09/12/2020
Biomechanics of the lumbar spine and sacrum (L4-L5 L5-S1)
The 3 movements in the spine are flexion, extension, rotation and lateral flexion. These movements occur as a combination of rotation and translation in the following 3 planes of motion: sagittal, coronal and horizontal . These movements result in various forces acting on the lumbar spine and sacrum: compressive force, tensile force, shear force, bending moment and torsional moment . For example, with lumbar flexion, a compressive force is applied to the anterior aspect of the disc and a distractive force is applied to the posterior aspect of the disc. The opposite forces occur with lumbar extension.
The lumbar spine complex forms an effective load-bearing system. When a load is applied externally to the vertebral column, it produces stresses to the stiff vertebral body and the relatively elastic disc, causing strains to be produced more easily in the disc. Pressure within the nucleus pulposus is greater than zero, even at rest, providing a “preload” mechanism allowing for greater resistance to applied forces. Hydrostatic pressure increases within the intervertebral disc resulting in an outward pressure towards the vertebral endplates resulting in bulging of the annulus fibrosis and tensile forces within the concentric annular fibres. This transmission of forces effectively slows the application of pressure onto the adjacent vertebra, acting as a shock absorber. The intervertebral discs are therefore an essential biomechanical feature, effectively acting as a fibrocartilage “cushion” transmitting force between adjacent vertebrae during spinal movement. The lumbar disc is more predisposed to injury compared with other spinal regions due to: the annular fibres being in a more parallel arrangement and thinner posteriorly compared with anteriorly, the nucleus being positioned more posteriorly, and the holes in the cartilaginous endplates.
When a load is applied along the spine, “shear” forces occur parallel to the intervertebral disc as the compression of the nucleus results in a lateral bulging of the annulus. Shear forces also occur as one vertebra moves, for example, forwards or backwards with respect to an adjacent vertebra with flexion and extension. Torsional stresses result from the external forces about the axis of twist and occur in the intervertebral disc with activity such as twisting of the spine
The zygapophysial or “facet” joints provide stability to the intervertebral joint with respect to shear forces, whilst allowing primarily flexion and extension movement
