Design of a musculoskeletal modeling pipeline to uncover the impact of spinal sagittal alignment on intervertebral joint loads

Background: Sagittal alignment describes the curvature of the spine in the sagittal plane. Despite the high variability in the normal sagittal alignment, a previous study conducted by Roussouly et al. identified four different types of sagittal profiles (type 1-4) in which human spine can be classified. Since sagittal alignment degeneration can lead to spinal diseases and deformities, causing pain and disability, clinical and numerical research have focused on understanding the relation between sagittal alignment, spinal loading and pathology. Aim: As there is no database of musculoskeletal models associated with different Roussouly types in literature, an algorithm was specifically developed to generate models classifiable according to Roussouly types. Materials and Methods: Two hundred models were created for each type, starting from a validated female reference model characterized by a fully articulated thoracolumbar segment and Millard formulation for muscular activity (Burkhart K., 2021, Thoracolumbar spine and rib cage model in OpenSim, OpenSim). The effect of spinal alignment on muscular forces and intervertebral joint compression and shear forces were evaluated during neutral standing, 15° trunk extension, 45° trunk flexion, 20° lateral bending and 30° trunk axial twist. A total of 4000 static optimizations were conducted. Finally, a multivariate analysis was performed to compare Roussouly types, using compression and shear forces in each intervertebral joint as dependent variables. Results: We assessed spinal load distribution through the whole spine and muscle forces for the transversospinalis, psoas major, quadratus lumborum and sacrospinalis fascicles. Regardless of motion, each Roussouly type is associated with decreasing compression forces and increasing shear forces from the head to the sacrum. To gain an insight into the differences that exist between Roussouly types, spinal loads acting on types 1, 2 and 4 were compared with those acting on type 3, as this is the most common profile and it is rarely related to spinal diseases. For all the motions except trunk extension, each Roussouly type shows compression forces with a local minimum at T10/T11 or T11/T12 and types 1, 2 and 4 show a percentage change in compression forces of less than 30% compared to type 3 at each intervertebral joint. In addition, shear forces in each Roussouly type exhibit a local maximum at T12/L1 or L1/L2 and types 1, 2 and 4 show similar values compared to type 3, with double values at L2/L3, halved values at L5/S1 and quadruple values at L3/L4, respectively. In trunk extension, compression local minimum and shear local maximum are located more caudally (at L1/L2 and L2/L3, respectively). Moreover, at each intervertebral joint compression forces of types 1, 2 and 4 do not differ by more than 30% from type 3, while shear forces were found to be three times higher at L1/L2 in type 1, halved at L5/S1 in type 2 and double at T12/L1 in type 4 compared to type 3. For each motion investigated, type 4 shows higher forces in the fascicles examined than the other types, but not in the psoas major muscle. Finally, statistically significant differences were found between Roussouly types, especially at lumbar levels.