The Z-joints are innervated by two lumbar medial branches (MB) of the dorsal rami. Both MBs must be anesthetized to achieve the diagnostic block of the associated zygapophysial joint. The target for blockade or neurotomy is the red region illustrated on the respective nerves. MB nerves are not labeled for the transverse process (TP) that they cross but rather for their originating spinal nerves. Using this nomenclature the L3 MB travels across the L4 TP at the base of the L4 SAP. The L4-L5 Z-joint would therefore be blocked by anesthetizing MBs L3 and L4. The L5-S1 Z-joint is a special case in which the L4 MB and the L5 dorsal ramus (DR) are anesthetized. A, Anteroposterior (AP) view. B, Oblique view.