Skull Base Segment

2.1Anatomic Layout

The petrous portion of the internal carotid artery (ICA) is extradural and intraosseous. It extends from the carotid foramen to the superior margin of the petrolingual ligament (PLL). The carotid foramen is anteromedial to the styloid process, posteromedial to the temporomandibular joint. The PLL, placed between the sphenoidal lingula and the petrous apex, represents the posteromedial border of the cavernous sinus (CS). Once it enters in the petrous bone, the ICA presents a short vertical course, and then it turns in front of the cochlea, forming the posterior genu. From here, it runs anteriorly and medially. The petrous ICA then curves upward above the foramen lacerum (FL), thus giving the anterior genu. The segment above the FL is not truly intrapetrous, and it has been called the lacerum segment by some authors (Bouthillier et al. 1996). These segments, the anterior genu and the anterior vertical segment, are placed above the FL, and the artery does not cross the foramen. In this sense, it is better called the supralacerum segment (Herzallah and Casiano 2007). Anatomically, the FL is an opening in the dry skull that in life is filled by fibrocartilagineous tissue (fibrocartilago basalis). Its borders are the petrous apex, the body of the sphenoid, and medially the occipital bone. It is passed by tiny branches of the periosteal branches of the petrous carotid, the meningeal branches of

the ascending pharyngeal artery, and small veins (Ziyal et al. 2005). The ICA passes above it and turns superiorly. At this level, the vidian canal is always lateral to the vertical segment. The lateral part of its terminal part is in the extradural space superior to the PLL. A little superior to the PLL, the abducens nerve crosses the lateral surface of the artery (Osawa et al. 2008). As stated, the horizontal portion of the petrous ICA runs within the temporal bone in an anteromedial direction, posterior to the tensor tympanic muscle, eustachian tube (ET) and spinosum and ovale foramina. When the artery enters into the carotid canal, the carotid sheath divides into 2 layers: 1 layer continues with the periostium of the carotid canal and the other as the periostium lining the lower surface of the skull base (Bouthillier et al. 1996). The bone above the horizontal segment thins progressively as the artery runs medially, so that, usually (80 %), only dura separates the horizontal portion of the ICA (ICAh) from the trigeminal ganglion (Osawa et al. 2008). Regarding the nervous network, the carotid branches of the cervical sympathetic ganglia form a plexus on the lateral surface of the artery. The largest bundle of this plexus usually is located anteroinferiorly and gives rise to the deep petrosal nerve (DPN), which joins the greater petrosal nerve (GPN) to become the vidian nerve. The vidian nerve passes through the pterygoid canal running from the anterior genu of the ICA to reach the pterygopalatine