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Concept
Facet joint
Summary
The facet joints (or zygapophysial joints, zygapophyseal, apophyseal, or Z-joints) are a set of synovial, plane joints between the articular processes of two adjacent vertebrae. There are two facet joints in each spinal motion segment and each facet joint is innervated by the recurrent meningeal nerves.
Innervation to the facet joints vary between segments of the spinal, but they are generally innervated by medial branch nerves that come off the dorsal rami. It is thought that these nerves are for primary sensory input, though there is some evidence that they have some motor input local musculature. Within the cervical spine, most joints are innervated by the medial branch nerve (a branch of the dorsal rami) from the same levels. In other words, the facet joint between C4 and C5 vertebral segments is innervated by the C4 and C5 medial branch nerves. However, there are two exceptions:
The facet joint between C2 and C3 is innervated by the third occipital nerve and the C3 medial branch nerve.
The facet joint between C7 and T1 is innervated by the C7 and C8 medial branch nerves.
In the thoracic and lumbar spine, the facet joints are innervated by the medial branch nerves from the vertebral segment above the upper segment and the upper segment. For example, the facet joint between T1 and T2 is innervated by C8 and T1 medial branch nerves. Facet joint between L1 and L2; the T12 and L1 medial branch nerves. However, the L5 and S1 facet joint is innervated by the L4 medial branch nerve and the L5 dorsal ramus. In this case, there is no L5 medial branch to innervate the facet joint.
The biomechanical function of each pair of facet joints is to guide and limit movement of the spinal motion segment. In the lumbar spine, for example, the facet joints function to protect the motion segment from anterior shear forces, excessive rotation and flexion. Facet joints appear to have little influence on the range of side bending (lateral flexion). These functions can be disrupted by degeneration, dislocation, fracture, injury, instability from trauma, osteoarthritis, and surgery.
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