Prosthetic Hip dislocations remain one of the most common major complications after total hip arthroplasty procedures, which has led to much debate and refinement geared to the optimization of implant and bearing options, surgical approaches, and technique. The implementation of larger femoral heads has afforded patients a larger excursion distance and primary arc range motion before impingement, leading to lowered risk of hip dislocation. However, studies suggest that while the above remains true, the use of larger heads may contribute to increased volumetric wear, trunnion related corrosion, and an overall higher prevalence of loosening, pain, and patient dissatisfaction, which may require revision hip arthroplasty. More novel designs such as the dual mobility hip have been introduced into the United States to optimize stability and range of motion, while possibly lowering the frictional torque and modes of failure associated with larger fixed bearing articulations. Therefore, the aim of this study is to compare the effect of bearing design and anatomic angles on frictional torque using a clinically relevant model8.

Two bearing designs at various anatomical angles were used; a fixed and a mobile acetabular component at anatomical angles of 0°,20°,35°,50°, and 65°. The fixed design consisted of a 28/56mm inner diameter/outer diameter acetabular hip insert that articulated against a 28mm CoCr femoral head (n=6). The mobile design consisted of a 28mm CoCr femoral head into a 28/56mm inner diameter/outer diameter polyethylene insert that articulates against a 48mm metal shell (n=6). The study was conducted dynamically following a physiologically relevant frictional model8.

A statistical difference was found only between the anatomical angles comparison of 0vs65 degrees in the mobile bearing design. In the fixed bearing design, a statistical difference was found between the anatomical angles comparison of 20vs35 degrees, 20vs50 degrees, and 35vs65 degrees. No anatomical angle effect on frictional torque between each respective angle or bearing design was identified. Frictional torque was found to decrease as a function of anatomical angle for the fixed bearing design (R2=0.7347), while no difference on frictional torque as a function of anatomical angle was identified for the mobile bearing design. (R2=0.0095)

These results indicate that frictional torque for a 28mm femoral head is not affected by either anatomical angle or bearing design. This data suggests that mobile design, while similar to the 28mm fixed bearing, may provide lower frictional torque when compared to larger fixed bearings >or= 32mm8. Previous work by some of the authors [8] show that frictional torque increases as a function of femoral head size. Therefore, this option may afford surgeons the ability to achieve optimal hip range of motion and stability, while avoiding the reported complications associated with using larger fixed bearing heads8. It is important to understand that frictional behavior in hip bearings may be highly sensitive to many factors such as bearing clearance, polyethylene thickness/stiffness, polyethylene thickness/design, and host related factors, which may outweigh the effect of bearing design or cup abduction angle. These factors were not considered in this study.

Introduction

Many tests have been published which measure frictional torque [1–4] in THR. However, different test procedures were used in those studies. The purpose of this study was to determine the effect of test setup on the measured friction torque values.

Burroughs et al showed that frictional torque increases with increasing head size in a simple in vitro model and showed differences in frictional torque with different polyethylene materials [1]. Therefore, the purpose of this study was to evaluate the influence of bearing material and bearing size on the frictional torque of hip bearings utilizing a more physiologically relevant hip simulator model.

A total of four hip bearing combinations (Crosslinked PE/CoCr, Conventional PE/CoCr, Crosslinked PE/Delta and Alumina /Alumina) with various bearing sizes were evaluated. The sizes tested in this study range from 22 mm to 44 mm; it is important to note that the study only evaluated bearing combinations (size and material combination) currently commercially available. A total of three samples per bearing combination were tested, with the exception of conventional PE, which included a total of 4 samples. A MTS hip joint simulator was used. All components were oriented anatomically with the femoral head mounted below on a rotating angled block which imparts a 23° biaxial rocking motion onto the head. Loading was held constant at each load level (500N, 1000N, 1500N, 2000N, 2450N) for at least two rotational cycles while all 3 axes of load and all 3 axes of moments were measured at 10 khz. Fresh Alpha Calf Fraction serum was utilized as a lubricant.

Results show that frictional torque increases with the increase of head size regardless of head material for all polyethylene combinations (p > 0.05), as shown in Figure 1 and 2. However, results showed no change in frictional behavior for the Alumina/Alumina combination regardless of the bearing size. The results of this test did not show any significant difference between crosslinked PE and conventional PE materials for sizes 28 mm and 32 mm when paired against a CoCr head (p > 0.05) (Figure 3). The Alumina/Alumina bearing combination had the lowest frictional torque among all the bearing material combinations evaluated in this study.

This data suggests that there is a strong correlation between increased head size and increased frictional torque (R² = 0.6906, 0.8847) for the polyethylenes evaluated here regardless of head material. No correlation can be concluded for the Alumina /Alumina bearing combination (R² = 0.0217). The combination of Alumina /Alumina seems to have the most favorable frictional properties. This data also suggests no effect on frictional properties regardless of the polyethylene material (crosslinked and conventional) for sizes 28 mm and 32 mm. The frictional torque values recorded in this study are different than those published by Burroughs et al [1]. This difference may be attributed to the testing methodology. The current study utilizes a hip simulator, which closely mimics the natural joint providing a more physiologically relevant model whereas the Burroughs et al study utilizes a single axis machine. It is important to understand that frictional behavior in hip bearings may be highly sensitive to bearing clearance, cup thickness, and stiffness, which may outweight the effect of head diameter. Further evaluation is necessary to isolate and investigate those parameters.

Demand for TKR surgery is rising, including a more diverse patient demographic with increasing expectations [1]. Therefore, greater efforts are being devoted to laboratory testing. As a result, laboratory testing may set a clinical performance presumption for surgeons and patients. For example, oxidized ZrNB (Oxinium) femoral components have been projected to show 85% less wear than CoCr femoral components in bench-top testing [2]. However, recent clinical data show no difference in outcomes between Oxinium® and CoCr for the same design [3]. While it does not show lagging peformance for the Oxinium components, it does call into question the predictive ability of simulation. To better understand the performance of these two materials, a non standardized simulator evaluation was conducted.

One commercially available design (Legion PS) was evaluated with two variations of femoral component material (n = 3/material) Oxinium® and Cobalt Chromium. All testing was conducted using a 7.5 kGy moderately crosslinked UHMWPE (XLPE). A 6-station knee simulator was utilized to simulate stair-climbing kinematics. The lubricant used was Alpha Calf Fraction serum which was replaced every 0.5 million cycles for a total of 5 million cycles. Soak controls were used to correct for fluid absorption and statistical analysis was performed using the Student's t-test.

Total wear rate results for the tibial inserts are shown in Figure 1. There was no statistical difference in volume loss (p = 0.8) or wear rate (p = 0.9) for the Oxinium® system when compared to the CoCrsystem under stair-climbing kinematics. Visual examination revealed typical wear scars and features on the condylar surfaces, including burnishing.

These results corroborate the recent clinical data showing no difference between Oxinium® components and their CoCr analogs [3]. The kinematics used here are not a combination of normal level walking with stair-climbing conditions as was published originally for the Oxinium® material [2], but stair-climbing kinematics only. Even though the stair-climbing profile utilized here does not represent standardized kinematics, it provided results that are in line with clinical observations for these femoral materials. Logic suggests that a combined duty cycle is more representative of patient behavior so there must be additional test factors contributing to the prediction previously reported. The goal of bench top testing is to simulate actual clinical performance so test models must be validated as clinicaly relevant in order to be predictive. Furthermore, the results of this test indicate that the different femoral materials evaluated in this study do not alter the wear characteristics of this TKR. This is further supported by a similar previous study showing the relative contribution of design versus materials in terms of wear behavior [4]. The main determination comes from clinical evidence, and as it has been demonstrated by Kim, et al [3], there is no significant difference in the clinical results of the two TKR devices analyzed.

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.

The dual mobility hip incorporates a femoral head mated within a spherical polyethylene liner which also has an unconstrained outer articulation with a polished metal shell. An additional wear surface is introduced at the outer articulation, however, the mobility of the polyethylene insert does allow for femoral-neck/acetabular-insert impingement by allowing the insert to displace upon contact. We evaluated the wear performance of a dual mobility hip during abrasive and impingement conditions independently. Three abrasive conditions were evaluated; abraded acetabular cup, abraded femoral head, and both abraded cup and head. Two impingement conditions were evaluated; impingement of the unconstrained acetabular insert against the femoral neck, and acetabular-insert/femoral-neck impingement when the insert becomes immobilized at the outer articulation.

Wear testing was conducted using a hip stimulator. The simulator applied physiologic loading with a maximum load of 2450 N and serum as the lubricant. Components were abraded at the pole according to a published method. Abraded samples were tested at 0° of inclination. The unconstrained impingement condition was created by adjusting the femoral neck angle to achieve impingement with 45° of acetabular inclination. Neck to liner impingement can occur at either the superior or inferior surface of the femoral neck, with subsequent impingement occurring randomly as the insert is allowed to re-align itself throughout testing. The fixed impingement conditions was created by locking the outer bearing through fixturing and inducing impingement as previously described. Dual mobility control components were tested at 0° and 50° of inclination. Inserts were sequentially crosslinked GUR 1020 polyethylene.

Results are shown in Figure 1. Abrasion testing results correlated to a combination of friction at the abraded articulation and bearing size. Abrasion at only the inner bearing had a larger effect on wear when compared to abrasion of only the outer bearing. When both sides were damaged, femoral head abrasion led to an increase in friction and resistance to movement at the inner articulation, thereby forcing an increase in overall movement of the outer articulation. This increased the contact area subject to motion across a scratched metal surface, which increased the wear rate of the system. Unconstrained impingement samples impinged during the first cycle and then randomly throughout testing, while the fixed impingement samples had predictable impingement at the same location every cycle of testing. The unconstrained impingement model was designed to replicate an instance where the dual mobility hip would run in a near/intermittent impingement condition where the polyethylene insert displaces upon contact with the femoral neck. Unconstrained impingement wear rates were not statistically different than the ideally aligned control. The fixed impingement samples wore at a higher rate than the unconstrained impingement and control groups. The insert encountered resistance to movement upon impingement resulting in wear and deformation at the point of contact. Additional intended bearing wear was also generated by head sliding and translation of the load path upon impingement of the rim. Note that this condition is difficult to envision clinically and all wear rates, even under adverse conditions, were acceptably low.

Pin-on-disk studies have demonstrated the role that cross-shear plays in polyethylene wear. It has been found that applying shear stresses on the polyethylene surface in multiple directions will increase wear rates significantly compared to linear sliding. Hip and knee joint replacements utilize polyethylene as a bearing surface and are subjected to cross-shear motions to various degrees. This is the mechanism that produces wear particles in hip and knee arthroplasty bearings and if excessive may lead to osteolysis, implant loosening, and failure. The amount of cross-shear is dependent on the bearing diameter and the angular motion exerted onto the bearing due to the gait of the patient. This study will determine the effect of sliding curvature (angular change per linear sliding distance) on the wear rate of polyethylene. Virgin polyethylene blocks were machined with a 28mm diameter bearing surface and against 28mm cobalt chromium femoral heads in a hip simulator. Dynamic loading was applied simulating walking gait but the motion differed between testing groups. Typical walking gait testing utilizes 23° biaxial rocking motion, in this study, 10°, 15°, 20°, and 23° biaxial rocking motions resulting in various sliding curvatures. Sliding motion path is described in Figure 1 and is a function of the bearing radius and the rocking angle. With increased rocking angle, the sliding distance reduces per cycle and the sliding path becomes more curved (more angular change per linear distance of sliding). Despite a significant increase in sliding distance at higher rocking angles, wear rates were relatively unchanged and ranged from 57mm3/mc to 62mm3/mc. Wear rates per millimeter increased exponentially with reduced sliding arc radius (smaller rocking angle) as shown in Figure 2. This study suggests that wear of polyethylene is highly dependent on sliding path curvature. The sliding path is largely a function of the bearing diameter and the patient activity. Large bearing diameter implants have been recently introduced to increase joint stability. Sliding distance increases proportional to the bearing radius which has led to some concerns regarding increased wear in larger bearings. However, in vitro wear studies have not shown this trend. Increased bearing diameter also increases the sliding path curvature which this study has shown to cause a reduction in wear roughly proportional to the radius of the bearing. Therefore, the increase in wear due to sliding distance is offset by the reduction in wear caused by the sliding curvature resulting in no significant change in wear with increased bearing diameter. Curved sliding path causes a change in surface shear direction which has been shown to increase wear of polyethylene. This study confirms that increased cross-shear in the form of more angular change per linear sliding distance can increase wear of polyethylene exponentially

Osteolysis induced by UHMWPE debris has historically been one of the major causes of long term failure of TJR. An increase in concentration of polyethylene particles in the peri-prostheic tissue has been linked to an increased incidence of osteolysis. The dual mobility hip bearing concept mates a femoral head into a polyethylene liner which has an unconstrained articulation into a metal shell. The wear mechanism of the dual mobility hip bearing is distinct from a constrained single articulation design, which may result in a difference in wear debris particles. The aim of this study is to evaluate wear debris generated from a dual mobility hip and compare it to a conventional single articulation design when both are manufactured from sequentially crosslinked and annealed polyethylene. The dual mobility hip (Restoration ADM) incorporated a 28mm CoCr femoral head into a polyethylene liner that articulates against a metal shell (48mm ID). The conventional hip (Trident®) mated a 28mm CoCr femoral head against a polyethylene liner. The polyethylene for all liners was sequentially crosslinked and annealed (X3). A hip joint simulator was used for testing at a rate of 1 Hz with cyclic Paul curve physiologic loading. A serum sample from each testing group was collected. Serum samples were protein digested following the published process by Scott et al. The digested serum was then filtered through a series of polycarbonate filter papers of decreasing size and sputter coated with gold for analysis using SEM. Image fields were randomized and wear debris was compared in terms of its length, width, aspect ration, and equivalent circular diameter (ECD). A total of 149 conventional hip particles and 114 dual mobility hip particles were imaged. Results show a majority of particles are of spherical nature and images do not indicate the presence of fibrillar or larger elongated polyethylene debris. Particle length between designs is not statistically different, while all other comparisons show statistical significance (p<0.05). It is hypothesized that the dual mobility hip system reduces the total amount of cross-shear motion on any one articulation, which aids in the reduction in wear. This design feature may be responsible for the slight difference in morphology of dual mobility wear debris when compared to the constrained hip design. The length of the particles was similar, simply indicating a different shape rather than a marked reduction in overall size. The debris generated is this study was from highly crosslinked polyethylene in two different designs, which produced a very significant decrease in quantity of particles when compared to the quantity of debris from conventional polyethylene. The wear debris was of similar length in both designs and so we do not expect any difference in biological response to debris from either device. The dual mobility design has also shown no effect of cup abduction angle on wear demonstrating forgiveness to implant positioning. This advantage, combined with the low wear rate and similar length wear particles, should lead to good clinical performance of dual mobility cups with sequentially irradiated and annealed polyethylene.

INTRODUCTION

For cementless TKA, highly crosslinked UHWMPE is traditionally used with modular components because of manufacturing and sterilization complexities of monoblock metal-backed components. However, it would be very useful to have a highly crosslinked UHMWPE monoblock metal-backed cementless component to address historical clinical issues. The purpose of this study was to evaluate the wear properties of a unique process for achieving a monoblock metal-backed cementless component featuring highly crosslinked polyethylene to standard highly crosslinked UHWMPE.

The introduction of highly crosslinked PE with improved wear performance has allowed for the marketing of thin liners. Previous studies have shown that steep angles reduce femoral head coverage thereby decreasing contact area and can subject the acetabular rim to excessive stresses. This can be especially concerning for thinner PE constructs. Previous work with thicker (9.9mm) non-crosslinked PE show a correlation of decreased wear with increased abduction angle. Therefore, the objective of this study was to isolate and examine the effects of varying cup abduction angles on the wear of a thin second generation highly crosslinked polyethylene. Five sets of sequentially crosslinked Trident^® design inserts with a wall thickness of 3.9mm were evaluated. Sequentially crosslinked liners were machined from compression molded GUR1020 UHMWPE that had been γ-irradiated followed by annealing 3 times (X3). Testing was conducted using a hip joint simulator for 3 million cycles. All cups were fixed, positioned superiorly at a neutral version angle, and divided into five groups of varying inclination angles: 0°, 20°, 30°, 50° and 70°. A physiological load was applied to each couple at a rate of 1Hz using Alpha Calf Fraction serum. Weight was converted to volume and plotted as a function of cycle count. In addition, all PE inserts were microscopically analyzed for any gross damage and areas of deformation. Wear rates plotted against inclination angle exhibited poor correlation between wear rate and angle (R2=0.253). Student’s t-tests revealed significant differences (p< 0.05) between 0° and 70°, and between 50° and 70° angles. There was no statistical differences for any of the other tested angles. Visual inspection of the tested liners revealed wear scars of increased areas of polishing on inserts positioned at lower abduction angles. No deformation, cracking or pitting of the liners was observed. Visual inspection of the liners revealed an increase in overall area of polishing with a reduction in abduction angle. This indicates that load is concentrated over a smaller area for higher angles resulting in increased contact stress for steeper cups; however, this did not translate into a correlation of high abduction angle and high wear. These results do not correlate with our previous work, however that study was conducted on smaller diameter thicker non-highly crosslinked material. We believe the difference in results is due to fundamental material response. Although visual burnishing indicates a trend in contact area, there may be a role of deformation in the results. Future work will involve finite element analysis to study these differences. The results in this study suggests that the sequentially crosslinked polyethylene is able to maintain its low wear characteristics at various abduction angles even with a thin (3.9 mm) liner.

Steep angles (> 55°) reduce femoral head coverage decreasing contact area and can subject the acetabular rim to excessive stresses. In the case of metal-metal implants it has been shown that at steep angles there is no bedding-in of the implants and run-away wear occurs. The dual mobility bearing concept mates a metal femoral head with a polyethylene liner that is free to articulate inside a polished metal shell. Previous work has shown acetabular wear can be minimized with this design, possibly through reduction of total amount of cross-shear motion in the joint. An additional potential benefit may exist through the maintenance of conforming contact and head coverage even under high inclination angle. This study evaluates the influence of inclination angle on the wear performance of three hip bearing designs. Four sets of dual mobility implants, three sets of metal-on-metal hip implants, and five sets of fixed hip implants were evaluated per inclination angle. All polyethylene components were made of GUR 1020 UHMWPE that was sequentially crosslinked and annealed three times (X3). The MoM components were fabricated from high carbon cast CoCr as per ASTM F75 (no heat treatment). A hip joint simulator was used for testing for a total of 2.5 million cycles with the cups oriented at either 35° or 65° of abduction. Testing was run at 1Hz following Paul curve physiologic loading and statistical analysis was performed using the Student’s t-test (p< 0.05). results for the 35 degrees of inclination angle condition show no statistical difference between any of the testing combinations with X3 polyethylene showing immeasurable wear. At this angle wear of the MoM devices was similar, although ion levels were not measured. results for the 65 degree condition showed an increase for the fixed PE and MoM systems. The increase in fixed PE bearing wear is consistent with previous findings and still within noise level values. The increase in MoM wear was substantial with both heads and cups showing scratches and abrasion damage related to edge contact. There is a statistically significant wear rate reduction (p< 0.05) of over 94% for both the dual mobility and fixed bearing PE constructs when compared to MoM. When comparing wear rates of the dual mobility system to the standard fixed acetabular bearing, the dual mobility device exhibited an 85% (p< 0.05) reduction in wear rate. The results of this study support our hypothesis that acetabular wear at high angles can be diminished through design. This is likely due to maintenance of the nature of the primary inner bearing contact regardless of shell positioning. Based on these results this dual mobility construct can be expected to outperform a conventional fixed construct and a metal-on-metal construct in terms of wear at high inclination angles, without any of the metal ion release concerns.

Ultra-high molecular weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint arthroplasty. However, longevity of these implants has been compromised by wear and fatigue damage of the polyethylene. The addition of vitamin E to the polyethylene is a process recently introduced in the market to stabilize free radicals produced during radiation crosslinking. The objective of the present study is to investigate the effect of the addition of vitamin E on the wear characteristics of UHMWPE. Sequentially cross-linked and annealed UHMWPE material (X3™, Stryker Orthopaedics, Mahwah, NJ) was used as a control.

Trident™ acetabular cups (Stryker Orthopaedics, Mahwah, NJ) with inner diameters of 36 mm and 44 mm and a wall thickness of 3.8 mm were tested on a 12 station MTS hip joint simulator. The simulator used a physiologic loading pattern with a maximum load of 2450N. The test was conducted under standard clean conditions with alpha calf fraction serum diluted to a protein concentration of 20 g/l for a total of three million cycles. All cups ran against CoCr femoral heads, and gravimetric measurements were taken every half-million cycles.

Results show that sequentially crosslinked components, size 3 6mm, had an average volume loss of 9.4 ± 2.5 mm3, while vitamin E components of the same size had an average of 16.5 ± 3.1 mm3. This represents a 75% increase for vitamin E components that is statistically significant (p = 0.039). Size 44 mm sequentially crosslinked components had an average volume loss of 6.8 ± 3.7 mm3, while vitamin E components had an average of 19.7 ± 3.2 mm3. This denotes a statistically significant increase of 192% for material with vitamin E (p = 0.011). Linear regression analysis yielded wear rates of 4.1 ± 0.9 mm3/mc and 6.1 ± 1.3 mm3/mc for size 36 mm sequentially crosslinked and vitamin E components, respectively, which represents a non-significant increase of 49% for vitamin E components. Size 44 mm sequentially crosslinked components had a wear rate of 3.8 ± 1.3mm3/mc, while vitamin E components had a wear rate of 8.1 ± 0.7 mm3/mc. This represents a statistically significant increase of 117% in wear rate for vitamin E components (p = 0.013).

The results of this testing indicate that the addition of vitamin E degrades wear performance relative to sequentially crosslinked material. Research shows that the introduction of Vitamin E affects the ability to create crosslinks during irradiation by reacting with some of the free radicals. Oral et al have shown that the crosslink density decreases when Vitamin E is blended into UHMWPE. Their research has also shown that a decrease in crosslink density causes an increase in wear rate. The results of the current testing show that the addition of vitamin E to polyethylene reduces the wear resistance of highly crosslinked polyethylene.