Abstract
Summary
The required torque leading to an abrasion of the passive layer in the stem-head interface positively correlates to the assembly force. In order to limit the risk of fretting and corrosion a strong hammer blow seems to be necessary.
Introduction
Modular hip prostheses are commonly used in orthopaedic surgery and offer a taper connection between stem and ball head. Taper connections are exposed to high bending loads and bear the risk of fretting and corrosion, as observed in clinical applications. This is particularly a problem for large diameter metal bearings as the negative effects may be enhanced due to the higher moments within the taper connection. Currently, it is not known how much torque is required to initiate a removal of the passive layer, which might lead to corrosion over a longer period and limits the lifetime of prostheses. Therefore, the purpose of this study was to identify the amount of torque required to start an abrasion of the passive layer within the interface dependent on the assembly force and the axial load.
Materials and Methods
Titanium hip stems (Furlong H-AC, JRI, UK) and cobalt-chromium heads (⊘ 28mm, size L, JRI, UK) were assembled using a drop rig with peak forces of 4.5 kN (FP,1, n = 4) or 6.0 kN (FP,2, n = 4). The prostheses were inverted and then mounted with the head rigidly fixed to the base of a materials testing machine using a non-conducting (nylon) jig while submerged in Ringer's solution. The stems were attached to the machine actuator via non-conductive plates. An axial load (FA,1 = 1 kN, FA,2 = 3 kN, n = 4 each) was applied to the stems along the taper axis. After a period of equilibration a torque, increasing from 0 up to 15Nm, was manually applied. The galvanic potential at the taper interface was continuously recorded using a titanium electrode. The torque required to cause a drop in the potential of 5% was identified. For statistical analyses non-parametric tests were performed (α = 0.05).
Results
Four different phases of the potential could be clearly differentiated during testing: equilibrium, removal of the passive layer leading to a drop of the potential, repassivation and then a second equilibrium. Prostheses assembled with a force of 6 kN required a significantly higher torque to start a removal of the passive layer compared to those with 4.5 kN (7.2 ± 0.5 Nm vs. 3.9 ± 1.0 Nm for FA,1, p = 0.029). In contrast, no influence of the axial load on the fretting behaviour of the prostheses could be found (8.0 ± 1.6 Nm for FP,2, p = 0.486).
Discussion
Changes in the galvanic potential were observed at low torque levels for a small head diameter. With increasing head diameter the tangential force leading to a removal of the passive layer in the stem-head interface decrease resulting in a higher risk for corrosion. Component assembly with a high force reduces the risk of fretting and corrosion in the taper interface; however, it is feasible that the determined torque levels can still be reached, particularly in situations of large weight and high activity of the patient or malpositioning of the prosthesis in the body.
