## 7 Conclusions  
This review explored the potential of 3D-printed PEEK and PEKK as promising alternatives to titanium as orthopedic implants, with a particular focus on spinal fusion applications. This review evaluated multiple aspects, including porous scaffold designs, advanced AM techniques, and the incorporation of bioactive agents such as HA into PEEK and PEKK. The primary objective was to understand how these factors contribute to improving the biological and mechanical properties of spinal fusion implants, with the goal of enhancing the patients' outcomes. This review identified significant progress in the development of PEEK/HA and PEKK/HA composites. HA, known for its ability to support bone growth and promote osseointegration, has shown potential in enhancing the bioactivity of PEEK and PEKK implants. The integration of HA into these polymers improves their compatibility with bone tissue, addressing one of the key limitations of the traditional polymers. Additionally, advances in additive manufacturing, particularly 3 DP, have enabled the creation of complex porous scaffold designs. These scaffolds are critical for facilitating tissue integration and bone regeneration, both of which are vital for achieving successful spinal fusion.  
The porous scaffold designs discussed in the review are essential for enhancing the interaction between the implant and surrounding bone tissue. By allowing for improved tissue ingrowth and bone formation, these designs are helping to overcome challenges associated with osseointegration. However, despite these advancements, this review emphasized the need for a deeper understanding of how the scaffold microstructure affectsthe balance between bioactivity and mechanical strength. While porous structures can promote tissue integration, they may also compromise the mechanical stability of the implant. Striking the right balance between bioactivity and a good mechanical performance remains a critical challenge in the design of PEEK/HA and PEKK/HA implants. To ensure the long-term success and durability of these implants, more research is needed to address the gaps in knowledge, particularly regarding the mechanical performance of PEEK/HA and PEKK/HA scaffolds under physiological loads. In-depth studies focusing on the interplay between the scaffold microstructure, bioactivity, and mechanical strength are crucial for refining implant design. These studies should involve a combination of biological research, including in vitro and in vivo experiments, as well as comprehensive mechanical testing. The results will provide valuable insights into the clinical performance of 3D-printed PEEK and PEKK scaffolds, helping to optimize their use in spinal fusion and other orthopedic applications.  
Moving forward, continued innovation in scaffold design and material composition will be the key to ensuring that 3D-printed PEEK and PEKK implants meet the stringent biological and mechanical requirements needed for successful orthopedic procedures. As research in this field progresses, these materials hold the potential to further transform spinal fusion and other orthopedic surgeries, providing more effective and personalized treatment options for patients.  
Conceptualization, A.R.B., J.W., A.G., K.S.-A., K.R. and R.M.; methodology, A.R.B., R.M., A.G. and J.W.; validation, A.R.B., J.W., A.G., K.S.-A. and K.R.; formal analysis, A.R.B. and R.M.; investigation, A.R.B., J.W., A.G., K.S.-A., K.R. and R.M.; resources, A.R.B.; data curation, A.R.B. and R.M.; writing--original A.R.B. and R.M.; writing--review and editing, A.R.B., J.W., A.G., K.S.-A., K.R. and R.M.; visualization, A.R.B. and R.M.; supervision, A.R.B., A.G., J.W., K.S.-A. and K.R.; project administration, A.R.B. All authors have read and agreed to the published version of the manuscript.  
[PERSON] acknowledges the Royal Academy of Engineering for supporting him with the award of an Industrial Fellowship (IF2122\(\backslash\)130).  
Not applicable.  
The authors declare no conflicts of interest.