Quantitative analysis of polyethylene wear debris, wear rate and head damage in retrieved Charnley hip prostheses.

Submicrometer- and micrometer-sized ultra-high molecular weight polyethylene (UHMWPE) wear particles have been associated with osteolysis and failure of total artificial joints. Previous studies have isolated predominantly submicrometer-sized particles at the expense of larger particles (>10 microm). This study aimed to isolate and characterize quantitatively all sizes of UHMWPE wear particles generated in 18 Charnley hip prostheses. In addition, to analyze the wear debris with respect to the total volumetric wear of the cup and damage to the femoral head. Particle size distributions ranged from 0.1 to ->1000 microm. A significant proportion (3-82%) of the mass of the wear debris isolated was>10 microm. The mode of the frequency distribution of the particles was in the range 0.1-0.5 microm for all patients. However, analysis of the mass of wear debris as a function of its size allowed differentiation of the wear debris from different patients. Femoral head damage was associated with high volumetric wear and increased numbers of biologically active submicrometer-sized particles.

Wear of ultra-high molecular weight polyethylene against damaged and undamaged stainless steel and diamond-like carbon-coated counterfaces.

The wear of ultra-high molecular weight polyethylene (UHMWPE) in artificial joints and the resulting wear debris-induced osteolysis remains a major clinical concern in the orthopaedic sector. Third-body damage of metallic femoral heads is often cited as a cause of accelerated polyethylene wear, and the use of ceramic femoral heads in the hip is gaining increasing favour. In the knee prostheses and for smaller diameter femoral heads, the application of hard surface coatings, such as diamond-like carbon, is receiving considerable attention. However, to date, there has been little or no investigation of the tribology of these coatings in simulated biological environments. In this study, diamond-like carbon (DLC) has been compared to stainless steel in its undamaged form and following simulated third-body damage. The wear of UHMWPE was found to be similar when sliding against undamaged DLC and stainless steel counterfaces. DLC was found to be much more damage resistant than DLC. Under test conditions that simulate third-body damage to the femoral head, the wear of UHMWPE was seven times lower against DLC than against stainless steel (P < 0.05). The study shows DLC has considerable potential as a femoral bearing surface in artificial joints.

A study of the combined effects of shelf ageing following irradiation in air and counterface roughness on the wear of UHMWPE.

Damage to polished femoral heads in vivo can cause increased wear of acetabular cups. Oxidation and ageing after sterilisation by gamma irradiation in air, can also change the mechanical properties and wear resistance of ultra high molecular weight polyethylene (UHMWPE). This study investigated the combined effect of these changes in material properties on the wear of UHMWPE for different counterface roughnesses, representative of new femoral heads and those damaged in vivo. Wear rates were studied on a tri-pin-on-disc tribometer in a protein-containing solution. A comparison was made of the wear, using three different counterface roughnesses, of specimens that were manufactured from polyethylene acetabular cups of different shelf ages (3-120 months) after gamma irradiation in air but never implanted. These were compared to the wear of control specimens that were manufactured from cups that had not been sterilised. The wear surfaces were tested 1 mm below the initial articulating surface of the cup, the position of high degradation. The wear rate of UHMWPE which had been sterilised by gamma irradiation in air was shown to increase significantly with ageing time on the shelf for all counterface conditions. The wear rate of all materials increased markedly as the counterface roughness increased, but to different extents depending on the age of the material. The combined effect of ageing and increase in counterface roughness had a dramatic effect (as high as 2000 fold increase) on the wear rate. Both ageing of the polymer and damage to the femoral head have been cited as causing increased wear in vivo. The results of this study demonstrate that these variables can act synergistically to markedly effect UHMWPE wear rate.

Ultra-high molecular weight polyethylene wear debris generated in vivo and in laboratory tests; the influence of counterface roughness.

The objective of this study was to investigate the effect of counterface roughness and lubricant on the morphology of ultra-high molecular weight polyethylene (UHMWPE) wear debris generated in laboratory wear tests, and to compare this with debris isolated from explanted tissue. Laboratory tests used UHMWPE pins sliding against stainless steel counterfaces. Both water and serum lubricants were used in conjunction with rough and smooth counterfaces. The lubricants and tissue from revision hip surgery were processed to digest the proteins and permit filtration. This involved denaturing the proteins with potassium hydroxide (KOH), sedimentation of any remaining proteins, and further digestion of these proteins with chromic acid. All fractions were then passed through a 0.2 micron membrane, and the debris examined using scanning electron microscopy. The laboratory studies showed that the major variable influencing debris morphology was counterface roughness. The rougher counterfaces produced larger numbers of smaller particles, with a size range extending below 1 micron. For smooth counterfaces there were fewer of these small particles, and evidence of larger platelets, greater than 10 microns in diameter. Analysis of the debris from explanted tissues showed a wide variation in the particle size distribution, ranging from below 1 micron up to several millimetres in size. Of major clinical significance in relation to osteolysis and loosening is roughening of the femoral components, which may lead to greater numbers of the sub-micron-sized particles.

Preliminary study of the effect of aging following irradiation on the wear of ultrahigh-molecular-weight polyethylene.

Several studies have indicated that degradation of ultrahigh-molecular-weight polyethylene following gamma irradiation in air adversely affects the mechanical properties of the material; however, it is not known how this subsequently affects its wear rate. Wear studies have therefore been performed on three groups of ultrahigh-molecular-weight polyethylene; unirradiated material, recently irradiated material (aged for 2 months), and aged irradiated material (aged for 5 years). The aging took place in sterile packaging on the shelf. The wear studies were carried out on a tri-pin-on-disk wear tester, with a pin from each type of material being studied in each test. In each test the wear rate of the nonirradiated material was slightly lower than the 2-month-aged, irradiated material. The 5-year-aged, irradiated material had the highest wear rate, and this was significantly greater than that of the unirradiated material (P < .05).

The influence of scratches to metallic counterfaces on the wear of ultra-high molecular weight polyethylene.

A number of studies of explanted metallic femoral heads have shown scratches or damage caused by bone cement, bone or metallic particles. This damage has been cited as a cause of increased wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. In this laboratory study, small scratches 2 mu m deep were made on smooth stainless steel surfaces at a spacing of 10 mm. These individual scratches were found to increase the wear rate of UHMWPE by a factor of 30 in unidirectional sliding and a factor of 70 in reciprocating motion. It is of particular concern that a single small scratch, which is not detected by the average surface roughness measurement Ra can cause such a dramatic increase in the wear of ultra-high molecular weight polyethylene.

A tribological study of a series of retrieved accord knee explants.

A tribological study of a series of 27 retrieved Accord meniscal bearing knee joints has been carried out. The roughness of the articulating surfaces of the meniscal and the femoral components was investigated, and the penetration into the polyethylene meniscal component was determined. There was generally little damage to the highly polished metallic femoral components. A general polishing and smoothing of the ultra high molecular weight polyethylene meniscal components was observed, although there was some subsequent deterioration of the polyethylene surfaces in certain cases. The penetration rate of the femoral components into the menisci was low, and was found to be similar to that reported for other meniscal knee joints.

An in vitro study of the effect of environment and storage time on the fracture properties of bone cement.

Changes either within the bone cement or at the cement-bone interface are known to contribute to loosening and hence failure of many cemented joint replacements. This study examines the in vitro changes in the fracture properties of bone cement as a result of storage, at both 21 and 37 degrees C, in air, water, Ringer's solution and lipid over a period of 2 years. Specimens stored in the fluid media were found to behave in a more ductile manner than those stored in air. Samples stored at 37 degrees C behaved in a more brittle manner than those stored at 21 degrees C. Although the work of fracture values measured for the samples stored in the water-based media increased during the first 18 months, this was followed by a decrease in the subsequent 6 months.