Hodroxyapatite Coating Improves Implant Performance

Richard Gawel


Researchers have developed a technique for coating polymer implants with a bioactive film that significantly increases bonding between the implant and surrounding bone in an animal model. The development could dramatically improve the success rate of these implants, including dental implants.

Known as polyether ether ketone (PEEK), the polymer has mechanical properties similar to bone but doesn’t bond well with it. So, the researchers coated it with hydroxyapatite (HA), a calcium phosphate that bonds well with bone.

“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,” said Afsaneh Rabiei, a professor of mechanical and aerospace engineering at North Carolina State University and a corresponding author of a paper on the work.

The researchers begin by coating a PEEK implant with a thin film of yttria-stabilized zirconia (YSZ) and then applying a coat of HA. Next, they heat the HA layer using microwaves. The YSZ layer acts like a heat shield and prevents the PEEK from melting. The heat gives the HA a crystalline structure that makes it more stable in the body, so the calcium phosphate will dissolve more slowly, promoting bonding with surrounding bone.

Three types of PEEK implants were tested in a rabbit model: PEEK implants with no coating, PEEK implants with an HA coating treated only with microwaves, and PEEK implants with an HA coating treated with both microwaves and brief exposure to an autoclave to enhance the HA’s crystalline structure.

The researchers used microscopic evaluations of tissue cells and 3-D x-ray imaging to assess the implants’ performance. After 18 weeks, both types of HA-coated implants had more than double the bone formation of PEEK alone, as well as higher bone-to-implant contact ratios, with comparable bone density.

“These results indicated an improved implant fixation in the body, decreasing the chances of loosening of the implant after surgery and the need for revision surgery to remove and replace the implant,” said Rabiei. “This improvement is due to increased regenerated bone volume around coated implants compared to uncoated PEEK.”

Additionally, the researchers assessed how well the implants bonded to the surrounding bone via biomechanical push-out testing, which applied force to the implants until they were dislodged. At 18 weeks, it took approximately 299.1 Newton millimeters (N-mm) of work to dislodge implants coated with microwave-treated HA and about 312.5 N-mm to dislodge implants coated with microwave and autoclave-treated HA. The unmodified implants were dislodged with only 183.9 N-mm of work.

“It is notable that these results were achieved on completely smooth surfaces of PEEK, while our subsequent studies have indicated that by slightly increasing the surface roughness of PEEK prior to coating, we can accomplish even higher adhesion strength of 2-layer HA/YSZ coatings that would require even higher work to dislodge,” Rabiei said.

“Whether looking at bone growth or toughness, HA-coated samples outperformed uncoated PEEK implants. This treatment will probably increase the cost of an implant marginally, but should help minimize the need for follow-up surgeries, which means HA-treated implants will more than pay for themselves over time,” said Rabiei.

“And the extent of the cost increase remains unclear. We are not aware of any health risks associated with HA or YSZ, both of which are used in devices already approved by the US Food and Drug Administration for long-term implantation,” said Rabiei.

“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,” Rabiei said.

The paper, “Hydroxyapatite coating on PEEK implants: biomechanical and histological study in a rabbit model,” was published by Materials Science and Engineering: C. The National Institute of Dental and Craniofacial Research supported the work.

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