In vivo evaluation of edge-loading in metal-on-metal hip resurfacing patients with pseudotumours

Abstract

Objectives: Pseudotumours (abnormal peri-prosthetic soft-tissue reactions) following metal-on-metal hip resurfacing arthroplasty (MoMHRA) have been associated with elevated metal ion levels, suggesting that excessive wear may occur due to edge-loading of these MoM implants. This study aimed to quantify in vivo edge-loading in MoMHRA patients with and without pseudotumours during functional activities.

Methods: The duration and magnitude of edge-loading in vivo was quantified during functional activities by combining the dynamic hip joint segment contact force calculated from the three-dimensional (3D) motion analysis system with the 3D reconstruction of orientation of the acetabular component and each patient's specific hip joint centre, based on CT scans.

Results: Edge-loading in the hips with pseudotumours occurred with a four-fold increase in duration and magnitude of force compared with the hips without pseudotumours (p = 0.02).

Conclusions: The study provides the first in vivo evidence to support that edge-loading is an important mechanism that leads to localised excessive wear (edge-wear), with subsequent elevation of metal ion levels in MoMHRA patients with pseudotumours.

Keywords: Edge-loading; Hip resurfacing; In vivo evaluation; Metal ions; Metal-on-metal; Pseudotumours.

Fig. 1

Coronal Short TI Inversion Recovery (STIR) MRI image of a typical example of predominantly solid pseudotumour with low signal intensity (arrows).

Fig. 2

Diagram showing force paths projected on the acetabular component viewed in the direction through the centre of the component. The inner bearing surface was divided into concentric zones defined in 10% increments of the component face radius, with the zone at the edge designated as zone 1. In hip A (in blue), the force path does not enter the outer most radial zone (zone 1), thus no edge-loading is observed. In hip B (in red), during walking, the force path enters the outer most radial zone (zone 1), indicating edge-loading. The black circles (●) indicate force path at heelstrike and the black triangles (▲) indicate force path locus at toe-off.

Figs. 3a - 3c

Graphs showing the distribution of ‘zone duration’ (the percentage of total stance time spent by the force path in each zone) during a) walking, b) stair climbing and c) rising from a chair. Zone 1 is defined as the edge-loading zone. The error bars represent standard errors of mean. An asterisk (*) indicates significant difference between the two MoMHRA patient groups.

Figs. 3a - 3c

Graphs showing the distribution of ‘zone duration’ (the percentage of total stance time spent by the force path in each zone) during a) walking, b) stair climbing and c) rising from a chair. Zone 1 is defined as the edge-loading zone. The error bars represent standard errors of mean. An asterisk (*) indicates significant difference between the two MoMHRA patient groups.

Figs. 3a - 3c

Graphs showing the distribution of ‘zone duration’ (the percentage of total stance time spent by the force path in each zone) during a) walking, b) stair climbing and c) rising from a chair. Zone 1 is defined as the edge-loading zone. The error bars represent standard errors of mean. An asterisk (*) indicates significant difference between the two MoMHRA patient groups.

Figs. 4a - 4c

Graphs showing the distribution of normalised hip joint ‘force impulse’ (the cumulative magnitude of the segment force throughout activity over time estimated by calculating the area under the force/time curve normalised to patient body weight) in each zone during a) walking, b) stair climbing and c) rising from a chair. Zone 1 is defined as the edge-loading zone. The error bars represent standard errors of mean. An asterisk (*) indicates significant difference between the two MoMHRA patient groups.

Figs. 4a - 4c

Graphs showing the distribution of normalised hip joint ‘force impulse’ (the cumulative magnitude of the segment force throughout activity over time estimated by calculating the area under the force/time curve normalised to patient body weight) in each zone during a) walking, b) stair climbing and c) rising from a chair. Zone 1 is defined as the edge-loading zone. The error bars represent standard errors of mean. An asterisk (*) indicates significant difference between the two MoMHRA patient groups.

Figs. 4a - 4c

Graphs showing the distribution of normalised hip joint ‘force impulse’ (the cumulative magnitude of the segment force throughout activity over time estimated by calculating the area under the force/time curve normalised to patient body weight) in each zone during a) walking, b) stair climbing and c) rising from a chair. Zone 1 is defined as the edge-loading zone. The error bars represent standard errors of mean. An asterisk (*) indicates significant difference between the two MoMHRA patient groups.

Fig. 5

Scatter graph showing the angles of inclination and anteversion of the acetabular component for the pseudotumour group (9 hips in 6 patients) and the non-pseudotumour group (21 hips in 13 patients). Lewinnek’s safe zone is outlined by the dotted rectangle.

Figs. 6a - 6b

Boxplots showing the median serum cobalt (Co) (a) and chromium (Cr) (b) level measurements in the six patients with psuedotumour and the 13 patients without. The boxes represent the median and interquartile range, and the whiskers denote the range of data excluding outliers (°, between 1.5 and 3×IQR) and extremes (*, > 3×IQR).

Figs. 6a - 6b

Boxplots showing the median serum cobalt (Co) (a) and chromium (Cr) (b) level measurements in the six patients with psuedotumour and the 13 patients without. The boxes represent the median and interquartile range, and the whiskers denote the range of data excluding outliers (°, between 1.5 and 3×IQR) and extremes (*, > 3×IQR).