Hip Wear Testing Series: Test Fluid Setup in Wear Measurement Testing

Hip Wear Testing Series: Test Fluid Setup in Wear Measurement Testing


One variable in total hip replacement wear
testing that can be overlooked is test fluid composition and environment. In our first
video, we mentioned that the ISO and ASTM standards differ greatly in their requirements
for test fluids. To summarize, both ISO 14242 specs call for
the use of bovine serum, however, -1 requires a protein concentration of 30g/l while -3
calls for no less than 17g/l. On the other end of the spectrum, ASTM F1714 makes no suggestion
regarding test fluid composition. The purpose of test solution for wear studies
is to mimic the in-vivo conditions of the human body as closely as possible, so that
results will give manufacturers and reviewers an accurate depiction of expected wear rates
when implants are under normal operating conditions. So how, and to what extent, does the test
fluid used affect overall wear results? Various independent studies have been conducted to
provide insights into this question, and while no solid consensus has been reached, various
trends have been observed. Bovine serum, as required by the ISO specs,
is generally considered superior to other possible lubricant alternatives such as saline,
distilled, and de-ionized water. Beyond this, the protein concentration of bovine serum
can have a significant impact on the wear rates of implants. Studies have shown that
the highest wear rates were discovered when using a median protein concentration, around
50%. When the concentration was lowered, UHMWPE
transfer was believed to have created an artificial film between the head and liner that slowed
wear action. Conversely, insoluble proteins in higher concentrations of lubricants precipitate
when degraded and act as a lubricant between the polyethylene bearing and the femoral head,
resulting in artificially lower wear rates. Fluid temperature is another potential variable
affecting wear rates, because temperature is a dictating factor in how much protein
precipitates from the fluid. When temperatures are elevated, more protein precipitates, which
causes an artificial barrier between the liner and the bearing. This barrier could also artificially
decrease reported wear rates. So, how do you decide what test fluid to use
for your study? The most important consideration is to determine what concentration and temperatures
most accurately reflect in-vivo loading conditions. This way, the data obtained is accurate and
relevant. It is important to speak with your regulatory consultant on this topic, because
he or she will have a good idea of what FDA and other bodies are currently requesting.
In addition, it’s critical to ensure that predicate devices are tested side-by-side,
using the same test fluid to decrease potential variables and allow for accurate comparisons
between devices. Please feel free to contact us with any questions
regarding this or any other wear testing topic, and stay tuned for the next video in our hip
wear testing series, where we will discuss yet another potential variable and how to
minimize the effects.

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