High cycle in vitro hip wear of and in vivo biological response to vitamin E blended highly crosslinked polyethylene

Statistical analysis of VEXLPE wear against alumina produced by a new 200-station, multidirectional pin-on-disk device

With low wear rates shown by contemporary bearing materials of total hip prostheses, the standard deviation of wear rate is relatively high. Therefore, large sample sizes are needed for an adequate power of test. Because wear tests take a long time, it is practical to test several samples simultaneously. A new high-capacity, multidirectional wear test device, called the SuperCTPOD-200, was introduced. A 3 million-cycle wear test with an unprecedented sample size of 200 was performed for VEXLPE. The duration of the test was 6 weeks. The wear factor was normally distributed with a mean ± SD of 1.64 × 10-7 mm3/Nm ± 0.22 × 10-7 mm3/Nm (n = 200). The observation that SD was 13.1% of the mean can be useful in power analyses of future tests with other highly cross-linked polyethylenes. Burnishing was the most typical feature on the worn pins, which was in agreement with clinical findings on retrieved acetabular liners. The present study emphasizes statistics that often plays a minor role only in wear studies.

Effect of synovial fluid temperature on wear resistance of different polymer acetabular materials

In order to investigate the effect of frictional heat on the wear resistance characteristics of polymeric acetabular materials, the tribological tests and wear numerical analysis of three common polymer acetabular materials were carried out under different synovial fluid temperatures. The study results show that XLPE and VE-XLPE exhibit superior wear resistance compared to UHMWPE in high-temperature, heavy load environments. The coefficient of friction of three materials gradually decreases as the temperature of the synovial fluid increases. The wear depth and wear volume of the three materials increased with the increase of the temperature of the synovial fluid, and the forms of wear at 46°C and 55°C were mainly adhesive wear and plastic deformation. The higher temperature of the synovial fluid accelerates the oxidative degradation of the material surface and generates oxidation functional groups, which leads to the breakage of C-C bonds in the surface molecular chains under the sliding shear effect, thus reducing the mechanical properties of the material. Specifically, the surface of the polymer material will soften at a higher ambient temperature, mainly due to the decrease of hardness, and then deteriorate in the friction property, and finally increase the wear rate. Ansys results showed that the volume wear of the three materials increased with the increase of synovial fluid temperature, and the trend could be approximately linear. Numerical calculations predict that VE-XLPE has the highest wear of 0.693 mm³ among the three materials at 37°C, followed by XLPE at 0.568 mm³ and UHMWPE with the lowest wear of 0.478 mm³. At higher synovial fluid temperatures (46°C, 55°C), VE-XLPE still has the largest wear volume among the three materials, while XLPE and UHMWPE have similar wear. The wear cloud pictures showed that the maximum wear volume occurred near the edge of the acetabulum.

How does lubricant viscosity affect the wear behaviour of VitE-XLPE articulated against CoCr?

Using a 50-station pin-on-disc (SuperCTPOD) machine, the influence of lubricant viscosity on the wear of vitamin E blended crosslinked polyethylene was investigated. Five different test lubricants were prepared by mixing different concentrations of carboxymethyl cellulose powder with deionised water. The viscosity range of the lubricants was 0.002-0.155 Pa, a range that represents the viscosities of diseased and healthy synovial fluids. Five groups of pins (10 pins in each group) were articulated against cobalt chromium discs. Wear was measured in terms of weight loss from each pin and disc for every group. Every 500,000 cycles the experiment was stopped to take gravimetric measurements along with roughness measurements of the articulating surfaces. The test discs did not show a significant change in weight after 2.5 million cycles of testing (p > 0.05). For the pins, the group tested with the lowest viscosity (0.002 Pa) produced the highest wear rate, namely 0.931 mg/million cycles, and the wear rates of the other groups were 0.074, 0.027, 0.034 and 0.021 mg/million cycles respectively. The wear rates calculated for the five groups were all lower than the wear rates recorded for ultrahigh molecular weight polyethylene and not significantly different to crosslinked polyethylene. In addition, apart from group 1 pins (tested with the lowest lubricant viscosity (0.002 Pa)), the machining marks on the other pins were still present after 2.5 million cycles of testing, indicating low wear.