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

Introduction: 3D-printed scoliosis orthoses provide improved comfort, customization, and
aesthetics compared to conventional brace designs. However, their mechanical properties
and behaviour remain difficult to model due to the anisotropic and heterogeneous shell–
infill architecture. Despite increasing clinical use, validated modelling frameworks are still
limited. This study developed and validated a multi-scale finite element (FE) framework for
3D-printed orthoses with experimental testing.

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.