What is the anatomy of the Z-joint relevant 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 Z-joint is a common pain generator in the lower back. The 2 common mechanisms for this generation of pain are either (1) direct, from an arthritic process within the joint itself, or (2) indirect, in which overgrowth of the joint (eg, Z-joint hypertrophy or a synovial cyst) impinges on nearby structures.

The Z-joints are diarthrodial joints with a synovial lining, the surfaces of which are covered with hyaline cartilage, which is susceptible to arthritic changes and arthropathies. Repetitive stress and osteoarthritic changes to the Z-joint can lead to zygapophyseal hypertrophy. Like any synovial joint, degeneration, inflammation, and injury can lead to pain with joint motion, causing restriction of motion secondary to pain and, thus, deconditioning. In addition, Z-joint arthrosis, particularly trophic changes of the superior articular process, can progress to narrowing of the neural foramen. In addition, as is the case for any synovial joint, the synovial membrane can form an outpouching and, thus, a cyst. Z-joint cysts are most commonly seen at the L4-L5 level (65%), but they are also seen at the L5-S1 (31%) and L3-L4 (4%) levels. These synovial cysts can be clinically significant, particularly if they impinge on nearby structures (eg, the existing nerve root).

The neural foramen is bordered by the superior articular process, pars interarticularis, and posterior portion of the vertebral body. Z-joint hypertrophy or a synovial cyst can contribute to lateral and central lumbar stenosis, which can lead to impingement on the exiting nerve root. Thus, Z-joint pain can occasionally produce a pain referral pattern that is indistinguishable from disc herniation.

To understand the pattern of pain generation from the Z-joint, knowledge of the innervation pattern is essential. This pattern is frequently misunderstood even by experienced practitioners. Each Z-joint is innervated by branches of the dorsal ramus, termed the medial branch. The medial branch is 1 of 3 branches of the dorsal ramus, with the other 2 being the lateral branch (which does not exist for the L5 dorsal ramus) and the intermediate branch. The lateral branch innervates the iliocostalis muscle, and the intermediate branch innervates the longissimus muscle. The medial branch innervates many structures, including the Z-joint, but it also innervates the multifidus, interspinales, and intertransversarii mediales muscles, the interspinous ligament, and, possibly, the ligamentum flavum (see image below).

Dorsal ramus innervation (medial and lateral branc Dorsal ramus innervation (medial and lateral branches). MAL23 = mamillo-accessory ligament bridging the mamillary and accessory processes of L2 and L3; Z-joint = zygapophyseal joint.

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