What is the anatomy of Z-joints innervation 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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Answer

Understanding of this anatomy is crucial for procedures that attempt to obliterate Z-joint–mediated pain by blunting the innervation, whether through anesthesia (eg, a medial branch block) or denervation (eg, medial branch radiofrequency ablation [RFA]). [16] Practitioners commonly make the mistake of thinking that each Z-joint is innervated by the 2 adjoining medial branches (eg, that the L4-L5 Z-joint is innervated by the L4 and L5 medial branches of the dorsal rami, when it is actually innervated by the L3 and L4 medial branches). Two common reasons are cited for why practitioners make this mistake.

First, in the cervical region, the Z-joints are innervated by the 2 medial branches of the same name (eg, the C3-C4 Z-joint is innervated by the C3 and C4 medial branches), with the transition occurring at the T1-T2 Z-joint, which is innervated by the C8 and T1 medial branches. The second reason practitioners commonly confuse the innervation pattern is because they fail to recognize that the medial branch descends one level to reach the Z-joint. For example, the L2 medial branch courses around the L3 superior articular process, crosses underneath the L3 MAL, and then sends branches to the L2-L3 and L3-L4 Z-joints. Therefore, in a medial branch block, the medial branches closest to the Z-joint are targeted; they simply descended from a higher level.

Moreover, it is important to note that the medial branch of the posterior rami also innervates other posterior back structures. This has several important clinical implications. First, pain relief from anesthetizing the medial branch does not necessarily implicate the Z-joints as the primary pain generator, because one of the other structures innervated by the medial branch may have been the pain generator. Second, denervation of the medial branch by RFA may affect the nerve supply to the multifidus muscle. This is important because lumbosacral radiculopathy is often another consideration in the differential diagnosis of LBP.

One test to confirm the diagnosis of a lumbosacral radiculopathy is electromyography (EMG) of the multifidus muscle. Normally, denervation potentials in the multifidus muscle of a patient with LBP might be interpreted as evidence of a lumbosacral radiculopathy. However, in the context of a patient who has had RFA of the medial branch of the dorsal rami for the treatment of Z-joint pain, an alternative explanation for the denervation potentials in the multifidus would be denervation from the RFA, not from a lumbosacral radiculopathy.

The Z-joints contain nociceptive nerve fibers from nerves of the sympathetic and parasympathetic ganglia, which can be activated by local pressure and capsular stretch. Nociceptive type IV receptors have been identified in the fibrous capsule and represent a plexus of unmyelinated nerve fibers and type I and II corpuscular mechanoreceptors. In addition, encapsulated type I and II nerve endings have been found to be primarily mechanosensitive and likely provide proprioceptive and protective information to the central nervous system.

In addition, the Z-joints have been found to undergo sensitization of neurons by naturally occurring inflammatory mediators such as substance P and phospholipase A2. Peripheral nerve endings release chemical mediators such as bradykinin, serotonin, histamine, and prostaglandins, which are noxious and can cause pain. Substance P has been implicated because of its ability to act directly on nerve endings or indirectly through vasodilation, plasma extravasation, and histamine release. Phospholipase A2 hydrolyzes phospholipids to produce arachidonic acid, causing an inflammatory reaction, edema, and prolonged nociceptive excitation.

In all, many sources of pain can be found at the Z-joint, ranging from degenerative changes to irritated nerve endings (chemical and mechanical) to concomitant nerve root entrapment.


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