Not just alignment: body shape influences adult spinal deformity biomechanics

Adult spinal deformity (ASD) alters sagittal alignment and trunk centre of mass location, increasing spinal loading. Current clinical assessment of ASD primarily focuses on alignment parameters disregarding patient's body shape variability. This study aimed to evaluate the relative effects of spinal sagittal alignment and body mass distribution (i.e., body shape) on spinal loads and trunk centre of mass location in upright-posture and forward flexion.

Sagittal alignments of 754 patients with ASD were obtained from clinical data. Trunk mass distributions were assessed in adult male volunteers (10 healthy weight, 9 obese). Using an established musculoskeletal model of the spine, inverse dynamics simulations were performed for each combination of sagittal alignment and trunk mass distribution profile. A factorial analysis using generalised linear models elucidated the relative effects of alignment vs. body shape.

Sagittal alignment primarily determined the antero-posterior positioning of trunk centre of mass; body shape, its cranio-caudal location. In the healthy weight cohort, the relative effect of mass distribution on compressive loads was 14-98% in the thoracic and 42-57% in the lumbar region. In the cohort with obesity, it was 24-98% and 19-27%, for thoracic and lumbar regions, respectively. Body shape effects on shear forces were greater in dynamic forward flexion than in static upright-posture.

The considerable impact of mass distribution on spinal loads suggests that patient body shape should be considered in ASD surgical planning to avoid segmental overloading. Future research should evaluate body shapes in patients with ASD, including females and elderly subjects.