Abstract
Background
Polyethylene (PE) wear is known as a limiting factor for total knee replacements (TKR). Thus, preclinical wear testing is an important tool to assess the suitability of new designs and new materials. However, standardized testing (e.g. according to ISO 14243) does not cover the individual situation in the patient. Consequentially, this study investigates the following two parameters:
- a)
Testing-Frequency: Patients with TKR's show a humiliated walking frequency (down to 0,5Hz) compared to standardized testing (1Hz±0.1). In the first part of this study, the influence of a decreased test frequency on the PE wear behavior is investigated
- b)
Interval of lubricant replacement: For in-vitro testing bovine serum is used as a substitute for the synovial fluid. Physiologically a continuous regeneration and removement of destructed components is taking place. In contrast, for simulator testing the bovine serum is typically changed completely every 500.000 cycles/steps. Therefore the goal of the second part of this study was to test if the serum replacing interval affects the PE wear behavior.
Material and Methods
Wear tests were conducted on an AMTI force controlled knee simulator. A cruciate substituting (ultracongruent) implant design (TC Plus, Smith & Nephew, Rotkreuz, Switzerland) was used.
First, a reference wear study with a test frequency of 1Hz and a lubricant replacement interval (RI) of 500.000 cycles according to ISO 14243-1:2009 was carried out. Tests were run to a total of 5 million cycles.
A second wear test was run with a reduced frequency of 0.5 Hz. The reduced frequency resulted in an extended testing period for the same number of cycles. To exclude an influence of the extended time period, the lubricant was changed, in the first half of testing every 500.000 cycles corresponding to 12 days (cycle depending (CD)), and in the second part every 250.000 cycles corresponding to 6 days (time depending (TD)). Tests were run to a total of 3 million cycles.
A third test was run with a frequency of 1 Hz. For this test a reduced serum RI of 150.000 cycles was choosen. This test was run to a total of 1.500.000 cycles.
Results
The results of wear testing are given in Fig. 1 & 2. There was no difference for testing at a lower frequency in the case, that the serum replacement occurred at the same time interval (p=0.234). However, if the replacement interval is extended or reduced, the wear rate decreases (2.69mg/Mc) or increases respectively (15.87mg/Mc) (p=0.001;p≤0.001). There is a great time depending influence of the serum RI. This influence is shown in Fig. 3, comparing the wear rates in dependency to the time period of lubricant change.
Conclusion
This study shows that there is no influence of the reduced testing frequency in a TKR wear simulator study on the PE wear rate. However, there is a massive influence of the lubricant replacement interval. Thus the interval of replacing is of crucial importance for the interpretation of wear simulator studies. The reasons for the changes in the tribological behavior of the bovine serum have to be investigated in further studies.
