The high anterior cervical retropharyngeal approach to the upper cervical spine has been previously described in detail [10-12]. Russo et al. used this approach to the clivus and foramen magnum . The procedure is performed with the patient positioned supine. The head is extended 20° to 30°and rotated 30° to 45° away from the side of the approach. The mandible is displaced superiorly. An incision is made approximately 3-4 cm inferiorly and parallel to the mandible to avoid injury to the marginal mandibular branch of the facial nerve (Figure 1A). The platysma is divided and retracted superiorly. The submandibular gland is exposed and elevated upward (Figure 1B). The posterior belly of the digastric muscle is brought into view under the submandibular glands. The anterior belly of the digastric muscle is retracted medially, and the facial artery and vein laterally. The posterior belly and tendons of the digastric muscle are elevated superiorly. When the posterior belly is divided deep, the hypoglossal nerve is revealed, which passes inferiorly to the muscle (Figures 1C, 1D). The hypoglossal nerve is carefully dissected and retracted rostrally. The external carotid artery and the facial artery branch is retracted laterally and superiorly and the lingual artery is retracted inferiorly (Figures 1B and 1C). The pharyngeal muscles are retracted medially and thus the retropharyngeal space is opened. The pharyngeal muscles are further separated deeply and the anterior tubercle of C1 and anterior surface of the cervical vertebrae are exposed (Figure 1E). Next, the prevertebral fascia and the anterior longitudinal ligament in the midline are resected, exposing the entire arch of C1 and the body of C2. The anterior atlantooccipital membrane, as well as the longus capitis muscles are detached from the anterior rim of the foramen magnum and midlateral portion of the clivus (Figure 1F). The upper boundary is the vomer and pterygoid process medial plate. The bilateral boundary is the petroclival fissure.
We performed distance measurements of the anatomical structures of the free clivus. In another specimen, the mean distance from the anterior rim of the foramen magnum to the vomer was 27.1 mm (range 26-28.9 mm). The width of the clivus at the pterygoid process medial plate level was 21.6 mm (range 21.0-22.3 mm). The width of the clivus at the pharyngeal tubercle level was 27.8 mm (range 26.9-28.4 mm). The width of the clivus at the hypoglossal canal outside the hole midpoint level was 28.7 mm (range 27.6-29.1 mm). The width of the clivus at inferior margin of the hypoglossal canal outside the hole was 29.6 mm (range 28.7-31.1 mm; Figure 2A).
We also performed distance measurements of the previous step on the heads, which had good exposure of the clivus. The anterior rim of the foramen magnum adheres to the longus capitis muscle, rectus capitis anterior muscle, pharyngobasilar fascia, and mucosa. It is difficult to dissect from the anterior margin of the foramen magnum, especially the fascia on the supracondylar groove; it is difficult to dissect from the cortical bone surface of the groove, which is an important landmark on the clivus for localizing the hypoglossal canal. It is located deep, at the same level. In this stage, the mean distance from the pharyngeal tubercle to the vomer was 19.1 mm (range 18.7-20.9 mm). The width of the clivus at the pterygoid process medial plate level was 20.1 mm (range 19.3-20.9 mm). The width of the clivus at the pharyngeal tubercle level was 25.9 mm (range 24.9-26.4 mm; Figure 2B). There was not an obvious difference compared with the measurements of the free clivus. This indicated that it was sufficient for the mucosa of the clivus to be separated bilaterally from the petroclival fissure.
For the CVJ, the hypoglossal canal and nerve are very important structures. The hypoglossal canal is directed posteriorly and medially at a 45° angle with the sagittal plane; its extracranial outside hole is located immediately above the junction of the anterior and middle third of the occipital condyle and medial to the jugular foramen. In the interior view of the clivus, the hypoglossal canal is located at the back of the free anterior rim of the foramen magnum; the medial border of intracranial hole and the petroclival fissure have the same vertical line (Figure 2C). In the outside view of the clivus, the hypoglossal canal is located at the front of the free anterior rim of the foramen magnum. The midpoint of the hypoglossal canal outside the hole acts as the lateral limiting points. We dotted a black line as the lateral limit of the medial condylectomy (petroclival fissure to the midpoint of the hypoglossal canal outside the hole to hypoglossal canal inside the hole (Figure 2D).
Exposure of the CVJ
The clivus and the high cervical area are exposed. First, clivus resection is performed. A high-speed drill or rongeur is used. Drilling is performed in the midline of the inferior portion of the clivus. The upper boundary is the sphenoidal sinus, which is limited laterally by the medial border of the petroclival fissure. The lower boundary is the anterior rim of the foramen magnum. In this resection, the atlantooccipital anterior membrane, anterior longitudinal ligament, and apical ligament of the dens of C1 are reserved. There is a 20 x 30 mm bony window through the clivus, as Russo previously reported . After opening the dura mater, we could observe the ventral aspect of the brainstem and the related vascular and neural structures (Figure 3A). The proximal segment of the basilar artery (BA) and bilateral vertebral artery (VA) can first be observed. In the upper view, the abducens nerve could be observed from its origin. The pontomedullary sulcus is exposed in the middle of the view. In the midline, the anterior spinal artery is observed. Posterior inferior cerebellar artery (PICA) and cranial nerve (CN) XII are bilaterally partly observed.
When performing the endoscope-assisted high anterior cervical approach and expanding the bone window, the anterior rim of the foramen magnum, atlantooccipital anterior membrane, and anterior longitudinal ligament are resected. After drilling the medial third of the lateral mass of C1 and the anteromedial third of the occipital condyle, we observe the CVJ vascular and neural structures using 0° and 30° rod-lens endoscopes. The BA, VA, abducens nerve, and pontomedullary sulcus can be observed. Exposure of the inferior field is increased. The hypoglossal canal, CN XII, and P.I.C.A. are still bilaterally partially observed using 0° endoscopes, with a greater observation range (Figure 3B). However, it is observed completely using 30° endoscopes (Figures 3C-F). It is noted that the occipital condyle anatomic levels in turn are cortical bone, soft cancellous bone, hard cortical bone, and hypoglossal canal. It is important to protecting the hypoglossal nerve when drilling deeper layer of hard cortical bone of occipital condyle which surrounds the hypoglossal nerve.
When drilling out the dens down to the level of the body of C2, and continuing to expand inferior field exposure. After opening the dura, then we exposed the cervicomedullary junction. It is worth noting the CVJ area is sufficiently exposed after drilling out the dens (Figure 4A). In addition to the aforementioned structure, others such as the BA, VA, abducens nerve, pontomedullary sulcus, hypoglossal canal,CN XII, and PICA are exposed. More CVJ region vascular and neural structures is observed using 0°endoscopes. Upper cervical spinal cord is also exposed. The vertebral artery intradural segments are observed completely, especially from their dural entrance points to the supramedial rising segment. More importantly, the spinal nerve C1 could be bilaterally observed completely using 0° endoscopes. Using 30°endoscopes results in greater exposure for the CVJ region (Figures 4B-E).
Exposure degree and resection range of the CVJ
The relationship of exposure degree and resection range of the CVJ are summarized in Figure 5. The Figure 5A, 5B, 5C is the endoscopic view of the resection of the clivus in the endoscope-assisted high anterior cervical approach. The Figure 5D, 5E, 5F is the endoscopic view of resection of the clivus and atlas. There is not an obvious increase in exposure degree of the CVJ. The Figure 5G, 5H, 5I is the endoscopic view of the resection of the clivus, atlas, and odontoid. There are not obvious increases in the exposure degree of the CVJ when the atlas is drilled out based on the resected clivus, in the anterior cervical approach (Figures 5C, 5F). After removing the odontoid, greater exposure of the CVJ area is obtained (Figures 5C, 5F, 5I). It is imperative to resect the dens in order to sufficiently expose the CVJ in the anterior cervical approach, especially for large lesions involving the high cervical spinal region.