Abstract
Metal-metal total hip replacements were commonly used in the 1960’s and early 1970’s. Failures usually occurred as a result of aseptic loosening although many of these implants provided long term function similar to metal-UHMWPE bearings.
Metal-metal bearings used in total hip arthroplasty are made of cobalt chrome. The volume of wear generated from a metal-metal bearing is considerably less than that from a metal-UHMWPE bearing. Factors which may affect the metal-metal wear rates include the clearance (difference in radius between the femoral head and acetabular bearing surface), surface roughness, and carbon content of the cobalt chrome alloy. A small clearance provides more contact area between the two surfaces which decreases contact stress while a large clearance permits more fluid flow into the joint. If the clearance is too small, and exceeds manufacturing tolerances, the joint articulation may become excessively tight and equatorial rather than polar contact occurs between the bearing surfaces which can increase frictional torque and cause loosening. This has been implicated as a cause of failure of the McKee-Fararr metal-metal hip replacements. Wear is also increased with increased surface roughness but the effect of other material variables such as cast vs. forged and carbon content are less clear.
Early clinical results with modern metal-metal hip replacements demonstrate clinical results which are comparable to metal-UHMWPE bearings and less aseptic loosening than metal-metal designs used in the 1960’s and 1970’s. However, concerns with metal-metal hip replacements include the generation of metal particulate debris which may travel to the distal sites as well as local osteolysis.
In patients with metal-polyethylene total joint replacements, metal as well as polyethylene particles can be found at distant sites. Urban et al. (1) reported that in a study of postmortem specimens from patients with metal-polyethylene total joint replacements, metallic wear particles were identified in the para-aortic lymph nodes in 68 percent and in the liver or spleen in 38 percent of the patients. The serum and urine levels of cobalt and chromium are elevated in patients with metal-metal articulations. Metals can travel to distant sites in ionic form and little is known about the long term clinical effects of elevated serum and urine metal levels. Cancer risk has not been shown to be increased in patients who have received metal-metal hip replacements. However, long term studies with large numbers of patients are needed to accurately asses this risk.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
