Effects of Low Bone Mineral Status on Biomechanical Characteristics in Idiopathic Scoliotic Spinal Deformity

Background

Low bone mass in patients with adolescent idiopathic scoliosis has been well reported. Poor bone quality was regarded as a new and unique prognostic factor in aggravating curve progression. However, the potential biomechanical correlation between them remains unclear.

Methods

Three-dimensional finite element models of idiopathic scoliotic spine with different bone mineral status were created for axial loading simulation. An axial load of 3 different body weights was applied on different bone mineral mass models. The mechanical responses of the vertebral cortical and cancellous bone, facet joints, end plate, and intervertebral disc were analyzed.

Results

Accompanied with the low bone mineral status, thoracic scoliosis produced asymmetric and higher stress in the cortical bone, lumbar facet joints, and end plate at the concave side of the thoracic structure curve. Stress increased in the disc at the apex of the scoliosis, whereas it mildly decreased in the L4-5 and L5-S1 disc. Body weight gain increased the stress in scoliotic spine structures in all bone mineral statues.

Conclusions

Biomechanical simulations indicated that low bone mineral mass might aggravate curve progression and induce more serious lumbar compensatory scoliosis in patients with adolescent idiopathic scoliosis. Weight gain was also a risk factor for curve progression.

Introduction

The cause of adolescent idiopathic scoliosis (AIS) remains unknown. However, a substantial component of scoliosis progression during the adolescent growth stage is biomechanically mediated.¹ The natural physiologic curves of the spine are designed to transfer loads to the pelvis and the lower limbs. Curves act as a cushion for load buffering. Idiopathic scoliotic spine is a complex three-dimensional deformity. A deformed spine has specific biomechanical characteristics.²

The data of our systematic literature review indicated that low bone mineral status was a generalized phenomenon and a systematic disorder in patients with AIS.³ Derangement of bony mechanical stability is quantified by bone quality. Abnormal bone quality and bone matrix mineralization have been detected in patients with AIS.4, 5 Bone quality is an important determinant of bone biomechanics.⁶ Changes of bone quality in patients with AIS might have distinct influences on mechanical features of deformed anatomic structures. Low bone quality might be associated with many scoliosis parameters.⁷ Reduction in trabecular bone modulus could substantially increase end-plate and cortical shell stresses.⁸ The spinal architecture weakened by osteopenia in patients with AIS might aggravate the spinal deformity. Clinical studies have advocated that poor bone quality is a new and unique prognostic factor in curve progression7, 9; the underlying mechanism related to the potential biomechanical correlation between poor bone quality and curve progression is still not so clear.

Finite element analysis has been used for biomechanical comparison between healthy and osteoporotic spine8, 10, 11, 12, 13, 14, 15 and used for biomechanical studies of scoliotic spine.16, 17, 18, 19 In the present study, effects of abnormal bone quality on biomechanical characteristics of the single thoracic idiopathic scoliosis were investigated using our previously reported three-dimensional finite element model.17, 18 The finite element method allows for flexible alteration of material parameters and supports the full field of mechanical results.10, 20 It is suitable for investigating issues related to bone quality.21, 22 Different models of normal, osteopenia, middle-grade osteoporotic AIS spine were developed in the current study. An axial loading simulation was performed. The mechanical responses of the vertebral cortical and cancellous bone, facet joints, end plate, and intervertebral disc were analyzed.