## 3 Results and Discussion
### Medical Aspects
#### 3.3.2. Biofunctionalization Approaches in Foot Arthrodesis  
Revision ankle arthrodesis following failed surgery is usually a technically difficult task because of the large bone defects left behind, combined with the biomechanical demands of the area during walking. The current common practice is to use bulk grafts to fill the defect, and most of the research is oriented towards failures of TAR. Initially, titanium foam blocks provided by the implant manufacturer were introduced to make complex surgeries simpler, while still maintaining good clinical results. These blocks aimed to improve the stability of the implant and enable patients to bear weight sooner. However, more extensive patient trials with a larger series of patients are needed to fully understand their effectiveness and limitations, and only a randomized controlled trial, which has not yet been conducted, can accurately compare these blocks to other bone defect treatments [51]. Another advancement in the development of special support techniques was marked by the idea of the use of spinal cages [52] or trabecular metal augments from knee revision, with some commercial products developed that were dedicated to foot and ankle surgery [53]. Therefore, research has been conducted on porous titanium and tantalum spacers, which are especially suitable for large, structural, weight-bearing bone defects. These materials provide the necessary strength to support the restored height and correct the ankle's alignment until bone fusion occurs between the metal implant and the surrounding bone tissue. This approach differs significantly from bone autografts or allografts, which are prone to collapse due to absorption after implantation. The explanation of these failures was deficient construct stability and/or insufficient implant porosity [54], while more recent studies indicate that one of the primary reasons for these failures is the weak fixation strength provided by the fixation nails, especially when patients begin early walking. This issue is particularly pronounced in diabetic or overweight patients, where the increased biomechanical stress exacerbates these challenges, even when the implants are inserted with the correct surgical techniques [55]. The solution to these challenges lies in biofunctionalization, which, in the reviewed papers, was implemented by tailoring the implant geometry to meet specific anatomical and biomechanical constraints. Traditionally, biofunctionalization also encompasses surface modifications, such as coatings or chemical treatments, designed to enhance cellular attachment and promote integration. However, the reviewed studies focused mainly on geometry-based strategies, emphasizing the role of advanced lattice structures, such as gyroid and truss designs, to mimic cancellous bone. These structures enable bone tissue ingrowth, improve load distribution, and enhance mechanical strength, thereby reducing the risk of collapse, nonunion, or implant failure. These 3D-printed titanium cages with biofunctionalized designs not only offer an anatomical fit but also promote osseointegration, as their geometry ensures bone tissue ingrowth, structural integrity, and stability throughout the fusion process. These implants are designed to match the mechanical demands of the region, ensuring long-term durability and reducing the risk of stress shielding--a phenomenon where excessive load bearing by the implant leads to bone resorption. Additionally, such designs can effectively integrate autografts or allografts, as reported in the reviewed papers, supporting faster healing and reducing the risks associated with traditional implants. Another important general aspect of such applications is ensuring the cytocompatibility of the implant materials--in this case, Ti6 Al4V--which directly influences osseointegration and the risk of postoperative infection.