## 4 Conclusions and Further Work  
This study assessed the feasibility of 3D-printed EF rings made from PLA-CF as a lower-cost alternative to metallic fixators. FEA was conducted to evaluate their mechanical performance, focusing on stiffness, stress distribution, wire pre-tensioning effects, and callus formation impact, for different scenarios.  
The main findings show that the PLA-CF rings exhibit approximately 25% lower stiffness than metallic rings at 0.1 g loading and 21% lower stiffness with wire pre-tension. For the metallic EF, the axial displacement under 1 g loading reached 8.41 mm without wire pre-tension, but this was reduced to 7.57 mm with 500 N pre-tension. However, transverse displacements remained significant, requiring design modifications to improve stability.  
The early healing stage analysis (0-4 weeks) confirmed that callus formation significantly improves the load-bearing capacity of the EF-bone configuration. Prior studies indicated that callus stiffness progressively increases, ranging from about 0.04% at 0.5 weeks to about 60% of intact bone stiffness at 12 weeks. This gradual stiffening transfers mechanical stress from the fixator to the bone, reducing stress on EF components. In this study, the fracture displacements were about 90% at four weeks under 0.1 g loading, demonstrating improved stability. However, excessive displacement persisted at 1 g loading, suggesting that full weight-bearing should be delayed beyond two weeks post-surgery. The transition in the load transfer from the fixator to bone can serve as an objective criterion for fixator removal, as prolonged use of external fixation beyond the necessary period may hinder natural bone remodeling.  
In the ring design assessment, the baseline PLA-CF ring exceeded its ultimate strength at 1000 N pre-tension, indicating a need for structural reinforcement. A full-ring configuration with two wires per ring improved performance, reducing von Mises stress by 38% compared with the three-wire configuration, keeping stress levels within acceptable limits. Additionally, increasing the transverse net section and redistributing wire attachment points further enhanced structural integrity. Thus, for 3D-printed rings, the use of full rings is found to be a better solution that may be suitable for the design of EF. However, a further redesign of the ring is needed to sustain preloads of 1000 N.  
Given that PLA-CF stiffness is 42 times lower than that of metallic components, further geometric modifications are required in order to optimize load distribution and minimize displacements. Future studies should explore topological optimization, experimental validation under dynamic loading conditions, and stress-displacement analysis at anchor-bone interfaces, as anchor loosening remains a common failure mode in external fixators.  
Overall, this study demonstrates the potential of 3D-printed PLA-CF fixator rings but underscores the need for design improvements to accommodate higher preloads, enhance transverse stiffness, and ensure long-term mechanical stability.  
Conceptualization, D.P.; data curation, D.P.; formal analysis, T.-G.A., D.P. and I.B.; investigation, I.B.; methodology, T.-G.A., D.P. and I.B.; software, I.B.; validation, T.-G.A. and I.B.; writing--original draft, T.-G.A., D.P. and I.B.; writing--review and editing, T.-G.A., D.P. and I.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The datasets presented in this article are not readily available because of an ongoing doctoral study. Requests to access the datasets should be directed to the corresponding authors.  
The authors declare no conflicts of interest.