Introduction: Crosslinking has been extensively introduced to reduce the wear of UHMWPE. Zero wear of highly crosslinked UHMWPE has been reported by some groups (1) in hip simulators, clinical studies have reported finite wear rates (2). The aim of this study was to compare the wear rates produced by UHMWPE with different levels of crosslinking.

Materials and Methods: Studies were carried out using 28mm diameter cobalt chrome femoral heads. These were articulated against UHMWPE in the Leeds ProSim hip joint simulator. The acetabular cups were manufactured from UHMWPE GUR 1050. The GUR 1050 was highly crosslinked with 10MRad or 7.5MRad of gamma irradiation in nitrogen followed by re-melting at a temperature above 150°C. Slightly crosslinked GUR 1050 was also tested (gamma irradiated with 2.5MRad in air). Non-crosslinked GUR 1050 UHMWPE was used as a control. Five cups of the materials were tested with one station from each set of five being used for creep data. Wear measurements were taken every million cycles using a coordinate measuring machine and tests were run to 5 million cycles. The tests were carried out in low serum concentrations of 25% (v/v) bovine serum diluted with 0.1% (w/v) sodium azide in water. At each million cycles a 3D measurement was taken of the contact region of the acetabular cups using a Form Talysurf profilometer.

Results and Discussion: The wear rate decreased as crosslinking levels increased. The non-crosslinked material had an overall average wear (mm3/million cycles) determined by volume change of 45.6+/−1.35, the 2.5MRad material 46.9+/−9.4, the 7.5MRad 15.04+/−4.28 and the 10MRad material 8.7+/−3.11. The intentionally cross-linked materials showed a significantly lower volume change than the other two materials, with the 10MRad polyethylene having a slightly lower volume change than the 7.5MRad polyethylene. All four polyethylenes showed greater volume change in the first million cycles than the subsequent four and this was associated with initial creep deformation in the first million cycles. The individual creep deformation cups confirmed this with volume changes in the first million cycles followed by stability. Creep volumes of between 10 and 25 mm3 total were measured with the lowest value being for the 10MRad polyethylene. The steady state wear rates for the PE’s between one and five million cycles were 0MRad 36.9+/−1.92 mm3/million cycles, 2.5MRad 44.12+/−10.09, 7.5MRad 7.89+/−2.32 and 10MRad 4.62+/−2.73. The results of the surface topography of the acetabular cups showed that the highly crosslinked materials became smoother than the other materials as the test progressed. This would benefit the crosslinked materials in aiding lubrication and could have contributed to the lower wear rate seen with these materials.

Conclusion: The highly crosslinked UHMWPE gave lower wear volumes than the noncrosslinked materials. This could have been due to the smoother surfaces of the cups as the study progressed which resulted in better lubrication of the components. Finite wear rates have been recorded for the first time with highly cross-linked polyethylene, that compare with clinical observations.

Viscosupplementation is the current treatment modality for early stage arthritis and in some cases for delaying joint replacement procedures. Rheological properties similar to that of synovial fluid and high molecular weight have been recognized as the determining factors in hyaluronic acids (HA) therapeutic and analgesic value (1). In this study, the self assembly of peptides into beta-sheet structures in solution (2–4) is explored to develop novel biocompatible injectable joint lubricants. These peptides can be delivered into the joint easily in their low viscosity monomer form, while they are designed to self-assemble in situ under physiological conditions. Four different peptides P11-4, P11-8, P11-9, and P11-12 were designed based on the chemical motif of hyaluronic acid and were found to self-assemble into nematic fluids and gels under physiological conditions. Friction characteristics of these peptides as lubricants were evaluated in a bovine cartilage on cartilage model using a simple pin on plate geometry and under various sliding conditions. Friction tests were carried out using both healthy and damaged bovine cartilage samples, to study the therapeutic effect of these peptides as lubricants. Further, a rheometer with cone-on-plate configuration was used to study these peptides in shear viscosity and oscillatory shear modes to determine their viscoelastic properties. Both the friction properties and rheological behaviour of the peptides were compared to that of a commercially available hyaluronic acid preparation that was tested along with the peptides. Peptide P11-9 was found to have very similar viscoelastic properties to that of HA, and was also the most effective in friction level reduction among the four peptides tested. When compared to HA, P11-9 showed slightly better friction characteristics in all the healthy cartilage models, while HA was the best lubricant in damaged cartilage models when compared to P11-9 and other peptides. The results indicate that these novel self assembling peptides can be developed as a new generation of synthetic viscosupplements for the treatment of early stage arthritis.

There is currently much interest in the wear of metal-on-metal THRs and potential concerns about elevated metal ion levels. Generally, wear of metal-on-metal THR’s has been low in simulator studies. Slightly higher and more variable wear has been found clinically. Variations in surgical approach, technique and fixation method may influence the level of force applied across the prosthesis during gait. It is hypothesised that increased joint tensioning may increase loading of THR’s during the swing-phase; leading to elevated wear and friction due to depleted fluid film lubrication. This study aimed to assess the effect of swing-phase load on the friction, lubrication and wear of metal-on-metal THR’s.

Cobalt-chrome 28mm metal-on-metal THR’s were tested in a physiological hip simulator, loading was modified to provide; (1) ISO swing-phase load (280N, as per ISO 14242-1) and (2) low swing-phase load (< 100N). Friction testing was conducted using a pendulum friction simulator, with 280N and 100N swing-phase loads. Theoretical lubrication modelling was carried out using elastohydrodynamic lubrication theory.

The overall mean volumetric wear rates was 10-times greater in THR’s tested with an ISO swing-phase load in comparison to THR’s tested with low swing-phase loads (0.58±0.49 compared to 0.06±0.039mm³/million cycles). The friction factors were 0.129 and 0.173 respectively under low and ISO swing-phase conditions. A decrease in the predicted lubricant film thickness when the swing-phase load was increased was observed; at the start of stance phase this was 0.12microns and 0.07microns under low and ISO swing-phase conditions respectively.

The results demonstrate that the performance of metal-on-metal THR’s is highly dependent on swing-phase load conditions. It is postulated that fixation method and surgical technique can affect the swing-phase load. This study has demonstrated that over-tensioning of the tissues may also accelerate wear. These observations may explain some of the variations reported clinically.

Introduction: Following hip replacement surgery the tension of the soft tissues and the laxity of the joint may vary. Variations in surgical approach, technique and fixation method may influence the effective joint laxity and the level of force applied across the prostheses during the swing phase of gait. The aim of this study was to investigate the effect of different swing phase load conditions on the wear metal-on-metal hip prostheses using a hip simulator.

Methods: Cobalt chrome metal-on-metal bearings, 28mm in diameter were tested for five million cycles in a Prosim hip simulator with flexion-extension and internal-external rotation kinematic inputs. A Paul-type twin peak loading curve was applied, which was modified to provide three different swing phase load conditions;

Low positive swing phase load (< 100N)

All tests were carried out in 25% (v/v) new-born bovine serum, with gravimetric wear measurements completed every million cycles.

Results: The wear rates for the different swing phase conditions are shown in Figure I. Elevating the swing phase load from 100N to 300N (ISO load) increased the overall wear rate by 10-fold. Introducing microseparation into the gait cycle increased wear by a further 3-fold. These results indicate the sensitivity of metal-on-metal bearing wear to swing phase load conditions and joint laxity.

Discussion: Little attention to date has been paid to the importance of joint laxity and swing phase load on the wear rate of hip replacements. Elevation of wear rates with increased swing phase load was probably due to the depletion of fluid film lubrication. This was consistent with the findings under stop-start motion [Medley et al., 2002] and demonstrates the dependency of metal-on-metal hip replacements on fluid film lubrication conditions. Testing with a negative swing phase load elevated wear due to microseparation of the components, the head contacted the insert rim at heel strike which caused a stress concentration and damage to the insert rim. The results demonstrate that the wear performance of metal-on-metal hip replacements is highly dependent on swing phase load conditions. It is postulated that the fixation method and surgical technique can effect the swing phase load; over tensioning of the soft tissue may increase the swing phase load, whereas joint laxity will cause a negative swing phase load and possibly microseparation.

Ultra high molecular weight polyethylene (UHMWPE) wear debris induced osteolysis is a major cause of long term failure of total hip replacements. Particles in the 0.1–1.0_m size range are believed to have greater osteolytic potential than larger wear debris. Crosslinked polyethylenes have been shown to have improved wear resistance compared to non-crosslinked materials on smooth counterfaces, however wear debris from cross-linked UHMWPE has been shown to be smaller than that produced from non-crosslinked materials. The aim of this study was to compare the wear, wear debris and biological activity of non-crosslinked and crosslinked polyethylenes when worn against smooth and scratched counterfaces.

Materials and Methods: Test pins were machined from non-crosslinked GUR1050 and GUR1050 crosslinked with either 5 or 10Mrad of gamma irradiation. Sterile endotoxin free clinically relevant wear debris was generated using a bi-directional pin-on-plate test rig. Tests were performed on scratched (Rp=1.0mm) or smooth (Ra=0.02mm) counterfaces. Particles were cultured with murine macrophages at particle volume (mm³): cell number ratios of 50:1,10:1,1:1 and 0.1:1. The levels of TNF-a produced were determined by ELISA following 0,2,4,6,8,24 and 48 hours of culture.

Results: On both smooth and scratched counterfaces crosslinked polyethylene had lower wear than non-crosslinked polyethylene. Determination of the volume distribution of the wear debris demonstrated a greater percentage of wear debris in the submicrometre size range from crosslinked material when worn on scratched counterfaces. Analysis of the debris when worn on smooth counterfaces showed a further reduction in size of debris with particles observed below 100nm in size which reduced the percentage of debris in the sub-micrometre size range for both materials. Crosslinked material worn against scratched counterfaces generated wear debris that was able to stimulate macrophages to produce significant levels of TNF-a after just six hours of co-culture at the highest volumetric concentration and after 24 hours at lower volumetric concentrations. The non-crosslinked material was able to stimulate macrophages only after 24 hours at the highest volumetric concentration. There were no differences between the biological activity of the particles from the 3 materials articulating on the smooth counterfaces they were only able to stimulate significant TNF-a release following 24 hour with the highest volumetric concentration.

Discussion: Although wear resistance is increased by cross-linking on both smooth and scratched counterfaces, when worn against a scratched counterface crosslinked polyethylene generated a greater percentage of debris in the 0.1–1.0mm size range than non-crosslinked polyethylene and this led to an increase in biological activity. However when worn against smooth counterfaces the production of nanometre size wear particles by both materials reduced the volume of debris in the 0.1–1.0mm size range which in turn lead to a lower biological activity.

Following total hip replacement surgery, fluroscopy studies have shown that a mean separation of 2 mm can occur between CoCr femoral heads and UHMWPE acetabular cups during the swing phase of gait [1]. In vivo and in vitro studies [2, 3] of alumina ceramic on ceramic hip replacements have demonstrated that swing phase microseparation followed by the impact of the femoral head on the acetabular insert rim can lead to accelerated wear. However, wear remains low. A similar trend was observed when metal on metal hip replacements were tested under microseparation conditions [4]. The purpose of the current study was to examine the wear of ceramic on polyethylene bearings under standard and microseparation conditions.

A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. The alumina ceramic heads and polyethylene cups were 28 mm in diameter and were tested for 5 million cycles in 25% new born calf serum at 1 Hertz. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, where the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation.

The volumetric wear of the polyethylene inserts was approximately four times less under microseparation conditions (5.6 ± 5.3 mm3 per million cycles), in comparison to standard conditions (25.6 ± 5.3 mm3 per million cycles). Deformation of the cup rim was observed, but some of this was attributed to creep. It is postulated that this reduction in wear was due to the separation of the components in swing phase improving the entrapment of lubricant, hence reducing wear via a squeeze film lubrication mechanism. In conclusion, surgical procedures that produce a small and controlled amount of joint laxity and microseparation may lead to a reduction in wear of the polyethylene acetabular cups.

In vivo and in vitro studies of ceramic on ceramic (COC) bearings have demonstrated that swing phase microseparation followed by the impact of the femoral head on the superior acetabular insert rim leads to accelerated wear. However, resultant wear remained low. The wear of ceramic on polyethylene (COP) and metal on metal (MOM) couples under swing phase microseparation is unknown, this study aimed to compare the wear of these total hip replacements under standard and microseparation conditions.

A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation. Components (as shown in table 1) were tested for 5 million cycles, at a frequency of 1 hertz in 25% (v/v) new born calf serum. Under standard conditions, wear of COC and MOM bearings was significantly lower than wear of COP couples. Under microseparation conditions the COC and MOM wear increased by 4 and 25 times respectively. Microseparation conditions reduced wear of COP couples by a factor of 4. Creep deformation and damage to the UHMWPE cup rim was observed, however, wear remained low. It is postulated that this reduction in polyethylene wear is due to the separation of the components in swing phase improving the entrapment of lubricant, hence wear is reduced via a squeeze film lubrication mechanism.

Introduction: UHMWPE wear debris is known to be a major cause of periprosthetic osteolysis and the long-term failure of total joint replacements by a macrophage-mediated mechanism. The aim of this study was to compare the in vitro response of mononuclear phagocytes from patients undergoing total hip arthroplasty to challenge with polyethylene particles or stimulation with lipopolysaccharide (LPS).

Methods: Peripheral blood was taken from 2 healthy donors and 16 patients admitted to hospital to undergo total hip arthroplasty. Human mononuclear phagocytes were isolated by density centrifugation. Polyethylene particles were sequentially filtered to obtain biologically active particles (0.1–0.6 μm diameter). Cells plus particles, cells plus LPS and cells only were co-cultured in supplemented RPMI-1640 culture medium. Culture supernatants were harvested and the concentration of TNFα quantified by ELISA. Mean specific activity was calculated.

Results:.

When considering all the subjects, no correlation was found between the response of their cells to polyethylene particles and LPS stimulation. However the cells of four subjects gave a much higher response to LPS than the rest and when these where excluded the correlation between the response to LPS and PE particles was significant with an R² value of 0.9076.

Discussion: Despite the different mechanisms by which PE particles and LPS activate macrophages, the patient group with ‘normal’ or low response to LPS had a significant correlation in their response to LPS and particle stimulation. Why a small number of subjects had a much higher response to LPS without a proportional response to PE particles is not known, but it could be due to an increased expression of LPS receptors or genetic polymorphism. A greater than ten-fold difference in the patient response to particles may be of clinical importance in their potential susceptibility to loosening through osteolysis.