Abstract
To further extend the longevity of total hip replacement, bearings with better wear characteristics are desirable. Despite generally inferior clinical results with metal-on- metal total hip replacements, many metal-on-metal implants lasted over two decades or are still functioning in patients who received the implant at a young age. Acetabular component wear and loosening limit the survival of hips. Because of this, long-term survival rates of the Charnley hip and the McKee-Farrar are similar. Consequently, there is renewed interest in metal-on- metal bearings for total hip arthroplasty.
Aseptic loosening of early metal-on-metal prostheses was not uniform due to the metal-on-metal bearings or due to wear. There is evidence indicating that some metal-on-metal bearing couples were poorly designed and/or manufactured leading to high frictional torque and/or excessive wear which could have been the cause of failure. Very low wear has been observed on metal-on- metal bearings retrieved after more than 20 years of use. The wear characteristics are a function of materials, design, and manufacturing. Polar bearing is preferred to reduce frictional torque, but excessive radial clearance is associated with higher wear. As has been demonstrated in hips with metal-on-plastic bearings, clinical success and failure are multifactorial. The chance of success with new metal-on-metal bearings is increased when the bearing is combined with well-established femoral stems and acetabular shells. As has been seen in hips, which have metal-on-polyethylene bearings, loosening of hips with metal-on-metal bearings can occur due to other factors such as suboptimal stem and/or cup design, manufacturing or implantation technique.
Studies of the levels of cobalt and chromium in the hair, blood, and urine have shown that metallic content in patients with metal-metal total hips are generally higher than in patients with metal-UHMWPE articulations. While the release of cobalt and chromium ions from metal-metal total hips has been verified, the clinical significance of this finding is still unclear. Cobalt and chromium wear particles have been shown to induce carcinoma in animal models, giving rise to the concern that such alloys could have the same effect if present in sufficient amounts in human tissue for a sufficient length of time. Overall, the available epidemiological data do not demonstrate an increase in cancer risk following total hip replacement. At the same time, it is important to recognise the limitations of the available data with regard to sample size, length of follow-up and lack of stratification for other co-morbidities.
The issue of delayed-type hypersensitivity (DTH) to the main elements in metal-metal total hips – cobalt, chromium, and nickel. Skin patch testing is unreliable for the assessment of hypersensitivity to implants, however, this type of testing has shown that cobalt, chromium, and nickel are associated with contact dermatitis. Because there is a higher reported incidence of metal sensitivity in patients with loose components, the association between metal sensitivity and loose implants has fuelled a long-standing debate: does hypersensitivity cause loosening or does loosening cause hypersensitivity? A small number of patients with metal-metal total hips develop an adverse local tissue response and present with unexplained pain and chronic effusions that resolve when the metal-metal bearings are exchanged for metal-UHMWPE hips. The histology of abundant lymphocytes and plasma cells is highly suggestive of an immune response. Caution should be taken in the implantation of a metal-metal bearing in patients with a known sensitivity to metals.
Further investigations of the local and systemic effects related to the wear of the primary articulating surfaces are needed. It should be emphasised, however, that clinical success is multifactorial. Patient selection, surgical technique, component fixation, and the other aspects of the prosthetic joint will influence the clinical performance of any articulation.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
