## 3 Results and Discussion  
### Narrative Review  
Table 1 presents the synthesis of the data extracted from the reviewed papers, including both medical and engineering categories. It should be noted that, despite the predominance of publications in medical journals, the use of 3D printing technology for the manufacturing of customized implants for foot arthroresis is also an emerging field from an engineering design perspective. Only three studies were classified as providing a level of evidence (LoE) of III, with the remainder being LoE IV and V, consisting mostly of case reports (10) and case series (7). A total of 148 patients were treated with these implants, with the majority of cases reporting CT-confirmed successful union. The highest number of patients was reported in 2022, with 60 patients; however, this case series reviewed data gathered from prior periods. [PERSON] et al. [15] presented the only comparative study that evaluated the outcomes of 3D-printed customized truss implants versus femoral head allografts. The results  
Figure 1: The PRISMA flowchart for this systematic search. showed that 75% of patients in the 3D-printed group achieved successful fusion across three articulations (talar neck, tibia, calcaneus). The researchers also reported a union rate of 43% in the femoral head allograft group, which they considered lower than what is typically found in the literature, noting that many referenced studies do not include all three joints in the evaluation. Additionally, the total fusion rate across all articulations was significantly higher in the 3D-printed spherical implant group (92%) compared to the group with femoral head allografts (62%). The rate of graft resorption was significantly higher in the femoral head allograft group (57.1%) compared to the 3D-printed implant group. No significant differences in complications were observed between the two groups. The largest number of patients included in a study was 39 ([PERSON] et al., 2014), between June 2014 and September 2019, among which 74% of patients did not require subsequent surgical interventions or amputation--in [PERSON] et al. (2017), a teenager suffering an ATV accident avoided amputation due to the use of a 3D-printed implant. The first reported use of a 3D-printed customized truss implant for foot arthrodeis dates back to 2015 ([PERSON] et al., 2015). This case report described an implant with a polyhedron shape, produced by 4 WEB Medical (Frisco, TX, USA) ([PERSON] et al., 2016), featuring a central hole that accommodated a retrograde TTC nail. Three different implants were produced (small, medium, and large) and their fit was verified intraoperatively. The outcomes were positive, with CT scans confirming osseointegration. The same manufacturer provided customized implants based on their patented truss technology ([PERSON] et al., 2015) for the majority of the implants included in this review. [PERSON] et al. reported a titanium spherical implant manufactured by Shenshi Medical Device (Shanghai, China) ([PERSON] et al., 2016). [PERSON] et al. (2016) mentioned that, among the six cases that they reported, four patients received implants from 4 Web Medical, while the other two patients received implants produced by Meshworks, primarily due to lower costs. Another producer was Restor3D (Durham, NC, USA) ([PERSON] et al., 2016), as described by [PERSON] et al. (2014), with designs based on a patented gyroid lattice structure (TIDAL technology) ([PERSON] et al., 2014) optimized for bone growth and graft retention ([PERSON] et al., 2014), guiding the bone attachment through the surface topography and curvature and featuring 100% interconnectivity and up to 80% porosity. LRS Implants (Cape Town, South Africa) ([PERSON] et al., 2014) is another producer of implants for foot arthrodeis ([PERSON] et al., 2014), produced with the DMLS process. As the reviewed papers did not provide detailed information on the manufacturing processes, additional sources were consulted.