Supplementary Materials: Multi-scale finite element modeling of 3D-printed orthoses

Methods

Orthotropic infill properties were derived via numerical homogenization and combined with isotropic shell properties in a hybrid FE model. At the microscale, tensile and compression tests of standardized 3D-printed specimens were compared with FE predictions. Sensitivity analyses were conducted to assess the structural contributions of the shell and infill on overall stiffness.

At the mesoscale, experimental nonhomogeneous strain distributions in organic subunits were analysed using digital image correlation and compared with FE simulations. At the macroscale, the model’s predictive accuracy was evaluated against full-brace experimental data. Results: The hybrid FE model accurately reproduced elastic behaviour in both tension and compression, with stiffness differences of 1.8% and 3.7%, respectively.

Sensitivity analysis confirmed the dominance of shell stiffness over infill but also the need to represent both components within the hybrid model. Mesoscale validation showed close agreement between experimental and simulated strain fields (82% histogram overlap). At the macroscale, predicted reaction forces deviated by less than 2% from averaged experimental measurements, and stress analysis identified regions consistent with observed plastic deformation.

Conclusions: The validated framework enables accurate and computationally efficient FE modelling of 3D-printed orthoses, supporting performance-driven design by redistributing material based on stress while maintaining structural functionality. This approach facilitates lighter, more breathable, and patient-compliant brace designs. Beyond scoliosis treatment, the methodology is broadly applicable to orthotic, prosthetic, and other lattice-based engineering structures.