Project Uses PEEK in Attempt to Solve Metal-on-Metal Problem

ENDURE—a project funded by the European Commission—may have developed a solution to the metal-on-metal (MoM) hip implant disaster. The project, entitled the Enhanced Durability Resurfacing Endoprosthesis, has come up with an implant made of carbon-fiber reinforced polyetheretherketone, also known as PEEK.

The problem with MoM implants, according to an ENDURE report, are the high levels of cobalt and chromium measured in the blood and serum samples of patients: “These concerns relate to the possible systemic effect of metal ions released by MoM prosthesis in terms of genomics as well as immunology and histology, and reviews have recently been published into the ion levels and their biological effects were marked when wear was abnormally increased, with low carbon (less than 0.2 percent) implants and loose or poorly positioned implants.”

PEEK, though expensive, offers a strong and more importantly biocompatible option for hip implants, researchers claim.

Partners in the research project are Finsbury Development Ltd., Medicoat AG, Hunt Developments UK Ltd., ALA Ortho SRL, CeramTec AG, Invibio Biomaterials Solutions Ltd., Fraunhofer IPA, Biomatech and the Universities of Gothenburg and Southampton.

On May 2, the Fraunhofer Institute for Manufacturing Engineering and Automation IPA released a statement explaining the results of the ENDURE project: Ceramic is used for the femoral head, and a hydroxylapatite coating at the interface to the bone helps ensure that the bone tissue will fuse thoroughly with the surface structure of the implant. Hydroxylapatite is a naturally occurring mineral form of calcium apatite.

“The cobalt-chromium implants in use to date are very rigid, and the load transfer to the bone is non-optimal leading to potential adverse bone adaptation,” said IPA engineer Jasmin Hipp. “Thanks to the new combination of materials, the transmission of force through the PEEK hip socket to the pelvic bone is modeled on natural conditions. And there are no metal ions released.”

Hipp and her team were able to confirm good wear resistance in initial tests of the new hip replacement using a robot that simulated various series of movements such as walking or climbing and descending stairs. The experiments used a prototype of the implant. Researchers have also redesigned the way the prosthesis is mechanically attached to the bone. Without cement, and using a press-fit and an integral scaffold-type structure on the surfaces of the implant that contact the bone, the hemispherical ball and socket are tapped onto the prepared femoral head and into the acetabulum—the natural, concave surface of the pelvis—and anchored in place.

Plastics Today reported that it isn’t clear how the PEEK implant would be manufactured. Possibilities lie in injection molding and additive manufacturing. Surgeons at Walter Reed Army Medical Center in Washington, D.C., have pioneered work on cranial implants, but chose not to use PEEK because of its high cost. Titanium is being used in custom implants produced through additive manufacturing.

According to ENDURE, in Europe alone there are 800,000 hip operations yearly costing an average of $2,479 per implant, worth more than $1.9 billion, and the number of hip operations is rising by an estimated 15 percent per year. At present, 10 percent of all the prostheses implanted are in patients younger than 55 years of age, of which 33 percent will fail within 16 years with a further 20 percent of implants fitted to patients older than 55 years failing within 20 years requiring revisional surgery. PEEK implants, therefore, look to be an expensive proposition.

Currently, researchers are seeking a patent on the PEEK prototype, which only has been tested on cadavers thus far.