Knee Wear Research Articles

Wear Distribution Detection of Knee Joint Prostheses by Means of 3D Optical Scanners.

The objective of this study was to examine total knee polyethylene inserts from in vitro simulation to evaluate and display—using a 3D optical scanner—wear patterns and wear rates of inserts exposed to wear by means of simulators. Various sets of tibial inserts have been reconstructed by using optical scanners. With this in mind, the wear behavior of fixed and mobile bearing polyethylene knee configurations was investigated using a knee wear joint simulator. After the completion of the wear test, the polyethylene menisci were analyzed by an innovative 3D optical scanners in order to evaluate the 3D wear distribution on the prosthesis surface. This study implemented a new procedure for evaluating polyethylene bearings of joint prostheses obtained after in vitro wear tests and the proposed new approach allowed quantification of the contact zone on the geometry of total knee prostheses. The results of the present study showed that mobile TKPs (total knee prosthesis) have lower wear resistance with respect to fixed TKPs.

Are Instrumented Knee Forces Representative of a Larger Population of Cruciate-Retaining Total Knee Arthroplasties?

BackgroundIt is not known if the loads and motions reported for instrumented knees are generalizable to a larger population of total knee arthroplasty (TKA) patients. The purpose of this study is to (1) report axial implant force data for chair and stair activities for a population of cruciate-retaining TKA patients and (2) compare the population forces to those measured with instrumented TKAs. MethodsTwenty-three subjects with a cruciate-retaining TKA underwent motion analysis during stair ascending, stair descending, chair sitting, and chair rising activities after informed consent in this institutional review board approved study. Axial TKA forces were calculated using a previously validated computational model. Differences between the mean and variability of population instrumented TKA peak forces and force impulses were tested using t tests and Levene test. ResultsPeak axial forces were 3.06, 2.74, 2.65, and 2.60 kN for stair ascent, stair descent, chair rising, and chair sitting, respectively. Force impulses were 123.3, 123.4, 153.5, and 154.0 kN*% activity cycle for stair ascent, stair descent, chair sitting, and chair rising, respectively. Population TKA and instrumented TKA peak forces were different for stair ascent (P = .03) and stair descent (P = .03) in the second half of the activity cycles. The variability of the peak forces and impulses were not different (P = .106 to P = .99). ConclusionThe forces and motions presented in this study represent cruciate-retaining TKA patients and could be used for displacement-driven knee wear testing. The forces are similar to those in the literature from instrumented prostheses of an ultracongruent cruciate-sacrificing TKA.

Design and analysis of a novel parallel mechanism for prosthetic knee wear test simulators

Using a parallel mechanism in the wear test simulators of prosthetic knees provides an easy access to the femoral and tibial components during tests. Moreover, because of its higher accuracy and load capacity, in comparison with serial mechanisms, it allows simulation of heavier tasks, such as climbing stairs, to be performed. This paper describes a new 3 Degree of freedom (DoF) spatial 2T1R (T: Translational DoF; R: Rotational DoF) parallel mechanism, for reproducing the force and motion of the tibial component of the knee prostheses in a wear test simulator. Kinematics and dynamics analysis of the mechanism indicated that it can satisfy the required DoFs, workspace and load capacity prescribed in the standard. Also, evaluation of the performance of the mechanism using the simMechanics toolbox of MATLAB revealed that it can reproduce the prescribed forces/torques with relatively high accuracy. However, the mechanism’s force outcomes were found to be affected by inaccuracies in mounting the prosthesis components, although less than the previous serial simulators. In comparison of the different possible architectures, a lateral leg angulation of 45 degrees provided the best efficacy, considering the size of actuators, the overall size of the mechanism and the accessibility of the test sample.

PEEK and CFR-PEEK as alternative bearing materials to UHMWPE in a fixed bearing total knee replacement: An experimental wear study

New bearing materials for total joint replacement have been explored as the need to improve longevity and enhance performance is driven by the changing demands of the patient demographic. Carbon-reinforced PEEK has demonstrated good wear characteristics in experimental wear simulation in both simple geometry pin-on-plate studies and in total hip joint replacement. Carbon reinforced PEEK CFR-PEEK has the potential to reduce tibial insert thickness and preserve bone in the knee.This study investigated the wear performance of PEEK and CFR-PEEK in a low conformity total knee replacement configuration. Custom-made flat inserts were tested against cobalt-chromium femoral bearings in a knee wear simulation for a period of three million cycles. Wear was assessed gravimetrically at intervals throughout the study.The wear rates of both PEEK and CFR-PEEK were very high and almost two orders of magnitude higher than the wear rate of UHMWPE under comparable conditions. Evidence of mechanical failure of the materials, including surface cracking and delamination was observed in both materials.This study highlights that these materials may not be suitable alternatives for UHMWPE in low-conformity designs.

Vitamin E stabilised polyethylene for total knee arthroplasty evaluated under highly demanding activities wear simulation

As total knee arthroplasty patients are getting more active, heavier and younger and structural material fatigue and delamination of polyethylene tibial inserts becomes more likely in the second decade of good clinical performance, it appears desirable to establish advanced pre-clinical test methods better characterizing the longterm clinical material behaviour. Various studies reported in literature attempted to artificially create delamination during in vitro knee wear simulation. We combined artificial ageing to clinically observed oxidation of gamma inert and vitamin E stabilised polyethylene inserts and highly demanding patient activities knee wear simulation based on in vivo load data. With this new method we were able to create clinically relevant subsurface delamination and structural material fatigue on standard polyethylene inserts in vitro.

Stair climbing kinematics measured in the gait lab as input for total knee wear testing

Stair climbing kinematics measured in the gait lab as input for total knee wear testing

Effect of CoCr Counterface Roughness on the Wear of UHMWPE in the Noncyclic RandomPOD Simulation

With the random motion and load pin-on-disk (RandomPOD) wear test system, conventional and highly crosslinked ultrahigh molecular weight polyethylenes (UHMWPE) were run against CoCr counterfaces with different surface roughnesses. The unique 16-station, computer-controlled pin-on-disk device produced noncyclic motion and load. With appropriate specimen shapes, simulations of wear mechanisms of both hip and knee prostheses were performed. Against polished counterfaces, the crosslinked UHMWPE showed negligible wear. Its wear against severely roughened counterfaces was close to that of conventional UHMWPE against polished counterfaces. The reduction in wear with crosslinked versus conventional UHMWPE was 80–86% in the hip and 87–96% in the knee wear simulation. The wear particles were of clinically relevant size and shape which indicated realistic wear mechanisms.

The influence of lubricant temperature on the wear of a metal-free knee implant

Event Abstract Back to Event The influence of lubricant temperature on the wear of a metal-free knee implant Raelene Cowie1*, Adam Briscoe2*, John Fisher1 and Louise M. Jennings1* 1 University of Leeds, Institute of Medical and Biological Engineering, United Kingdom 2 Invibio Biomaterials Solutions, United Kingdom Introduction: PEEK Optima® has been considered for use as an alternative arthroplasty bearing material due to its biocompatibility and favourable material properties[1]. In this study, the wear of all polymer tibial components was assessed articulating against PEEK Optima femoral components under different environmental conditions; experimental wear simulation of conventional metal-on-polyethylene implant materials was carried out in parallel. Materials and Methods: Six size C cruciate retaining PEEK Optima femoral components (Invibio Biomaterials Solutions, UK) and six cobalt chrome femorals of similar initial surface topography and geometry were tested against GUR 1020 all polyethylene tibial components (conventional, EO sterilised) in a 6 station ProSim Knee simulator (Simulation Solutions, UK). N=3 of each material type was tested under room temperature conditions as per standard practice at Leeds and n=3 under elevated temperature conditions where the bulk lubricant temperature was maintained at approximately 33°C by incorporation of a heater system into the simulator. The lubricant used was 25% serum supplemented with 0.03% sodium azide and the simulation was run under Leeds high kinematics[2]. 5 million cycles (MC) of wear simulation was carried out under room temperature and 9MC under elevated temperature. The wear of the UHMWPE tibials was assessed by their loss in mass by gravimetric analysis with unloaded soak controls used to compensate for moisture uptake. Statistical analysis was carried out using ANOVA with significance taken at p<0.05 Results: Figure 1 shows the wear of the UHMWPE tibials under the different test conditions. At room temperature, the wear of the UHMWPE tibial was similar against the different femoral materials (p>0.05). Under elevated temperature conditions, the wear rate of UHMWPE was lower than at room temperature for both femoral materials. A high density of light scratches was observed on all the PEEK Optima implants but this did not influence the wear rate which remained constant over the duration of the study. Discussion: The study shows a similar rate of wear of UHMWPE tibials against PEEK Optima and cobalt chrome femorals of similar initial geometry and surface topography independent of the temperature of the bulk lubricant. Testing at an elevated temperature reduced the wear of the UHMWPE against both femoral materials, which may be attributed to deposition of protein from the lubricant onto the articulating surfaces artificially protecting the implants. Conclusion: This study shows that PEEK Optima has the potential for use as a metal-free knee and that testing under elevated lubricant temperature conditions may have introduced a test artefact which would not occur under room temperature conditions or in vivo. Invibio Biomaterials Solutions Ltd. provided funding and materials for this study.

Wear simulation of total knee prostheses using load and kinematics waveforms from stair climbing

Knee wear simulators are meant to perform load cycles on knee implants under physiological conditions, matching exactly, if possible, those experienced at the replaced joint during daily living activities. Unfortunately, only conditions of low demanding level walking, specified in ISO-14243, are used conventionally during such tests. A recent study has provided a consistent knee kinematic and load data-set measured during stair climbing in patients implanted with a specific modern total knee prosthesis design. In the present study, wear simulation tests were performed for the first time using this data-set on the same prosthesis design. It was hypothesised that more demanding tasks would result in wear rates that differ from those observed in retrievals.Four prostheses for total knee arthroplasty were tested using a displacement-controlled knee wear simulator for two million cycles at 1.1Hz, under kinematics and load conditions typical of stair climbing. After simulation, the corresponding damage scars on the bearings were qualified and compared with equivalent explanted prostheses.An average mass loss of 20.2±1.5mg was found. Scanning digital microscopy revealed similar features, though the explant had a greater variety of damage modes, including a high prevalence of adhesive wear damage and burnishing in the overall articulating surface.This study confirmed that the results from wear simulation machines are strongly affected by kinematics and loads applied during simulations. Based on the present results for the full understanding of the current clinical failure of knee implants, a more comprehensive series of conditions are necessary for equivalent simulations in vitro.

Increased antibiotic release and equivalent biomechanics of a spacer cement without hard radio contrast agents

We compared a novel calcium carbonate spacer cement (Copal® spacem) to well-established bone cements. Electron microscopic structure and elution properties of the antibiotics ofloxacin, vancomycin, clindamycin, and gentamicin were examined. A knee wear simulator model for articulating cement spacers was established. Mechanical tests for bending strength, flexural modulus, and compressive and fatigue strength were performed. The electron microscopic analysis showed a microporous structure of the spacer cement, and this promoted a significantly higher and longer antibiotic elution. All spacer cement specimens released the antibiotics for a period of up to 50days with the exception of the vancomycin loading. The spacer cement showed significantly less wear scars and fulfilled the ISO 5833 requirements. The newly developed spacer cement is a hydrophilic antibiotic carrier with an increased release. Cement without hard radio contrast agents can improve tribological behaviour of spacers, and this may reduce reactive wear particles and abrasive bone defects.

Prediction of Wear in Crosslinked Polyethylene Unicompartmental Knee Arthroplasty

Wear-related complications remain a major issue after unicompartmental arthroplasty. We used a computational model to predict knee wear generated in vitro under diverse conditions. Inverse finite element analysis of 2 different total knee arthroplasty designs was used to determine wear factors of standard and highly crosslinked polyethylene by matching predicted wear rates to measured wear rates. The computed wear factor was used to predict wear in unicompartmental components. The articular surface design and kinematic conditions of the unicompartmental and tricompartmental designs were different. Predicted wear rate (1.77 mg/million cycles) was very close to experimental wear rate (1.84 mg/million cycles) after testing in an AMTI knee wear simulator. Finite element analysis can predict experimental wear and may reduce the cost and time of preclinical testing.

Wear Testing of Moderate Activities of Daily Living Using In Vivo Measured Knee Joint Loading

Resumption of daily living activities is a basic expectation for patients provided with total knee replacements. However, there is a lack of knowledge regarding the impact of different activities on the wear performance. In this study the wear performance under application of different daily activities has been analyzed. In vivo load data for walking, walking downstairs/upstairs, sitting down/standing up, and cycling (50 W & 120 W) has been standardized for wear testing. Wear testing of each activity was carried out on a knee wear simulator. Additionally, ISO walking was tested for reasons of comparison. Wear was assessed gravimetrically and wear particles were analyzed. In vivo walking produced the highest overall wear rates, which were determined to be three times higher than ISO walking. Moderate wear rates were determined for walking upstairs and downstairs. Low wear rates were determined for standing up/sitting down and cycling at power levels of 50 W and 120 W. The largest wear particles were observed for cycling. Walking based on in vivo data has been shown to be the most wear-relevant activity. Highly demanding activities (stair climbing) produced considerably less wear. Taking into account the expected number of loads, low-impact activities like cycling may have a greater impact on articular wear than highly demanding activities.

Analysis of UHMWPE wear particles produced in the simulation of hip and knee wear mechanisms with the RandomPOD system

If the wear rate of ultrahigh molecular weight polyethylene (UHMWPE) components of prosthetic joints is high, the microscopic UHMWPE wear particles that are produced in large numbers are known to cause osteolysis. This may lead to the loosening of fixation of the implant. Conventional UHMWPE GUR 1020 wear particles produced with the novel RandomPOD wear test system were analyzed by scanning electron microscopy. Worn UHMWPE surfaces were analyzed as well. The wear tests included the simulation of both hip (flat-on-flat) and knee (ball-on-flat) wear mechanisms against polished CoCr in serum. The same non-cyclic motion and load input were used in both cases. The diameter of the hip wear particles was 0.30μm±0.15μm. The knee wear particles were on the average five-fold larger, 1.5μm±0.9μm in diameter. The principal wear mechanism was moderate adhesive wear, which was macroscopically manifested as burnishing. The sizes of the particles and the burnishing were in agreement with clinical findings. The RandomPOD was shown to be the first pin-on-disc wear test device to meet these principal validation criteria regarding simulation of wear mechanisms for both the prosthetic hip and the prosthetic knee.

Increase in the tibial slope reduces wear after medial unicompartmental fixed-bearing arthroplasty of the knee.

Introduction. Unicompartmental arthroplasty of the knee in patients with isolated medial osteoarthritis gives good results, but survival is inferior to that of total knee prosthesis. Knees may fail because positioning of the prosthesis has been suboptimal. The aim of this study was to investigate the influence of the tibial slope on the rate of wear of a medial fixed-bearing unicompartmental knee arthroplasty. Materials and Methods. We simulated wear on a medial fixed-bearing unicompartmental knee prosthesis (Univation) in vitro with a customised, four-station, and servohydraulic knee wear simulator, which exactly reproduced the walking cycle (International Organisation for Standardisation (ISO) 14243-1:2002(E)). The medial prostheses were inserted with 3 different posterior tibial slopes: 0°, 4°, and 8° (n = 3 in each group). Results. The wear rate decreased significantly between 0° and 4° slope from 10.4 (SD 0.62) mg/million cycles to 3.22 (SD 1.71) mg/million cycles. Increasing the tibial slope to 8° did not significantly change the wear rate. Discussion. As an increase in the tibial slope reduced the wear rate in a fixed-bearing prosthesis, a higher tibial slope should be recommended. However, other factors that are influenced by the tibial slope (e.g., the tension of the ligament) must also be considered.

PMMA third-body wear after unicondylar knee arthroplasty decuples the UHMWPE wear particle generation in vitro.

Introduction. Overlooked polymethylmethacrylate after unicondylar knee arthroplasty can be a potential problem, since this might influence the generated wear particle size and morphology. The aim of this study was the analysis of polyethylene wear in a knee wear simulator for changes in size, morphology, and particle number after the addition of third-bodies. Material and Methods. Fixed bearing unicondylar knee prostheses (UKA) were tested in a knee simulator for 5.0 million cycles. Following bone particles were added for 1.5 million cycles, followed by 1.5 million cycles with PMMA particles. A particle analysis by scanning electron microscopy of the lubricant after the cycles was performed. Size and morphology of the generated wear were characterized. Further, the number of particles per 1 million cycles was calculated for each group. Results. The particles of all groups were similar in size and shape. The number of particles in the PMMA group showed 10-fold higher values than in the bone and control group (PMMA: 10.251 × 1012; bone: 1.145 × 1012; control: 1.804 × 1012). Conclusion. The addition of bone or PMMA particles in terms of a third-body wear results in no change of particle size and morphology. PMMA third-bodies generated tenfold elevated particle numbers. This could favor an early aseptic loosening.

Variability of TKR knee kinematics and relationship with gait kinetics: implications for total knee wear.

Several factors, including compressive load and knee kinematics, have been shown to influence wear. External knee moments (a surrogate for load) have recently been correlated with the medial and lateral wear scar areas of an unconstrained, PCL retaining knee design. Therefore, the purpose of this study was to determine whether differences in kinetics during level walking are accompanied by specific differences in relative knee kinematics. Thirty TKR patients were gait tested using the point cluster technique to obtain 3D motions of the knee. External knee moments were calculated from ground reaction forces recorded with a multicomponent force plate. The subjects were separated into two distinct anteroposterior (AP) motion categories: a low motion group and a high motion group. Similarly, the low and high motion groups for internal-external (IE) rotation were also identified. For the IE motion, there was no significant difference between the transverse internal rotation moments between the two IE motion groups. However for the AP motion groups, a higher external peak flexion moment was found for the group displaying less AP motion. These observations suggest that subjects with higher joint moments execute smaller ranges of AP motion and thus are likely to incur less wear.

Synovial fluid replication in knee wear testing: an investigation of the fluid volume.

Wear testing cannot replicate the variations in wear rates and wear mechanisms seen in vivo, which may be related to differences between in vivo and in vitro conditions. A considerable difference exists between the in vivo synovial fluid volume (few milliliter) and the in vitro substituted bovine serum volume (several hundred milliliter). The aim of this study was to analyze the effects of a reduced fluid volume on the wear behavior in a knee wear simulator study. Four wear tests with decreasing fluid volumes (250, 150, 75, and 45 ml) were carried out. Using a large fluid volume of 250 ml for wear testing resulted in a wear rate of 9.7±1.2 mm3/10(6) cycles. Decreasing the fluid volume consecutively reduced the wear rate to down to 8.8±1.4 mm3/10(6) for 150 ml (p=1.00), 5.6±1.2 mm3/10(6) for 75 ml (p=0.01), and 1.0±0.2 mm3/10(6) cycles for 45 ml fluid volume (p≤0.01). Additionally, higher serum degradation and larger wear particles were observed with smaller fluid volumes used for testing. This study demonstrates the high relevance of the protein-based lubricant on the wear behavior and the technical limitation to replicate the synovial fluid in simulator tests. Wear testing should be carried out using larger fluid volumes (e.g., 250 ml) to generate physiological relevant wear masses.

A new protocol from real joint motion data for wear simulation in total knee arthroplasty: Stair climbing

In its normal lifespan, a knee prosthesis must bear highly demanding loading conditions, going beyond the sole activity of level walking required by ISO standard 14243. We have developed a protocol for in vitro wear simulation of stair climbing on a displacement controlled knee simulator. The flexion/extension angle, intra/extra rotation angle, and antero/posterior translation were obtained in patients by three-dimensional video-fluoroscopy. Axial load data were collected by gait analysis. Kinematics and load data revealed a good consistence across patients, in spite of the different prosthesis size. The protocol was then implemented and tested on a displacement controlled knee wear simulator, showing an accurate reproduction of stair climbing waveforms with a relative error lower than 5%.

The effects of contact area and applied load on the morphology of in vitro worn ultra‐high molecular weight knee prostheses: a micro‐Raman and gravimetric study

Ultra‐high molecular weight polyethylene knee prostheses of two different sizes (i.e. characterised by different contact areas) were run on a knee wear simulator in two tests differing for the applied load. Gravimetric and micro‐Raman spectroscopic analyses were performed to investigate at macroscopic and molecular levels the effects of contact area and applied load on the morphology of the knee components. The larger prostheses showed a higher mass loss in both tests, while Raman spectroscopy showed a more significant polymer degradation in the smaller prostheses, suggesting that a lower contact area is responsible for a higher wear at the molecular wear, but for a lower gravimetric wear. Raman measurements showed that in the second test (using a higher applied load), the wear mechanism changed with respect to the first test: the less severe conditions used in the latter resulted in an increase in the orthorhombic content, while the more severe conditions used in the former determined an increase and rearrangement of the amorphous phase. The results here reported allow to gain more insights into the effects of contact area, applied load and contact stress on wear. Preliminary micro‐Raman analyses on in vivo worn knee prostheses confirmed the results obtained in vitro: the retrievals of smaller size were found more degraded at molecular level than those of larger size. These findings showed that the conditions used in the in vitro tests well simulate the knee kinematics of the knee joint and thus the in vivo wear. Copyright © 2014 John Wiley &amp; Sons, Ltd.

In vitro wear simulation on the RandomPOD wear testing system as a screening method for bearing materials intended for total knee arthroplasty

The 16-station RandomPOD wear test system, previously validated for prosthetic hip wear, was used in the simulation of knee wear mechanisms with a ball-on-flat test configuration. This consisted of a CoCr pin with a ground and polished spherical bearing surface (radius 28mm) against a conventional, gamma-sterilized UHMWPE disk in serum lubrication. The biaxial motion, consisting of x and y translations, and the load was non-cyclic. Relative to the disk, the center of contact wandered within a circle of 10mm diameter, and the average sliding velocity was 15.5mm/s (ranging from 0 to 31mm/s). The load varied non-cyclically between 0 and 142N (average 73N). In the 60-day test with 16 similar wear couples, moderate adhesive wear, the principal wear mechanism of a well-functioning prosthetic knee, dominated. This showed as a burnished, circular wear mark (diameter 13.2mm, area 137mm2). The wear factor was 2.04±0.03×10−6mm3/Nm (mean±95 percent confidence limit). For the first time a truly multidirectional, realistic and uniform, large capacity pin-on-disk simulation of knee wear mechanisms was implemented.