Thermal localization improves the interlayer adhesion and structural integrity of 3D printed PEEK lumbar spinal cages

Abstract

Additive manufacturing (AM) is a potential application for polyetheretherketone (PEEK) spinal interbody fusion cages, which were introduced as an alternative to titanium cages because of their biocompatibility, radiolucency and strength. However, AM of PEEK is challenging due to its high melting temperature and thermal gradients. Although fused filament fabrication (FFF) techniques have been shown to 3D print PEEK, layer delamination was identified in first generation FFF PEEK cages [1]. A standard cage design [2] was 3D printed with a second generation FFF PEEK printer. The effect of changing layer cooling time on the FFF cages’ mechanical strength was investigated by varying nozzle sizes (0.2 mm and 0.4 mm), print speeds (1500 and 2500 mm/min), and the number of cages printed in a single build (1, 4 and 8). To calculate the porosity percentage, FFF cages were micro-CT scanned prior to destructive testing. Mechanical tests were then conducted on the FFF cages according to ASTM F2077 [2]. Although altering the cooling time of a layer was not able to change the failure mechanism of FFF cages, it was able to improve cages’ mechanical strength. Printing a single cage per build was associated with a higher ultimate load than printing multiple cages per build. Regardless of the cage number printed per build, cages printed with bigger nozzle diameter achieved higher ultimate load compared to cages printed with smaller nozzle diameter. Printing with a bigger nozzle diameter resulted in less porosity, which might have an additional benefit on the interlayer delamination failure mechanism.