In vivo osseointegration property of a novel PEEK cage with a wavy surface coated with hydroxyapatite particles by a cold spraying technique

Abstract

Background: A novel surface treatment for polyetheretherketone (PEEK) material, which coats it with hydroxyapatite (HA, Ca10(PO4)6(OH)2) using a low temperature spraying technique, has recently been introduced to overcome the limitation of the materials bioinert property which does not allow osseointegration. The HA-coated PEEK material using the aforementioned technique has demonstrated improved osseointegration properties in a previous study following implantation in the ilium of rabbits. This study was undertaken to assess the efficacy of a novel PEEK cage coated with HA on a surface area enlarged by wavy engraving in a large animal interbody spinal fusion model. Materials and methods: A novel cage, the surface of which was coated with HA via a cold spraying technique after the engraving of wavy lines to increase the contact area, was compared to a conventional PEEK cage with the same configurations using a large animal interbody spinal fusion model. Twelve mini-pigs underwent 4-level lumbar interbody fusion surgery via a retroperitoneal approach, with the relevant cage for implantation in each segment randomly allocated. In total, 48 empty cages (24 control and 24 experimental) were implanted without any additional graft material after disc removal and endplate preparation. Four months after surgery, a micro-computed tomography (micro-CT) scan, histologic review, and biomechanical test under tensile stress were performed. Results: All 12 animals were kept alive for 4 months without any significant complications after surgery: 9 segments (5 control and 4 experimental) in which the cage was dislodged or improperly placed were excluded from the analysis. The micro-CT scan and histologic analysis were performed in 35 segments (17 control and 18 experimental) and the biomechanical test was performed in 4 segments (2 control and 2 experimental). Histologic osseointegration was observed in only five segments of the experimental group and in none of the segments of the control group (p=0.04). The fusion rate, assessed with multi-plane-reformatted micro-CT imaging, did not differ between the groups (64.7% vs. 72.2% in the control and experimental groups, respectively

p=0.72). The bone volume and bone volume fraction inside and around the cage, as measured on the micro-CT scan, were not significantly different. The structural model index calculated from the micro-CT data showed an improved remodeling status for the trabecular bone in the experimental group (p=0.032). Biomechanical testing failed to reveal any difference between the groups. Conclusions: Enhanced osseointegration properties of the implant surface and improved microarchitectural quality of the bridging bone were observed in the experimental group, using a large animal interbody spinal fusion model.