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
• There is a lot of variation in vertebral body strength between people, ranging from 3 to 12kN, and is directly related to increased bone density and increased physical activity.
• The blood within the vertebral body marrow and intra-osseous veins buffers the compressive loads. This is because of the energy required when compression causes blood to be extruded from the vertebra.

Vertebral Discs

• During compression, after initial rapid deformation of the disc, deformation then slows down reaching a peak at 90 minutes.
• Over 16 hours there is a 10% loss of disc height and 16% loss of disc volume, and people are 1-2% shorter at the end of the day. Height is restored during sleep or reclined rest. Resting with the knees and hips flexed results in a more rapid return to full disc height than being completely supine.
• During standing the load on a disc is around 70kPa, and increases to 700kPa while holding a 5kg weight.

Facet Joints

• The exact amount of force that the facet joints bear is not conclusively known. Reports have varied - 0%, 20%, 28%, 40%.
• The differences are due to differences in measurement techniques, and not completely appreciating anatomy.
• In the neutral position the joint runs vertically in the sagittal and coronal planes. (They are curved in the transverse plane) Therefore in the neutral position they cannot resist vertical load, and will simply slide past one another.
• Two mechanisms can operate in isolation or in combination to allow facet joint weight-bearing
  • With backward rocking (extension) of a vertebra, without backward sliding, the inferior articular process tips are driven into the superior articular facets of the caudal vertebra (inferior medial portion is the site of impaction and maximal pressure). In this position, axial compression results in load transference through the facet joints.
  • With severe or sustained axial compression, the disc height may be reduced to such as extent that the tips of the inferior articular processes of the cranial vertebra impact on the laminae of the caudal vertebra.

References

These are study notes taken from Chapter 8 of:

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