A painful facet joint can be blocked by injecting into the joint itself or by blocking the nerves that supply the painful joint. The medial branch of the posterior primary ramus of the spinal nerve innervates the facet joint. The medial branch of the adjacent dorsal rami carries the nociceptive fibers supplying the facet joint. Because each facet joint is dually innervated by the medial branch above and below the joint, it can be blocked by injecting the medial branch above and below the joint. For example, the L4-L5 facet is innervated at its upper aspect by branches from L3 and at its lower aspect by branches from L4. Therefore, two injections are necessary to block the innervation of this single facet joint. To block the medial branch, particular attention should be paid to needle placement to avoid inadvertent injection into the neuroforamen (Fig. 16-4).
Figure 16-4. Posterior view of lumbar spine showing location of medial branches (mb) of dorsal rami, which innervate lumbar facet joints (a). Needle position for L3 and L4 medial branch blocks shown on left half of diagram would be used to anesthetize L4-L5 facet joint. Right half of diagram shows L3-L4, L4-L5, and L5—S1 intraarticular facet joint injection positions. (From Canale S, Beaty J. Campbell's Operative Orthopaedics. 11th ed. Philadelphia: Mosby; 2007. Redrawn from Boduk N. Back pain: zygapophyseal blocks and epidural steroids. In: Cousins MJ, Bridenbaugh PO, editors. Neural Blockade in Clinical Anesthesia and Management of Pain. 2nd ed. Philadelphia: Lippincott; 1988.)
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