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.