Why don't all malpositioned resurfaced cups have high wear? A dynamic, gait lab study

INTRODUCTION: Metal-on-Metal Hip Resurfacing Arthroplasty (MoMHRA) implants can show low wear rates, secondary to fluid film lubrication. Under edge-loading conditions, lubrication is disturbed and wear dramatically increases. Malorientated cups are at greater risk of edge-loading, leading to increased wear, elevated serum metal ion levels and pseudotumour development. However, not all mal-orientated cups have increased wear and some patients with optimally orientated cups can have increased wear. These observations suggest that additional factors such as an individual’s activity patterns may be involved in the wear process. To-date however, all studies have reviewed MoMHRAs statically (Xrays and CTs) and have not considered the dynamic conditions.
OBJECTIVES: The aim of this in vivo, prospective study was to identify whether patient-specific activity patterns influence the risk of edge loading.
METHODS: One hundred and fifty eight (201 hips) MoMHRA patients had serum metal ion levels and cup orientations measured in this IRB-approved study. From this cohort, fourteen unilaterally resurfaced subjects were invited to participate in a motion analysis study. Patients were selected based on their metal ion levels (high ions if Cr>4.4 g/l and Co> 4.0 g/l) and cup orientations as per the previously defined optimum cup orientation (inclination/anteversion: 45°/20° ±10°). Subjects were divided into three groups: Well-orientated cups-low metal ions (n=6), mal-orientated cups-high metal ions (n=4) and mal-orientated cups-low metal ions (n=4). The theoretical contact patch to rim distance was determined for each patient using the radiographic cup orientation, hence representing static conditions.
RESULTS: There were no differences in gender, age, component size and weight between the groups (p=0.4 – 0.9).There was no difference in the theoretical contact patch to rim distance between the two groups with mal-orientated cups (p=0.5) (static representation). The HRF vectors of subjects with mal-positioned cups and high ion levels were closest to the edge of the acetabular component during both activities tested. The smallest distance of the HRF from the edge during STS for this group was 10.1 mm (SD:2.6), in comparison to 16.2 mm (SD:2.4) for the mal-positioned low ions groups and 16.9mm (SD:2.5) for the well positioned low ions group (p<0.001). Similar, but less significant differences in distance to edge (8.9/12.6/13.1) were observed during gait (p=0.05).
CONCLUSION: Findings of this study suggest that edge-loading is influenced by both cup orientation and intrinsic subject-specific activity patterns. Although subjects with mal-orientated cups are more likely to edge-load, intrinsic activity patterns of patients during functional activity may prevent edge-loading and explain why some subjects with malpositioned components have low ion levels. Greater differences were observed during STS, which may represent a more important action as the hip enters a loading state following a period of rest.