Lumbar Instability

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The term instability as it pertains to the lumbar spine is controversial.

Biomechanics

Left: Continuous movement is approximated by taking multiple radiographs from full extension to full flexion. Incremental movements for rotation and translation are calculated, and the instability factor is the sum of the incremental movements. Right: Nonlinear instability factor.

Instability is when a small load causing an abnormally large or catastrophic displacement. There is no gold standard definition for instability of the lumbar spine. The concept has been viewed through different lenses

Stiffness: A loss of spinal motion segment stiffness such that a force application to the structure produces a greater displacement(s) than would be seen in a normal structure, resulting in a painful condition, the potential for progressive deformity, and that places neurologic structures at risk. This doesn't cover the idea that a small load causes a large displacement, and loss of stiffness may simply be hypermobile segments not at risk of catastrophic collapse. Bogduk suggests the term hypermobility or segmental looseness be applied here instead.

Neutral zone: A significant decrease in the capacity of the stabilising system of the spine to maintain the intervertebral neutral zones within the physiological limits so that there is no neurological dysfunction, no major deformity, and no incapacitating pain. The neutral zone is similar to the length of the toe phase of the stress strain curve (see Lumbar Spine Biomechanics). The joints are loose in early range with excessive displacement, but their ultimate strength may be normal.

Instability factor: Terminal instability is the behaviour of a system at its endpoint with a sense of impending failure. This can also be seen as the quality of movement during complete range, not just at terminal range. This can be viewed through a lens of the amount of translation and rotation that occurs during sagittal movement of the lumbar spine. Normal lumbar segments have a uniform ratio of translation to rotation. Instability could be seen as an abnormal change of this ratio at any point in movement within normal range, but it may be stable at end range. This type of instability can be viewed by taking serial radiographs during range of motion, and calculating the ratio of the sums of translation to the sums of rotation. Mean and standard deviations have been reported. and an abnormal instability factor can be defined as being outside two standard deviations.

References

Bogduk. Instability In: Clinical and radiological anatomy of the lumbar spine. Elsevier 2012