Researchers at North Carolina State University revealed test results of a heat-treated bioactive coating for use on polymer bone implants to improve bonding to surrounding bone, known as osseointegration. The implant is made of polyether ether ketone, or PEEK—a material that has found common use in bone implants because it has similar mechanical properties. It also resists degradation in aqueous and organic environments. But since it doesn’t bond to bone, the implant can be coated with a bonding material like hydroxyapatite (HA). In fact, a form of HA occurs naturally in bone, making up 70% of human bone by weight.
The bone implants were tested in mice animal subjects, and the results were published in the paper “Hydroxyapatite coating on PEEK implants: biomechanical and histological study in a rabbit model, which appeared in the journal, Materials Science and Engineering. The scientists expect that these findings will benefit bone surgery in many different spheres, including spinal and dental surgeries.
“We can now use our technique to coat the entire surface of an implant, and testing HA-coated implants in an animal model has given us very promising results,” says Afsaneh Rabiei, a professor of mechanical and aerospace engineering at NC State and corresponding author of a paper on the work. One of the highlights of the team’s coating technique is that it enables them to coat irregular surfaces for better osseointegration.
In the first step of the coating process, the implant is coated with a ceramic called yttria-stabilized zirconia (YSZ) to protect the polymer as it is heated in the next step. Then, HA is applied and heated with microwaves, giving it a crystalline structure that is more stable in the body—calcium phosphate degrades at a slower rate as the implant bonds to the bone. A third step includes putting the HA-coated implant in an autoclave for further heating and enhancement of the crystalline structure.
The scientists tested an uncoated PEEK implant, a microwave-treated HA-coated implant, and a microwave/autoclave-treated HA-coated implant in mice subjects. Using three-dimensional X-ray imaging and a microscope to monitor the tissue, the scientists assessed the bonding to bones in mice. Eighteen weeks after surgery, the researchers found that bone formation improved two-fold with both HA-coated implants when compared to non-coated PEEK. The HA-treated implants also had higher bone-to-implant contact ratios than PEEK alone.
The scientists then used push-out testing to see how well the bones performed under loading. Applying mechanical loading, it took 299.1 N-m of work to dislodge implants coated with microwave-treated HA. Meanwhile, HA-coated implants that underwent microwave and autoclave treatment were dislodged under loads as high as 312.5 N-m. The scientists also found out that rougher PEEK surfaces coated with treated HA had even better performance under push-out testing.
While this coating process may increase the cost of implants, Rabiei notes that that the coating will improve the success of surgeries and reduce the need for follow-up surgeries.
“The extent of the cost increase remains unclear,” says Rabiei. “We are not aware of any health risks associated with HA or YSZ—both of which are used in devices already approved by the FDA for long-term implantation. As a result, we may not need additional clinical trials before HA-coated implants can be used in clinical practice. We’re investigating that now, and are looking for industry partners to help us commercialize the technique.”