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

• Reports on the compressive load borne by facet joints have varied - 0%, 20%, 28%, 40%. These differences are due to differences in measurement techniques, and researchers 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. This also can occur if an intervertebral joint is axially compressed during extension - i.e. the weightbearing lordotic lumbar spine.
• In erect sitting, the facet joints do not bear any vertical load. During prolonged standing with a lordotic lumbar spine, the facet joints bear around 16% of axial load - the lower joints bear slightly more weight than the upper joints (19% vs 11%).
• In degenerative disc space narrowing, around 70% of the compressive load is borne by the inferior articular processes and laminae.

Fatigue Failure

• Repetitive compression of lumbar interbody joints can cause fractures of the subchondral trabeculae, fracture or impressions of one or both endplate, and even fracture of the cortical bone.
• Fracture from this type of repetitive compression can occur within the range of forces of normal activities of daily living, work, and sporting.
• The probability of fracture is related to the load and number of repetitions. With increases in load above 30% of ultimate stress there is an increase in the probability of fracture with fewer repetitions.
• Endplate fracture leads to disc height loss.

With endplate fracture stress over the NP and anterior anulus decreases, and the stress over the posterior anulus increases. This causes the lamellae of the anulus to collapse inwards towards the NP, disrupting the internal disc architecture.

• Even a small lesion can substantially compromise the normal biomechanics of a disc.

Axial Distraction

• Discs are not as stiff in distraction compared to compression
• Tensile loads occur with tree-climbing, not a common activity in humans. Humans spend more time bearing compressive loads.
• Tensile properties vary with location, and results between studies are conflicting.
  • In the intact specimen, the outer posterior AF is stronger and stiffer than the outer anterior AF.
  • In isolated fibres, the anterior AF region is stiffer and stronger than the posterolateral region. The outer regions of the AF is stiffer than the inner regions.
• Facet joint capsules are very strong against axial distraction. A single level of two facet joints can bear twice the body weight subjected to axial distraction.
• With traction, lumbar spine lengthening reduces with increasing age.
• With traction, healthy discs distraction more than degenerated discs.
• With traction, 40% of lengthening is due to flattening of the lumbar lordosis, and 60% is due to vertebral body separation.
• With traction using a 9kg load, there is only about 0.9mm of separation per intervertebral joint
• Upon release of traction, but before reloading of body weight, there is a residual 0.1mm residual separation per intervertebral joint. There is likely no maintained lengthening of the spine once the patient starts bearing axial compression with rising.

Flexion

• Flexion involves unfolding of the lumbar lordosis, at full forward flexion the lumbar spine is straight or curved slightly forwards (curve reversal).
• Curve reversal occurs mainly at the upper lumbar segments
• Movement is anterior sagittal rotation, plus a component of forward translation.
• The facet joints are very important for maintaining flexion stability
  • Anterior sagittal translation is resisted by direct impaction of the inferior articular facets of a vertebra against the superior articular facets of the vertebra below. The greatest force is at the anteromedial portions of the superior and inferior articular facets. This bony locking mechanism is more has a greater contribution than the mechanism below.
  • Anterior sagittal rotation is resisted by tension in the joint capsules, and tension in the ligaments of the intervertebral joints.
• The posterior ligaments protect the disc and resist 80% of the flexion moment and restrict the segment to 80% of the range of flexion that will damage the disc.

Failure

• Ligaments alone are not enough to support the flexed lumbar spine, and the back muscles need to help during heavy lifts.
• The disc fails by horizontal tears across the middle of the posterior AF or by avulsion of the AP from the ring apophysis.
• Prolonged flexed positions and reduce resistance of the ligaments of the spine to flexion. Reducing the duration of movement increases resistance.
• Repetitive flexion can also induce fractures of the pars interarticularis. The facet joints resist forward translation during flexion, and the force passes into the pars interarticularis.

Extension

• Extension involves posterior sagittal rotation and a small posterior translation
• Bony impaction of the inferior articular processes and spinous processes limit extension (cf ligamentous tension with flexion)
• The interspinous ligament buckles and becomes trapped between the spinous processes, and the spinous processes virtually come into contact with further extension.
• With active extension (with back muscle action) the inferior articular processes are also drawn downwards due to the action of the back muscles, and in this position the facet joints become weight-bearing.
• Resection of the facet joints has little impact of a lumbar segment to bear an extension load. In this situation the anterior AF limits extension adequately.

Axial Rotation

• Lumbar intervertebral discs resist torsion more strongly than bending.
• Discs contribute 35% of torsional resistance (quantitative analysis)
  • During axial rotation, all the AF fibres that are inclined towards the direction of rotation are strained, while the other half are relaxed.
  • The maximum range of rotation is about 3 degrees without causing microscopic injury to the fibres.
  • Total macroscopic failure of the disc is at 12 degrees.
• The posterior elements contribute the remaining 65% of torsional resistance (quantitative analysis, 42%-54% in experimental studies)
• The facet joints also protect against torsion, one of the inferior articular facets of the upper vertebra impacts against its apposing superior articular facet.
• The facet joint cartilage is able to compress, providing a buffer, before the critical range of 3 degrees.
• Rotating beyond this point means that this site of impaction becomes the new axis of rotation.
• The posterior element ligaments also help a small amount to protect against torsion, but the role is not great (supraspinous, interspinous, and opposite tensed facet joint capsule).

Fatigue Failure

• Failure is unlikely with repeated repetitions with rotation up to 1.5 degrees
• Fatigue failure is more likely with less repetitions with initial larger ranges of motion.
• Failure occurs in the form of fractures of the facets, laminae or vertebral bodies; and tears in the AF and facet joint capsules

Lateral Flexion

• Lateral flexion involves lateral bending, and rotatory movements of the interbody joints, and diverse movements of the facet joints
• There is no detailed biomechanical analysis due tot he complex and variable combination of elements invovled.

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.