Abstract
Introduction
Support of appositional bone ingrowth and resistance to bacterial adhesion and biofilm formation are preferred properties for biomaterials used in spinal fusion surgery. Although polyetheretherketone (PEEK) is a widely used interbody spacer material, it exhibits poor osteoconductive and bacteriostatic properties. In contrast, monolithic silicon nitride (Si3N4) has shown enhanced osteogenic and antimicrobial behavior. Therefore, it was hypothesized that incorporation of Si3N4 into a PEEK matrix might improve upon PEEK's inherently poor ability to bond with bone and also impart resistance to biofilm formation.
Methods
A PEEK polymer was melted and compounded with three different silicon nitride powders at 15% (by volume, vol.%), including: (i) α-Si3N4; (ii) a liquid phase sintered (LPS) ß-Si3N4; and (iii) a melt-derived SiYAlON mixture. These three ceramic powders exhibited different solubilities, polymorphic structures, and/or chemical compositions. Osteoconductivity was assessed by seeding specimens with 5 × 105/ml of SaOS-2 osteosarcoma cells within an osteogenic media for 7 days. Antibacterial behavior was determined by inoculating samples with 1 × 107 CFU/ml of Staphylococcus epidermidis (S. epi.) in a 1 × 108/ml brain heart infusion (BHI) agar culture for 24 h. After staining with PureBlu™ Hoechst 33342 or with DAPI and CFDA for SaOS-2 cell adhesion or bacterial presence, respectively, samples were examined with a confocal fluorescence microscope using a 488 nm Krypton/Argon laser source. Images were also acquired using a FEG-SEM in secondary and backscattered modes on gold sputter-coated specimens (∼20–30Å). Hydroxyapatite (HAp) deposition was measured using a laser microscope. Raman spectra were collected for samples in backscattering mode using a triple monochromator using a 532 nm excitation source (Nd:YVO4 diode-pumped solid-state laser).
Results
PEEK composites with 15 vol.% α-Si3N4, LPS ß-Si3N4, or the SiYAlON mixture showed significantly greater SaOS-2 cell proliferation (>600%, p<0.003, cf., Fig. 1(a)) and HAp deposition (>100%, p<0.003, cf., Fig. 1(b)) relative to monolithic PEEK. The largest increase in cell proliferation was observed with the SiYAlON composite, while the greatest amount of HAp was found on the LPS ß-Si3N4 composite. Following exposure to S. epidermidis, the composite containing the LPS β-Si3N4 powder showed one order of magnitude reduction in adherent live bacteria (p<0.003, cf., Fig. 1(c)) as compared to the PEEK monolith. It is interesting to note that the composite containing α-Si3N4 exhibited the worst bacterial resistance (i.e., ∼100% higher than monolithic PEEK), suggesting that the bacteriostatic effectiveness of Si3N4 bioceramics is apparently dependent upon the presence of selective sintering additives, viz. yttria and alumina.
Conclusions
The addition of 15 wt.% of specific Si3N4 powders to PEEK showed enhanced SaOS-2 cell adhesion, proliferation, and HAp deposition when compared to monolithic PEEK. These same composites also showed resistance to S. epi. adhesion and biofilm formation.. Although improvements in osteoconductivity have been previously observed by compounding or coating PEEK with HAp, titanium, or tantalum, these approaches did not provide anti-microbial properties. Compounding PEEK with Si3N4 represents a significant advancement due to its ability to provide both improved bone apposition and resistance to biofilm formation.
For any figures or tables, please contact the authors directly.
