The effect of three-dimensional geometrical changes during adolescent growth on the biomechanics of a spinal motion segment.

During adolescent growth, vertebrae and intervertebral discs undergo various geometrical changes. Although such changes in geometry are well known, their effects on spinal stiffness remains poorly understood. However, this understanding is essential in the treatment of spinal abnormalities during growth, such as scoliosis. A finite element model of an L3-L4 motion segment was developed, validated and applied to study the quantitative effects of changing geometry during adolescent growth on spinal stiffness in flexion, extension, lateral bending and axial rotation. Height, width and depth of the vertebrae and intervertebral disc were varied, as were the width of the transverse processes, the length of the spinous process, the size of the nucleus, facet joint areas and ligament size. These variations were based on average growth data for girls, as reported in literature. Overall, adolescent growth increases the stiffness with 36% (lateral bending and extension) to 44% (flexion). Two thirds of this increase occurs between 10 and 14 years of age and the last third between 14 years of age and maturity. Although the height is the largest geometrical change during adolescent growth, its effect on the biomechanics is small. The depth increase of the disc and vertebrae significantly affects the stiffness in all directions, while the width increase mainly affects the lateral bending stiffness. Hence, when analysing the biomechanics of the growing adolescent spine (for instance in scoliosis research), the inclusion of depth and width changes, in addition to the usually implemented height change, is essential.