Metal-on-metal retrieval studies indicated that MOM wear-rates could rise as high as 60–70mm3/year in short-term failures (Morlock, 2008). In contrast, some MOM and ceramic-on-ceramic (COC) devices of 1970's era performed admirably over 2–3 decades (Schmalzreid, 1996; Shishido, 2003). While technology has aided analysis of short-term MOM and COC failures (Morlock 2008; Lord 2011), information on successful THA remains scant. Lack of long-term data creates difficulties in setting benchmarks for simulator studies and establishing guidelines for use in standards. In this study we compared clinical and wear histories for a 30-year MOM and a 32-year COC to establish such long-term, wear-rates.

The McKeeTM retrieval was cemented and made 100% of CoCr alloy (Fig. 1a). This patient had a right femoral fracture at 47 years of age, treated by internal-fixation, which failed. Her revision with a Judet implant also failed, leaving her right hip as a Girdlestone. At the age of 68, she had a McKee THA implanted in left hip, and used it until almost 98 years of age (Campbell, 2003). The COC case was a press-fit AutophorTM THA, head and cup made of alumina ceramic, with the only metal being the CoCr stem (Fig. 1c). This was implanted in a female patient 17-years of age active in sports (water-skiing). This modular THA was revised 32-years later due to hip pain from cup migration. Wear on these implants was identified by stereomicroscopy and stained red for photography (Fig. 1). Cup-to-neck impingement was denoted by circumferential neck notching, roughness was assessed by interferometry, and wear determined by CMM (Lord, 2011).

McKee head wear covered 1092mm2 area (Figs. 1a, 2: hemi-area ratio 58%). There was no stripe wear and head roughness was 36nm (Ra). Cup wear covered an area of 1790mm2 (hemi-area 63%). Circumferential damage was noted on the supero-posterior femoral neck with scuff marks also on posterior collar (Fig. 2c). Head and cup wear amounted to 37.7 and 25.2mm3, respectively. Total MOM wear was 62.9mm3, indicating a wear-rate of 2.1mm3/year.

Ceramic head wear consisted of two circular patterns (Fig. 1c), the major one of area 1790mm2 (hemi-area 79%). No wear stripes were identified. Non-worn and extensively worn surfaces had roughness (Ra) 17nm and 123nm, respectively. The cup showed 360o circumferential arc of rim wear with a small, non-wear zone inferiorly (Fig. 1c). Gray metallic transfer was evident, EDS revealing Co and Cr (Fig. 3a). Head and cup wear volumes were 77.2 and 54mm3, respectively. Total COC wear amounted to 131.2mm3 indicating a wear-rate of 4.1mm3/year.

These two THA functioned successfully over 3 decades. The McKee retrieval had minor signs of impingement but no adverse “stripe wear”. This MOM performed satisfactorily due to good positioning and patient's advanced age (68 to 98Yrs of age). The COC patient was 17 years of age at index surgery and active. The ceramic cup showed 360o of edge wear, CoCr transfer and a COC wear-rate double that of the MOM retrieval. Thus the high ceramic wear-resistance protected this youthful patient.

Although bovine serum is the lubricant recommended by several international standards for the wear testing of orthopedic biomaterials there are issues over its use. The inherent batch variation in protein content means that two bovine serum lubricants can give different wear rates. Due to degradation, the lubricant needs to be changed regularly, so that any third body wear particles are removed, thus potentially influencing wear regimes. There are also cost and safety issues with the use of bovine serum. For these reasons, alternative lubricants were investigated.

A 50-station wear test rig was used, which applied multi-directional motion to each ultra-high molecular weight polyethylene (UHMWPE) test pin. Each pin articulated against a cobalt chrome plate polished to better than 0.05 microns Ra. The following lubricants were used: 50% dilute bovine serum; soy protein; olive oil; wheatgerm oil; soya oil; albumin and globulin (AG) mix; albumin, globulin and chondroitin sulphate (AGC) mix; whole milk; Channel Island milk; 11 mg/ml protein egg white; 20 mg/ml egg white; and 40 mg/ml egg white. A minimum of 6 UHMWPE pins per lubricant were wear tested and the tests ran to 2.5 million cycles. Gravimetric measurements were taken throughout the test to determine the volume of wear and at the end of the test the samples were examined using a SEM.

The lubricants giving the closest results to bovine serum were 20 and 40 mg/ml egg white, with mean UHMWPE total wear volumes of 17.4 mm3 and 17.8 mm3 compared to bovine serum which gave 20.7 mm3. Surface topographies showed similar features too. The 11 mg/ml egg white lubricant and the AG and AGC lubricants were next closest in terms of wear. An UV absorbance assay found that all the protein based lubricants suffered from a high degradation rate, and the rate increased with increasing protein content.

Egg white may offer a less expensive alternative to dilute bovine serum as a test lubricant although it is likely that it too would need to be changed as regularly as bovine serum.

In designing artificial joints the main criteria are to reduce the wear-rate of the material and the reaction of the body to the wear particles produced. These can be achieved by using harder materials (metals, ceramics) or by reducing the chances of producing a wear particle by the choice of material and design. This paper will look at combinations of PEEK-OPTIMA against different counterfaces with the aim of reducing the wear-rate of artificial hips and knees.

Pin-on-Plate Studies Twenty-six different sets of experiments combining PEEK-OPTIMA in different formulations and against different counterfaces were conducted to evaluate the lowest wear combination.

The lowest wear-rate combination was CFR-PEEK PAN against low carbon CoCrMo alloy (K=0.144×10-6 mm3/Nm) which is only about 1/8th of the wear of UHMWPE against stainless-steel (1.1×10-6 mm3/Nm). Gamma radiation sterilisation did not seem to affect the PEEK wear-rate.

Hip Simulator Studies A 25 million cycle wear study has been conducted on the Durham Hip Simulator using 54 mm diameter alumina heads against CFR-PEEK thin-walled acetabular cups (MITCH). Five joints were in active stations and one acted as a loaded control. Wear was measured gravimetrically.

Particles were analysed using a NanoSight LM10 instrument at 0.5, 10 and 25 million cycles. Also an Atomic Force Microscope (AFM) was used to look at particles above 2μm which is the limit of the NanoSight instrument.

The wear-rate was linear over the whole 25 million cycle test at 1.16 mm3/ million cycles (range 0.811–1.392 mm3/million cycles). As the test progressed, the number of particles reduced and the dominant particle size increased from about 40nm to circa 200 nm. The AFM showed some particles as large as 3μm to exist also. No fluid film lubrication was observed to be generated in these joints so the low wear-rate was due to the inherent low-wear properties of the material combination.

Knee Simulator Studies CFR-PEEK was moulded into the interpositional bearings for experimental lateral and medial unicompartmental knee designs and tested for 5 million cycles using 5 pairs of active joints and one pair of loaded controls in the Durham Knee Simulator. Wear was measured gravimetrically.

Whilst the CFR-PEEK components gave a total wear-rate (both medial and lateral) of 2.72 mm3/million cycles, UHMWPE inserts in a similar application¹, gave 9.67mm3/million cycles. This represents a reduction of 72 per cent for the wear of the CFR-PEEK components.

Conclusions CFR-PEEK used in the correct combination and application can give a much reduced wear-rate compared with UHMWPE. It does not have the problem of metal ion release and has been shown by others not to exhibit cytotoxicity.²

Introduction: In an attempt to prolong the lives of implantable devices, several ‘new’ materials are undergoing examination to determine their suitability as joint couplings. As part of a series of tests, polyetherether-ketone (PEEK) against cobalt chrome molybdenum (CoCrMo) and carbon fibre reinforced-PEEK against CoCrMo were tested on a multidirectional pin-on-plate machine.

Materials and methods: The two four station pin-on-plate machines used in this study applied both reciprocation and rotational motion. Each material combination was tested individually on separate machines. Four samples of PEEK pins against CoCrMo plates were tested and eight samples (two tests) of CFR-PEEK pins against CoCrMo plates were tested. The pins were supplied by Invibio Ltd. A 40 N load was provided to each station. The lubricant used was 24.5 % bovine serum (protein content: 15 g/l) and this was heated to 37 degrees C. The wear was assessed gravimetrically and the tests each completed 2 million cycles.

Results: On average, the pin and plate wear factors were 7.37 and 0.010 x 10 -6 mm3/Nm for PEEK-CoCrMo and 0.144 and 0.011 x 10 -6 mm3/Nm for the CFR-PEEK against CoCrMo specimens respectively. These results show the wear of the components corrected relative to the control specimens that therefore took into account the weight gain due to lubricant absorption.

Discussion: The CFR-PEEK pins gave considerably lower wear against CoCrMo than the PEEK pins. It is interesting to note that the total wear factor provided by high carbon CoCrMo pins articulating against high carbon CoCrMo plates (which is known as a low wearing material combination in hip implants) was found to be 0.84 x 10 -6 mm3/Nm (1) which is actually higher than that found in these studies for CFR-PEEK against CoCrMo tested under the same conditions.

Conclusions: CFR-PEEK articulating against CoCrMo provided much lower wear than the PEEK-CoCrMo samples. This material combination also gave lower wear than metal-on-metal samples. This, therefore, indicates that this material combination may perform well in joint applications.

New generation alumina-on-alumina (A-A) prostheses have been introduced to try and overcome the problem of osteolysis often attributed to polyethylene wear particles liberated within conventional metal-on-ultra high molecular weight polyethylene (UHMWPE) joints. This study uses a hip simulator to compare the volumetric wear rates of five different radial clearances of A-A joints. Atomic force microscopy (AFM) provided topographic characterisation of the prosthesis surfaces throughout the wear test.

Materials and methods: The wear test was performed on the Durham hip joint wear simulator. The 28 mm diameter alumina ceramic couples investigated were manufactured by Morgan Matroc Ltd., in accordance with ISO 6474. Four radial clearances (33, 40, 48 and 74 μm) of A-A joints were tested to one million cycles with 25% new-born calf serum as the lubricant. Contact mode AFM (TopoMetrix Explorer SPM) was used to produce a topographical map of the poles of the four alumina heads every 0.1 million cycles. Every 0.2 million cycles the wear was assessed gravimetrically using a Mettler AE 200 balance (accurate to 0.1 mg).

Results and discussion: There was no measurable wear of either the heads or cups during this one million cycle wear period. Throughout the wear test the alumina equiaxed grain structure became apparent on the AFM images, the mean alumina grain size was 2 μm. The grain surfaces were below the mean femoral head surface height. Such topography was not observed on an as-received head. Some granular pull-out also took place. As the wear test proceeded, the average area surface roughness increased from 2.33 nm to 4.42 nm for the heads but stayed relatively constant for the cups (from 2.75 nm to 2.97 nm).

Conclusions: The very low wear produced by these A-A hip joints is very difficult to measure gravimetrically as it is close to the limits of resolution of the weighing equipment. The surface topography analysis, however, shows changes to the ceramic surfaces during the wear test and gives an indication of the wear processes and lubrication regimes acting within such joints. The authors wish to thank EPSRC for funding this research and Morgan Matroc Ltd. for supplying the joints.

Modular knee bearings typically consist of a femoral component, a tibial base-plate and a polyethylene insert, which is located in the tibial base-plate using some sort of locking mechanism. Although modular knee bearings offer many advantages there is the potential for micro-motion between the tibial insert and the base-plate.

Tests were performed on six large Kinemax Plus knee bearings (snap-fit design) to evaluate the amount of movement between the tibial inserts and the tibial base-plates. The knee bearings were tested up to one million cycles on the Durham Six-Station Knee Wear Simulator which subjected the bearings to similar motion and loading profiles that would be experienced by the natural knee during walking. The movement of the tibial inserts was measured with dial gauges (accuracy of ±0.01 mm) before and after the bearings were tested on the simulator, when unloaded, and throughout the tests whilst the bearings were being dynamically loaded in the simulator. Movement occurred between the tibial insert and the tibial base-plate after initial assembly due to the snap-fit mechanism used to locate the tibial insert within the tibial base-plate. However, this decreased appreciably when the bearings were loaded in the simulator. The amount of movement did not change with time when the bearings were continuously loaded in the simulator. However, after each test the amount of movement of the tibial inserts, when unloaded, was less than before the test. This was thought to be due to creep of the UHMWPE inserts. The movement between the tibial insert and tibial base-plate in-situ is likely to be much less than that observed by a surgeon at the time of assembly due to loading of the knee bearing in the body. However, the amount of movement when the tibial inserts are loaded may still be great enough to produce a second interface where wear of the tibial insert may take place.