Correlation Between Immediate In-Brace Correction and Biomechanical Effectiveness of Brace Treatment in Adolescent Idiopathic Scoliosis

Abstract

Multiple brace designs were simulated using a finite element model and their biomechanical effect was evaluated. To study correlations between immediate in-brace correction of coronal curves and bending moments acting on the apical vertebrae. Immediate in-brace correction has often been deemed as fundamental to long-term brace effect but the biomechanical explanation is unclear. Three-dimensional geometry of 3 patients was acquired using multiview radiographs and surface topography techniques. A finite element model of the patients' trunk including gravitational forces and a parametric brace model were created. Two sets of mechanical properties of the spine (stiff and flexible) were tested. Installation of the brace on the patients was simulated. Using an experimental design framework including fourteen design factors, 1024 different virtual braces were tested for each patient. For each brace, immediate in-brace correction of the coronal Cobb angles and the bending moment acting on the apical vertebrae were computed and their correlation was studied. Immediate correction of coronal curves and corresponding impact on the apical vertebrae bending moments were linearly correlated (mean R = 0.88). The amount of immediate correction necessary to nullify the bending moment ranged between 19% and 61% with average 48% (flexible spine model) and 27% (stiff spine model). The braces corrected the apical vertebrae bending moment more in the flexible spine model. In the framework of the Hueter-Volkmann principle, the correlation between coronal immediate in-brace correction and corresponding apical bending moment can be interpreted as a correlation between immediate in-brace correction and long-term treatment outcome. The amount of immediate correction necessary to invert the bending moments, and in theory counteract the progression of the scoliotic deformity, depends on spine stiffness and spine segment. This study confirms the importance of immediate in-brace correction to predict long-term outcome of the treatment and provides insights in the understanding of brace biomechanics.