In Vivo Damage of the Head-Neck Junction in Hard-on-Hard Total Hip Replacements: Effect of Femoral Head Size, Metal Combination, and 12/14 Taper Design

Abstract

Recently, concerns have been raised about the potential effect of head-neck junction damage products at the local and systemic levels. Factors that may affect this damage process have not been fully established yet. This study investigated the possible correlations among head-neck junction damage level, implant design, material combination, and patient characteristics. Head-neck junctions of 148 retrieved implants were analysed, including both ceramic-on-ceramic (N = 61) and metal-on-metal (N = 87) bearings. In all cases, the male taper was made of titanium alloy. Damage was evaluated using a four-point scoring system based on damage morphology and extension. Patient age at implantation, implantation time, damage risk factor, and serum ion concentration were considered as independent potential predicting variables. The damage risk factor summarises head-neck design characteristics and junction loading condition. Junction damage correlated with both implantation time and damage factor risk when the head was made of ceramic. A poor correlation was found when the head was made of cobalt alloy. The fretting-corrosion phenomenon seemed mainly mechanically regulated, at least when cobalt alloy components were not involved. When a component was made of cobalt alloy, the role of chemical phenomena increased, likely becoming, over implantation time, the damage driving phenomena of highly stressed junctions.

Keywords: fretting corrosion; hard-on-hard bearings; head-neck junction; total hip replacements.

Figure 1

Hip joint load and frictional moment acting on the femoral head. Left: the force acting on the centre of the femoral head is split into two components, an axial force and an orthogonal force to the head-neck junction (HNJ) axis; Right: the frictional moment about the instantaneous rotation axis is split into two components, a torsional moment and a bending moment.

Figure 2

Left: head-taper offset when the head is assembled on a 12/14 male taper; Right: head-adapter sleeve and head-taper offset when an adapter sleeve is included.

Figure 3

The dimensions of the 12/14 taper were measured by means of a digital calliper: proximal diameter of the contact area, i.e., the smallest diameter of the male taper that was engaged with the head bore; contact length i.e., the axial length of the male taper that was engaged with the head bore. All measurements were rounded to 0.1 mm. The head diameter was also measured. The taper angle was achieved from the manufacturer’s specifications. The centroid height was calculated.

Figure 4

Plot of residuals versus implantation time and DRF. R² and root mean square error (RMSE) calculated for each material combination are reported.

Figure 5

The bore of a ceramic head and the male taper of its Ti-alloy neck. Metal transfer is visible in the head bore. The male taper is damaged specularly. EDX spectra of ceramic and material spread on ceramic surface (a), and Ti-alloy and fretted area (b) are shown.

Figure 6

The bore of a Co-alloy head, its adapter sleeve (Co-alloy), and its male taper made of Ti-alloy. Damaged areas are visible in the 12/14 junction. The head-adapter sleeve junction is very slightly damaged. EDX spectra of Co-alloy and slightly damaged area on the head bore (a); Co-alloy and deposits on the sleeve male taper (b); Co-alloy and damaged area on the sleeve bore (c); and Ti-alloy and deposits between the ridges on the male taper (d) are shown.