116 - 3D Printed Silicon Nitride-PEEK Composite Cervical Cages Demonstrate Antimicrobial Effects In Vitro

Introducton: PEEK is widely used for cervical spinal cages. PEEK composites have the potential to combine the favorable mechanical properties and 3d printability of PEEK with the bioactive components of other materials. One such material is silicon nitride (SiN), a ceramic which has demonstrated its ability to support bone growth and resist bacterial infection in vivo. The goal of this work to test whether composite SiN/PEEK may be suitable for 3D-printed (3DP) cervical cages. We aimed to determine: (1) will 3DP SiN/PEEK cervical cages have the strength of conventional cages and (2) will 3DP SiN/PEEK composites exhibit antimicrobial properties?

Methods. Virgin PEEK and 15 vol% SiN/PEEK composite filament were 3DP using a medical PEEK printer. Generic cage designs were based on the literature, including a solid cage and a design with 500 micron endplate porosity for bone ingrowth. The porosity was generated with a diamond TPMS topology. Five cages of each design were tested in compression and shear per ASTM F2077 (Fig 1).

For antibacterial testing, virgin and composite filament samples were placed in sextuplicate in a 48-well plate with media containing 10% human serum, 1X PBS, and 7 mg/mL dextrose. Samples were inoculated with S. epidermidis (ATCC 14990) at a concentration of 104 CFU/mL per well. The plate was then incubated at 37°C and 95 rpm for 24 hours. Samples were aseptically removed, rinsed 3X in PBS, and placed in 10% Trypsin. Serial dilutions were plated on Petrifilms to quantify adherent bacteria. The antimicrobial experiment was performed with 3 replicates. Statistical significance for antibacterial testing was determined using a Kruskal-Wallis test.

Results. The 3DP PEEK and SiN/PEEK cages exhibited gradual yielding behavior for both designs. There was no brittle fracture or load drop for any of the tests. Since there was no clear failure on the force/displacement curves, we tested the individual cervical cages up to benchmark loads per Peck et al (2017). The mean strength of the generic 3DP cage designs exceeded the 75% percentile benchmarks reported by Peck (2017): 14.7kN in compression and 6.9kN in compression-shear. Control SiN/PEEK was found to have 93.9% lower levels of bacterial adhesion than the Control PEEK (p < 0.01), corresponding to a 1.21 log reduction.

Discussion: Infection is a serious cause of spinal implant failure, demonstrating the need for biomedical materials with strong antimicrobial properties in addition to their strong mechanical properties. A composite material of SiN/PEEK would allow for custom 3D-printed spinal implants that reduce the risk of infection when compared to traditional implant materials such as titanium alloy and unfilled PEEK. We observed antimicrobial properties at the lower end of expected 1-1.5 log reduction for a candidate antimicrobial. This suggests that the presence of Si3N4 was able to impart the PEEK with some of its antimicrobial properties. Increasing the concentration of Si3N4 to increase the antimicrobial effect will need to be balanced with the composite material’s ability to be 3D printed.

Reference: Peck JH et al. Mechanical performance of cervical intervertebral body fusion devices. J Biomech 54: 26, 2017