PEEK implants for cranioplasty: a mapping review of 3D printing, surface functionalisation, and clinical performance gaps

Cranioplasty remains associated with significant intra- and postoperative complications, with infection as the leading cause of implant failure. Autologous bone grafts, in particular, may fail in up to 50% of cases due to bone resorption. These clinical challenges highlight the need for implants that combine mechanical reliability, biocompatibility, and patient-specific precision. Among candidate materials, polyether ether ketone (PEEK) has gained increasing attention, especially when fabricated via fused deposition modeling (FDM). The first successful cranioplasty using an FDM 3D-printed PEEK implant was reported in 2022, demonstrating recent clinical translation of this technology. Despite well-documented structural differences between cranial and femoral bones, as well as intra-/interpersonal cranial bone variation, the mechanical properties of 3D-printed PEEK are often benchmarked against femoral bone or averaged values. While the effects of FDM process parameters have been extensively studied, knowledge gaps remain regarding less-explored parameter combinations, printer variability, and their effects on implant performance. Surface functionalisation and drug-loading strategies offer potential to enhance PEEK's bioactivity and antimicrobial properties, yet their mechanical consequences are not fully understood. This mapping review highlights current limitations in the understanding of PEEK cranial implants and emphasises the need for region-specific bone data, systematic evaluation of printing parameters, and integrated functionalisation approaches to optimise implant performance and improve clinical outcomes.