The use of spinal braces is the most common solution to address adolescent idiopathic scoliosis by limiting the progression of spinal inclination. From a mechanics point of view, spinal braces are shell-type structures, relying on the fundamental engineering principles of three-point bending and inversion forces. A research project is ongoing to develop a procedure comprising laser scanning, advanced numerical simulation, topology optimization and additive manufacturing, to design and 3D print lightweight, personalized braces. In the present paper recent advances in the numerical simulation procedure are reported. Finite element simulation of the brace and its interaction with the patient’s body is employed, to evaluate the developing deformations and stresses during the brace’s use. For that purpose, the brace is modeled with shell finite elements and the body with a mirror surface of appropriate geometry, considering deformability and detachment by means of contact elements with an appropriate pressure – overclosure relation, employing realistic values of human body stiffness at its different parts. The mechanical properties of the brace material employed in the simulation are adopted from experimental tensile tests on 3D-printed Polylactic Acid (PLA) specimens. From the numerical simulation, von Mises stresses and displacements of the spinal brace are obtained and are used to interpret its structural behavior. To understand how the patient’s body is affected by the use of the brace, contact pressures are presented, reflecting the interaction between the brace and the body.