Abstract
Summary
Five year migration results of 49 large-head metal-metal (MoM) total hip arthroplasties show good implant stability and no association between implant migration and metal-ions levels, stem and cup position, or femoral bone mineral density.
Introduction
The failure mechanism of metal-metal (MoM) total hip arthroplasty has been related to metal wear-debris and pseudotumor, but it is unknown whether implant fixation is affected by metal wear-debris.
Patients and Methods
In July-August 2012 41 patients (10 women) at a mean age of 47 (23–63) years with a total of 49 MoM hip arthroplasties (ReCap Shell/M2a-Magnum head/Bi-Metric stem; Biomet Inc.) participated in a 5–7 year follow-up with blood tests (chrome and cobalt serum ions), questionnaires (Oxford Hip Score (OHS) and Harris Hip Score (HHS), measurement of cup and stem position and periprosthetic BMD. Further the patients had been followed with stereo-radiographs post-operative and at 1, 2 and 5 years for analysis of implant migration (Model-Based RSA 3.32).
Results
4 patients (6 hips) had elevated metal-ion levels (>7ug/l). The mean cup inclination was 45°(sd 6), the mean cup anteversion was 17°(sd7), and the mean stem anteversion was 19°(sd7). The difference between genders was statistically insignificant (p>0.09).
At 5 years follow-up total translation (TT) for the stems (n=39 hips) was a mean 0.79mm (sd 0.53) and total rotation (TR) was a mean 1.99° (sd 1.53). Between 1–2 years there was no significant difference in mean TT (p=0.49)for the stems and between 2–5 years TT was mean 0.13 mm (sd 0.35) which was significant (p=0.03) but clinically very small and within the precision limits of the method. We found no significant migration along the 3 separate axes. There was no significant association between stem migration and metal ion levels >7ug/l (p=0.55), female gender (p=0.86), stem anteversion > 25° (p=0.29), T-scores < −1 (p=0.23), total OHS < 40 (p=0.19) or total HHS < 90 (p=0.68). Between 1–5 years there was no significant change in neither subsidence (p=0.14) nor in version (p=0.91) of the stems.
At 5 years TT for the cups (n=36) was mean 1.21 mm (sd 0.74) and TR was mean 2.63° (sd 1.71). Between 1–2 years cup migration along the z-axis was mean 0.29 (sd 0.73) (p=0.03), which was also within precision limits of the method. There was a positive association between total OHS below 40 (n=4) and cup migration (p=0.04), but no association between cup migration and metal ion levels >7ug/l (p=0.80), female gender (p=0.74), cup inclination > 50° (p=0.93), cup anteversion > 25° (p=0.88) or HHS < 90 (p=0.93). Proximal cup migration at 5 years was mean 0.46 mm (sd 0.47), which was similar to the cup migration at 1 year (p=0.91) and 2 years (p=0.80) follow-up.
No patients were revised before the final 5–7 year follow-up. Patient satisfaction was high (94%).
Conclusion
All cups and stems were well-fixed between 1–5 years. We found no statistical significant correlation between implant migration and other factors that have been associated with failure of MoM hip arthroplasty such as elevated metal ion levels, component position, and female gender. Cup migration was higher in patients with a total OHS below 40. In conclusion, metal wear-debris does not seem to influence fixation of hip components in large-head MoM articulations at mid-term follow-up.
