Modelling of Annulus Fibrosus Imbalance as an Aetiological Factor in Adolescent Idiopathic Scoliosis

Objective: 1) To assess the potential influence of collagen fibre imbalance within the Annulus Fibrosus on the progression of Adolescent Idiopathic Scoliosis deformity. 2) To model the progression of adolescent idiopathic scoliosis deformity due to collagen fibre imbalance in the intervertebral disc in the thoracic region during rapid growth.

Background: A number of aetiological factors have been proposed for idiopathic scoliosis. Some have indicated a contribution from the intervertebral disc, but the specific influence of the annulus fibrosus has yet to be effectively modelled.

Method: A mathematical model was used to determine the contribution of collagen fibre orientation and directional imbalance within the annulus fibrosus to vertebral rotation and overall deformity of thoracic spine. Combined rotational (due to the collagen fibre imbalance) and translational (due to the rapid growth) movements were applied to a simplified model of the thoracic spine, using a three-dimensional transformation matrix approach.

Result: Alterations of the ratio of clockwise to anti-clockwise fibres in the intervertebral disc (from unity to 0.80) demonstrated a considerable influence on the spatial curvature. The three-dimensional model illustrates the initiation and progression of the scoliotic deformity during adolescent growth. Conclusion: Imbalance in the ratio of clockwise and anti-clockwise collagen fibre within the annulus fibrosus of the intervertebal disc has been demonstrated to have the potential to contribute(s) to the progression of the spinal deformity. The influence is not governed by the rate of increase in height, being a constant for any given fibre ratio.

Relevance: Knowledge concerning the aetiology of adolescent idiopathic scoliosis can help to predict the likelihood of the curve progression. Also the pathomechanical mechanism driving the curve may assist in the search for the responsible genes. The proposed model specifies the role of collagen fibre imbalance within the annulus fibrosus to adolescent idiopathic scoliosis.