Socket wear in ceramic-on-polyethylene total hip arthroplasties: fixed versus rotating heads

The long-term outcome of the cemented Weber acetabular component in total hip replacement using a second-generation cementing technique

We report the long-term outcome of a modified second-generation cementing technique for fixation of the acetabular component of total hip replacement. An earlier report has shown the superiority of this technique assessed by improved survival compared with first-generation cementing. The acetabular preparation involved reaming only to the subchondral plate, followed by impaction of the bone in the anchorage holes. Between 1978 and 1993, 287 total hip replacements were undertaken in 244 patients with a mean age of 65.3 years (21 to 90) using a hemispherical Weber acetabular component with this modified technique for cementing and a cemented femoral component. The survival with acetabular revision for aseptic loosening as the endpoint was 99.1% (95% confidence interval 97.9 to 100 after ten years and 85.5% (95% confidence interval 74.7 to 96.2) at 20 years. Apart from contributing to a long-lasting fixation of the component, this technique also preserved bone, facilitating revision surgery when necessary.

Properties of crosslinked ultra-high-molecular-weight polyethylene

Substantially reducing the rate of generation of wear particles at the surfaces of ultra-high-molecular-weight polyethylene (UHMWPE) orthopedic implant bearing components, in vivo, is widely regarded as one of the most formidable challenges in modern arthroplasty. In the light of this, much research attention has been paid to the myriad of endogenous and exogenous factors that have been postulated to affect this wear rate, one such factor being the polymer itself. In recent years, there has been a resurgence of interest in crosslinking the polymer as a way of improving its properties that are considered relevant to its use for fabricating bearing components. Such properties include wear resistance, fatigue life, and fatigue crack propagation rate. Although a large volume of literature exists on the topic on the impact of crosslinking on the properties of UHMWPE, no critical appraisal of this literature has been published. This is one of the goals of the present article, which emphasizes three aspects. The first is the trade-off between improvement in wear resistance and depreciation in other mechanical and physical properties. The second aspect is the presentation of a method of estimating the optimal value of a crosslinking process variable (such as dose in radiation-induced crosslinking) that takes into account this trade-off. The third aspect is the description of a collection of under- and unexplored research areas in the field of crosslinked UHMWPE, such as the role of starting resin on the properties of the crosslinked polymer, and the in vitro evaluation of the wear rate of crosslinked tibial inserts and other bearing components that, in vivo, are subjected to nearly unidirectional motion.

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.