Polyether-ether-ketone (PEEK) is believed to be the next-generation biomedical material for orthopaedic implants that may replace metal materials because of its good biocompatibility, appropriate mechanical properties and radiolucency. Additive manufactured (AM) PEEK orthopaedic implants offer new opportunities for bone substitutes. However, the poor integration between PEEK with hard and soft tissue represents a major challenge of PEEK orthopaedic implant owing to its chemical inertness. Comparing with strategies such as chemical modification, surface coating, biomolecular coating and surface morphology, bioceramic enhanced composition was believed to be feasible from the perspective of engineering implementation and clinical application to promote the integration of PEEK with both bone and soft tissue. However, it is still difficult to obtain a coherent conclusion about the effect of bioceramic fillers on the integration of bone and soft tissue with PEEK-based implant.
Herein, the influences of hydroxyapatite (HA) fillers and pore size of AM HA/PEEK composites scaffolds on the integration with bone and soft tissues were systematically investigated through cellular experiments and animal experiments. The composite scaffolds with HA contents from 0, to 40 wt% and pore sizes from 0.4 mm to 1.6 mm were manufactured by fused filament fabrication (FFF). In the in-vitro experiments, the adhesion, proliferation, osteogenic differentiation and mineralization ability of mesenchymal stem cells (BMSCs) and myofibroblasts on scaffolds with different HA contents were investigated. In the in-vivo experiments, HA/PEEK composited scaffolds were implant into the lateral femoral condyles of rabbit to investigate the osseointegration, and were placed between chest muscles and muscular superficial fascia on both sides of rabbit to investigate the integration with soft tissue. CT evaluation and histological characterization were carried out to evaluate the ingrowth of bone and soft tissue. The mechanical bonding strength of HA/PEEK scaffolds with surrounding bone and soft tissue was evaluated by push-out test and peeling test, respectively.
The cell experiment showed that the adhesion, proliferation, osteogenic differentiation and mineralization ability of mesenchymal stem cells (BMSCs) as well as the proliferation and adhesion of myofibroblasts on the PEEK/HA scaffolds were significantly improved. The in-vivo experiment of osseointegration showed that PEEK/HA scaffold exhibited a higher bone ingrowth volume than those of pure PEEK scaffold. Specially, the ingrowth volume (24.3%) of 40 wt% HA/PEEK scaffold after implantation for 12 weeks was comparable to the bone ingrowth reported for metallic or ceramic scaffolds. Tighter adhesion of soft tissue in the scaffolds with higher HA contents. The bonding strength between bone tissue and the scaffolds showed that PEEK/HA composites scaffolds exhibited better mechanical integration than that of the pure PEEK scaffold. The macroscopic bonding force between soft tissue and scaffolds was dominated by the pore size of the scaffolds but was hardly affected by the HA content. The maximum bonding force of soft tissue was 5.61 ± 2.55 N, which was higher than that between natural bone and soft tissue of rabbits.
The present study provides engineering-accessible design principles on material components and geometry of AM PEEK-based composites orthopaedic implant for improving the integration with bone and soft tissue.