# A Novel Digitized Method for the Design and Additive Manufacturing of Orthodontic Space Maintainers
## 2 Materials and Methods
### Generative Design  
Based on the previous step's selections, a manifold model must be created for 3D printing. The digital approach and additive manufacturing technology enable the production of custom space maintainers with innovative shapes, while adhering to medical limits and requirements. In this regard, the generative design approach was used to explore multiple solutions.  
The generative design utilizes computer algorithms and artificial intelligence to automatically explore and produce multiple design alternatives. It generates innovative and optimized solutions to complex problems in fields like engineering, architecture, and product design. By setting constraints, goals, and parameters, the algorithm iteratively evaluates numerous design possibilities, uncovering solutions not readily apparent to human designers. Techniques like parametric modeling, genetic algorithms, machine learning, and optimization are employed to identify efficient designs. This methodology yields groundbreaking outcomes with enhanced efficiency, cost-effectiveness, and performance, while minimizing material usage and time expenditures [24].  
Figure 4 illustrates the main phases of the process using the software Autodesk Fusion 360. Initially, the meshes of the two input parts are converted into T-Spline surfaces, as shown in Figure 4a, and thickened by 1 mm in the normal direction, as shown in Figure 4b. These two solid borders represent the crucial connection between the space maintainer and the child's teeth; thus, their shape and thickness must be preserved. Generative design algorithms can be guided by an initial design, but this may lead to bias, causing the algorithm to favor outcomes aligned with the proposed preferences and restrict the exploration of diverse possibilities. To prevent such influence on the results, it was decided to avoid providing an initial design to the algorithm.  
Additionally, it was essential to define a volume to be excluded during the automatic generative process to prevent interference with the actual oral cavity. Therefore, a mesh was created to cover the internal space and the lateral sides of each thickened element, highlighted in yellow in Figure 4c. The load is applied to the internal surface of the molar tooth, while the other internal surface is fixed in space. The load's direction aligns with a line connecting the centers of mass of the two solid borders. Another design criterion is the adoption of additive manufacturing as the production technology, with a minimum element thickness of 1 mm. The optimization objectives focus on reducing mass while maintaining a minimum factor of safety equal to 2. A factor of safety refers to an additional margin of strength or performance that is intentionally incorporated into the generated design solutions. It serves to enhance the reliability and robustness of the final design by providing a safety margin against uncertainties and unexpected conditions. The factor of safety is used to ensure that the selected design, among those generated, has a higher capacity to handle loads or stresses than the predicted maximum operational demands. Itis typically expressed as a ratio, representing the factor by which the predicted maximum load or stress is multiplied to determine the final design's capacity.  
The generative design produces multiple shapes, but the selected one, shown in Figure 4d, strikes the best tradeoff between mass reduction and strength. Adjustments to the output are easily made due to the T-Spline nature of the body. Figure 4e displays the space maintainer model placed on the intraoral scan of the child.