Serum-cobalt levels with metal-on-metal bearings in the cement-free total hip arthroplasty results covering two years; prospective study.

PURPOSE OF THE STUDY: Total hip arthroplasty increases the use of alternate bearings to prevent polyethylene wear as the number of younger and more active patients has drastically risen. We carried out a prospective randomized study, to assess and compare clinical results and radiological changes, serum-cobalt- and serum-aluminium-levels when metal-on-metal and ceramic-on-ceramic bearings are applicated.

MATERIAL AND METHODS: After giving informed consent 80 consecutive patients were included in this prospective randomized study. They were randomly assigned to receive either a metal-on-metal or a ceramic-on-ceramic bearing in their total hip replacement. Eligible were patients with a primary coxarthrosis or an avascular necrosis of the head of femur. Of the 80 patients 54 were females and 26 males. 42 patients were randomized to a metal-on-metal bearing and 38 patients were randomized to a ceramic-on-ceramic bearing. The average patient-age was 65,8 years and the mean body mass index was 27,7 at the time of operation. Surgery was performed through a transgluteal approach in supine position under general or spinal anaesthesia. A forged conical threaded acetabular component made of titanium-aluminium-niobium alloy was used in all patients. The metal inlays and the 28 mm metal heads were made of Co-28Cr-6Mo alloy with a carbon content of 0,2%. The ceramic inlays and the 28 mm ceramic heads are Al2O3 implants. We used as femoral component a conical rectangular stem of a titanium-aluminium-niobium alloy. Cup and stem werde implanted cementfree. Clinical data werde obtained at a follow up at a minimum of two years after implantation. Patients were assessed with the Harris Hip Score and the University of California at Los Angeles activity scale. 72 of the 80 patients could be explored clinically and radiologically.

RESULTS: The 2 year follow up check showed clinically and radiologically no difference between the two groups. The median Harris HipScore was above 90 points and the UCLA score was about 7 points. The medium serum-cobalt level was in the metal-on-metal group about 1.2μg/L and in the ceramic-on-ceramic group below the detection limit. The medium serum aluminium level showed values of 1.2 respectively 1.3 μg/L. The luminescencies in the metal-on-metal group were increased, but all components of the prosthesis could be regarded as stable.

DISCUSSION: It was the goal of our prospective study to compare clinical and radiological results of hip arthroplasty in metal-on-metal and ceramic-on-ceramic bearings and assess the serum-cobalt and the serum-aluminium levels. The socalled "biocorrosion" is still a high risk element for loosening of implants because of aseptic osteolysis. A higher release of polyethylene and metal particles is triggered, which leads to a slow but continuous process of inflammation. Apart from the debris also metal ions in a higher concentration are released, so that we could detect after some years in patients with artificial implants a higher level of metal in blood and urine. So far we did not recognize any carcinogenity or the appearance of renal disorders, or other mutagene effects in our patients. We could also not observe any fractures in ceramic implants. We are convinced that short time and intermediate reports are necessary, although final judgement can only be based on long term data.

CONCLUSION: Our prospective randomized study showed after two years no difference clinically between the two groups of metal-on-metal and ceramic-on-ceramic bearings with total endoprostheses of the hip. Although medium serum-cobalt level in the metal-on-metal group with 1,2u/L is a significant higher value, whereas it lies in the ceramic-on-ceramic group below the detectable limit.