FEMORAL COMPONENT WEAR IN RETRIEVED HIP PROSTHESES

On the roughness measurement on knee prostheses

The majority of total knee replacements currently implanted present an articulation composed of two metal parts, femoral and tibial components, between which there is a polyethylene insert serving as a bearing surface. The finishing surface of the metal components is a very important factor in minimizing the polyethylene wear rate and, later, the gradual production of metal and plastic debris. Considering the role of surface roughness on volumetric wear rates, the purpose of this study is to develop a protocol for the roughness characterization of total knee prosthesis (TKP) metal components, taking into consideration a limited number of points on each surface. Six mobile TKP of different sizes (three size 2 and three size 6) were tested on a knee joint simulator to compare the wear behavior of each group. After 2 million cycles the weight loss by the polyethylene inserts was measured with gravimetric method and the surface roughness of the metallic components was assessed in terms of average surface roughness, Ra, skewness, Rsk, and total roughness, Rt. Roughness measurement involved 29 points on each femoral condyle and 26 points on each metal tibial plate. The data collected has shown an increased roughness upon wear testing for both the investigated TKP sizes. No statistical differences were observed between the two groups for both the parameters Ra, Rsk, and Rt. The surface of all metallic components became more negatively skewed, indicating diminishing peaks. The various parameters were correlated to the volumetric loss using a linear regression analysis.

Ceramic bearing surfaces

Despite more than 25 years of clinical experience with ceramic materials as bearing surfaces, their role in modern joint replacement surgery remains to be clearly defined. The two primary materials are alumina and zirconia. The application of these materials is primarily as a femoral head bearing surface against polyethylene, but alumina also is used as a femoral head and an acetabulum to provide a polyethylene-free bearing surface. Important issues that must be clarified for these materials to gain wide acceptance are the material properties, wear rates against polyethylene and alumina, the biologic response to ceramic wear debris, and cost in relation to indications. The bulk materials are biocompatible, hard, wettable, high-strength, and can yield good surface finishes. Linear polyethylene wear against alumina heads is reported to be as much as a factor of 5 to 10 lower than metal versus polyethylene. Thus, the ceramic femoral head may be a good choice for the younger patient in whom it seems necessary to use a larger head for stability reasons with a polyethylene cup. Ceramic-on-ceramic wear rates are in the range of 0.003 mm/year, a factor of 10 less than the lowest polyethylene wear rates. These costly materials are limited in head and neck sizes because of statistical variation in strength that can lead to fracture. Occasional reports of high alumina-on-alumina wear have appeared. Many of the problems of the past have been design, manufacture, or application related, and have been improved or eliminated. Proper clinical technique in the use of ceramic femoral heads is crucial to prevent fracture. The materials hold high promise and should continue to be used so that additional experience can help define the clinical indications for components made of these materials.