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Osseous Relationships

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A, Inferior view of the occipital condyles and foramen magnum. The occipital condyles are ovoid structures located along the lateral margin of the anterior half of the foramen magnum. Their articular surfaces are convex, face downward and laterally, and articulate with the superior facet of C1. A probe inserted through the hypoglossal canal passes forward approximately 45 degrees from the midsagittal plane in an anterolateral direction. The hypoglossal canal is located above the middle third of the occipital condyle and is directed from posterior to anterior and from medial to lateral. The intracranial end of the hypoglossal canal (small oval) is located approximately 5 mm above the junction of the posterior and middle third of the occipital condyle and approximately 8 mm from the posterior edge of the condyle. The extracranial end of the canal is located approximately 5 mm above the junction of the anterior and middle third of the condyle. The average length of the longest axis of the condyle is 21 mm. The large arrow shows the direction of the transcondylar approach and the cross-hatched area shows the portion of the occipital condyle that can be removed without exposing the hypoglossal nerve in the hypoglossal canal. The condylar fossa is frequently the site of a canal, the condylar canal, which transmits the posterior condylar emissary vein that connects the vertebral venous plexus with the sigmoid sinus just proximal to the jugular bulb. The condylar canal passes above and usually does not communicate with the hypoglossal canal. The jugular process of the occipital bone extends laterally from the posterior half of the occipital condyle to form the posterior margin of the jugular foramen. The portion of the jugular process located immediately behind the jugular foramen serves as the site of attachment for the rectus capitis lateralis muscle. The stylomastoid foramen is situated lateral to the jugular foramen. The styloid process is located anterior and slightly medial to the stylomastoid foramen. B, Inferolateral view. A probe has been passed through the hypoglossal canal, which passes above occipital condyle. From its intracranial to its extracranial end it is directed forward, lateral, and slightly upward. C, Superior view. The occipital condyle projects downward from the lateral margin of the anterior half of the foramen magnum. The intracranial entrance of the hypoglossal canal is located above the condyle. The jugular tubercles are located above and anterior to the hypoglossal canals. The jugular process of the occipital bone extends laterally from the condyles to form the posterior margin of the jugular foramen. The sigmoid sinus crosses the occipitomastoid suture and turns in a hooklike groove on the upper surface of the jugular process to reach the jugular foramen. Drilling the occipital condyle increases access to the anterolateral margin of the foramen magnum. Drilling in a supracondylar location below the hypoglossal canal accesses the lateral edge of the clivus. Drilling in the supracondylar location above the hypoglossal canal accesses the jugular tubercle, which projects upward and often blocks visualization of the junction of the middle and lower clivus and the region of the pontomedullary junction during the far-lateral approach. Drilling the jugular process in a paracondylar location accesses the posterior margin of the jugular bulb, which is situated in the sigmoid portion of the jugular foramen. D, Medial aspect of the occipital condyle and supracondylar region. The inner surface of the mastoid portion of the temporal bone is grooved by the sulcus of the sigmoid sinus. The asterion, the site of the junction of the lambdoid, parietomastoid, and the occipitomastoid sutures, is an important landmark used to define the transition between the transverse and sigmoid sinuses. The sigmoid sulcus crosses the occipitomastoid suture just behind the jugular foramen. The intracranial end of the hypoglossal canal is located above the junction of the posterior and middle thirds of the occipital condyle. The external occipital protuberance is located an average of 2 cm below the apex of the internal occipital protuberance and 1 cm below the lower margin of the torcular herophili. The parietal notch, located at the junction of the squamosal and parietomastoid sutures, defines the upper limit of the petrous portion of the temporal bone and the floor of the posterior portion of the middle fossa. The midportion of the parietomastoid suture approximates the anterior edge of the junction of the transverse and sigmoid sinuses. (Images courtesy of AL Rhoton, Jr.)

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