISO 14243–1.

Contact areas increased with increasing knee congruency, whereas the peak surface contact stresses decreased. The wear rates for the knee design configurations differed substantially between the three test groups (flat, curved, and dished).

Our observations demonstrate that increased congruency in conjunction with decreased surface contact stresses significantly contributes to reducing wear in fixed bearing knee articulations.

Influence of humeral head material on wear performance in anatomic shoulder joint arthroplasty

BACKGROUND

The number of total shoulder arthroplasties has increased in the past years, with encouraging results. However, the survival of anatomic total shoulder arthroplasty (aTSA) is lower compared with that of knee and hip replacements. Wear-associated problems like loosening are well-known causes of long-term failure of aTSA. The main purpose of this study was to investigate the wear behavior of ceramic-polyethylene bearings compared with the standard metal-polyethylene bearings. Because there is a lack of valid experimental wear testing methods, the secondary aim was to develop a validated wear simulation.

METHODS

The wear assessment was performed using a force-controlled joint simulator for 3 × 10⁶ cycles, and polyethylene wear was assessed gravimetrically and by particle analysis. Kinetic and kinematic data were adopted from in vivo loading measurements and from several clinical studies on shoulder joint kinematics. The reaction of the rotator cuff was simulated on the basis of a virtual soft tissue model. As activity, an abduction-adduction motion of 0°-90° lifting a load of 2 kg superimposed by an anteversion-retroversion has been chosen.

RESULTS

The studied aTSA resulted in a polyethylene wear rate of 62.75 ± 1.60 mg/10⁶ cycles in combination with metallic heads. The ceramic heads significantly reduced the wear rate by 26.7% to 45.99 ± 1.31 mg/10⁶. There were no relevant differences in terms of the particle characteristics.

CONCLUSION

This is the first study that experimentally studied the wear behavior of aTSA based on patient-related and biomechanical data under load-controlled conditions. Regarding polyethylene wear, the analyzed aTSA could benefit from ceramic humeral heads.

Effect of rotational prosthetic alignment variation on tibiofemoral contact pressure distribution and joint kinematics in total knee replacement

In total knee replacement surgery, implant alignment is one of the most important criteria for successful long-term clinical outcome. During total knee replacement implantation, femoral and tibial alignment are determined through appropriate bone resections, which could vary based on patient anatomy, implant design and surgical technique and further influence loading conditions and clinical outcomes. The current research focused on three critical alignment parameters for total knee replacement insertion: femoral component internal/external (I/E) rotation, varus-valgus tibiofemoral angulation and posterior tibial slope. A computational finite element model of total knee replacement implant was developed and validated comparing with kinematic outputs generated from experimentally simulated knee joint motion. The FE model was then used to assess 12 different alignment scenarios based on previous case reports. Postoperative knee kinematics and joint contact pressure during simulated gait motion were assessed. According to the parametric study, FE model cases with femoral rotation revealed extra tibial I/E rotation in the predefined direction but negligible change in tibial anterior-posterior translation; cases with increased tibial slope showed notably increased tibial external rotation and anterior translation; cases with varus tibiofemoral angle presented slightly more tibial external rotation, whereas cases with valgus angle presented an observable increase in tibial internal rotation at the middle phase of the gait cycle. Finally, the response surface obtained from the postprocessing study demonstrated good statistical correlation with existing case study results, providing reliable estimation of peak tibiofemoral contact pressure affected by combinations of alignment parameters. The observations indicate that femoral external alignment should be favored clinically for enhanced patellar tracking and reduced contact pressure concentration for better long-term performance. Posterior tibial slope enables deep knee flexion. Extra femoral internal rotation as well as tibiofemoral varus-valgus alignment could be avoided in surgery due to deficiency in patellar tracking and high pressure concentration.

Evaluation of the relationship of tibiofemoral kinematics before and after total knee replacement in an in vitro model of cranial cruciate deficiency in the dog

OBJECTIVE

To investigate the relationship between tibiofemoral kinematics before and after total knee replacement (TKR) in vitro.

ANIMALS

Eight canine hemipelves.

METHODS

A modified Oxford Knee Rig was used to place cadaveric limbs through a range of passive motion allowing the kinematics of the stifle to be evaluated. Four measurements were performed: a control stage, followed by a cranial cruciate transection stage, then following TKR with the musculature intact stage, and finally TKR with removal of limb musculature stage. Joint angles and translations of the femur relative to the tibia, including flexion-extension versus adduction-abduction, flexion-extension versus internal-external rotation, as well as flexion-extension versus each translation (cranial-caudal and lateral-medial) were calculated.

RESULTS

Significant differences were identified in kinematic data from limbs following TKR implantation as compared to the unaltered stifle. The TKR resulted in significant decreases in external rotation of the stifle during flexion-extension compared to the limb prior to any intervention, as well as increasing the abduction. The TKR significantly increased the caudal translation of the femur relative to the tibia compared to the unaltered limb. When compared with the cranial cruciate ligament-transection stage, TKR significantly decreased the ratio of the external rotation to flexion.

DISCUSSION

All three test periods showed significant differences from the unaltered stifle. The TKR did not completely restore the normal kinematics of the stifle.

Displacement or Force Control Knee Simulators? Variations in Kinematics and in Wear

The problems associated with prosthetic failure and revision surgery still constitute the main clinical problem of prosthetic surgery. The objective of wear evaluation is to determine the wear rate and its dependence on the test conditions. To obtain realistic results, a wear test can be performed to reproduce in vivo working conditions and compare the wear characteristics of various total knee prostheses designs. At the state of the art, two simulation concepts are available and defined in ISO 14243 standards series. In both these guidelines, level walking is the sole activity of daily living that is represented for testing. With so many variables and so many sources of error and the sensitivity of the output to these errors, can the motion determined in the simulator be representative of the in vivo motion? This article goes beyond the current status of these knee simulations comparing literature results.

Effects of kneeling on tibiofemoral contact pressure and area in posterior cruciate-retaining and posterior cruciate-sacrificing total knee arthroplasty

The objective of this study was to measure the effect of kneeling on tibiofemoral contact following cruciate-retaining and posterior-stabilized total knee arthroplasty. Five cadaveric knees were tested on a custom testing system that allowed physiologic muscle loading. Three forces were used to simulate nonkneeling, double-stance kneeling, and single-stance kneeling at flexion angles of 90°, 105°, 120°, and 135°. Tibiofemoral contact areas and pressures were measured using the Tekscan (South Boston, MA) system. Kneeling increased contact areas and pressures in both designs with variable significance (P < .05). Moving from double- to single-stance kneeling increased pressures in the cruciate-retaining group but decreased pressures in the posterior-stabilized group (P < .05). Chronic, repetitive kneeling after total knee arthroplasty may increase polyethylene wear due to increased contact areas and pressures.

Can the method of fixation influence the wear behaviour of ZrN coated unicompartmental mobile knee prostheses?

BACKGROUND

Modern unicompartmental knee prostheses represent a valid alternative to total knee replacement. It is known that variations in clinical alignment lead to altered biomechanics and abnormal wear. The aim of this study was to assess the influence, on wear behaviour, of two different cementing interfaces of the femoral components tested on a knee joint wear simulator.

METHODS

The wear tests were run in a knee wear simulator at a frequency of 1.1 Hz for 3 million cycles in accordance with ISO 14243-3. Twelve commercial mobile GUR 1020 UHMWPE meniscus specimens articulated in between 12 cobalt-chromium-molybdenum alloy femoral and tibial components covered by a multi-layer of chromium nitride and a final layer of zirconium nitride ceramic coating to prevent ion release from the substrate. Two wear tests were performed: in the first test, each femoral component was cemented into a custom made metallic-block shaped to perfectly host it. In the second test, synthetic composite femurs received the femoral components on the basis of guidelines used in current surgery.

FINDINGS

The two cementing interfaces showed a significantly different wear behaviour, quantified as mean weight loss (P<0.001). Scanning electron microscope examinations of new and tested metallic components showed macro- and micro-pores of few microns on both configurations.

INTERPRETATION

The wear pattern observed at 3 million cycles showed differences between the two methods of fixation for the meniscus femoral components.

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.

Comparison of polyethylene tibial insert damage from in vivo function and in vitro wear simulation

Function and wear of total knee arthroplasties were compared by analysis of damage patterns on polyethylene tibial inserts retrieved from patients (Group R) with inserts obtained after in vitro force-controlled knee joint wear simulation. Two simulator input profiles were evaluated, including standard walking (Group W), and combined walking and stair descent (Group W + S), simulating varied activities and a more severe physiological environment. Damage regions on all inserts were quantitatively assessed. On average, inserts in all groups had internally rotated damage patterns and the greatest articular deformation in the lateral compartment. These patterns were more pronounced in Group W + S compared to Group W. Deformation rates of simulated inserts were analogous to about six years of physiologic function. However, both groups of simulated inserts generally underestimated the magnitude of damage area and extent observed on retrieved inserts, consistent with differences in the simulator's tibiofemoral contact mechanics and those known to occur in patients during functional activities. Modification of simulator inputs, such as the increased anteroposterior excursion and more severe loading conditions in Group W + S, can generate greater wear volume, larger damage areas, and increased surface deformation rates compared to standard inputs.

Two-Dimensional Surrogate Contact Modeling for Computationally Efficient Dynamic Simulation of Total Knee Replacements

Computational speed is a major limiting factor for performing design sensitivity and optimization studies of total knee replacements. Much of this limitation arises from extensive geometry calculations required by contact analyses. This study presents a novel surrogate contact modeling approach to address this limitation. The approach involves fitting contact forces from a computationally expensive contact model (e.g., a finite element model) as a function of the relative pose between the contacting bodies. Because contact forces are much more sensitive to displacements in some directions than others, standard surrogate sampling and modeling techniques do not work well, necessitating the development of special techniques for contact problems. We present a computational evaluation and practical application of the approach using dynamic wear simulation of a total knee replacement constrained to planar motion in a Stanmore machine. The sample points needed for surrogate model fitting were generated by an elastic foundation (EF) contact model. For the computational evaluation, we performed nine different dynamic wear simulations with both the surrogate contact model and the EF contact model. In all cases, the surrogate contact model accurately reproduced the contact force, motion, and wear volume results from the EF model, with computation time being reduced from 13minto13s. For the practical application, we performed a series of Monte Carlo analyses to determine the sensitivity of predicted wear volume to Stanmore machine setup issues. Wear volume was highly sensitive to small variations in motion and load inputs, especially femoral flexion angle, but not to small variations in component placements. Computational speed was reduced from an estimated 230hto4h per analysis. Surrogate contact modeling can significantly improve the computational speed of dynamic contact and wear simulations of total knee replacements and is appropriate for use in design sensitivity and optimization studies.

Y-TZP zirconia run against highly crosslinked UHMWPE tibial inserts: knee simulator wear and phase-transformation studies

BACKGROUND

Zirconia (ZrO(2)) ceramics combined with highly cross-linked polyethylene appears to be a promising approach to minimize wear in artificial knee joints. The wear performance of yttria-stabilized zirconia (YZr) femoral condyles on 7-Mrad tibial inserts was compared in a knee simulator to CoCr bearing on 3.5-Mrad inserts.

METHODS

The knee design was the Bi-Surface type with a 9-year clinical history in Japan (JMM, Japan). A displacement-controlled knee simulator was used with kinematics that included 20 degrees flexion, +/-5 degrees rotation, and 6 mm anterior/posterior translation. Lubricant was alpha-calf serum, test duration was 10 million cycles (10 Mc), and wear was measured by weight-loss techniques. The wear zones were studied by laser interferometry, scanning electron microscopy, and Raman microprobe spectroscopy.

RESULTS

At 10 Mc the wear rates of the CoCr controls averaged 4.5 mm(3)/Mc. This was within 7% of the prior estimate at 5-Mc duration and comparable to Bi-Surface wear data from another laboratory. The CoCr condyles increased in roughness (R(a)) from <50 nm to average R(a) = 250 nm due to linear scratching. The ceramic condyles remained pristine throughout the wear study (R(a) <7 nm). With the YZr/7-Mrad combination, the weight change had a positive slope over at 10 Mc, which meant that the actual polyethylene wear was unmeasurable. Microscopic examinations at 10 Mc showed that the zirconia surfaces were intact and there was no detectable change from tetragonal to monoclinic phase.

INTERPRETATION

Our laboratory knee wear simulation appeared very supportive of the 9-year YZr/PE clinical results with Bi-Surface total knee replacements in Japan.

Wear in molded tibial inserts: knee simulator study of H1900 and GUR1050 polyethylenes

Hi-fax 1900 tibial inserts were used in the IB-1 total knee replacement (TKR) beginning 1978, soon followed by the AGC design. Such direct compression molded (DCM) inserts was relatively immune to oxidation. Unfortunately the Hi-fax 1900 resin (H1900) was taken off the market in year 2004. As an alternate, GUR1050 was introduced in the Vanguard TKR. However there appeared to be little or no wear comparisons of molded inserts. Therefore the study aim was to compare wear performance of GUR1050 to the historical H1900. The hypothesis was that Hi-fax and GUR1050 would show comparable wear performance. The Vanguar was a posterior-cruciate sacrificing design (Biomet Inc.). All tibial inserts were sterilized by gamma-radiation (3.2 Mrad) under argon. A 6-channel, displacement-controlled knee simulator was used with serum lubricant (protein concentration 20 mg/mL). Wear assessments were by gravimetric methods and linear regression techniques. The gross weight-loss trends over 2.5 Mc duration demonstrated excellent linear behavior with good agreement between TKR sets (<+/-10%). Fluid sorption artifacts in control represented less than 5% of gross wear magnitudes. Thus suitable corrections could be made in determining net wear. The H1900 and GUR1050inserts demonstrated net wear-rates of 3.6 and 3.4 mm(3)/Mc, respectively. This difference was not found to be statistically significant. This wear study demonstrated that GUR1050 inserts were indistinguishable from the Hi-fax 1900 in terms of laboratory wear performance, proving our hypothesis. Given the excellent clinical history of DCM Hi-fax 1900, the GUR1050 should be an ideal candidate for TKR.

A highly crosslinked UHMWPE for CR and PS total knee arthroplasties

X3 is a highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) produced by a sequential irradiation and annealing process. The sequential process results in a material with a free radical content that is 1% that of conventional UHMWPE gamma sterilized in nitrogen resulting in an oxidation resistance similar to that of virgin UHMWPE. Yield strength and ultimate tensile strength exceed American Society for Testing and Materials minimum rates for UHMWPE. Simulator testing of contemporary cruciate retaining (CR) and posterior-stabilized knee inserts (Triathlon) manufactured by the sequential process demonstrated 68% and 64% less wear, respectively, compared to conventionally processed inserts. The wear and mechanical integrity of sequentially processed posterior-stabilized inserts was unaffected by accelerated aging, whereas conventional UHMWPE exhibited increased wear, cracking, and delamination.

Investigation on wear of knee prostheses under fixed kinematic conditions

Although polyethylene components have been used in the human joint for over 30 years, wear simulation studies are fundamental to assess wear resistance of total joint replacements. This assessment will help to obtain quality control and acquire further knowledge of the tribological processes that involve joint prostheses. As a result, the risk of implant failure of innovative prostheses will be reduced. The aim of the present study was to investigate the influence on wear of the polyethylene tibial inserts against metallic femoral components using a knee simulator following a standardized kinematics protocol (ISO/CD 14243-3.2). Four intact "small" size specimens of the tibial MP 913 were tested in a four-station knee wear simulator for two million cycles. The volumetric weight loss for the ultra-high-molecular-weight-polyethylene (UHMWPE) tibial inserts was 44 +/- 2, 45 +/- 3, and 47 +/- 3 mg for the specimens #1-3, respectively. This study demonstrated a good repeatability among the stations of the knee simulator obtaining weight loss values congruent with those found by other authors using similar test conditions.

Dose effects of cross-linking polyethylene for total knee arthroplasty on wear performance and mechanical properties

Wear performance and mechanical properties of cross-linking polyethylene (XLPE) tibial inserts were investigated using a knee simulator, scanning electron microscopy (SEM), and a small punch test (SPT). Ultrahigh molecular weight PE made from GUR1050 resin was irradiated at doses ranging from 0 to 200 kGy and then machined into tibial inserts followed by annealing. The knee simulator was run for up to four million cycles. As the radiation dose increased from 0 to 100 kGy, the wear rate decreased dramatically, yielding 95% wear reduction at 100 kGy. The microwear features observed by SEM supported the dose-dependent wear reduction. The SPT for XLPE after the simulation test showed that, as the radiation dose increased from 0 to 200 kGy, the ultimate displacement decreased dose-dependently, while the ultimate load increased from 0 to 75 kGy and decreased from 75 to 200 kGy. The resulting toughness of the PE increased to its maximum at a dose of 50 kGy and then decreased with higher doses up to 200 kGy. PE cross-linked with radiation doses from 25 to 75 kGy had greater toughness than virgin, nonirradiated PE. However, PE irradiated with 100 kGy or more had lower toughness than virgin PE. These data suggest that a certain amount of irradiation enhances both wear performance and toughness of PE tibial inserts. Although a certain amount of cross-linking would be effective for clinical application of PE tibial inserts, an optimal radiation dose should be much smaller than that used in current XLPE in total hip arthroplasty.

Effect of variability in anatomical landmark location on knee kinematic description

Small variability associated with identifying and locating anatomical landmarks on the knee has the potential to affect the joint coordinate systems and reported kinematic descriptions. The objectives of this study were to develop an approach to quantify the effect of landmark location variability on both tibiofemoral and patellofemoral kinematics and to identify the critical landmarks and associated degrees of freedom that most affected the kinematic measures. The commonly used three-cylindric open-chain kinematic description utilized measured rigid body kinematics from a cadaveric specimen during simulated gait. A probabilistic analysis was performed with 11 anatomical landmarks to predict the variability in each kinematic. The model predicted the absolute kinematic bounds and offset kinematic bounds, emphasizing profile shape, for each kinematic over the gait cycle, as well as the range of motion. Standard deviations of up to 2 mm were assumed for the anatomical landmark locations and resulted in significant variability in clinically relevant absolute kinematic parameters of up to 6.5 degrees and 4.4 mm for tibiofemoral and 7.6 degrees and 6.5 mm for patellofemoral kinematics. The location of the femoral epicondylar prominences had the greatest effect on both the tibiofemoral and patellofemoral kinematic descriptions. A quantitative understanding of the potential changes in kinematic description caused by anatomical landmark variability is important not only to the accuracy of kinematic gait studies and the evaluation of total knee arthroplasty implant performance, but also may impact component placement decision-making in computer-assisted surgery.

The influence of femoral condylar lift-off on the wear of artificial knee joints

In vivo fluoroscopic studies of patients with total knee replacements (TKRs) have shown lift-off of the femoral condyles from the tibial insert. This study investigated the influence of femoral condylar lift-off on the ultra-high molecular weight polyethylene (UHMWPE) wear of fixed bearing (FB) and rotating platform mobile bearing (RP MB) total knee replacements, using a physiological knee joint simulator. In the absence of lift-off, the RP MB knees exhibited a lower wear rate of 5.2 +/- 2.2 mm3 per million cycles (mm3/MC) compared with 8.8 +/- 4.8 mm3/MC for the FB knees. The presence of femoral condylar lift-off was found to accelerate the wear of the FB and RP MB knees tested in this study to 16.4 +/- 2.9 and 16.9 +/- 2.9 mm3/MC respectively. For the RP MB knees the increase in wear rate was more marked, resulting in a similar wear rate for both designs of knee under lift-off conditions. In both cases the medial condyle displayed more wear damage. This study has shown that a small amount of abduction/adduction lift-off and medial-lateral shift increases wear and that the increase in wear is design dependent. In this simulator test, lift-off was simulated on every cycle, whereas the amount of wear and effect of lift-off clinically would depend on the frequency of occurrence of lift-off in vivo.

Effects of knee simulator loading and alignment variability on predicted implant mechanics: a probabilistic study

Inherent variability in total knee arthroplasty loading and alignment, present in vivo and in simulator testing, may ultimately influence polyethylene tibial insert wear and long-term performance. The effect of this variability was quantified on implant kinematics and contact mechanics during simulated gait loading conditions using semi-constrained and unconstrained fixed bearing, cruciate retaining implants. A probabilistic finite element model of the Stanmore knee wear simulator was utilized to estimate the envelope of anterior-posterior (AP) and internal-external (IE) position and contact pressure and to evaluate the variability in corresponding ranges of motion (ROM). Variability levels were represented by standard deviations of up to 10% of the maximum value for load inputs and 0.25 mm and 0.5 degrees for component alignment inputs. Model predictions compared well with experimental simulator results for the semi-constrained implant, with predicted positional envelopes of up to 1.8 mm (AP) an 3.4 degrees (IE) for the semi-constrained and up to 2.6 mm (AP) and 3.7 degrees (IE) for the unconstrained implant at the variability levels evaluated. ROM varied by up to 22%, while peak contact pressure variations averaged less than 2 MPa for both designs. For each implant, loading variability was more influential during the swing phase of gait, while alignment variability affected kinematics more during stance. The relative rank of sensitivities showed differences between the two designs, providing insight into critical parameters affecting kinematics and contact characteristics.

Wear and toughness of crosslinked polyethylene for total knee replacements: a study using a simulator and small-punch testing

Severe loading and complex kinematics in total knee replacement make wear performance and toughness of the polyethylene in tibial inserts important. We investigated wear of crosslinked polyethylene using a knee simulator and measured toughness using small-punch testing. GUR1050 rods were gamma-irradiated in air at doses from 0 to 200 kGy, annealed in nitrogen, and machined into tibial inserts. The simulator was run to 4 million cycles, and wear rates determined from weight loss. Wear rate decreased by 54, 78, and 95% as radiation dose increased from 50 to 75 to 100 kGy, respectively. At every dose, toughness was significantly less after simulator testing, but the difference between control and wear-tested polyethylene, considered to be due to fatigue damage accumulation, was smallest at 50 kGy. The simulator-tested polyethylene that received 35 to 75 kGy had slightly higher toughness than equivalent material that received no irradiation. However, the toughness of simulator-tested polyethylene that received 150 and 200 kGy was lower than that of the simulator-tested polyethylene that received no irradiation. Our results suggest that an optimal irradiation dose may exist for crosslinked polyethylene for use in TKR and that the optimum dose would be less than the 100 kGy or more that are used in some current crosslinked polyethylene for hip replacement.

Probabilistic finite element prediction of knee wear simulator mechanics

Computational models have recently been developed to replicate experimental conditions present in the Stanmore knee wear simulator. These finite element (FE) models, which provide a virtual platform to evaluate total knee replacement (TKR) mechanics, were validated through comparisons with experimental data for a specific implant. As with any experiment, a small amount of variability is inherently present in component alignment, loading, and environmental conditions, but this variability has not been previously incorporated in the computational models. The objectives of the current research were to assess the impact of experimental variability on predicted TKR mechanics by determining the potential envelope of joint kinematics and contact mechanics present during wear simulator loading, and to evaluate the sensitivity of the joint mechanics to the experimental parameters. In this study, 8 component alignment and 4 experimental parameters were represented as distributions and used with probabilistic methods to assess the response of the system, including interaction effects. The probabilistic FE model evaluated two levels of parameter variability (with standard deviations of component alignment parameters up to 0.5mm and 1 degrees ) and predicted a variability of up to 226% (3.44mm) in resulting anterior-posterior (AP) translation, up to 169% (4.30 degrees ) in internal-external (IE) rotation, but less than 10% (1.66MPa) in peak contact pressure. The critical alignment parameters were the tilt of the tibial insert and the IE rotational alignment of the femoral component. The observed variability in kinematics and, to a lesser extent, contact pressure, has the potential to impact wear observed experimentally.

Wear debris associated with a large lateral femoral condyle cyst following an Oxford medial unicompartmental knee replacement

A 63-year-old gentleman presented with a history of worsening left knee pain almost 5 years following bilateral uni-compartmental knee replacements. X-Rays revealed a fracture of the medial tibial plateau and revision surgery was undertaken. A large cyst was found in the lateral femoral condyle, which extended from the weight bearing area in the lateral compartment to the margin of the trochlea. Histology of the lesion revealed a central degenerate cyst with multiple particles of polyethylene and cement debris around its periphery. Cyst progression in association with wear debris is a rare problem following unicompartmental knee replacement. The findings at the time of revision in this case demonstrate that the complications of unicompartmental arthroplasty may affect all compartments of the knee.

Polyethylene wear characteristics in vivo and in a knee stimulator

Validation of a wear simulator requires that the device produce a similar type and amount of wear and particles of a comparable morphology as occurs clinically. Using techniques previously established to compare polyethylene particles from hip simulators to those from retrieved tissues, particles isolated from six revised posterior stabilized knee replacements were characterized and compared to particles generated from the same knee design worn in a knee simulator. The particles produced in the knee simulator were of comparable size but had less variability in their form factor compared to the particles produced in vivo. Comparable wear features were seen on the articulating surfaces in both groups. These results indicate that this knee joint simulator is able to reproduce a baseline type of wear that is similar to that in vivo and should encourage further use of this device to better understand knee component wear and function.