What is the anatomy of the spine relative to lumbosacral facet syndrome?

Updated: Nov 19, 2018
  • Author: Gerard A Malanga, MD; Chief Editor: Craig C Young, MD  more...
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The spine is composed of a series of functional units. Each unit consists of an anterior segment, which is made up of 2 adjacent vertebral bodies and the intervertebral disc between them, and the posterior segment, which consists of the laminae and their processes. One joint is formed between the 2 vertebral bodies, whereas the other 2 joints, known as the Z-joints, are formed by the articulation of the superior articular processes of one vertebra with the inferior articular processes of the vertebra above. Thus, the Z-joints are part of an interdependent functional spinal unit consisting of the disc-vertebral body joint and the 2 Z-joints, with the Z-joints paired along the entire posterolateral vertebral column.

In the lumbar spine, the superior articular processes face anterolaterally, whereas the inferior articular processes face posteromedially. The superior articular process has a concave orientation in order to accommodate the more convex orientation of the inferior articular process. The upper lumbar Z-joints are oriented in a sagittal plane, whereas the lower lumbar Z-joints approach a more frontal orientation. Thus, as the lumbosacral Z-joints maintain a progressive coronal orientation, greatest at the S1 level, they are functionally able to resist rotation in the upper lumbar region as well as resist forward displacement in the lower lumbosacral region.

The Z-joint is considered a motion-restricting joint, able to resist stress and withstand both axial and shearing forces. In back extension, the Z-joints, along with the intervertebral discs, absorb a compressive load. In addition, the transmission of the Z-joint load occurs through contact of the tip of the inferior articular process with the pars of the vertebra below. The overloaded Z-joint then causes posterior rotation of the inferior articular process, resulting in stretching of the joint capsule.

If one considers the disc and each of the adjacent Z-joints as an interdependent functional spinal unit, degenerative changes within this 3-joint complex can influence each of the segments. Thus, degeneration of the discs can lead to loss of disc height, resulting in a relative increase in Z-joint load that is found in compression and extension maneuvers. One theory is that these excessive Z-joint loads cause the inferior articular process to pivot about the pars and stretch the joint capsule, in addition to causing rostrocaudal subluxation (ie, Z-joint malalignment). Thus, some authors postulate that Z-joints undergo osteoarthritic changes in response to disc degeneration secondary to changes in loading. 80% of the loading weight is carried through the vertebral bodies and intervertebral discs and 20% by the Z-joints. However, thinning of the intervertebral discs from wear and tear causes more weight bearing through the Z-joints. This in turn results in arthropathy and slight angulation deformity, or a more horizontal orientation, which allows for degenerative spondylolisthesis to occur. Most commonly, this occurs at L4-5 and to a lesser degree at the superior levels.

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