Cervical pain is a common issue, and one of its causes is zygoapophyseal joint pain. Radiofrequency ablation of the medial branches that supply the pathological zygoapophyseal joints is a standard form of treatment for cervical pain that previously responded to a medial branch block. While ultrasound-guided medial branch block is admitted in skilled hands by some interventional pain treatment societies, radiofrequency ablation of cervical medial branch should be actually done under X-ray guide.

The cervical zygoapophyseal joint located beyond C2–C3 is innervated by the corresponding medial and upper branch. The cervical medial branch arises from the dorsal ramus of the segmental spinal nerve and runs along the long axis of the articular pillar with an oblique cranial to caudal direction. To adequately denervate a joint is thus necessary to ablate both medial branches [1]. According to basic principles in thermal radiofrequency lesioning, the active tip of the cannula must run as close and parallel as possible to the long axis of a nerve to effi ciently burn it [2]. Therefore, placing an active tip needle along the pillar with an oblique direction will result in an effective thermal lesion of the medial branch [3]. We propose an ultrasound-guided technique to correctly align the radiofrequency cannula parallel to the medial branch in the center of the articular pillar.

After obtaining a written consent form, the patient was placed in a lateral decubitus with the affected side upward. Standard monitoring was applied with strict aseptic techniques, the linear transducer (4–15 MHz; 4, 5 cm array) was positioned in a long axis on the neck, and the mastoid process was identified. The inferior articular process of C2 was then identifi ed as a drop-off, moving the probe anteriorly and posteriorly. After moving the probe caudally, the junction between C2 and C3 was marked and from that point caudally, we identified a “sawtooth” line in which the tops are the zygoapophyseal joints, and the bottom is the center of the pillar where the medial branch lies [4]. The C2–C3 joint is the only one supplied by a single nerve which is the third occipital nerve, running along the surface of the joint itself. The C7 medial branch is variably located and often runs onto the superior articular process of C7 [5]. We identifi ed the medial branch we wanted to treat, slid the probe to position it in the center of the screen, and measured the depth at which it was located compared to the adjacent shallow zygoapophyseal joints. We then started to rotate the probe to around 45° angle (clockwise for a right-sided procedure and anticlockwise for a left-sided procedure) while keeping the target medial branch at the middle of the ultrasound image. The endpoint was the change from a roundish bony surface to a fl at one, signifying the long axis of the articular pillar where the medial branch ran along (Figure 1).

We checked if the depth of the pillar from the skin corresponded to the one measured in the sawtooth line; otherwise, the procedure was carefully repeated. We chose a skin entry point 2 or 3 cm from the posterior edge of the probe, thus ensuring a fi nal needle position as parallel as possible to the medial branch. After some local anesthesia, we placed an 18 G, 10 mm curved tip needle with an in-plane technique alongside the articular pillar advancing up until its anterior third [5]. The ultrasound probe was then moved anteriorly to scan the corresponding spinal nerve, evaluating its safe distance from thermal cannula (Figure 2), and then an out-of-plane dynamic scan was performed over the long axis of the cannula to check its precise position onto the groove of the articular pillar. Since the cannula was placed under direct ultrasound guide along the bony surface, antero-posterior X-ray view was therefore not mandatory. Instead, a lateral X-ray view could confi rm the parallel placement of the needle along the long axis of the articular pillar, and if this was not the case, its position was adjusted accordingly.

After appropriate sensory and motor testing, 1 mL lignocaine 2% was administered before thermal lesioning of the medial branch which was done with a target temperature of 90°C for 120 seconds.

Figure 1. 45° Scan
The image shows the spinal nerve with its posterior tubercle and the flat surface of the articular pillar, where the medial branch runs. The dura mater is recognizable at the bottom of the image.

Figure 2. 45° Scan, Needle Position Along the Flat Surface of the Pillar and its Safe Distance From the Spinal Nerve

Figure 3. Cervical Spine
Yellow line: sawtooth line. Green: medial branches. Red: 90° angle rotation from sawtooth line, with the trajectory just crossing the medial branch (green). Blue: 45° angle rotation from sawtooth line, with the trajectory lying on the path of the medial branch. Image from Dixon A. Normal cervical spine radiographs. Case study, Radiopaedia.org. https://doi.org/10.53347/rID-32505

Rotating the probe with a 90° angle from the sawtooth line as typically carried out, and without carefully tuning the rotation of the ultrasound beam to correctly scan the articular pillar with its own medial branch, will result in an X-ray lateral image showing the active tip of the cannula just “crossing” the site of the medial branch without being parallel to it (Figure 3). This needle positioning may still have a positive sensory testing on the medial branch, but the thermal lesioning may not be able to provide adequately long-lasting pain relief.

X-ray is the current standard guidance for cervical medial branch radiofrequency neurotomy. Since spinal nerves are undetectable by X-ray, this tool can avoid any damage to the spinal nerve only by an indirect measuring between the needle tip and the intervertebral foramen. Ultrasounds instead can visualize both the spinal nerve and the needle tip, allowing even to measure the effective distance between them. Once we can reasonably estimate the size of the lesioning we are making, by choosing accurately the “factors that affect radiofrequency heat lesion size” (tip gauge, tip length, temperature, and time) [2], and since we can effectively measure the real distance between the spinal nerve and the needle tip, we can actually state if our neurotomy will damage any spinal nerve or not (Figure 2).

Ultrasound-guided cervical medial branch neurotomy using a 45° angle ensures a more parallel placement of the cannula to the long axis of the articular pillar, resulting in an efficient lesion of the medial branch. It can reduce X-ray exposure and save time by avoiding continuous shifting from antero-posterior to lateral view. Direct ultrasound measuring of the distance between active needle tip and the spinal nerve may add further safety to standard care in thermal radiofrequency, preventing unwanted spinal nerve lesions.

We declare that the contents have not been published elsewhere and the paper is not being submitted elsewhere. Also the manuscript has been read and approved by all co-authors. Furthermore, the authors declared no conflict of interest and no sources of support.