Abstract
INTRODUCTION
Metallic ion release may be related to bearing surface wear and thus serve as an indicator of in-vivo performance of metal on metal (MOM) articulations.
OBJECTIVES
Compare large head MOM hip components with modular MOM and metal on polyethylene (MOP) to determine their relative effects on serum metal ion levels.
METHODS
A prospective controlled trial to compare clinical, radiographic, and serum metal ion concentration (Co and Cr) results between the Large Head ASR XL System (MOM-1), the Ultamet Advanced Modularity System (MOM-2), and the Pinnacle Acetabular Cup System with polyethylene liner (MOP). We enrolled 151 consecutive patients (MOM-1 = 97, MOM-2 = 22, MOP = 32). Radiographs, clinical scores (Harris Hip and WOMAC), and serum ion level assessments (in Parts Per Billion - PPB) were performed pre-operatively and post-operatively (6, 12, and 24 months for all patients and 60 months for MOM-1). At 60 months, we compared WOMAC, Harris Hip scores and serum ion levels (Co and Cr) from MOM-1 (revisions excluded) to 24 month scores from MOM-2 and MOP. After revision, serum ion levels were measured at 1, 3, 6, and 12 months.
RESULTS
In MOM-1, 11 patients had significantly elevated ion levels at all postoperative periods (Co AVG 130.35 PPB and Cr AVG 61.46 PPB) after 2 years. Excluding outliers and revised patients, average serum ion levels in the 86 remaining MOP-1 patients at 24 and 60 months were statistically higher than MOP and MOM-2 patients. However, this difference was not clinically relevant (no symptoms, no revisions, Co and Cr AVG below 5 PPB). Table 1. Nine hips (9.3%) in 8 MOM-1 patients required revision and serum ion levels decreased rapidly post-up, but have still not returned to baseline after 1 year. Serum ion levels were not significantly different between MOM-2 and MOP groups at any time. Clinical scores improved after surgery in all groups and continued to improve in MOM-2 and MOP patients after 2 years, but decreased slightly in the MOM-1 patients at 2 and 5 years. Average cup inclination angle did not differ significantly between the groups: MOM-1 50.2, MOM-2 47.8, and MOP 51.7.
CONCLUSIONS
We are presenting 5 years of prospectively collected data comparing ion levels among 2 MOM and 1 MOP group, as well as post revision ion levels at 1 year. Average serum ion levels were elevated at all post-operative periods in the MOM-1 group, but this was attributable to significantly elevated levels in a subset of outliers that ultimately required revision. Excluding these outliers, there is a statistically significant but clinically irrelevant difference in post-operative ion levels between MOM-1 vs. MOM-2 and MOP. There was no radiographic evidence of component malposition or aseptic loosening in any group. MOM-2 and MOP performed comparatively across all variables. We present an algorithm to diagnose and manage patients with MOM THA (Yable 2.) and offer evidence that metal ion levels do decrease after revision. Table 3.
