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:

The solvent extraction step applied in conventional oxidation measurement protocols for UHMWPE retrievals resulted in an elevated oxidation index (OI) in remelted highly cross-linked UHMWPE (RM-HXLPE). The present study seeks to confirm the effect of solvent extraction on OI measurement and to understand the relationships among soak-aging, fluid uptake, and resulting OI from various test protocols.

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.

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

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.

Acetabular rim damge due to rim impingement is frequently found on retrievals and may be associated with increased wear and contact stresses, instability, and implant loosening of total hip replacement devices. Large X3 bearings (> 36mm) from Stryker have increased implant range of motion and improved polyethylene material (sequentially crosslinked and annealed). A hip simulator wear study was performed with and without femoral neck to acetabular rim impingement to determine the wear performance of these new bearings under aggressive impingement conditions. Two sizes of these new components were tested (36mm with 3.9mm thickness and 40mm with 3.8mm thickness) with two standard sized controls (28mm with 7.9mm thickness in X3 and conventional polyethylene. The 36mm component was chosen to be the largest component utilizing the same shell as the standard 28mm size components while the 40mm component was chosen to be the thinnest bearing currently offered.

Impingement significantly increased wear for all bearings (p< 0.05) but no cracking or failures of the rim occurred. Wear rates for all X3 bearings were statistically indifferent under each testing condition despite bearing size and thickness. Average wear rates for X3 bearings were 0.3mm3/million cycles (mc) under standard conditions and 3.5mm3/mc under impingement conditions. Average wear rates for conventional bearings were 19.5mm3/mc under standard conditions and 48.3mm3/mc under impingement conditions. Overall the X3 bearings exhibited a 93% reduction in wear under impingement conditions and 99% reduction in wear under standard conditions.

Increased bearing range of motion reduces the chance of impingement. This study shows the simulated outcome even if these larger bearings were to impinge. We conclude that these larger X3 bearings exhibits the same wear performance as standard X3 bearings and significantly superior wear performance compared to conventional polyethylene bearings under standard and impingement conditions.

Femoral head roughening is a clinically observed phenomenon that is suspected to cause increased wear of acetabular inserts. Two approaches have been taken to reduce hip bearing wear. Improved femoral head materials may decrease the impact of roughening and reduce the effect of abrasion. Additionally, improved polyethylene materials may be utilized to reduce wear against smooth or roughened femoral heads. This study looks at these two approaches in the form of a toughened alumina femoral head (Biolox Delta) and a sequentially crosslinked and annealed polyethylene (X3). A wear study was performed with new and artificially scratched ceramic femoral heads (28mm Biolox Delta) as compared to new and artificially scratched Cobalt Chromium femoral heads. These femoral heads were articulated against both conventional (N2\Vac) and highly crosslinked (X3) polyethylene acetabular cups. Artificial scratching utilized a Rockwell C indentor loaded at 30N to scratch a multidirectional scratch pattern on the articulating surface of the femoral head to simulate in vivo roughening.

Delta femoral heads exhibited superior resistance to scratching. Peak to valley roughness for CoCr heads was 7.1um while Delta heads only roughened to 0.4um. Head material under standard conditions (no scratch) had no effect on PE wear (p=0.31 and p=0.53). Under abrasive conditions, the Delta femoral head exhibited a clear advantage over CoCr heads (65–97% reduction in wear rate; p< 0.007). X3 polyethylene also showed a clear advantage over conventional PE against either CoCr or Delta heads and under both conditions (all p < 0.012).

This study clearly demonstrates that X3 polyethylene has a clear wear advantage over conventional polyethylene despite head material or abrasive conditions. Secondary to the polyethylene choice, the use of a ceramic femoral head leads to superior performance under abrasive conditions.