Serum Level of Cobalt and Chromium After Ceramic on Metal Articulation Total Hip Arthroplasty

Peri-prosthetic osteolysis is associated with aseptic loosening of a total hip replacement (THR). It is understood to be a biological response to wear particles, with polyethylene debris thought to be the major cause of the long-term failure.

With total hip replacement (THR) patients now being younger, more active younger, and anticipating much greater longevity, reduced bearing wear becomes increasingly important. Compared to the wear and osteolysis incidence with metal-polyethylene bearings, the historical results of alternate bearings such as metal-on-metal (MOM) and ceramic-on-ceramic (COC) generally showed much lower wear rates and osteolysis was seemingly quite rare. The laboratory wear-rates for metal-metal bearings have generally fallen into the range 1-7 mm3/million cycles (Mc) and 0.1-1 mm3/Mc for run-in and steady-state phases, respectively.

Ceramic-on-ceramic and metal-on-metal bearing surfaces are often employed for total hip replacement because of their resistance to wear. Alternative bearing surfaces such as ceramic-on-ceramic and metal-on-metal eliminate polyethylene wear. Explanted metal-on-metal articulations made of cobalt-chromium-molybdenum (Co-Cr-Mo) alloy have been shown to have extremely low rates of linear wear, and ceramic-on-ceramic bearings even less. A ceramic-on-ceramic articulation was first implanted in 1970 in France. Long-term reports have shown excellent clinical results.

Ceramic-on-ceramic bearings have the advantage of low wear and high biocompatibility. Alumina-ceramic particles are essentially insoluble in organic media and ionisation is therefore unimportant. However, they have some limits: brittleness is a major concern for ceramic, which could lead to clinically relevant failure rates (0.004% head fracture and 0.22% liner fracture. Although Metal-on-metal (MOM) articulations have been seen as one potential solution to the problems associated with UHMWPE-induced osteolysis, metal ions present in the serum and their potential toxic effects both locally and systemically are a cause for concern.

A low wear rate is believed to be critical for extending the implant life of a prosthetic joint, and wear volumes produced by MOM articulations have been estimated to be 40-100 times lower than metal-on-polyethylene bearings. The wear of MOM prostheses is known to be highly dependent upon the materials, tribological design and finishing technique. Clinical studies of retrieved first and second-generation MOM hip prostheses have shown linear penetrations of approximately 5 μm/year. This is equivalent to a wear volume of approximately 1 mm3/year, two orders of magnitude lower than conventional polyethylene acetabular cups.

The observation that a small number of patients with first-generation MOM prostheses exhibited good clinical and radiographical results after 20 years in vivo led to the development of second-generation MOM hip prostheses, and in 1988 the Metasul prosthesis was introduced into clinical practice. This comprised of a cobalt chrome alloy femoral head articulating on a cobalt chrome alloy acetabular cup. Over 200,000 Metasul combinations have been implanted to date. Short-term clinical performance has been encouraging; with low wear rates, and few prostheses requiring revision. However, long-term clinical performance is as yet unknown.

Although MOM hip prostheses produce significantly lower wear rates than conventional UHMWPE-on-metal couples, and lower wear rates than highly crosslinked UHMWPE-on-metal hip prostheses, there are concerns associated with these bearings. Wear particles have been reported to be in the nanometer size range, an order of magnitude smaller than UHMWPE particles. Therefore, despite a lower wear volume than UHMWPE bearings the number of particles produced are estimated to be greater, possibly between one and ten million particles per step. These small particles have the potential to distribute throughout the body via the lymphatic system, with particles found in the lymph nodes, liver, spleen and bone marrow. As MOM hip prostheses are indicated for younger more active patients these particles will be present at these sites for a long period of time, possibly 30-40 years.

Concern has been expressed about the biological response to metal particles released from metal-on-metal couplings. The levels of cobalt and chromium ions have been measured in the blood and serum in patients with successful THR, but information about the possible release of aluminum from ceramic-on-ceramic articulations is lacking.

A better understanding of the influence of the metallurgy and tribology on wear of the component and improved manufacturing technology have allowed the re-introduction of metal-on-metal articulations in total hip replacement. Excellent early clinical outcome has been reported with the new generation of metal-on-metal components. Retrieval studies of previous metal-on-metal bearings and contemporary implants have shown low rate of wear. These hard bearing surfaces are especially promising for young and active patients. Although metal-on-metal bearings produce significantly less wear debris than metal-on-polyethylene bearing surfaces, the debris generated results in the body being exposed to metal ions for prolonged periods. Concerns exist over metal hypersensitivity, osteolysis, chromosomal mutation, carcinogenicity and fetal exposure to high ion levels. These matters require further investigation, particularly the long-term exposure in younger patients and the recognition that some patients may have unusually high ion levels. From tribological studies, joint simulation testing and clinical trials, it has been proposed that the extent of wear can be reduced by using components with a larger diameter with a high carbon content in the alloy (0.2%), well-adjusted clearance for the components, better implant sphericity and lower surface roughness.

Whilst research has shown that high concentrations of nanometre-sized metal wear particles are cytotoxic to human fibroblasts and macrophages in vitro, there are also concerns about the release of metal ions from these small particles, and the potential effect that these ions have on cells and tissues. Elevated ion concentrations have been reported in both the blood and urine of patients with metallic implant components. In addition, contrary to what might be expected, there is no conclusive evidence that ion levels fall in vivo after the bedding in period of the MOM prostheses, during which wear of the prostheses are higher. Cobalt and chromium ions have high toxicity, and there are very real concerns about the effects of sub-lethal doses of metal ions, which have been shown to cause DNA damage. This damage takes the form of chromosome aberrations and chromosome translocations. Over long periods of exposure the worry is that this will lead to the development of certain types of cancer, such as leukaemia and lymphoma. In animal models elevated levels of cobalt, chromium and nickel have been correlated with increased carcinoma rates. However, reports in the literature of malignancies developing after total hip or knee replacement surgery are exceedingly rare. Out of eight epidemiological studies on the relative risk of cancer after TJR, only one study looked specifically at MOM implants. This study did not find an increased risk of cancer development in the subjects receiving MOM hip prostheses compared to those receiving metal-on-polyethylene hip prostheses. However, all the studies to date have been substantially underpowered in terms of patient numbers required to show a difference between the two prosthesis types.

High levels of cobalt and chromium ions are detected in the blood and urine of patients with metal-on-metal (MOM) hip replacement despite, even with these elements are released as a result of wear at the bearing surfaces. The clinical effects of elevated metal ion levels include local tissue reactions, characterized by increases in T and B cells in the tissues surrounding the hip, and are associated with premature failure of the hip. Systemically, one report suggests reduced numbers of CD8-positive T cells. Metal sensitivity is also a potential problem. Metal ions, whether produced secondary to wear debris or via corrosion can initiate a hypersensitivity response. A delayed cell-mediated response, or delayed-type hypersensitivity response can occur, in which cytokines are released by T-lymphocytes and increased activation of macrophages is seen, which may result in T-cell mediated periprosthetic osteolysis. Many metals can initiate a hypersensitivity response, the most common is nickel followed by cobalt and chromium. Therefore, with the well-documented elevation of cobalt and chromium ions in patients with MOM hip implants, there is a theoretical risk of developing hypersensitivity reactions. Recently, an immune response exclusively associated with second-generation MOM hip prostheses, has been described. Histomorphological changes suggest a type of hypersensitivity reaction to the all-metallic implants. The hypersensitivity hypothesis was further strengthened by the observation that these patients experienced early clinical failure at 11-60 months (mean 29 months), and the fact that patients who received a second MOM prosthesis did not experience any relief of symptoms. Conversely, patients who received either metal-on-polyethylene or ceramic-on-polyethylene couples reported that their symptoms completely disappeared. In a control group of patients with joint prostheses not containing cobalt, chromium or nickel these signs of an immunological reactions were absent. Reports of this type of reaction are becoming increasingly common; however, more research is needed in this area. It is not known whether these patients experience prosthesis failure because of a pre-existing metal sensitivity, or whether metal sensitivity develops because of a high wearing bearing and elevated metal ion levels. Theoretically, there would be an increased probability of developing hypersensitivity to elevated metal ion levels, and hence an increased risk of implant failure.

To reduce the effect of these limitations, such as the breakage of the ceramic insert in COM bearing and to reduce ions release in MOM bearing a hybrid coupling of ceramic-on-metal has been proposed. The theoretical advantage of this new coupling might lead orthopedic surgeons to use it indiscriminately.