Background

Polyethylene (PE) wear is known as a limiting factor for total knee replacements (TKR). Thus, preclinical wear testing is an important tool to assess the suitability of new designs and new materials. However, standardized testing (e.g. according to ISO 14243) does not cover the individual situation in the patient. Consequentially, this study investigates the following two parameters:a)

Testing-Frequency: Patients with TKR's show a humiliated walking frequency (down to 0,5Hz) compared to standardized testing (1Hz±0.1). In the first part of this study, the influence of a decreased test frequency on the PE wear behavior is investigated

Background. Wear and fatigue damage to polyethylene components remain major factors leading to complications after total knee and unicompartmental arthroplasty. A number of wear simulations have been reported using mechanical test equipment as well as computer models. Computational models of knee wear have generally not replicated experimental wear under diverse conditions. This is partly because of the complexity of quantifying the effect of cross-shear at the articular interface and partly because the results of pin-on-disk experiments cannot be extrapolated to total knee arthroplasty wear. Our premise is that diverse experimental knee wear simulation studies are needed to generate validated computational models. We combined five experimental wear simulation studies to develop and validate a finite-element model that accurately predicted polyethylene wear in high and low crosslinked polyethylene, mobile and fixed bearing, and unicompartmental (UKA) and tricompartmental knee arthroplasty (TKA). Methods. Low crosslinked polyethylene (PE). A finite element analysis (FEA) of two different experimental wear simulations involving TKA components of low crosslinked polyethylene inserts, with two different loading patterns and knee kinematics conducted in an AMTI knee wear simulator: a low intensity and a high intensity. Wear coefficients incorporating contact pressure, sliding distance, and cross-shear were generated by inverse FEA using the experimentally measured volume of wear loss as the target outcome measure. The FE models and wear coefficients were validated by predicting wear in a mobile bearing UKA design. Highly crosslinked polyethylene (XLPE). Two FEA models were constructed involving TKA and UKA XLPE inserts with different loading patterns and knee kinematics conducted in an AMTI knee wear simulator. Wear coefficients were generated by inverse FEA. Results. Predicted wear rates were within 5% of experimental wear rates during validation tests. Unicompartmental mobile bearing back-side wear accounted for 46% of the total wear in the mobile bearing. Wear during the swing phase was 38% to 44% of total wear. Discussion & Conclusions. Crosslinking polyethylene primarily decreased (by nearly 10-fold) the wear generated by cross-shear. This result can be explained by the reduced propensity of crosslinked polyethylene molecules to orient in the dominant direction of sliding. A highly crosslinked fixed-bearing polyethylene insert can provide high wear performance without the increased risk for mobile bearing dislocation. Finite element analysis can be a robust and efficient method for predicting experimental wear. The value of this model is in rapidly conducting screening studies for design development, assessing the effect of varying patient activity, and assessing newer biomaterials. This FEA model was experimentally validated but requires clinical validation

Introduction. Wear of polyethylene tibial inserts has been cited as being responsible for up to 25% of revision surgeries, imposing a very significant cost burden on the health care system and increasing patient risk. Accurate measurement of material loss from retrieved knee bearings presents difficult challenges because gravimetric methods are not useful with retrievals and unworn reference dimensions are often unavailable. Geometry and the local anatomy restrict in vivo radiographic wear analysis, and no large-scale analyses have illuminated long-term comparative wear rates and their dependence on design and patient factors. Our study of a large retrieval archive of knee inserts indicates that abrasive/adhesive wear of polyethylene inserts, both on the articular surface and on the backside of modular knees is an important contributor to wear, generation of debris and integrity of locking geometry. The objective of the current study is to quantify wear performance of tibial inserts in a large archive of retrieved knees of different designs. By assessing wear in a large and diverse series, the goal is to discern the effect on wear performance of a number of different factors: patient factors that might help guide treatment, knee design factors and bearing material factors that may inform a surgeon's choice from among the array of arthroplasty device options. Methods. An IRB approved retrieval database was queried for TKA designs implanted between 1997 and 2017. 1385 devices from 5 TKA designs were evaluated. Damage was ranked according to Hood's method, oxidation was determined through FTIR, and wear was determined through direct measurement of retrieved inserts using a previously established protocol. Design features (e.g. materials, conformity, locking mechanisms, stabilization, etc.) and patient demographics (e.g. age, weight, BMI, etc.) were cataloged. Multivariate analysis was performed to isolate factors contributing to wear, oxidation, and damage. Results. Wear and oxidation were both found to scale with time in vivo in conventional and crosslinked polyethylene. Wear rate was also found to scale with time in vivo, but was not found to be a function of oxidation. Regression shows patient age and female sex to correlate negatively with wear rate. Polished trays, crosslinked polyethylene, and constrained knee designs are all correlated with decreased wear rates. Discussion. While this study indicates that loosening and infection are predominant causes for TKA revision, wear related failure remains common. We believe this to be the largest existing comparative study of modern TKA wear rates. Insert wear is shown to correlate with several patient factors. Wear performance also varies significantly between knee designs, polyethylene material choice and tray surface finish. When compared to a historical standard for knee wear rates, all designs evaluated in the current study exhibited significant improvements in wear rates. Retrieval analysis can provide insight into implant and patient related factors that contribute to knee wear, with the goal of improving patient outcomes and best matching design decisions to patient populations

Introduction. Orthopedic implants are subject to wear and release ultra-high molecular weight polyethylene (UHMWPE) debris. Analysis of UHMWPE wear particles is critical in determining the safety and effectiveness of novel orthopedic implants. Complete digestion of periprosthetic tissue and wear fluid is necessary to ensure accurate morphological and quantitative particle analysis. Acid digestion methods are more effective than enzymatic and base digestion approaches [Baxter+ 2009]. However, optimal digestion times, quantity, and type of acid are unclear for particle isolation. In addition, imaging and analysis techniques are critical to ensure accurate reporting of particle characteristics. Here, we 1) compared the efficacy of three acid-based digestion methods in isolating particles from a) bovine serum and b) animal/human tissue, and 2) analyzed the effects of imaging location on particle quantity/morphology results. Methods. 1a) UHMWPE (GUR 150) particles were generated by Mode I knee wear testing for 1 million cycles in bovine serum. Serum was digested in one of four solutions: 12.2M HCl, 15.8M HNO. 3. , a 1:1 volume ratio of HNO. 3. :HCl (aqua regia), or filtered H. 2. O (control). The serum:solution volume ratio was 1:5 [Niedzwiecki+ 2001, ISO 17853:2011]. Digestion occurred for 60min on a stir plate at 60°C. Each digest was combined with MeOH at a 1:5 digest:MeOH volume ratio and filtered using a 100 nm polycarbonate membrane. The particle-containing membranes were imaged (12 images/membrane) using scanning electron microscopy (SEM) to determine particle characteristics, including quantity, equivalent circular diameter (ECD) and aspect ratio (AR). 1b) Based on 1a, HNO. 3. was used to digest porcine and human tissue at concentrations of 1:40, 1:60, or 1:80 tissue:HNO. 3. volume ratios for either 1, 12, or 24 hours, followed by SEM analysis. 2) Particle characteristics were compared at nine locations (20 images/location) across a particle-containing membrane to determine the effects of imaging location. Results. 1a) HNO. 3. and aqua regia methods successfully digested the bovine serum, whereas the HCl and H. 2. O methods were unsuccessful (Fig.1A). Comparing HNO. 3. and aqua regia groups, particle characteristics and ECD frequency distribution were nearly identical (Fig.1B). 1b) Nitric acid did not fully digest porcine or human tissues. 2) Similar particle characteristics were observed in all nine locations analyzed across the polycarbonate membrane. The particle quantity, ECD, and AR for a representative center vs. intermediate location were 808 vs. 780 particles, 0.33±0.28 vs. 0.35±0.29 µm, and 1.57±0.56 vs. 1.51±0.4, respectively (Fig.2). Conclusions. Nitric acid and aqua regia are capable of digesting bovine serum using low quantities of acid for short duration, allowing precise analysis of UHMWPE particle debris from orthopedic implants. However, further optimization of digestion techniques for animal/human tissue is warranted. In addition, an accurate representation of particle distribution can be achieved without analyzing hundreds of images, because membrane location does not strongly influence particle results. Finally, ASTM F1877-16 – Standard Practice for Characterization of Particles – could benefit from adding software-based automated particle characterization as an optional method. An automated approach that uses k-means clustering image segmentation to identify particles and computer vision tools to extract relevant morphological features is under development and validation

Background. The anatomy of the human knee is very different than the tibiofemoral surface geometry of most modern total knee replacements (TKRs). Many TKRs are designed with simplified articulating surfaces that are mediolaterally symmetrical, resulting in non-natural patterns of motion of the knee joint [1]. Recent orthopaedic trends portray a shift away from basic tibiofemoral geometry towards designs which better replicate natural knee kinematics by adding constraint to the medial condyle and decreasing constraint on the lateral condyle [2]. A recent design concept has paired this theory with the concept of guided kinematic motion throughout the flexion range [3]. The purpose of this study was to validate the kinematic pattern of motion of the surface-guided knee concept through in vitro, mechanical testing. Methods. Prototypes of the surface-guided knee implant were manufactured using cobalt chromium alloy (femoral component) and ultra-high molecular weight polyethylene (tibial component). The prototypes were installed in a force-controlled knee wear simulator (AMTI, Watertown, MA) to assess kinematic behavior of the tibiofemoral articulation (Figure 1). Axial joint load and knee flexion experienced during lunging and squatting exercises were extracted from literature and used as the primary inputs for the test. Anteroposterior and internal-external rotation of the implant components were left unconstrained so as to be passively driven by the tibiofemoral surface geometry. One hundred cycles of each exercise were performed on the simulator at 0.33 Hz using diluted bovine calf serum as the articular surface lubricant. Component motion and reaction force outputs were collected from the knee simulator and compared against the kinematic targets of the design in order to validate the surface-guided knee concept. Results. Under deep flexion conditions of up to 140° of squatting the surface-guided knee implants were found to undergo a maximum of 22.2° of tibial internal rotation and 20.4 mm of posterior rollback on the lateral condyle. Pivoting of the knee joint was centered about the highly congruent medial condyle which experienced only 1.6 mm of posterior rollback. Experimental results were within 2° (internal-external rotation) and 1 mm (anteroposterior translation) agreement with the design target throughout the applied exercises (Figure 2). Conclusion. The results of this test confirm that by combining a constrained medial condyle with guiding geometry on the lateral condyle, deep knee flexion activities of up to 140° can be performed while maintaining near-natural kinematics of the knee joint. The authors believe that the tested surface-guided implant concept is a significant step toward the development of novel TKR which allows a greater range of motion and could improve the quality of life for active patients undergoing knee replacement. For any figures or tables, please contact the authors directly

Background. Additive manufacturing (AM) has created many new avenues for material and manufacturing innovation. In orthopaedics, metal additive manufacturing is now widely used for production of joint replacements, spinal fusion devices, and cranial maxillofacial reconstruction. Plastic additive manufacturing on the other hand, has mostly been utilized for pre-surgical planning models and surgical cutting guides. The addition of pharmaceuticals to additively manufactured plastics is novel, particularly when done at the raw material level. The purpose of this study was to prove the concept of antibiotic elution from additively manufactured polymeric articles and demonstrate feasibility of application in orthopaedics. Methods. Using patented processes, three heat-stable antibiotics commonly used in orthopaedics were combined with six biocompatible polymers (2 bioresorbable) into filament and powder base materials for fused deposition modeling (FDM) and selective laser sintering (SLS) AM processes. Raw materials of 1%, 2%, and 5% antibiotic concentrations (by mass) were produced as well as a blend of all three antibiotics each at 1% concentration. Thin disks of 25 mm diameter were manufactured of each polymer with each antibiotic at all concentrations. Disks were applied to the center of circular petri dishes inoculated with a bacterium as per a standard zone of inhibition, or Kirby-Bauer disk diffusion tests. After 72 hours incubation, the zone of inhibited bacterial growth was measured. Periprosthetic joint infection (PJI) of the knee was selected as the proof-of-concept application in orthopaedics. A series of tibial inserts mimicking those of a common TKR system were manufactured via SLS using a bioresorbable base material (Figure 1). Three prototype inserts were tested on a knee wear simulator for 333,000 cycles following ISO 14242–1:2014 to approximate 2–4 months of in vivo use between surgeries of a 2-stage procedure for PJI. Gravimetric measurement and visual damage assessment was performed. Results. Bacterial growth was inhibited to a mean diameter of 32.3 mm (FDM) and 42.2 mm (SLS) for nearly all combinations of polymers and concentrations of antibiotics. Prototype tibial inserts experienced an average of 200 mg of wear during testing and demonstrated no evidence of cracking, delamination or significant deformation (Figure 2). Conclusion. Bench-level testing of these novel antibiotic-eluting polymers demonstrates feasibility for their application in orthopaedic medicine. In particular, treatment of stubborn PJI with potential for increased and sustained antibiotic elution, patient-specific cocktailing, and maintenance of knee joint structure and function compared to existing PJI products and practices. Subsequent testing for these novel polymers will determine static and dynamic (wear-induced) antibiotic elution rates. For any figures or tables, please contact the authors directly

Introduction. Ideally, a patient receiving a unicondylar knee replacement will have fully functional anterior and posterior cruciate ligaments. When at least one of the cruciate ligaments is not fully functional, femoral and tibial implant contact position can potentially increase along the anterior-posterior (AP) axis. Where unicondylar implant wear testing typically uses AP resistance assuming fully functional cruciate ligaments, the authors used reduced AP resistance intended to simulate deficient cruciate ligaments. Methods. Optetrak Logic® Uni (Exactech Inc, Gainesville, FL USA) unicondylar test specimens featuring an all-UHMWPE tibial component and a cobalt chromium femoral component were used in this study. The system has a semi-constrained articular geometry. Testing was conducted at an independent testing facility (EndoLab GMBH, Thansau, Rosenheim, Germany). A four-station knee simulator was used (EndoLab knee simulator) with two unicondylar knee implants per station, giving a total of eight test specimens. Two different tibial fixation designs (keeled and peg) with identical articulating surfaces were tested. Tibial test specimens were 6 mm in thickness. Unloaded soak controls were stored in distilled water at 37°C. The test was conducted according to ISO 14243–1: 2009 [1]. Test specimens were immersed in calf serum (PAA GmBH, Cölbe, LOT B00111-5126) with a protein content of 20 g/l. Custom polyurethane molds allowed for individual component measurement. Per the ISO 14243-1, a 7% medial offset was incorporated into the set-up. The unicondylar knee implants were set at neutral position in extension. Tibial rotational restraint was 0.36 Nm/° and zero when the test specimen was within ± 6° of the reference position. This test was conducted with an AP resistance of 9.3N/mm to maximize AP displacement and simulate deficient cruciate ligaments. Typical unicondylar knee wear testing is conducted with an AP resistance of 44N/mm, which assumes functional cruciate ligaments. Results. Wear data was separated by component design (keeled and peg) as well as for medial and lateral placement [Table 1]. There was no significant difference between lateral components but there was for medial components. This difference could be due to the small sample size. Contact area of the UHMWPE tibial components was elliptical, with the longer portion along the AP axis. Mean wear rates were comparable to historical unicondylar knee systems tested at the same laboratory using the standard AP resistance (i.e., 44 N/mm). Discussion/Conclusion. This study demonstrated using an AP resistance 9.3 N/mm to simulate the presence of deficient cruciate ligaments in a unicondylar knee wear test produced similar wear rates and greater AP displacement when compared to testing using an AP resistance of 44 N/mm, which assumes functioning ligaments. This being said, design and material information about historical unicondylar knee systems tested are not known, so a direct comparison cannot be made. Performing unicondylar knee wear tests with reduced AP resistance could provide realistic wear information for devices implanted in patients without fully functioning cruciate ligaments

INTRODUCTION. Highly cross-linked polyethylene (XLPE) inserts have shown significant improvements in decreasing wear and osteolysis in total hip arthroplasty [1]. In contrast to that, XLPE has not shown to reduce wear or aseptic loosening in total knee arthroplasty [2,3,4]. One major limitation is that current wear testing in vitro is mainly focused on abrasive-adhesive wear due to level walking test conditions and does not reflect “delamination” as an essential clinical failure mode [5,6]. The objective of our study was to use a highly demanding daily activities wear simulation to evaluate the delamination risk of polyethylene materials with and without vitamin E stabilisation. MATERIALS & METHODS. A cruciate retaining fixed bearing TKA design (Columbus® CR) with artificially aged polyethylene knee bearings (irradiation 30 & 50 kGy) blended with and without 0.1% vitamin E was used under medio-lateral load distribution and soft tissue restrain simulation. Daily patient activities measured by Bergmann et al. [7] in vivo, were applied for 5 million knee wear cycles in a combination of 40% stairs up, 40 % stairs down, 10% level walking, 8% chair raising and 2% deep squatting with up to 100° flexion [8] (Fig. 1). The specimens were evaluated for gravimetric wear and analysed for abrasive-adhesive and delamination wear modes. RESULTS. The total amount of gliding surface wear was 28.7±1.9 mg for the vitamin E stabilised polyethylene irradiated with 30 kGy and 26.5±5.7 mg with 50 kGy irradiation, compared to 355.9±119.8 mg for the standard material. The combination of artificial ageing and high demanding knee wear simulation leads to visible signs of delamination in the articulating standard polyethylene bearing areas in vitro. Delamination began after 2 million test cycles for the standard polyethylene, indicated by the transition between linear and exponential slope in Fig. 2. Delamination was not found in the Vitamin E blended gliding surfaces. CONCLUSION. To evaluate moderately or highly cross-linked polyethylenes in regard to ageing and wear behaviour in vitro, conditions are simulated to create clinical relevant failure modes given in total knee arthroplasty. With the applied test protocol it is possible to discriminate between the polyethylene bearing materials with and without Vitamin E stabilisation. To view tables/figures, please contact authors directly

Generic walking profiles applied to mechanical knee simulators are the gold standard in wear testing of total knee replacements (TKRs). Recently, there was a change in the international standard (ISO) for knee wear testing (ISO 14243-3): the direction of motion in the anterior/posterior (AP) and internal/external (IE) directions were reversed. The effects of this change have not been investigated, therefore it is not known whether results generated by following this new standard can be compared to historical wear tests which used the old standard. Using a finite element analysis (FEA) model of a TKR in parallel with an energy based wear model and adaptive remeshing, we investigated differences in wear between the newest ISO standard developed in 2014, and the previous ISO standard developed in 2004. CAD models of a left sided NexGen Cruciate Retaining (CR) TKR (Zimmer, Warsaw, IN) were used to create the FEA model (Figure 1). The loads and motions specified by simulator standards ISO 14243-3(2004) and ISO 14243-3(2014) were applied to the model. Analyses were run using ABAQUS v6.13-2 Standard (Dassault Systèmes, Waltham, MA). 8 node hexahedral elements were used to model the UHMWPE component. The contact was modeled as penalty contact, with the friction coefficient set to 0.04 on the articular surface. The cobalt chromium molybdenum femoral component was modeled as a rigid surface, utilizing a mix of 2. nd. order quadrilaterals and tetrahedrons. Wear of the polyethylene (PE) component was predicted to 1,000,000 cycles using a previously published frictional energy-based wear model. The wear model, developed from data generated in wheel-on-flat tests, utilizes two parameters defining the frictional energy required to remove a unit volume of material both parallel (3.86E8 J/mm. 3. ) and perpendicular (3.55E7 J/mm. 3. ) to the primary polyethylene fibril direction. Primary fibril direction for the analysis was set to the AP direction. Wear for each simulation of a gait cycle was scaled to 500,000 cycles. Two gait cycles were simulated representing 1,000,000 cycles in total. Adaptive remeshing was driven by the wear model, with the mesh being updated every time increment to simulate material ablation. The time step size was variable with a maximum of 0.01s. The FEA predicted higher wear rates for the newest ISO standard (7.34mg/million cycles) compared to the previous standard (6.04mg/million cycles) (Figure 2). Comparing the predicted wear scars (Figure 3), the new version of the standard covered a larger percentage of the total articular surface, with wear being more spread out as opposed to localized. This is more similar to what is seen in patient retrievals. The results of the study suggest that major differences between the old and the new ISO standard exist and therefore historical wear results are not comparable to newly obtained results. In addition, this study demonstrates the utility of FEA in wear analysis, though the wear model needs further work and validation before it can be used as a supplement to simulator testing. Validation of the wear model against simulator tests and pin-on-disk experiments is currently underway. For figures/tables, please contact authors directly.

Introduction. Total knee arthroplasty (TKA) is a consolidated orthopaedic procedure and success of such operation depends on the prosthetic design [1]. Unfortunately, as there is a good survival rate of primary TKA, failures occur for factors concerning the polyethylene composition of the implants, secondary osteolysis, and ultimately loosening of the implants are the usual causes of failure after normal use [2]. Dynamic in vitro testing of the human knee continues to be an area of interest to the orthopaedic biomechanics community. The scope of this work was to assess pre-clinically the wear behaviour of polyethylene knee insert under a realistic stair climbing activity using a displacement knee simulator. Materials & Methods. Four commercial posterior-stabilized fixed-bearing component prosthesis for TKA were tested in this study (Stryker®-Orthopaedics, Mahwah, NJ-USA). These were new and delivered in sterilized packages. Particularly, corresponding UHMWPE tibial inserts (size #7) were made of conventional surgical grade polyethylene resin (GURâ�¨1020), consolidated by compression moulding (accordingly to ISO 5834/1-2), and EtO sterilized. These were tested in conjunction with corresponding CoCrMo alloy femoral components. For the implementation of realistic loading scenarios during in vitro wear testing for human joint prostheses, an in vitro protocol was designed to simulate the flexion/extension angle, intra/extra rotation angle, and antero/posterior translation. These movements were obtained in patients by three- dimensional video-fluoroscopy. Axial load data were collected by gait analysis [3]. Results. The components run under stair climbing simulation completed the planned two million cycles under bovine calf serum as lubricant. The volumetric wear trend is schematised in Figure 1. The wear patterns observed on the contact surfaces were similar over the three tested inserts. A few pitting phenomena were observed on the insert contact surfaces. In addition, unidirectional scratches were observed in both condyles along the AP direction. Conclusion. The knee wear simulator executed the imposed physiological gait condition, under stair climbing waveforms. This new approach opens the way to more scenarios able to give comprehensive answers to the wear behaviour of knee components. Hence, further development will be the definition of a global protocol, with the implementation of various motor tasks (chair sitting and rising, squatting), using the same approach

Introduction. Total knee replacement (TKR) implant designs and materials have been shown to have a significant impact on tibial insert wear. A medial-pivot (MP) design theoretically should generate less wear due to a large contact area in the medial compartment and lower contact stresses. Synovial fluid aspiration studies have confirmed that a first generation MP TKR system (ADVANCE®, MicroPort Orthopedics Inc., Arlington, TN, USA) generates less wear debris than is seen with other implant designs articulating against conventional polyethylene (CP). Objectives. The objective of this study was to evaluate the in vitro wear rate of a second generation MP TKR system (EVOLUTION® Cruciate-Sacrificing, MicroPort Orthopedics Inc., Arlington, TN, USA) using CP tibial inserts and compare to previously published values for other TKR designs with CP and first or second generation crosslinked polyethylene (XLPE) tibial inserts. Methods. In vitro wear was assessed for five MP CP tibial inserts, each loaded for 5 megacycles (Mc) of simulated gait in accordance with ISO 14243–3. Insert cleaning and wear measurements were performed every 0.5 Mc in accordance with ISO 14243–2. Manufacturer websites and the MEDLINE database were searched for previously published in vitro wear rates for other TKR designs used in combination with CP and first or second generation XLPE inserts. Second generation XLPE inserts are those with additives or additional manufacturing, such as sequentially annealed and irradiated XLPE (X3®, Stryker, Mahwah, NJ, USA) and vitamin E infused polyethylene (E1®, Biomet, Warsaw, IN, USA). All TKR designs utilized cobalt-chrome (CoCr) femoral components, except Legion-Verilast that included Oxinium™ femoral components (Smith & Nephew, Memphis, TN, USA). Results. The mean wear rate for the MP system (2.0+0.2 mg/Mc) was less than half the wear rates reported for other TKR designs using CP inserts (Figure 1). The wear was also reduced or similar to those reported for all but three designs used in combination with XLPE inserts (Figure 2). Interestingly, wear rates for the MP system were approximately one-third of those reported for E1 and X3 used in combination with the Scorpio and Triathlon CR TKR systems (Stryker, Mahwah, NJ, USA). The main limitation to the current study is the use of literature comparators. While the comparison studies were all conducted using similar methods on knee wear simulator machines, there were some experimental differences that could potentially impact wear rates (e.g. diluted vs. non-diluted serum, gait patterns, types of testing machines). Conclusions. In vitro wear for a second generation MP TKR system was similar or lower than what has been previously reported for other TKR systems used with CP or XLPE tibial inserts. These results suggest that implant design may play a larger role in TKR wear debris generation than the material used for the tibial insert

Introduction. There is a demand for longer lasting arthroplasty implants driving the investigation of novel material combinations. PEEK has shown promise as an arthroplasty bearing material, with potentially relatively bio inert wear debris [1]. When coupled with an all-polyethylene tibial component this combination shows potential as a metal-free knee. In this study, the suitability of PEEK Optima® as an alternative to cobalt chrome for the femoral component of total knee replacements was assessed using experimental knee wear simulation under two kinematic conditions. Methods. Three cobalt chrome and three injection moulded PEEK Optima® (Invibio Biomaterial Solutions, UK) femoral components of similar geometry and surface roughness (mean surface roughness (Ra) ∼0.02µm) were coupled with all-polyethylene GUR1020 (conventional, unsterilised) tibial components in a 6 station ProSim knee simulator (Simulation Solutions, UK). 3 million cycles (MC) of wear simulation were carried out under intermediate kinematics (maximum anterior-posterior (AP) displacement 5mm) followed by 3MC under high kinematics (AP 10mm) [2] with 25% serum as the lubricant. The wear of the tibial component was assessed gravimetrically. At each measurement point, the surface roughness of the femoral components was determined using contacting profilometry and throughout testing, the bulk lubricant temperature was monitored close to the articulating surfaces. Statistical analysis was carried out using ANOVA, with significance at p<0.05. Results. Figure 1 shows the wear rate of the all-polyethylene tibial components. After 3MC of intermediate kinematics, the mean wear rate of UHMWPE articulating against cobalt chrome was 1.0±2.3mm3/MC and against PEEK was similar (p=0.06) 2.5±0.8mm3/MC. Scratches were apparent on the surface of the PEEK implant in the AP direction significantly (p<0.05) increasing mean surface roughness of the PEEK components (Table 1) compared to pre-test values. The surface topography of the cobalt chrome components (Table 2) was similar to pre-test measurements. Increasing AP displacement caused no significant increase in the wear of the tibial inserts against either material. Under intermediate kinematics, the mean bulk lubricant temperature was 28.0±0.7°C for cobalt chrome and significantly higher (p<0.001) for PEEK, 29.5±0.1°C; kinematic conditions had no effect on the lubricant temperature. Conclusions. This study showed a similar wear rate of all-polyethylene tibial components against PEEK and cobalt chrome femoral components of similar initial surface topography and geometry. Wear simulation with a higher AP displacement did not increase the wear of the polyethylene, in contrast to other designs of knee replacements, potentially due to the low conforming geometry of the implant [3]. The linear scratching on the surface of the PEEK implants did not increase the wear rate of the tibial components and the surface did not deteriorate further between 3 and 6 MC. A higher mean lubricant temperature was measured with PEEK femoral components, which was attributed to the higher friction of the PEEK-PE bearing couple. However it is not known whether this is clinically relevant or an artefact of the continuous running of the simulator. PEEK Optima® shows promise as the femoral component in a metal-free knee

Introduction. Temporary use of antibiotic-impregnated polymethylmethacrylate (PMMA) bone cement spacers in two-stage revisions is considered to be standard of care for patients with a chronic infection of a joint replacement. Spacers should be wear resistant and load-bearing to avoid prolonged immobilisation of the patient and to reduce morbidity. Most cement spacers contain barium sulphate or zirconium dioxide as radio-opaque substrate. Both are quite hard materials that may negatively influence the wear behaviour of the spacer. Calcium carbonate is another radio-opaque substrate with lower hardness potentially increasing the wear resistance of the spacer materials. The purpose of the study was to compare a prototype PMMA knee spacer (calcium carbonate loaded) with a commercially available spacer (containing barium sulphate) regarding the wear performance and particle release in a knee wear simulator. Material and Methods. Spacer K (TECRES, Italy) was used as barium sulphate (10%) containing spacer material. A prototype material (Heraeaus Medical, Germany) with 15% calcium carbonate was compared. Both were gentamicin impregnated, ready-made for clinical application (preformed) and consist of a tibial and a femoral component. Force-controlled simulation was carried out on an AMTI knee simulator. The test parameters were in accordance to ISO 14243–1 with a 50% reduced axial force (partial weight bearing). Tests were carried out at 37 °C in closed chambers filled with calf serum. Tests were run for 500,000 cycles at a frequency of 1 Hz. For wear analysis, gravimetric wear measurements according to ISO 14243–2 and wear particle analysis according to ASTM F1877–05 were performed. Results. Fig. 1 presents the results of the gravimetric wear measurements. For the Spacer K cement a mean articular wear mass of 375.53±161.22 mg was determined after 500.000 cycles (femoral components: 149.55±17.30 mg, tibial components: 225.98±153.01 mg). The prototype cement showed lower mean total wear of 136.32±37.58 mg (femoral components: 74.32±33.83 mg, tibial components: 61.99±15.74 mg). However, a statistically significant lower wear rate was only seen for the femoral components (p=0,027). In Fig. 2 isolated PMMA wear particles are shown and the morphological characteristics are given in Tab. 1. Discussion and conclusion. The prototype material showed better wear performance in terms of gravimetric wear and particle release. Thus calcium carbonate seems to be a promising material as radio-opaque substrate in PMMA spacers. Nevertheless, the wear amount released from both spacer materials is much higher as compared to conventional total knee replacements with polyethylene inserts. In this context biological reactions against PMMA particles and an increased release of cytokines have been reported in vitro [1] and furthermore, the promotion of osteolysis has been shown in vivo in the presence of PMMA particles [2]. As a clinical consequence we suggest excessive debridement during removal of the cement spacer components to reduce the risk of third body wear for the final joint replacement. Beside the wear performance further studies are essential to prove the mechanical stability and the antibiotic release kinetics for the prototype cement

Introduction. Current pre-clinical testing is performed using knee wear simulators with standardized walking profiles. Differences in generated damage patterns to those observed on retrieved liners have been explained with the absence of activities other than walking, less severe loading conditions, and a discrepancy in the simulator's tibiofemoral contact mechanics and in vivo knee excursion. While it has been recognized that rotational alignment of the knee may also drive the location and shape of wear scars, to the best of our knowledge this parameter has not been investigated in knee simulator studies. Methods. Here, we use patient specific gait as input to the simulation to approximate the patient specific contact mechanics. Kinematic and kinetic input data was obtained from gait analysis of a patient with a MGII (Zimmer Inc.) prosthesis at 11 years post-op using the point cluster technique for tibiofemoral kinematics, and a mathematical model for internal force calculations. Using the identical type of prosthesis on the simulator, wear tests were conducted in displacement mode on a closed-loop controlled station. Because x-rays of the patient suggested an internal rotation of the tibial tray, it was varied form 0–10° and the effect on location and wear scar dimension was assessed. Results were compared with the retrieved liner (obtained after 13 years in vivo). Results. The simulator inputs generated from the gait data were compared with ISO 14243–3 (Figure 1). The first contact force peak of the patient was significantly lower, while second contact force peak similar to ISO. There were minimal differences in the flexion/extension profiles. For stance phase, the anterior/posterior translation and internal/external rotation kinematics did not show similar patterns, but they did fall within similar ranges from zero. There was little similarity for the swing phase. The total wear scar area of the retrieval was measured to be 919.8 mm. 2. The average total wear scar of the tested components was 853.0 ± 59.8 mm. 2. (p= 26.28%) The outcome values of the tested components compared to the retrieval are shown in Figure 2. All offsets produced a smaller wear scar than the retrieval, but the 7° offset produced the closest area which was within 1 mm. 2. of the retrieval. The 7° offset also had the closed centroid offset angle, which was within 0.2° of the retrieval (Figure 3). On the retrieval, a small wear scar was observed on the anterior- medial aspect of the intracondylar eminence (not shown). Among the tested components, the 7° and 10° offsets recreated damage at this location. Discussion. Rotational alignment affected the wear scar size by as much as 15% in this study. Only, the 7° offset produced outcome values very similar to the retrieval, highlighting the importance of rotational mismatch for wear. It should be noted that ± 10° of rotational mismatch is clinically well tolerated [5] and therefore may occur frequently. All tested components had smaller wear scar areas than the retrieved liner. This suggests that other activities other than walking may have contributed to wear in vivo

Introduction:. Microseparation has resulted in more than ten-fold increase in ceramic-on-ceramic and metal-on-metal bearing wear, and even fracture in a zirconia head [1–4]. However, despite the greater microseparation reported clinically for metal-on-polyethylene wear, less is known about its potential detrimental effects for this bearing couple. This study was therefore designed to simulate the effects of micromotion using finite element analysis and to validate computational predictions with experimental wear testing. Methods:. Experimental wear rates for low and highly crosslinked polyethylene hip liners were obtained from a previously reported conventional hip wear simulator study [5]. A finite element model of the wear simulation for this design was constructed to replicate experimental conditions and to compute the wear coefficients that matched the experimental wear rates. We have previous described out this method of validation for knee wear simulation studies [6,7]. This wear coefficient was used to predict wear in a Dual-Mobility hip component (Fig 1). Dual mobility total hip arthroplasty components, Restoration ADM (Fig 1), with highly crosslinked acetabular liners were experimentally tested: the control group was subjected to wear testing using the ISO 14242-1 waveform on a hip wear simulator. The microseparation group was subjected to a nominal 0.8 mm lateral microseparation during the swing phase by engaging lateral force springs and reducing the swing phase vertical force. Results:. The wear coefficients that matched experimental wear rates for the low and highly crosslinked polyethylene liners were 4.57×10. −10. and 5.89×10. −11. mm. 3. N. −1. mm. −1. , respectively. Introducing microseparation in the conventional hip increased the wear rate by 15.59 mm. 3. /million cycles in the low crosslinked liner and by 1.12 mm. 3. /million cycles in the highly crosslinked liner (Fig 2). Discussion:. Microseparation did increase predicted wear rates for the low crosslinked polyethylene liner and supports the hypothesis that microseparation can adversely affect the wear of hip arthroplasty. However, the predicted and experimental increase for the dual mobility highly crosslinked liners due to microseparation was low (3.3 mm. 3. and 2.9 mm. 3. /million cycles, respectively) and below the threshold for clinical relevance. The small increase in wear rate in our study supports the high wear tolerance to wear of a dual-mobility sequentially crosslinked polyethylene liner

A unique, laterally stabilized design concept (3D Knee-DJO Surgical, Inc) for total knee arthroplasty (TKA) without traditional post and cam construct was developed to allow surgeons to resurface the arthritic knee while choosing to maintain or sacrifice the posterior cruciate ligament (PCL). Reported complications with current ‘post and cam’ designs of PCL-substituting TKRs include higher polyethylene wear associated with cam-post impingement, increased bone interface shear stresses, and more distal femoral bone resection making revisions more complex and problematic. The effectiveness of this laterally stabilized TKA design has been extensively studied biomechanically using both in-vitro and in-vivo methods. It was hypothesized that for this total knee arthroplasty design; the mid-term clinical, radiographic and functional results would be the same for patients having two different surgical techniques in which the posterior cruciate ligament was either completely retained or completely resected. This study reports on eight year clinical results as well as in-vivo fluoroscopic results and retrieval data. Reported are 159 patients with 116 knees done by a surgeon who preserved the PCL with a bone block technique and 43 knees by a second surgeon who completely resected the PCL. Clinical results did not statistically differ between the two groups and found Knee Society Scores of 96 for Pain and 91 for Function. Average ROM was measured at 124 degrees. Comparative fluoroscopic imaging analysis of in-vivo dynamic flexion activities of thirty-three (20 PCL-preserved and 13 PCL resected) knees was performed demonstrating stable performance and only small (non-significant) mechanical differences. Analysis of two unrelated groups of tibial polyethylene inserts, the first retrieved from patients after 1–4 years in-vivo function (n = 14) and the second after in-vitro knee wear simulation (n = 4) showed low wear rates with no delamination. There was only one failure for mechanical loosening in the cruciate resected group and radiolucent lines of greater than 2 mm were only seen in 4% with none being progressive. Kaplan-Meier Survivorship, using mechanical loosening as the end point, was 99.2% at an average of 8.8 years. In summary, this laterally stabilized TKR design offers a very good alternative to standard ‘post and cam’ PCL sacrificing TKRs while still giving surgeons the ability to maintain the PCL if desired

It is difficult for surgeons to make the decision on which design or material to use given multiple available options for total knee arthroplasty. Due to the complex interaction of soft tissue, implant position, patient anatomy, and kinematic demands of the patient, the prosthetic design of a knee device has traditionally been more important than materials. The purpose of this study was to examine the overall influence of both implant design and materials on volumetric wear rates in an in vitro knee simulator study for two knee designs. Two different designs (single radius and J-curve) with two highly crosslinked materials (Sequentially crosslinked and annealed PE (X3®, Stryker Orthopaedics, Mahwah, NJ) (7.5 kGy moderately crosslinked UHMWPE (XLPE, Smith and Nephew, Memphis, TN)) were evaluated. The two designs tested were the Triathlon® CR knee system (single radius design)(Stryker Orthopaedics, Mahwah, NJ) and the Legion™ Oxinium® CR knee system (J-curve design) (Verilast™, Smith and Nephew, Memphis, TN). Three inserts per condition were tested in this study. This comparison incorporates the effects of both materials and designs: different femoral component materials, different tibial bearing materials, and implant geometry (J-curve vs. single radius saggital profile). All devices were tested under ISO 14243-3 normal walking using an MTS knee simulator for a total of 5 million cycles. Standard test protocols were used for cleaning, weighing and assessing the wear loss of the tibial inserts (ASTM F2025). Soak control specimens were used to correct for fluid absorption with weight loss data converted to volumetric data (by material density). Statistical analysis was performed using the Student's t-test. Total volume loss results are shown in Figure 1. Test results show a 36% reduction (p<0.05) in volume loss and a 30% reduction (p<0.05) in wear rate for the single radius design compared to the J-curve design, respectively. All comparisons are statistically significant by the t-test method (p<0.05). Visual examination of all worn inserts revealed typical wear scars and features on the condylar surfaces, including burnishing. Results indicate superior wear resistance for the single radius system. This finding indicates that a combination of implant design and prosthesis material plays a significant role in knee wear rates. The in vitro low volumetric wear observed in the single-radius prosthesis could theoretically influence long term survivorship in vivo, and supports the potential for improved durability and long term wear performance for this design when compared to a J-curved femoral component. Longer term clinical evidence such as published studies or outcomes reported in the available joint registries will be needed to establish whether any material or design can achieve a 30-year or longer outcome

Background. Polyethylene (PE) as a bearing material for total joint replacements (TJR) represents the golden standard for the past forty years. However, over the past decade it becomes apparent that PE wear and the biological response to wear products are the limiting factor for the longevity of TJRs. For this reason research has focused onto PE wear particle analysis. A particle analysis highly depends on the methodological work and results often show discrepancies between different research groups. From there, our hypothesis was, that an often unattended influencing factor is the optical magnification which has been used for particle analyses. Material and Methods. In the present study samples of a previous conducted knee wear simulator test were used. Wear particles were isolated from the bovine serum using an established method. 1. Briefly the serum was digested with hydrochloric acid and a continuous stirring and heating. Particles were filtered onto 20nm alumina filters and analyzed using high resolution field emission gun scanning electron microscopy (FEG-SEM). Filters were analyzed on the same points using three different magnifications: 5000, 15000 and 30000. To describe the size and morphology of the particles the equivalent circle diameter (ECD), aspect ratio (AR), roundness (R) and form factor (FF) were specified according to ASTM F 1877-05. The estimated total number (ETN) of particles was calculated based on the number of particles recovered on the filter, the analyzed area, the dilution, evaporation and the total serum volume. Results. The results showed significant differences between the different magnifications. Examples of the analyzed pictures are depicted in Fig. 1. The results are summarized in Tab. 1. In particular the size of the particles highly depends on the choosen optical magnification which becomes apparent in a more than twofold increase when comparing wear particles at magnification of 5k or 30k (p≤0.001). The mean particle diameter distribution (Fig. 2) also shows a shift in the distribution of wear particles: A higher magnification results in a higher fraction of smallest particles (e.g. over 50 percent between 0–0.2µm with magnification 30.000 compared to only 3 percent with a magnification of 5.000) and nearly no particles above 1µm could be verified. The results regarding the particle morphology show smaller but significant differences. The ETN of particles quadruplicates when comparing results with magnifications of 5.000 and 30.000. Conclusion. This study shows great differences in particle size, which can be directly attributed to the differences in optical magnification. According to ASTM F1877-05 a magnification of 10.000 for the analysis of wear particles between 0.1–1µm is recommended. However, results show that this magnification is not sufficient especially for particle sizes below 0.2µm, which account for the greatest number of particles. To the authors opinion a more detailed recommendation concerning the magnification is needed. Additional, a standardized evaluation system which takes the magnification into account is necessary to allow comparison of different research groups

INTRODUCTION. Knee simulators are being used to evaluate wear. The current international standards have been developed from clinical investigations of the normal knee [1, 2] or from a single TKA patient [3, 4]. However, the forces and motions in a TKA patient differ from a normal knee and, furthermore, the resulting kinematic outcomes after TKA will depend on the design of the device [5]. Consequently, these standard tests may not recreate in-vivo conditions; therefore, the goal of this study was to perform a novel wear simulation using design-specific inputs that have been derived from fluoroscopic images of a deep knee bend. METHODS. A wear simulation was developed using fluoroscopic data from a pool of eighteen TKA patients performing a deep knee bend. All patients had a Sigma CR Fixed Bearing implant (DePuy) and were well functioning (Knee Society Score > 90). A single patient was selected that represented the typical motions, which was characterized by early rollback followed by anterior motion with an overall modest internal tibial rotation (Figure 1). The relative motion between the femoral and tibial components was transformed to match the coordinate system of an AMTI knee wear simulator [6] and a compressive load input was derived using inverse dynamics [7]. The resulting force and motions (Figure 2) were then applied in a wear simulation with 5 MRad crosslinked and remelted polyethylene for 3 Mcyc at 1 Hz. Components were carefully positioned and each joint (n=3) was tested in 25% bovine calf serum (Hyclone Laboratories), which was recirculated at 37±2°C [3]. Serum was supplemented with sodium azide and EDTA. Wear was quantified gravimetrically every 0.5 Mcyc using a digital balance (XP250, Mettler-Toledo) with load soak compensation. RESULTS. The knee simulator was able to recreate the in-vivo input kinematics. The femoral low point location revealed good agreement between in-vivo and in-vitro conditions and the overall pattern of the motion from full extension to maximum knee flexion was replicated (Figure 3). The measured wear from these inputs was very low (0.7 ± 0.2 mg/Mcyc). DISCUSSION. We have performed a device-specific wear simulation for a deep knee bend. Surprisingly, the wear associated with this activity was very low. It is possible that abnormal kinematics, including paradoxical anterior slide and reverse rotation, would generate higher wear. The deviations the between in-vivo and in-vitro kinematics (Figure 3) are likely due to a size mismatch across the transformation process. In a previous study [7] we recreated the in-vivo motions with better fidelity (RMS error = 0.6mm) using size matched components. Further work is needed to improve the transformation technique for different sized components. Also, similar approaches will be used in future investigations to study the effect of abnormal kinematics as well as other designs including rotating platform and cruciate substituting devices

Introduction. Wear of polyethylene continues to be a significant factor in the longevity of total knee replacement (TKR). Moderately cross-linked polyethylene has been employed to reduce the wear of knee prostheses, and more recently anti-oxidants have been introduced to improve the long-term stability of the polyethylene material. This is the initial study of the wear of a new anti-oxidant polyethylene and a new TKR design, which has modified femoral condylar geometry. Materials and Methods. The wear of a new TKR the Attune knee was investigated using a physiological six station Prosim knee wear simulator (Simulator Solutions, UK). Six mid-size Attune fixed bearing cruciate retaining TKRs (DePuy Inc, Warsaw, USA) were tested for a period of 6 million cycles. The inserts were manufactured from AOX™, a compression moulded GUR1020 polyethylene incorporating Covernox™ solid anti-oxidant. The AOX polymer was irradiated to 8M Rad, to give a moderately cross-linked material. High and intermediate kinematics, under anterior-posterior displacement control were used for this study (McEwen et al 2005). The maximum femoral axial loading was 2600N, with flexion-extension of 0 to 58°, an anterior-posterior displacement of 0–10 mm for high kinematics and 0–5 mm for intermediate, and an internal-external rotation of ±5°. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (v/v) sodium azide solution in deionised water, as an antibacterial agent, and was changed approximately every 0.33 Mc. Wear was assessed gravimetrically and moisture uptake was assessed using unloaded soak controls. Volumetric wear was calculated using a density of 0.94 mg/mm. 3. , and compared with a previous study examining the Sigma XLK TKR design which uses moderately crosslinked polyethylene which is in current clinical use (Brockett et al 2012). Results. The mean wear as a function of cycles, under high and intermediate kinematics is indicated in Figure 1. There was a significant reduction in wear rate with intermediate kinematics. The mean wear rate for the Attune TKR under high kinematics (0–3 Mc) was 6.27±1.03 mm. 3. /Mc, and during the intermediate kinematics (3–6 Mc) was 4.63 ±1.01 mm. 3. /Mc. These wear rates were comparable to previously reported data for a moderately cross-linked UHMWPE Sigma TKR (Figure 2, Brockett et al 2012) under high kinematics. Under intermediate kinematics there was a small but significant difference between the wear rates (ANOVA, p<0.05). Discussion. A new Attune design TKR has a new bearing material and a new femoral geometry was examined through experimental wear simulator studies, and compared with a previously reported study conducted under the same test conditions. The wear performance of the new Attune TKR was comparable with the Sigma XLK bearing under high kinematics, but higher under intermediate kinematics. This study has examined the short term wear performance of the implant, and found it to be a low wear option for TKR. The longer-term potential advantages offered by a more oxidatively stable material will be investigated in the future