  The pathogenesis of adolescent idiopathic scoliosis (AIS) remains poorly
understood and biomechanical data are limited. A deeper insight into spinal
loading could provide valuable information for the improvement of current
treatment strategies. This work therefore aimed at using subject-specific
musculoskeletal full-body models of patients with AIS to predict segmental
compressive forces around the curve apex and to investigate how these forces
are affected by simulated load carrying. Models were created based on spatially
calibrated biplanar radiographic images from 24 patients with mild to moderate
AIS and validated by comparing predictions of paravertebral muscle activity
with reported values from in vivo studies. Spinal compressive forces were
predicted during unloaded upright standing as well as upright standing with
external loads of 10%, 15% and 20% of body weight (BW) applied to the scapulae
to simulate carrying a backpack in the regular way, in front of the body and
over both shoulders. The validation studies showed higher convex muscle
activity, which was comparable to the literature. The implementation of spinal
deformity resulted in a 10% increase of compressive force at the curve apex
during unloaded upright standing. Apical compressive forces further increased
by 50-62%, 77-94% and 103-128% for 10%, 15% and 20% BW loads, respectively.
Moreover, load-dependent compressive force increases were the lowest in the
regular backpack and the highest in the frontpack and convex conditions. The
predictions indicated increased segmental compressive forces during unloaded
standing, which could be ascribed to the scoliotic deformation. When carrying
loads, compressive forces further increased depending on the carrying mode and
the weight of the load. These results can be used as a basis for further
studies investigating segmental loading in AIS patients during functional
activities.
