Anatomy of cervical medial branch and third occipital nerve (TON). (Reprinted with permission from Philip Peng Educational Series)
(a) Transverse view of a C5 vertebra demonstrating the spinal nerve anatomy. Spinal nerve (1); medial branch of posterior ramus (2); lateral branch of posterior ramus (3); ventral ramus (4).
(b) Lateral view of a cervical spine model demonstrating the course of the cervical medial branches. A cervical medial branch traverses the articular pillar, supplying the zygapophyseal joint above and below (2); The C2/C3 joint is unique as it is innervated solely by the third occipital nerve (5), which emerges from the posterior ramus along with the medial branch of C3 (6). The latter innervates the C3/C4 joint with the C4 medial branch.
Facet joints are the most commonly implicated structure in chronic neck pain. Levels to be treated are determined by known pain referral patterns (Fig. 13.2).
Probe: linear 15-8 MHz
Position: lateral decubitus, neck in neutral position.
Coronal (Long Axis) Scan
This view is used for level confirmation in both the upper and lower cervical spines. In the long axis, the AP appear as a series of peaks (zygapophyseal joints lines) and valleys (convex shapes of the APs).
Figure 13.3 shows the coronal scan of the cervical spine demonstrating the articular pillars (AP) of C4, C5 and C6, as well as the inferior articular process (IAP) of C2. This view is used for level and needle position confirmation. The medial branches (mb) of C3, C4 and C5 can be seen, as well as the third occipital nerve (TON).
Above the C2–C3 joint, the slope of the inferior articular process of C2 creates a characteristic drop-off with the vertebral artery visible immediately cephalad to it (Fig. 13.4).
In the lower cervical spine, the TP of C7, which can be found anterior to the AP, provides a reference for needle positioning (Fig. 13.5).
Figure 13.5 shows a coronal scan of the lower cervical spine demonstrating the transverse process of C7 (C7 TP); superior articular process of C7 (SAP); articular pillar of (C5) and (C6). This view is used to confirm appropriate needle positioning for blocks of the lower cervical medial branches.
Transverse (Short Axis) Scan
This view is used for needle placement (Fig. 13.6). The targets are the C2–C3 zygapophyseal joint (TON) and the centroid aspect of the AP (C3–C6 medial branches MB). The latter appears as a distinctive flat hyperechoic line that can be appreciated when moving the probe in a cephalo-caudal direction. In contrast, the joints present a rounded and less echogenic contour (Fig. 13.7).
Figure 13.6 shows a transverse scan of the cervical spine at the level of C6. The centroid of the articular pillar (AP) forms a distinct flat hyperechoic line and constitutes the target for cervical medial branch blocks. The semispinalis capitis muscle (SSC) inserts on the posterior tubercle of the transverse process (PT) and courses over the AP. Further anterior, the nerve root (NR) and anterior tubercle (AT) can be seen. A CT overlay of the same bony landmarks is depicted in the right lower inset. When performing a cervical medial branch block, the injectate should be deposited along the periosteum of the AP, beneath the SSC.