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Adverse Wear Testing

Wear testing plays a critical role in ensuring the durability and safety of joint replacement implants. What is adverse wear testing and why is it important for new innovations in joint replacements?

Our team shares a strong focus on advancing the medical device industry through innovative research, design, and testing. We have investigated and developed effective ways to test and measure joint replacement wear that goes above and beyond traditional methods to ensure your device performs to the highest quality and safety standards, which includes adverse wear testing scenarios.
Figure 1: Photographic image of a retrieved knee polyethylene insert showing severe wear from delamination and embedded third-body particulate.

What is wear testing?

Wear testing has been an extremely valuable method of pre-clinical evaluation of joint replacement implants for more than 40 years. Wear testing stresses the implant as a complete system, whereas other testing methods focus on a particular component or aspect of the implant system. In many ways, wear testing is seen as the final trial before the real performance when implanted in a patient.

The method of wear testing has evolved significantly over time, but the key principles have remained constant:

  1. Apply a physiological testing environment
  2. Assess overall durability of the implant system
  3. Determine the rate of implant wear

These principles have been well documented and fleshed out in numerous iterations of international testing standards including ISO 14242-1 (hip), ISO 14243-1/-3/-5 (knee), ISO 22622 (ankle), and ISO 16436-1/-2 (shoulder). But in recent years, a new challenge in wear testing has arisen as a result of a selection of modern implant systems ending in catastrophic failures when subjected to ‘imperfect’ conditions in vivo. For example, retrieved knee polyethylene inserts have shown high levels of embedded particulate causing severe delamination (Figure 1).

What is adverse wear testing and why is it important?

Adverse wear testing evaluates implant performance under imperfect worst-case conditions that are more relatable to clinical scenarios. It is well-known in the medical industry that implant performance depends on so many variables, making it difficult to predict how it will truly perform. The most successful products are robust within their intended environment under the harshest conditions. Therefore, it is best engineering practice to design to a higher factor of safety by pushing the device to its testing limits.

Adverse wear testing can take many forms, however the most common form is third-body wear testing due to a number of factors. The main concern is over metal-ion release from joint replacements, which has heightened diligence of the orthopaedic community to produce implants with reduced likelihood of metal wear and subsequent ion release. A recent industry shift away from cobalt-chromium articulations has led to development of several new coating technologies, generally known as ceramicized metal coatings (Figure 2). A vital characteristic of these implants is their ability to resist coating delamination which is most directly challenged by abrasive particulate in the joint.

Figure 2: A ceramicized metal knee replacement component, retrieved from a patient.
Figure 3: Adverse ankle wear test using embedded particulate in the polyethylene bearing to increase the wear rate.

How have we modified existing test methods for adverse wear testing?

A specialized Working Group of ASTM is developing a standardized method for third-body particulate wear testing in hips to evaluate device performance under simulated adverse conditions. Our biomedical engineer and technical lead in joint replacements, Sarah Fay M.Sc., is an active member of this Working Group, and has performed many particulate wear tests for our clients’ using adaptations of the method for both knee and ankle wear testing (Figure 3).

The general process is to apply a small quantity of particulate to the surface of the bearing couple and run a defined motion pattern under load to both embed the particulate into the softer bearing component (typically polyethylene) and abrade the harder bearing component (typically metal or ceramicized metal). An adverse wear test is then performed with these components and the wear results are compared against a standard wear test (without particulate).

You may be wondering, what is the best particulate material to use? There are currently several options, but defining which is the most suitable particulate is still up for debate. The contenders include readily available particulate such as alumina oxide from sanding grit, clinically-relevant particulate such as bone cement granules, metal carbides which are occasionally released from metallic surfaces, and metal powder which can be residue from additive manufacturing processes.

We have tested and studied several of these particulate choices and other methods for adverse wear testing such as pre-roughened and metal-transfer surfaces. The results from these test methods are covered in one of our whitepapers.

Our expert testing support and services

Our technical team of engineering experts can support you through the myriad of choices concerning adverse wear testing to select the most suitable option for your unique device. The adverse wear testing for your joint replacement will be tested on either our hip or knee simulator equipment. Our expertise in this field is based upon the data we’ve accumulated, the experience we’ve amassed, and our proven ability to think beyond the standard and develop novel test methods for truly novel devices.

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