The Implant Testing Puzzle

When it comes to testing of implants and other products, there’s a definite difference between biocompatibility testing and impact or longevity testing. But there’s also a substantial relationship between the two types of testing, as representatives of two Boston, Mass.-area firms from the testing equipment sector told MedicalDeviceNow in recent conversations.

Bruce White, CEO of Watertown-based AMTI, said, “We don’t cover the biocompatibility aspect per se, but we provide a mechanical environment to evaluate some of the things that can go wrong downstream, and there is a definite relationship between biocompatibility and this testing.”

Photo caption: Pictured at left, an AMTI ADL Hip Simulator performs a wear test on a set of hip implants. The AMTI Hip Simulator allows implant manufacturers and researchers rto evaluate the performance and durability of implant designs before implantation by replicating the multi-axis motions and loads that occur in the joint during daily activities. Photo courtesy of AMTI.

He said the biocompatibility angle comes as implants wear. “Because they are mechanical systems and they will wear out eventually, as they begin to degrade they release particles into the body,” he said.

Polyethylene is a significant component of many orthopedic implants, and particulate wear debris is worn off the surface of those implants as they are in use.

“That particulate wear debris is actually quite biologically active,” White said, “and it has been a quest to remove that wear debris from implant wear for many years. At this point, manufacturers have come up with some really great new polyethylenes that probably will reduce that problem considerably.”

He said the amount of that wear debris that is generated “depends a great deal on the wear history, the actual motions and loading of each device. Different implants will release different amounts of biologically active materials, so it becomes very important to simulate the mechanical environment of the human body in order to recreate those loading scenarios that cause that type of particle release.”

White noted that another area that has been in the news in recent months due to recalls is the release of metal ions from metal-on-metal hip implants. “With the metal-on-metal implants, metal doesn’t release very large amounts of ion material during wear, but it releases small amounts, which can be critical to the immune system. It’s another case where the basic materials are fine, but the particles that are released through the mechanical wearing process become problematic.”

Debbi Cohen, a biomedical sales engineer at ADMET, a manufacturer of materials testing systems, had a similar basic message. “The ADMET testing systems focus on the mechanical properties of the devices or the biomaterials—stress and strain, durability, reliability, flexibility, and other measurable strength properties. Our systems are not designed for biocompatibility or cell science-type of testing.”

She said that ADMET, based in Norwood, Mass., is “known for being able to tailor customize our testing systems to meet specific requirements.” This is common in medical device manufacturing, because of the variety of product sizes and shapes and the need to test every aspect of their functioning.

Cohen noted that her company has “many customers in the orthopedic realm that are trying out new materials that haven’t been used before for implants, different types of bone screws, adhesives, cements, biodegradable bone screws. For example, we have a customer that is testing an acrylic bone implant. They find it challenging to use existing standards formaterials that are traditionally used for the same application. They need to have specific data that they can compare against for their submission to the [U.S. Food and Drug Administration], so we build systems that will allow the user to gain all kinds of mechanical property data about their product.”

Cohen noted that because of changing regulatory requirements, “we’re trying to add more and more to our software controller to document their testing so implant manufacturers can submit as much information as possible to the FDA. When the users don’t have an exact standard, or when they’re working with new materials, any documentation that they can provide helps their case.”

As for new demands from customers involved with development of biodegradable products, she said, “We have heard from our users, to have a bone implant that would eventually biodegrade is the ultimate solution, but it is very tricky and there is a lot of refining that needs to happen before it is a marketed product.”

Cohen said a big issue facing biocompatibility these days, one that concerns all scientists involved with the sector, is “the evolution of products, from the simple polymers and ceramics and metals into biodegradables and bioresorbables into nanomaterials and nanoproducts. There is concern that the methodologies that have been in place for so many years still will not be able to predict biocompatibility.”