Rheological Behavior of Different Calf Sera before, during and after Biomechanical Testing

Abstract

Due to different rheological behavior of human synovial fluid and the test mediums for in vitro examinations, wear tests cannot replicate the in vivo situation completely. The standards for wear testing indicate calf serum as in vitro test medium. However, these standards do not contain precise information on the main constituent components and the rheological properties. In this study, bovine calf serum and newborn calf serum with a protein concentration of 20 g/L, both approved for wear testing defined by the International Organization for Standardization (ISO), were characterized according to their rheological properties to detect differences before and during tribological simulation. The rheological behavior was determined at five defined intervals of a tribological test. The two test fluids differ in their rheological properties before and during the test and can therefore lead to deviating results in tribological testing. Furthermore, both test media changes considerably over test duration. At a test duration of 0.5 Mio cycles, both fluids have changed so that there is no longer any difference between them in terms of rheological properties. These changes could be attributed to denaturation and degradation of proteins. Thus, the choice of medium impacts tribological test results which should be considered for the interpretation of these studies.

In Vitro Wear Behavior of Knee Implants at Different Load Levels: The Impact of the Test Fluid

“…thickening of the test fluid at high shear forces may, thereby, distort the wear behavior of polyethylene inserts. A thickening of the test fluid during the test duration has already been observed by Uhler et al [10]. Wang et al also found protein precipitates in the test chambers when performing in vitro wear simulation on hip implants [14].…”

“…All the rheological investigations were carried out at 37 ± 0.04 • C, using a Physica MCR 702 rheometer (Anton Paar GmbH, Graz, Austria) and a double-gap measuring system. Similar to previous studies, the dynamic viscosity η was determined, as a function of the shear rate in the interval between 10 1 s −1 and 10 3 s −1 ramping up in logarithmic steps [10,18]. Each test fluid was swished gently, before pouring it into the measuring cup.…”

“…Instead, the test fluid remains in the test chamber and will be replaced every 0.5 × 10 6 gait cycles. A recent knee wear simulation study already showed that the rheologic properties of the calf serum changes during a test interval of 0.5 × 10 6 cycles [10]. Thereby, the unstressed test fluid (0 cycles) shows a shear-thinning behavior and a lower dynamic viscosity for high shear rates than the stressed test fluid after 0.5 × 10 6 cycles, which shows a near Newtonian behavior.…”

“…The measuring tools were cleaned with water and isopropyl alcohol before each measurement. The laminar flow could be ensured, because the Reynolds number of an identical newborn calf serum with a protein content of 20 g/L was determined in a previous study [10]. The Reynolds number was between 237 and 261, which is far below the critical Reynolds number of 1000 and above, where turbulent flows are expected.…”

The influence of lubricant temperature on the wear of total knee replacements

“…Changes in the rheological behaviour of serum over the duration of a 0.5 MC wear study of total knee replacements carried out under ISO 14243-1 [2] conditions at 37°C have been shown. The increase in lubricant viscosity with test duration is thought to be as a result of protein denaturation and degradation [35]. It is not known whether thermal or mechanical degradation of the lubricant has the greater influence on the change in properties.…”