Large-diameter metal-on-metal total hip arthroplasty: dislocation infrequent but survivorship poor

Abstract

Background: Use of large-diameter metal-on-metal (MoM) articulations in THA increased, at least in part, because of the possibility of achieving improved joint stability and excellent wear characteristics in vitro. However, there have been subsequent concerning reports with adverse reactions to metal debris (ARMD), pseudotumors, and systemic complications related to metal ions.

Questions/purposes: The purpose of this study was to determine at a minimum of 2 years' followup (1) the proportion of patients who experienced a dislocation; (2) the short-term survivorship obtained with these implants; (3) the causes of failure and the proportion of patients who developed ARMD; and (4) whether there were any identifiable risk factors for revision.

Methods: We reviewed the results of 1235 patients who underwent 1440 large-diameter MoM primary THAs at our institution using two acetabular devices from a single manufacturer with minimum 2-year followup. Large-diameter MoM devices were used in 48% (1695 of 3567) of primary THAs during the study period. We generally used these implants in younger, more active, higher-demand patients, in patients considered at higher risk of instability, and in patients with adequate bone stock to achieve stable fixation without use of screws. Clinical records and radiographs were reviewed to determine the incidence and etiology of revision. Patients whose hips were revised were compared with those not revised to identify risk factors; Kaplan-Meier survivorship analysis was performed as was multivariate analysis to account for potential confounding variables when evaluating risk factors. Minimum followup was 2 years (average, 7 years; range, 2-12 years); complete followup was available in 85% of hips (1440 of 1695).

Results: Dislocation occurred in one hip overall (<1%; one of 1440). Kaplan-Meier analysis revealed survival free of component revision was 87% at 12 years (95% confidence interval, 84%-90%). The two most common indications for revision were ARMD (48%; 47 of 108 hips revised) and loosening or failure of ingrowth (31%; 34 of 108). Risk factors for component revision were younger age at surgery (relative risk [RR] 0.98 per each increased year; p=0.02), higher cup angle of inclination (RR 1.03 per each increased degree; p=0.04), and female sex (RR 1.67; p=0.03).

Conclusions: Large-diameter MoM THAs are associated with a very low dislocation rate, but failure secondary to ARMD and loosening or lack of ingrowth occur frequently. Patients with MoM THA should be encouraged to return for clinical and radiographic followup, and clinicians should maintain a low threshold to perform a systematic evaluation. Early diagnosis and appropriate treatment are recommended to prevent the damaging effects of advanced ARMD.

Level of evidence: Level IV, Therapeutic study.

Fig. 1

The M2a-38 design (Biomet, Inc, Warsaw, IN, USA) was introduced at our center in October 2001. It is a monoblock CoCr acetabular component with a fixed 38-mm inner diameter, increasing outer diameter, and applied Ti-PPS fixation surface.

Fig. 2

The Magnum™ design (Biomet, Inc, Warsaw, IN, USA) was used in our center beginning in November 2004 through February 2010. It is a resurfacing style monoblock CoCr acetabular component coated with Ti-PPS and features a 3-mm fixed shell thickness, inner diameters up to 60 mm, and outer diameters up to 66 mm. It articulates with a modular CoCr head component that is effectively 6 mm smaller in diameter than the cup, which is mated with a titanium insert taper adapter.

Fig. 3

Kaplan-Meier analysis revealed survival free of component revision was 87% at 12 years for the 1235 patients with 1440 large-diameter MoM THA in our study.

Fig. 4

Kaplan-Meier analysis for the two acetabular devices in our study showed no significant difference in survival, with 87% survival at 12 years for the M2a-38 (Biomet, Inc, Warsaw, IN, USA) and 88% survival at 10 years for the Magnum™ (Biomet, Inc).