The atlanto-occipital joint is formed by the occipital condyles and the superior articular facets of the lateral mass of C1 (Fig. 17.3a, b). The lateral part of the foramen magnum comprises the jugular tubercle and occipital condyle, and HGC is located between the two (Fig. 17.3c) [8, 24, 34]. The left HGC is located at 10 o’clock, and the right HGC at the 2 o’clock position with respect to the foramen magnum [15, 16]. The jugular foramen and sigmoid sulcus are situated laterally and posterolaterally to these bony structures, respectively (Fig. 17.3c, d). When we approach via the lateral foramen magnum, the jugular tubercle and condyle form the biggest obstacles. The location and size of the condyle vary, which affects the selection of an approach. When the condyle is located more posteriorly, it often needs to be removed, whereas when it is located more anteriorly, it may be left intact.

A hollow—the condylar fossa—lies posterosuperior to the condyle (Fig. 17.3b, d). At its base, the extracranial orifice of PCC opens, in which the posterior condylar emissary vein runs. PCC courses from the condylar fossa to the inferior end of the sigmoid sulcus posterolateral to the jugular tubercle [23, 25–27]. Then, it communicates with the posterior part of the jugular foramen or HGC anteriorly. HGC is also called the anterior condylar canal, in contrast to PCC. A venous plexus and cranial nerve (CN) XII run in the canal. The occipital bone lateral to the condylar fossa covers the sigmoid sulcus, which contains the sigmoid sinus. The jugular process lateral to the condyle forms the posterior wall of the jugular foramen (Fig. 17.3d).

The extracranial portion of VA runs in a groove on the superior surface of the posterior arch of C1, between the occipital bone and the arch of C1, after passing through its transverse foramen (Fig. 17.4a). In its course, it runs around the atlanto-occipital joint and then passes through the dura mater to enter the intracranial region (Fig. 17.4b). VA runs inferior to the articular surface of the occipital condyle. This segment of VA gives rise to many muscular branches and is plentifully covered by the vertebral venous plexus (Fig. 17.4c). When VA penetrates the dura mater, it gives rise to the posterior spinal artery immediately before or after penetration. The intracranial segment of VA is covered with and fixed by the dentate ligament at the level of the foramen magnum (Fig. 17.4b). The nerve rootlets of CN XI cross the posterior surface of VA. When the dentate ligament is medially retracted, the C1 ventral root can be seen running ventral to VA [8, 27, 34, 43].

There are many veins in the lateral part of the foramen magnum, and they are connected to one another. There are three main venous structures: the jugular bulb in the jugular foramen; the vertebral venous plexus surrounding the extracranial segment of VA; and the marginal sinus running in the dura mater around the foramen magnum. These venous structures are connected with one another through the posterior and lateral condylar veins and the venous plexus of HGC (Fig. 17.4b–e) [8, 37]. The posterior view of the left lateral foramen magnum presented in Fig. 17.4c shows the left posterior condylar emissary vein emerging from PCC at the inferior edge of the foramen magnum and connecting with the vertebral venous plexus. The superior view of the left lateral part of the foramen magnum presented in Fig. 17.4d demonstrates the posterior condylar emissary vein in the axial section of the jugular tubercle, venous plexus of HGC, jugular bulb, and sigmoid sinus. Figure 17.4e demonstrates the venous channels in the left lateral foramen magnum, which include three main draining points from the jugular bulb to the extracranial veins. It is crucial to understand these venous structures to perform lateral foramen magnum surgeries.

As PCC and posterior condylar emissary vein coursing in the posterior part of the jugular tubercle represent good intraoperative anatomical landmarks when drilling during the lateral foramen magnum approaches, they will now be explained in more detail [23, 25, 31, 33]. The step-by-step dissection series presented in Fig. 17.5 shows the posterior and superior views of the left lateral foramen magnum, demonstrating the course of PCC. The extracranial orifice of PCC opens at the bottom of the condylar fossa just posterior to the condyle (Fig. 17.5a). With the occipital bone partially removed, PCC can be seen coursing in the posterior part of the jugular tubercle from behind (Fig. 17.5b). As HGC runs anterior to PCC, HGC is not exposed at this stage. HGC is exposed when the jugular tubercle is removed more anteriorly (Fig. 17.5c). The jugular tubercle is situated superior to HGC, and the condyle is inferior to it. On the resected surface of the jugular tubercle, PCC is not present anymore. In the superior view of the lateral foramen magnum, the intracranial orifice of PCC opens near the posterojugular ridge, which is the boundary between the distal end of the sigmoid sulcus and the jugular foramen (Fig. 17.5d). As reported in our previous study [23], the intracranial orifice of PCC can be classified into four types (Fig. 17.6). After removing the roof of PCC, its entire course, which runs in the posterior part of the jugular tubercle and opens at the condylar fossa, is visible (Fig. 17.5e). When the roof of HGC is partially removed, the canal can be seen anterior to the entire course of PCC. The canal is situated superior to the extracranial orifice of PCC and inferior to its intracranial orifice, in the so-called twisted position (Fig. 17.5f).

The step-by-step dissection series in Fig. 17.7 shows the left transcondylar fossa approach combined with the transcerebellomedullary fissure approach, demonstrating how PCC and posterior condylar emissary vein are considered as intraoperative anatomical landmarks, and that the condylar fossa and the jugular tubercle are obstacles.

Figure 17.8a demonstrates three different approaches via the lateral part of the foramen magnum to VA–PICA aneurysms, which can originate at multiple locations of VA [32]. The VA–PICA aneurysm inferolateral to the hypoglossal canal can be treated through the midline suboccipital approach with C1 laminectomy. The approach selected depends on the level and location of an aneurysm. The main obstacles to these approaches include the atlanto-occipital joint, the occipital condyle, and the jugular tubercle. Therefore, it is necessary to understand the relationships between the extensions of bone removal and surgical view in the two basic approaches, the transcondylar fossa and transcondylar approaches.

Figure 17.8b and c show the position of the left lateral foramen magnum in the skull, and the areas of bone removed in the two approaches are colored blue and green. The bony area removed varies depending on whether we observe superiorly or inferiorly through the operating microscope. It is necessary to remove the jugular tubercle superior to PCC to see the middle part of the clivus. To approach the anterior margin of the foramen magnum, the atlanto-occipital joint, including the condyle, must be partially removed. Recently we have opened the cerebellomedullary fissure to obtain a wide operative field for those vascular diseases as well.

PCC runs in the posterior part of the jugular tubercle toward the jugular foramen. Therefore, when PCC is followed by removing the bone superior to the canal, the transcondylar fossa approach permits partial removal of the posterior part of the jugular tubercle without damaging the atlanto-occipital joint (Fig. 17.8b, c

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