The role of muscles and effects of load on growth.

If mechanical modulation of growth explains progression of scoliosis during adolescent growth, two elements of the 'vicious cycle' must be quantified. Firstly we must identify the magnitude of the asymmetrical loading imposed on the spine as a function of the scoliosis curve magnitude. Secondly, we must characterize the growth response of vertebrae and remodeling response of discs to this asymmetrical loading. Animal models are helpful to quantify the former, and extrapolation to the human spine should be possible once the parameters that influence the growth modulation response have been identified. Direct measurement of spinal loading is not currently feasible, so analytical modeling to estimate plausible loading states is required. Our simulations using a model that represents the lumbar spinal musculature and a spine with increasing degrees of spinal curvature suggests that there is a range of muscle activation strategies that may predispose to progression by the 'vicious cycle' mechanism, but other strategies can load the spine uniformly, or even reverse the asymmetrical loading that would lead to progressive deformity. However, the latter strategies have a physiological 'cost' associated with increased muscle stress and increased spinal loading.