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Disc Height Loss and Preclinical Instability

annulus angle and disc height loss

Disc height loss is the common theme in back pain.

And with early disc height loss, hypermobility is related. But how are they associated to one another? Barr in 1948 (1) was the first to describe instability. His description of low back pain related to disc height and the passive stabilizers. A nice review of lumbar instability as an evolving concept was written by Beazell et al. These authors discussed the evolution through Farfan and his model, to Kirkadly-Willis and the three phases of degeneration as well Panjabi’s added concepts of neurological control. To understand the relationship between disc height loss and the development of hypermobility, it may be helpful to highlight the anatomy. In Panjabi’s and Adams “Biomechanics of the Spine”, there is a description of the annulus angles in alternating lemellae at 35 degrees from the horizontal explaining the tensile resistance with movement. What appears to not be described is the relationship of the annulus fibres when a disc loses its height. Below are a series of images, developed by Dr. Jerome Fryer, to help explain the displacement factor.

Annulus Fibrosus

Annulus Orientation

Each disc consists of concentric, alternating in orientation, fibrous sheets that encompass a hydraulic centre core named the nucleus pulposus. Once the disc height is reduced, the annulus angles change. If the length of the annulus fibres do not change, a displacement factor of hypermobility can occur.

annulus angle

Annulus angle change with disc height loss

 

disc height loss

Disc height loss leads to displacement

As it is often a therapeutic goal to increase spinal stability, increasing disc heights should be at the forefront.

The common theme seen in back or neck pain is intervertebral disc height loss. It is the earliest radiological finding in the degenerative cascade. This height loss leads to many geometrical and morphological changes that results, initially, in hypermobility which often leads to pain. This has been discussed thoroughly in the literature.

The three important changes related to disc height loss include:

  1. increased annular and endplate stress
  2. development of hypermobility
  3. facet approximation with reduced joint space width

These anatomical areas are important because this is where the innervation exists that contributes to pain.

Generally, spinal pain generators can be categorized into three distinct areas…all affected by disc height loss:

  1. discogenic and associated sinuvertebral nerve
  2. vertebral endplate disruption and associated basivertebral nerve
  3. facetogenic with medial branch and subchondrial innervation

A better understanding of the relationship between disc height loss and hypermobility will help us move toward developing models–as this relationship is thought to be the beginning stage of degeneration. A focus on the related biorheology to maintain disc height will be of paramount importance in the decades to come in the prevention of reduced joint space width of the opposing endplates and facet hyaline cartilaginous surfaces.

  1. Barr JS. Low-back and sciatic pain: results of treatment. J Bone Joint Surg Am 1951;33-A: 633–49.
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