Wear of PEEK All-Polymer Articulations for Cervical Spinal Disc Arthroplasty

Abstract

The conventional treatment for degenerative disc disease (DDD) and disc herniation is spinal fusion, a process consisting of fusing two segments of the spine together. Arthroplasty treatments that preserve the natural motion of the spine are still in the early stages of development. Cervical disc arthroplasty (CDA) involves removal of the existing damaged disc and replacement with an articulating implant. The materials used for implants must possess excellent biocompatibility, strength, and wear resistance properties. Spinal implants in particular should also allow precise post-operative imaging because surgeons rely on imaging tools to check for migration of the implant and nerve impingement post-operatively. The purpose of the current thesis is to investigate the wear behaviour of three different versions of poly-ether-ether-ketone (PEEK), a radiolucent polymer that does not distort MRI images, articulating against themselves. The materials tested include: PEEK OPTIMA (OPT), carbon-fiber reinforced (CFR) PEEK and carbon-nanofiber (CNF) PEEK. A series of wear tests were performed on a pin-on-plate apparatus that imposed reciprocating crossing-path motion to the articulating specimens. The first series of wear tests, “normal conditions tests”, consisted of application of 80 N for 2.0 million cycles (Mc). Continuation of testing was aimed at evaluating the tribological behaviour of the materials under “adverse conditions”. The adverse conditions involved increasing the load every 0.15 Mc until the material showed significant surface damage. The materials were tested in a 12g/L protein concentration alpha calf fraction serum, at 37˚C. The wear of the specimens were evaluated using volumetric wear calculations and microscopy. The lowest wear, at the end of the normal conditions test, occurred with the articulation of CFR PEEK-on-CFR PEEK, and the highest wear, after 2.0 Mc, occurred with CNF PEEK-on-CNF PEEK. The adverse conditions revealed the highest wear value for PEEK OPT. Surface damage was apparent on both the PEEK OPT and CFR PEEK specimens; however, volumetric wear measurements performed on the specimens did not indicate a rise in wear for CFR PEEK, though surface damage was visibly noted. CNF PEEK was not tested to failure, although surface damage was evident as the material neared the end of the adverse conditions test. The PEEK OPT wear values after the normal conditions test are similar to those reported for spine simulator studies on a PEEK OPT-on-PEEK OPT all-polymer lumbar nucleus implant. This tentatively suggests that the normal test conditions represent a clinically realistic range. CFR PEEK shows the most promise for application in cervical disc arthroplasty. The other versions of PEEK possess excellent imaging qualities but had inferior wear resistance compared with CFR PEEK. However, wear volumes found in the present thesis for all three versions of PEEK after the “normal conditions” test were considerably lower than those found for stainless steel (SS) in similar testing. Prestige® STLP, composed of SS, is an FDA approved product that is currently implanted in patients in the United States.