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Dissertation
Fused filament fabrication of polyether-ketone-ketone and polyether-ether-ketone lumbar interbody fusion devices: an investigation into process parameters that enhance static strength
Doctor of Philosophy (Ph.D.), Drexel University
Dec 2025
DOI:
https://doi.org/10.17918/00011249
Abstract
The implantation of lumbar interbody fusion (LIF) devices or spinal cages is currently the standard of care for treating chronic back pain that does not respond to conservative therapies. Due to the prevalence of this condition and its significant impact on individuals worldwide, the demand for effective spinal cages continues to surge. Ensuring successful outcomes requires careful selection of biomaterials and manufacturing methods used for creating these devices. To mitigate the risk of spinal fusion failure, the biomaterial must be capable of withstanding in vivo loading, providing stability, and encouraging bone ingrowth. The use of polyether-ether-ketone (PEEK) for these spinal cages attempts to overcome stress-shielding and subsidence issues associated with metallic devices, due to its bone-like mechanical properties and radiolucency, allowing post-implantation device monitoring. However, its hydrophobic nature hinders osseointegration, potentially compromising long-term clinical success. Polyether-ketone-ketone (PEKK), a newer high-performance thermoplastic in the same family as PEEK, has emerged as a promising alternative. It possesses similar bone-like mechanical properties with different surface chemistry that enhances its osseointegration potential. PEKK is also well-suited for fused filament fabrication (FFF), an extrusion-based additive manufacturing technique that is increasingly being adopted for polymeric orthopedic devices due to its cost-effectiveness, design flexibility, and low material waste. FFF enables the integration of porosity into high-performance thermoplastics like PEEK and PEKK to promote biological fixation. However, importantly, the mechanical performance of FFF-printed parts is highly dependent on process parameters; hence, identifying key parameter levels that preserve or improve mechanical integrity in load-bearing devices such as spinal cages with integrated porosity is critical. Given that PEKK shares PEEK's advantageous properties and is less challenging to process using FFF, this thesis explores PEKK's suitability for spinal cage applications by first identifying optimal FFF process parameters and then comparing its performance to that of the well-established PEEK biomaterial. Aims 1 and 2 focused on identifying FFF process parameters that maximize compressive mechanical properties of solid and porous PEKK and PEEK, respectively. The findings in Aim 1 indicated that elevated thermal conditions enhanced the strength of both materials. In PEKK, elevated thermal conditions increased the tendency for crystallization, while in PEEK, they may have slowed its crystallization kinetics; as a result, print conditions significantly influenced the overall crystallinity of PEKK but not PEEK. Additionally, the highest elastic moduli of solid PEKK and PEEK were 106% and 113% of the expected value of unreinforced PEEK (3.3 GPa), whereas the highest yield strengths were 120% and 132% of unreinforced PEEK (94 MPa), respectively. Consistent with Aim 1, Aim 2 demonstrated that elevated processing conditions improved heat retention, slowed crystallization, increased strut thickness, and enhanced bonding at strut junctions in porous PEKK and PEEK structures. These improvements increased their compressive load endurance. Like PEEK, the elastic moduli of the porous PEKK fell within the range of trabecular bone, while their yield strength surpassed that of trabecular bone. Notably, despite being amorphous and unannealed, the PEKK specimens achieved over 87-88% of PEEK's calculated elastic modulus and 87-90% of its yield strength. In Aim 3, the identified suitable FFF process parameters were used to fabricate idealized PEKK and PEEK spinal cages, and their mechanical performance was evaluated under static axial compression and compression-shear test configurations. The solid cages of both materials met the minimum mechanical requirements based on previously approved PEEK and metallic spinal cages. Fully porous spinal cages solely used for benchmarking purposes, achieved between 65-90% of these benchmarks for PEKK and 72-79% for PEEK. Additionally, the fully porous cages exhibited favorable characteristics, such as pore size, porosity, and interconnectivity, that would make them suitable for promoting bone ingrowth. Finally, in Aim 4, the axial compression experiments from Aim 3 were extended using finite element analysis (FEA) to examine how geometric variations affect the mechanical performance of lumbar spinal cages. The results revealed that changes in cage geometric features influenced their deformation patterns, stress distribution, and overall mechanical performance. While all simulated PEKK and PEEK idealized cage scenarios met the minimum mechanical performance threshold, they did so to varying degrees. Regardless of material, geometric features that significantly improved yield load, stiffness, and ultimate load included the largest medial-lateral and anterior-posterior size, with the shortest height, and the smallest graft area. Further simulations of expected best and worst-case cage scenarios confirmed these conclusions, with the former demonstrating the highest performance metrics, while the latter failed to meet all the minimum mechanical thresholds. These results highlight that, beyond optimizing FFF parameters, the selection of geometric features of cages plays an important role in ensuring spinal cages meet mechanical performance requirements. The culmination of the research findings, including the comparable compressive properties of solid and porous PEKK structures to PEEK, the static mechanical performance of the PEKK spinal cages surpassing the same performance thresholds as PEEK, and the porous PEKK cages exhibiting characteristics favorable for osseointegration, all support the hypothesis that PEKK remains a suitable non-metal alternative biomaterial for spinal cages.
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Details
- Title
- Fused filament fabrication of polyether-ketone-ketone and polyether-ether-ketone lumbar interbody fusion devices
- Creators
- Abigail E. Tetteh
- Contributors
- Steven M. Kurtz (Advisor)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University
- Number of pages
- 218 pages
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- School of Biomedical Engineering, Science, and Health Systems (1997-2026); Drexel University
- Other Identifier
- 991022150338604721