Abstract
Introduction
Friction between head and cup is a primary factor for survival of total hip joint replacement (THR) and its gliding surfaces. In up to 40% of all revisions, the cup or inlay must be replaced as result of friction-induced wear [1]. Aim of the study was to measure the friction-induced temperature increase in vivo in THR and to identify possible individual parameters of influence.
Methods
For the in vivo measurement, an instrumented implant with an Al2O3/XPE-pairing and an integrated temperature sensor was used [Fig. 1] [2]. Ten patients were provided with such an instrumented implant.
Up to now, long time measurements were performed on six of these patients (Ø63y, Ø89kg). During these measurements, the subjects walked Ø60min on a treadmill with 4km/h. The investigation was performed Ø61 (43–70) months post operatively. Short time (Ø3min) in vivo load measurements during walking on treadmill were already available from the other four patients. These data were used to calculate the peak temperatures after 60mins of walking by using a model, based on the long time measurements.
Results
The peak values of the friction-induced temperature increase were achieved in vivo after 30min (H7R) to 70min (H2R), with peak temperatures between 1.5°C (H6R) to 4.8°C (H7R) [Fig. 2]. These maximum values were similar to those already observed in other patients [3]. The in vivo measured peak values of the friction-induced temperature increase after long time walking on a treadmill with respect to the implant orientation are shown in Fig. 3 as points and the calculated peak values as circles.
First analyses have shown that the individual implant orientations seem to have an influence [Fig. 3] on the friction-induced increase of the joint temperature during walking, but also the patient's age.
Discussion
The gliding partners and joint lubrication directly influence friction in artificial hip joint replacements and thus the friction- induced temperature increase. Analyses of the in vivo acting joint friction during walking have shown that there is an increase in friction over the course of each gait cycle after contralateral toe off [4]. This can be explained by a decrease in the lubricating film thickness due to the pressing out of the synovia from the joint space. During load reduction of the joint in the swing phase, the fluids are transported back into the joint space. Thus, the level of joint friction at the beginning of the next gait cycle depends on the return transport of the synovia.
The influence of the sum anteversion angle (ΣAV) on friction-induced temperature increase (Fig. 3) can therefore be explained mechanically: The ΣAV determines the functional joint roofing and the position of the load-transferring zone into the joint socket. The larger the ΣAV, the more it shifts towards the edge of the socket, and the shorter the path for the return transport of the synovium.
