Abstract
Background:
Standardized preclinical wear testing cannot replicate the variations of wear rates and wear mechanisms seen in-vivo [1]. Therefore, a lot of studies focused on testing scenarios which replicate a wider range of patient-specific conditions such as different activities or malalignment of components. However, differences between the in-vivo and in-vitro situation regarding the environmental conditions are often neglected.
Considerable differences between the in-vivo and the in-vitro situation are related to the surrounding synovial fluid and its in-vitro substitute (bovine serum). For the in-vivo knee only small volumes (1–4 ml) of synovial fluid are reported [2]. However, for in-vitro testing several hundreds of milliliters of bovine serum are typically used. Therefore, the hypothesis of this study is that fluid volume significantly influences the wear rates in simulator tests.
Material and Methods:
For wear testing an established implant system (Sigma® PFC, DePuy, Warsaw, USA) has been used.
Four wear tests with four different testing volumes of bovine serum were carried out: 250 ml; 150 ml; 75 ml; 45 ml. The testing volume of the original chamber (250 ml) was reduced using solids (Fig. 1). Care was taken, that the contact surfaces of the implants were completely immersed and the fluid level was kept constant in all cases (Fig. 1). The testing fluid was maintained at a temperature of 37 ± 1°.
All wear tests were run displacement controlled according to ISO 14243-3: 2004 on an AMTI knee simulator.
Results:
No significant differences could be detected for testing with 250 ml or 150 ml testing fluid (wear rate 9.1 mg/Mc and 8.2 mg/Mc, respectively; p = 1.000). Further volume reduction leads to a significant decrease in wear rate: Using a testing volume of 75 ml the wear rate decreases to 5.2 mg/106 cycles (p = 0.010) and with a testing volume of 45 ml a wear rate of 0.9 mg/106 cycles (p ≤ 0.001) could be observed. A serum degradation during simulation was observed. The amount of degradation increased with decreased serum volume, as shown through precipitation and turbidity of the serum. A small increase in wear particles size with smaller used volumes could be observed.
Conclusion:
It has been shown, that the testing volume highly affects the PE wear rates. A larger testing volume caused a higher wear rate. Otherwise larger wear particles were observed when testing was carried out with a smaller testing volume. It is not known, which testing situation better replicates the in-vivo situation.
The lubricant affects the wear behavior in a permanent way. Meaning, that the lubricant impacts the wear behavior of joint replacements with every activity carried out. Considering this, the role of the lubricant is underestimated and may be a highly relevant factor for the in-vivo and in-vitro wear behavior.
