## 3 Results and Discussion
### Narrative Review
#### 3.1.1 Customized 3D-Printed Cage Implant Design Aspects  
The design of patient-specific Ti alloy (Ti6 Al4V) cage implants for foot arthrodsis is a customized process that integrates both anatomical and mechanical considerations to achieve positive outcomes. These outcomes are primarily evaluated in terms of implant osseointegration, evaluated on medical imaging, with secondary outcomes including Visual Analog Scale (VAS) pain scores, re-surgery rates (amputations, revisions), and pre- and postoperative validated clinical PROMs (such as the Manchester-Oxford Foot Questionnaire (MOXFQ), etc. [22]. Engineers use preoperative CT scans for anatomical reconstruction, and the implants are designed based on these reconstructions using a 3D modeling software, also incorporating the surgeon's feedback. However, among the analyzed papers, only one study mentioned the use of specific design software (UG 12.0) for the design of spherical-shaped customized implants [21]; most of the other studies primarily focused on communicating medical aspects. The collaboration between the surgeon and engineer is essential in discussing critical factors such as the necessary correction, acceptable shortening, and fixation options. Moreover, during the design process, the quality of the patient's soft tissue and biomechanical requirements should be taken into account. This means that the design considerations include ensuring that the implant can be successfully inserted through the planned surgical exposure, respecting the anatomical constraints and the presence of any previous implants. Furthermore, the design must accommodate additional fixation means that enhance the mechanical stability, such retrograde nails, plates, or screws, or holes or channels passing through the implant. Compression across the device is mandatory to promote bone in-growth, thus supporting the fusion process. Once the implant's design is set and approved by the surgeon, it is manufactured, sterilized, and sent to the healthcare institution along with patient-specific instrumentation (PSI), including implant replicas and surgical guides for surgery planning and rehearsal. As the implants are customized for each patient, their geometries vary, with manufacturers incorporating particular features such as truss structures, spikes, or gyroid lattices to enhance osseointegration. All analyzed implants had a cage-like design with a 100% open architecture, allowing for autologous (autograft) and allogeneic bone graft (allograft) and bone graft substitutes (calcium phosphate or sulfate, demineralized bone matrix, etc.) [42]. In the study by [PERSON] et al. [27], the implant's design evolved over three generations. Initially, it featured polyhedral shapes to fit the tibial and calcaneal profiles, with 4-mm-thick side walls and a 2 mm mesh for bone ingrowth, and incorporated mesh trabeculations and 2 mm spikes for osseointegration and stability, as well as slots for bone grafts. Then, thicker 3 mm mesh trabeculations with 1 mm spikes were used for improved bone ingrowth, with the implant having a C-shape in its axial profile. Spikes or struts are especially useful in ensuring the implant's stability under dynamic loads. The patient-specific design ensures that the implant corrects the bone length; maximizes contact with the surfaces of the tibia, talus, or calcaneus; and provides structural stability after means of internal fixation are applied. Structurally, titanium truss implants have similar characteristics to the cancellous bone. Still, the modulus of elasticity is about 50 times greater than that of natural bone, providing the implant with the structural integrity needed to prevent failure over time at fusion sites. The porous surfaces of the implants enhance their stability, resisting torsional and shear forces, thus supporting the fusion process. As mentioned, several companies are involved in the production of these custom implants.