IMPLICATIONS OF FOUR BEARING COMBINATIONS ON BONE MINERAL DENSITY ADJACENT TO HYBRID HIP ARTHROPLASTY, SERUM CONCENTRATIONS OF METALS, AND PLASMA CONCENTRATIONS OF CYTOKINES

Abstract

At present we conduct a clinical study on four bearing combinations in hip arthroplasty. Our main purpose is to assess changes in bone mineral density (BMD), function of the joint, and to monitor serum concentrations of prosthetic metals as well as plasma concentrations of a range of cytokines, chemokines, and related proteins during a ten-year follow-up. This is done in order to evaluate the potential role of these variables as predictors of dysfunction or loosening of the arthroplasty.

A total of 300 patients were randomly allocated to four bearing combinations. Four years after surgery the following number of patients were available for follow-up: Type A: Zirconia ceramic head, polyethylene cup insert in the Universal RingLoc metal backed shell (n=50); Type B: Cobalt-Chrome-Molybdenum head and cup insert in the Universal RingLoc metal backed shell (n=57); Type C: Zirconia ceramic head, polyethylene moulded on the Titanium shell of the Asian cup (n=55); Type D: Alumina head and cup insert in the Universal RingLoc metal backed shell (n=45). A BiMetric Titanium-Aluminium-Vanadium (Ti6Al4V) stem was used with all four combinations (n=207). All patients, but two with rheumatoid arthritis, had primary osteoarthritis or avascular necrosis of the femoral head. Five patients had astma and eight had diabetes.

At the time of surgery the groups were equal with regard to age, gender distribution, body height and weight, side of arthroplasty, and BMD in all seven Gruen zones. Harris Hip Score prior to arthroplasty was equally low in all groups (mean ± SD) (42 ± 17), and increased in all groups, with no significant differences between them (87 ± 10).

At follow-up there was a significant decrease in BMD in all Gruen zones ranging from −1.9 % in zone 4 in group C to −21.7% in zone 7 in group D. However, there were no significant differences between groups.

There were significantly higher blood concentrations of Chromium and Cobalt in group B patients compared to all other groups (p < 0.001).

Plasma concentrations of cytokines IL-1β, IL-6, IL-8, IL-10, TNF-α, TNF-R1, VEGF, OPG, GM-CSF, or TGF-β₁ did not differ significantly between groups. Instead, elevated levels of IL-1β, IL-10 and TNF-R1 were found in patients with asthma. IL-6, TNF-α and VEGF were elevated in patients with rheumatoid arthritis or asthma. IL-8 and TGF-β₁ were higher in patients with osteoarthritis, whereas GM-CSF was high in patients with asthma or diabetes.

CONCLUSION: The elevated serum concentrations of Chromium and Cobalt were seen in the group of patients using a bearing with these metals. It still remains to be elucidated, whether high serum concentrations of bearing surface metals signify a dysfunction of the prosthesis. Concentrations of plasma cytokines are mainly related to patient comorbidity when no sign of prosthetic loosening is present. Changes in BMD adjacent to the cemented femoral stem can not be attributed to any of the bearing materials used in this study.