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 studies have looked at the effect of titanium versus cobalt chrome baseplates on backside wear. However, the surface finish of the materials is usually different [1,2]. There may also be subtle locking mechanism design changes [2]. The purpose of this study was to evaluate the wear performance of polyethylene inserts when mated with titanium baseplates to cobalt chrome baseplates, where both have non-polished topside surfaces and an identical locking mechanism.

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.

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.

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.

INTRODUCTION

Wear and fracture of patellar components has been frequently reported as a failure mode for cemented and press-fit patellar components. Malalignment of the patellar components may cause higher contact stresses, which may lead to excessive wear, delamination, and/or component fracture. In vitro testing of the patella in a clinically relevant malaligned condition is necessary to demonstrate adequate performance of the patellar component and assess the endurance of its fixation features under severe loading conditions. The purpose of this study was to test in vitro the patellar components under malaligned conditions using a knee joint simulator.

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.