Abstract
Introduction
Hip implant research has been carried out for decades using hip simulators to reflect situations in vivo. With regards to metal on metal (MoM) implant testing, it has been reported that there is no significant difference between the wear generated by various cobalt chromium (CoCr) microstructures. On the contrary, higher wear, metal ion levels and subsequent failures have been reported in heat treated (high carbon, low carbide) devices compared to as cast (high carbon, high carbide) devices in vivo. During testing, the bearing surfaces may be masked from the effect of microstructure on wear under fast and continuous cycles, while in vivo, the extensive range of kinetics and kinematics, stop/start motion, varying walking frequencies could break down the fluid film, resulting in a less favourable lubrication regime. The aims of this study were to develop a more physiologically relevant hip simulator test protocol, and investigate the effect of microstructure on wear.
Materials & Methods
Three pairs of 50mm as cast (AC) and four pairs of 50mm double heat treated (DHT) CoCr MoM devices were tested in a ProSim hip simulator. In order to determine the frequency for testing, Patients' activities have been monitored using a Step Activity Monitor (SAM) device. The data showed a relatively slower walking pace (frequency) than that used in the hip simulator studies. The new frequency, along with stop/start motion and various kinetics and kinematics profiles have been used in putting together a more physiologically relevant hip simulator test protocol. The lubricant used in this study was new born calf serum with 0.2 % (w/v) sodium azide concentration diluted with de-ionised water to achieve an average protein concentration of 20 g/l. Gravimetric measurements have been taken at 0.5, 1, 1.5 & 2 million cycle (Mc) stages and ion analysis has been carried out on the serum samples.
Results & Discussions
A biphasic wear pattern similar to the parts in vivo was observed. Under the newly developed physiologically relevant test conditions, the DHT CoCr devices generated 40% higher wear than the AC CoCr devices (Figure 1).
The metal ion analysis results also showed a similar biphasic wear trend, however, the difference between the AC and DHT devices was further increased by approximately 30 % at 2 Million cycle stage (Figure 2).
It has been reported that the DHT devices generate smaller size particles and in much larger numbers compared to those generated by the AC devices. This would result in a larger net surface area of the wear particles exposed to corrosion and thus would contribute to a higher amount of metal ion levels with the DHT devices compared to AC devices.
Conclusion
The in vitro results obtained with the new test protocol correlate well with the in vivo results. The higher wear, metal ion levels observed with double heat treated CoCr devices compared to as cast CoCr in vivo were also represented in vitro, highlighting the effect of microstructure on wear.
