Movements of the Lumbar Spine

The principal movements of the lumbar spine are axial compression, axial distraction, flexion, extension, axial rotation, and lateral flexion. Horizontal translation does not occur naturally as an isolated movement, but it does occur with axial rotation.

Axial Compression

• Axial compression occurs during weight-bearing in the upright posture, or as a result of contraction of longitudinal back muscles
• The nucleus pulposus (NP) and anulus fibrosus (AF) cooperate to transmit weight from one vertebra to the next.
• The outermost AF fibres do not participate in bearing load
• The compression load is uniform across the inner, anterior AF and NP, but with peak stress over the inner, posterior anulus.
• In older adults, the posterior peak stress is larger.
• Compressive forces squeeze water out of the disc, resulting in an increased electrolyte concentration, which helps to reabsorb water back in the disc after the compression is gone.
• Under compression, the vertebral bodies approximate, and the disc bulges radially. NP pressure prevents buckling inwards. The bulging is greater anteriorly than at the posterolateral corner.
• Discectomy results in an increase in both the loss of disc height and an increase in the radial bulge.

Endplates

• Endplate loading during compression is distributed evenly over the NP and AF.
• The endplate bows during compression because of the slightly weaker central trabecular bone compared to the peripheral strong cortical bone.
• With excessive load, the trabecula under the endplates fracture, and the endplates themselves fracture, usually in their central region over the NP, rather than over the AF. The entire endplate may fracture with extreme loads.
• The endplates are the weakest components of the intervertebral disc with axial compression
• In the setting of a healthy AF, the endplate fractures before the AF ruptures. I.e. the AF is stronger than the endplates against axial pressure.

Vertebral Bodies

• In adults <40 years, 25-55% of the weight applied to a vertebral body is borne by the trabecular bone. The remaining force is borne by the cortical periphery. The proportion changes in older adults

References

Study notes taken from Chapter 8 of:

• Bogduk, Nikolai. Clinical and radiological anatomy of the lumbar spine. Edinburgh: Elsevier/Churchill Livingstone, 2012.