Summary Statement

Antioxidant containing UHMWPE particles induced similar levels of in vitro macrophage proliferation and in vivo inflammation in the mouse air pouch model as UHMWPE particles alone. Benefit of antioxidant in reducing wear particle induced inflammation requires further investigation.

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.

INTRODUCTION

Many studies have looked at the effects of titanium tibial baseplates compared to cobalt chrome baseplates on backside wear. However, the surface finish of the materials is usually different (polished/unpolished) [1,2]. Backside wear may be a function not only of tray material but also of the locking mechanism. The purpose of this study was to evaluate the wear performance of conventional polyethylene inserts when mated with titanium tibial trays or cobalt chrome tibial trays that both have non-polished topside surfaces.

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.

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.

Hip and knee wear simulators have been used by implant manufacturers and researchers for many years as a performance predictor and comparator for hip and knee implants. The clinical accuracy of these simulators in predicting wear depends heavily on the type of simulator as well as the methodology used. The joint lubricant used in the simulators is one crucial aspect that has been well studied in hip simulators. This study will compare the wear performance of a modern total knee replacement system using two commonly used simulator lubricants at various dilutions (Alpha Calf Serum and Bovine Calf Serum, Hyclone Labs). The Triathlon knee implant system (Stryker Orthopaedics) was used along with a six station knee wear simulator from MTS Systems to determine the effect of lubricant type and dilution.

Wear rates were found to be dependent on the type and dilution of the lubricant. At 0g/L protein concentration (100% water) wear rates were 4.8mm3/million cycles (mc). With the introduction of Bovine serum, wear rates increase to a peak of 24mm3/mc at 5g/L of concentration. Increased concentration of Bovine serum resulted in a decrease of wear rates. Wear rates for Alpha serum peaked at 28mm3/mc at 20g/L concentration with decreased wear rates at higher concentrations.

Knee implant wear performance is often characterized by wear simulation. As has been previously shown for hip simulations, this study shows the importance of choosing the correct lubricant type and dilution to correctly simulate wear performance. While this study cannot correlate any of the lubricants to the synovial fluid present in vivo, this study shows that 20g/L of Alpha serum produces the highest wear rates and should be used to determine worst case wear rates in the wear performance characterization of knee implants.