The biomechanical effects of spinal fusion on the sacral loading in adolescent idiopathic scoliosis

• Sacral loading before and after posterior spinal fusion were compared.
• Finite element analysis explained the biomechanics of spino-pelvic alignment.
• The pre- and post-operative sacral loading was related to postural balance.
• Pressure centroid on the sacrum was implemented in post-surgical evaluation.
• Compressive stress on the sacrum was related to pelvic anteversion/retroversion.

BackgroundPosterior spinal surgical correction is performed to correct spinal deformities in adolescent idiopathic scoliosis. Althoughthe relative spino-pelvic alignment changes after spinal surgery, pelvis remains unfused in idiopathic scoliosis surgery. The impact of the spinal fusion on the transferred load to the pelvis via sacrum is not documented in the scoliotic subgroups.MethodBi-planar radiographs of 9 scoliotic subjects before and in average 16 months after spinal instrumentation surgery, and 12 controls were selected retrospectively. Patient-specific 3D reconstruction and finite element models of the spine, ribcage, and pelvis were developed. Spinal parameters (Cobb angles, kyphosis, lordosis), sacro-pelvic parameters (pelvic incidence, pelvic tilt, sacral slope), frontal and sagittal balances, the position of the trunk center of mass, and the centroid of the stress distribution on the sacrum superior endplate were measured and computed before operation and in the last follow-up.FindingsThe position of the stress distribution centroid on the sacrum superior endplate with respect to the central hip vertical axis was significantly different between pre-operative and post-operative patients p < 0.05. The distance between the anterior–posterior position of the trunk center of mass and thecenter of pressure on the superior sacral endplate significantly decreased after the spinal surgery p < 0.05.InterpretationThe impact of the scoliosis spinal fusion on the transferred load between the spine and pelvis was evaluated. The biomechanical loading of the sacrum endplate was related to the post-operative postural balance and compensatory changes in the spino-pelvic alignment after scoliosis surgery.