Will we need patient specific spine models?

Background: The differences in vertebral size between patients can be large: between 11 and 18 years vertebrae grow 25% in size and interindividual differences in vertebral size can be up to 20%. Although it is generally recognised that this will have an effect on the mechanical behaviour of the spine, little research has been done on how large this effect is. These effects are important considerations in the treatment of spinal disorders such as scoliosis.

Purpose: We studied how differences in vertebral size influence the mechanical behaviour of the spine.

Methods: We used a computational model of a human motion segment. As the flexibility of the motion segments is dominated by the behaviour of the discs, the vertebral endplates were modelled as rigid bodies. The disc was modelled as an annulus of a base material with fibres in a symmetrical pattern around a hydrogel-like nucleus. The mechanical behaviour of the resulting model compared very well to data found in literature. Parameters that change during growth and are likely to have a mechanical effect on the spine (width, depth, disc height and nucleus size) were varied over a physiological range.

Results: A 12.5% (25%) wider disc increased the stiffness in axial compression and flexion-extension by approximately 10% (20%). Lateral bending stiffness increased by 38% (85%) and axial rotational stiffness increased by 28% (59%). A 12.5% (25%) deeper disc increased the stiffness in axial compression and lateral bending by approximately 13% (25%). Flexion-extension stiffness increased by 39% (86%) and axial rotational stiffness increased by 27% (54%). A 12.5% (25%) higher disc lowered the stiffness in axial compression, lateral bending, flexion-extension, and axial rotation all by approximately 14% (23%). A 40% larger nucleus lowered the stiffness in axial compression, lateral bending, flexion-extension, and axial rotation all by approximately 9%.

Conclusions: The changes in size that typically occur during growth or are present as interindividual differences cause dramatic changes in the stiffness of the spine. This presents some interesting challenges for the treatment of spinal disorders and biomechanical studies of the spine. As modern treatment techniques for scoliosis intend to stimulate the spine to regain normal curvature without severely impairing its flexibility, the forces applied to the spine must be well balanced with the mechanics of that spine, thus necessitating patient specific spine models.