Metal-on-metal (MOM) total hip arthroplasty and hip resurfacing using large diameter femoral heads offer clinical advantages, however the failure rates of these hips are unacceptably high. Retrieved MOM hips have a wide range of wear rates of their bearing surfaces and there is ongoing research to identify the causes of failure. Detailed visual inspection is the first step in the forensic examination of failed hip components, but there is no universally accepted description or process. Visible features may help explain the mechanism of failure of MOM hips. During our analysis of 2000 MOM hip components, we have developed protocols to undertake detailed, non-destructive macroscopic and stereomicroscopic examinations of each component, using quantitative assessment to document the presence of types of damage. We have established a systematic terminology to describe the types of damage that are observed, allowing for consistency and clarity in the vocabulary used. These include (but are not limited to): . Scratching – when there is an increase in the number and/or magnitude of scratches present on the surface, typically increasing measured roughness parameters. The severity of scratching is determined by rubbing a 0.18 mm thickness acetate gage or fingernail over the surface: . Light –visible but not detectable with gage. Moderate – visible and detectable with gage. Heavy – depth clearly visible and will catch a fingernail or gage. Pitting – indentations in the surface for which the dimensions are similar in all directions. Embedded Particles – hard particles that have become embedded in the bearing surface. Discolouration – observed as a change in the appearance of the surface, often as colour diffraction patterns. Polishing, gouges, etching (from corrosion), surface films, surface deposits and tribochemical reaction layers. To assess the distribution of these types of damage on the components, each surface is considered in terms of zones defined by quadrants (cup, head, stem and taper) and sub-quadrants (cup and head only), Figure 1. Each zone is scored on a scale of 0 to 3 by determining the percentage of the surface area of the quadrant that exhibits the feature in question: . a score of 0 indicates 0% . 1. indicates greater than 0% but less than 25%. 2. indicates between 25% and 75% inclusive. 3. indicates greater than 75%. The use of zones to differentiate between the polar and equatorial regions of the cup and head surfaces can offer important information relating to the articulation of the bearing surfaces. For example in a cohort of 90 inspections, a score of 3 for light scratching was recorded in all areas of the bearing surface in 40% of cups, whilst approximately 5% had evidence of pitting, occurring near the rim. Current research at our retrieval centre involves correlating the results of detailed inspections with a range of variables, including implant design, size and surgical position

Fluid film lubricating ability of a total hip prosthesis depends on the profile accuracies including surface-roughness or the sphericity of a head or a cup. Therefore, surface polishing is important. It was, however, difficult to polish the central portion of a cup or head using the conventional rotating machine. In the present study, we developed a polishing method combining a pendulum machine and a robotic arm. The effect of the accuracy improvement by this method was evaluated by the friction measurements on some test specimens.

Nine balls and a cup of Co-Cr-Mo alloy that were polished by a conventional process using a rotating machine were prepared for the prototype. The average diameter of the balls was 31.9648 mm with the sphericity of 0.0028 μm. The inside diameter of the cup was 31.9850 mm with the sphericity of 0.0044 μm. We combined a robotic arm and a pendulum apparatus to enable the further polishing. The ability of both automatic centering and change in the sliding direction was accomplished by this system. The sliding direction has been changed 180 times every ten degrees. The total distance of polishing was 120 m under vertical load of 100 N in a bath of saline solution containing abrasive grains of silicate of the diameter of 2μm. The surface roughness of the central portion of the cup, which is important area for the fluid film lubrication decreased from Ra 20.2 μm before the polishing to Ra 18.7 μm after the polishing.

A pendulum type friction tester was used for the assessment of the improvement of the lubricating ability by the polishing. The measurement was run over at 10 times under the conditions of the load of 600 N in a bath of saline solution. As the result, the frictional coefficients decreased from 0.1456–0.1720 before polishing to 0.1250–0.1300 after polishing. The polishing effect was, however, observed only at the specimens that radial clearances did not exceed the value of 50 μm.

The present results indicated that the surface polishing of the central portion of hip prostheses must improve the lubrication ability and the radial clearance before the finishing process should be chinked as possible.

Aims: The purpose of this study was to evaluate the patterns of wear of patella implants in total knee arthroplasty. Materials and Methods: The pattern of polyethylen wear of 17 patella components, 5 Kinematic and 12 Total Condylar, retrieved after an average of 80 month in situ, was studied. The primary diagnosis was osteoarthritis in all cases. The implants were retrieved from 6 men and 11 women with an average age of 64 years at the time of arthroplasty. Their average height was 168 cm and average weight was 68.5 kg. In all cases revision was performed for mechanical loosening of one or several components. Using light microscopy, the articulating surface of each patella component was analyzed for six modes of damage: Polishing, Delamination, Surface deformation due to cold flow, Scratching, Pitting and Abrasion. To describe the damage the surface of the patella component was divided into four sections. The contact stresses between the patellar and femoral components were calculated in relation to the areas of wear. Volumetric wear could not be accurately established. Results: The “Papion” pattern indicated higher rate of wear at the periphery of the patellar implants. Four modes of damage were observed: polishing in 13, delimitation in 12, cold flow in 6 and scratching in 3. The median total area of polyethylene damage was for polishing 76.5%, delimitation 70.6%, cold flow 35.3% and scratching 17.6%. The average contact stress on the nonconforming Total Condylar patella component was 12.9 Kgf/mm. 2. It was significantly higher (p< 0.002) than the average contact stress on the conforming Kinematic patellar component – 2.9 Kgf/mm. 2. The area of wear was smaller, 357.2 mm. 2. for the nonconforming Total Condylar, than for the conforming Kinematic patella 439.2 mm. 2. However, this difference was not statistically significant. The average weight of the patients with the Kinematic knee (74.5 kg) was higher as compared to the patients with Total Condylar knee (66 kg) but the difference was not significant. Conclusions: The high incidence and pattern of wear of the patella components indicates a basic fault in the design of the patellar implants. In our opinion there is insufficient thickness of the polyethylene in most points of contact specifically at the periphery. We suggest improving two mechanical properties: adequate thickness of the entire polyethylene implant including the periphery and conforming articulating surfaces of the periphery of the dome