Taper junction failure in large-diameter metal-on-metal bearings

Abstract

Objectives: An ongoing prospective study to investigate failing metal-on-metal hip prostheses was commenced at our centre in 2008. We report on the results of the analysis of the first consecutive 126 failed mated total hip prostheses from a single manufacturer.

Methods: Analysis was carried out using highly accurate coordinate measuring to calculate volumetric and linear rates of the articular bearing surfaces and also the surfaces of the taper junctions. The relationship between taper wear rates and a number of variables, including bearing diameter and orientation of the acetabular component, was investigated.

Results: The measured rates of wear and distribution of material loss from the taper surfaces appeared to show that the primary factor leading to taper failure is the increased lever arm acting on this junction in contemporary large-diameter metal-on-metal hip replacements.

Conclusions: Our analysis suggests that varus stems, laterally engaging taper systems and larger head diameters all contribute to taper failure.

Keywords: Adverse reaction to metal debris; Arthroplasty; Hip; Large diameter; Metal-on-metal; Taper.

Fig. 1

Photograph of an Articular Surface Replacement (ASR) XL head with Corail taper adaptor. The arrows represent the taper engagement level.

Fig. 2

Image from coordinate measuring machine (CMM) analysis of the Corail taper seen in Figure 1. The boldest red areas indicate the areas of maximal wear (i.e., the taper engagement level).

Fig. 3

Radiograph showing the measurement of the horizontal lever arm (HLA) distance (bold black line). The HLA is the horizontal distance (in mm) from a line through the axis of the neck to the tip of the bearing surface (broken line) and the centre of the taper engagement level.

Fig. 4

Scanning electron microscopy (SEM) images of an area of unworn manufactured taper surface (left) and an area deeper in the same taper that shows the imprint of the machining grooves of the trunnion (right). Note: images are at the same level of magnification.

Fig. 5

Scanning electron microscopy (SEM) images of a taper that has experienced greater wear, at the same level of magnification as in Figure 4 (left), showing that the peaks and troughs caused by the impression of the machining grooves have been sheared off, leading to significant material loss, and at a higher magnification (right), showing the formation of pits with inclusion bodies, probably as a result of mechanical wear

Fig. 6

Scatter graph showing the relationship between linear wear rates of the taper surfaces and the horizontal lever arm (HLA) distance (all taper components included) (ASR, Articular Surface Replacement).

Fig. 7

Boxplot showing the distribution of head taper angle in all retrieved implants in the study. The box represents the median value and interquartile range (IQR) and + the mean value. The whiskers correspond to the limits of the data, beyond which values are considered anomalous (°, outliers; *, extreme outliers; •, upper and lower values). The bold rectangle represents the area outside of which excessive micromotion has been shown to take place by finite element analysis.