New coating for polymer implants should prove success rate of bones bonding in spinal surgery, new research finds

Polymer implants, often used in spinal surgeries, have been found not to bond well with bone and other tissues, which can result in medical complications and the need for additional surgery.

However, researchers from North Carolina State University, have discovered a way to successfully coat polymer implants with a bioactive film which should improve the success rate of the implants, says a study first published in Journal of Biomedical Materials Research: Part A.

While using a high performance polymer, known as PEEK, has advantages over metallic biomaterials for application in spinal implants, research suggests that coating these implants with bioactive film will improve bonding with bone.

Dr Afsaneh Rabiei, an associate professor of mechanical and aerospace engineering at NC State led the research, in which he says that low back pain in one of the most prevalent and costly health problems in the world, resulting in more lost productivity than any other medical condition. The research reported that 85% of people in the US report having low back pain at some point in their lives and cites one study which reported that patients suffering from chronic low back pain increased from 3.9% to 10.2% between 1992 – 2006 and of those from suffering from chronic low back pain, 25% will undergo spinal surgery.

The research continues to say that as a result of the increased demand for spinal surgery, there has been an advancement in technology of spinal implants over the past 20 years and the global market for spine implants has been reportedly increasing at a rate of 20% per year. Spinal fusion procedures specifically are recommended for patients who suffer from persistent and untreatable back pain which has resulted from degenerative disk disease, spinal instability, or other spinal disorders.

Despite the significant increase in spinal fusion procedures in the US, rising from 121,400 to 281,300 between 1990 and 2003 for primary lumbar and cervical procedures, there is still a 9% revision rate among adults undergoing surgery for spinal deformations, and implant failure is one of the primary causes for revision surgery.

Dr Rabiei and his team have now discovered a way to coat polymer implants, often used in spinal surgery, to prove the success rate of the implants.  “We wanted to apply a bioactive coating that would allow the polymer implants to bond with surround tissues,” he says. “The challenge was that these coatings need to be heated to 500 degrees Celsius, but the polymer melts at 300 C. We’ve finally solved the problem.”

Dr Rabiei and his team first coated the implant with a thin film of yttria-stabilised zirconia (YSY). The second step in the new technique was to apply a coating of hydroxyapatite, which is a calcium that bonds well with bone. The researchers then found that if they heated the hydroxyapatite using microwaves, the YSY layer acted as a heat shield, thus preventing the PEEK from melting.

The team also discovered that the heat gives the hydroxyapatite a crystalline structure that makes it more stable in the body, meaning that the calcium phosphate dissolves more slowly, prompting bonding with surrounding tissue.

Dr Rabiei says, “We have received funding from the National Institutes of Health to proceed with animal testing to fine-tune this technique, then we will move on to clinical testing.”

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