Microsurgical anatomy of the transcondylar, supracondylar, and paracondylar extensions of the far-lateral approach

Transcription

1 J Neurosurg 87: , 1997 Microsurgical anatomy of the transcondylar, supracondylar, and paracondylar extensions of the far-lateral approach HUNG T. WEN, M.D., ALBERT L. RHOTON, JR., M.D., TOSHIRO KATSUTA, M.D., AND EVANDRO DE OLIVEIRA, M.D. Department of Neurological Surgery, University of Florida, Gainesville, Florida; and São Paulo Neurological Institute, São Paulo, Brazil Despite a large number of reports of the use of the far-lateral approach, some of the basic detail that is important in safely completing this exposure has not been defined or remains poorly understood. The basic far-lateral exposure provides access for the following approaches: 1) the transcondylar approach directed through the occipital condyle or the adjoining portions of the occipital and atlantal condyles; 2) the supracondylar approach directed through the area above the occipital condyle; and 3) the paracondylar exposure directed through the area lateral to the occipital condyle. The transcondylar approach provides access to the lower clivus and premedullary area. The supracondylar approach provides access to the region of, and medial to, the hypoglossal canal and jugular tubercle. The paracondylar approach, which includes drilling of the jugular process of the occipital bone in the area lateral to the occipital condyle, provides access to the posterior portion of the jugular foramen and to the mastoid on the lateral side of the jugular foramen. In this study, the anatomy important to completing the far-lateral approach and these modifications was examined in 12 cadaveric specimens. In the standard posterior and posterolateral approaches, an understanding of the individual suboccipital muscles is not essential. However, these muscles provide important landmarks for the far-lateral approach and its modifications. Other important considerations include the relationship of the occipital condyle to the foramen magnum, hypoglossal canal, jugular tubercle, the jugular process of the occipital bone, the mastoid, and the facial canal. These and other relationships important to completing these exposures were examined in this study. KEY WORDS anatomical study microsurgical anatomy vertebral artery occipital condyle occipital bone temporal bone craniocervical junction skull base cranial nerve foramen magnum surgical approach L ESIONS located along the lower clivus and the anterior portion of the foramen magnum have presented a surgical challenge associated with significant morbidity and mortality. Most lesions in the area have been treated via a posterior or posterolateral approach. The anterior approaches directed through the oral and nasal cavities and the paranasal sinuses, although offering a direct route to the clivus, have the disadvantages of the great depth of the surgical field, limited lateral exposure, and an increased risk of postoperative cerebrospinal fluid leakage, especially when dealing with intradural lesions. 8,34 Several modifications of posterolateral approaches that increase surgical exposure and reduce retraction on the neuraxis have been presented. 2 4,7,11,12,14,16,24,27 29,31,32 These modifications can be divided into two groups. 21 The first, the far-lateral approach, is completed without removal of the occipital condyle, 11,13,14 and the second, the transcondylar approach, involves removal of some or all of the occipital condyle. 2 4,7,12,13,16,27 29,31 33 The transcondylar approach consists of 1) dissection of the muscles along the posterolateral aspect of the craniocervical junction to permit an adequate lateral approach while reducing the depth of the surgical field; 2) early identification of the vertebral artery (VA) either above the posterior arch of the atlas or in its ascending course between the transverse processes of the atlas and axis; 3) a suboccipital craniectomy or craniotomy with removal of at least one-half of the posterior arch of the atlas; and 4) removal of the posterior portion of the occipital condyle to allow a more lateral approach. The approach can also include removal of more of the occipital condyle and the adjacent portion of the superior facet of the atlas, with additional optimum exposure of the hypoglossal canal, and removal of the jugular tubercle in the supracondylar area, with exposure of the jugular foramen and mastoid portion of the facial nerve in the paracondylar area. The current study was undertaken to define the muscular relationships important to completing the exposure; to identify the VA, the occipital artery (OA), and the lower cranial and upper cervical nerves; and to examine the relationship of the occipital condyle to the hypoglossal canal, jugular tubercle, jugular bulb, and jugular process of the occipital bone, facial nerve, and stylomastoid foramen. Previous descriptions vary according to how much occip- 555

2 H. T. Wen, et al. FIG. 1. Photographs displaying osseous relationships. A: Inferior view of the occipital condyles and the foramen magnum. The foramen magnum is ovoid, having its long axis in the anteroposterior direction. 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 C-1. A probe inserted through the hypoglossal canal passes forward approximately 45 from the midsagittal plane in an anterolateral direction. The hypoglossal canal is located above the middle one-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 a depression located behind the occipital condyle. It is frequently the site of a canal, the condylar canal, that transmits the posterior condylar emissary vein. The condylar canal is directed almost straight anteriorly and slightly upward, and it 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 Fig. 1 continued 556

3 Transcondylar, supracondylar, and paracondylar approaches ital condyle can be removed without entering the hypoglossal canal; what structures indicate that the hypoglossal canal is being approached when drilling the occipital condyle; what is the relationship of the hypoglossal canal to the posterior ring of the foramen magnum and to the jugular tubercle; and what structures define the position of the facial nerve and jugular bulb (Fig. 1). 1 3,12,15,18,19,27 29,31,32 Materials and Methods With the aid of magnification ( 3 40) the microsurgical anatomy of the region was examined in 12 dried skulls of adults and five cadaveric specimens. Vessels were injected with colored silicone to facilitate their exposure. Results The results are arranged into three stages (Fig. 2). The first stage, muscular dissection, includes skin incision; reflection of muscles, including those forming the suboccipital triangle; and examination of the relationship of the muscles to the OA and VA, the vertebral venous plexus, the transverse process of the atlas, and the upper cervical nerves. The second stage, the extradural dissection, examines landmarks for the suboccipital craniectomy; the extent of occipital condyle removal; and exposure and identification of the hypoglossal canal, jugular process, jugular tubercle, and facial nerve. The final stage, the intradural exposure, reviews the relationships of the intradural segment of the VA and its branches, including the posterior inferior cerebellar artery (PICA), to the lower cranial and upper cervical nerves and the dentate ligament. Muscular Stage For this study, the region was exposed using a horseshoe scalp flap because it provides a better display of the muscular layers and their relationships to neural and vascular structures (Fig. 2A). The incision began in the midline, approximately 5 cm below the external occipital protuberance, and was directed upward to just above the external occipital protuberance, turning laterally just above the superior nuchal line, reaching the mastoid, and turning downward in front of the posterior border of the sternocleidomastoid muscle onto the lateral aspect of the neck to approximately 5 cm below the mastoid tip and 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 mastoid notch, the site of attachment of the digastric muscle and the occipital groove, in which the OA courses, are located lateral to the jugular process. The occipitomastoid suture, which separates the occipital bone medially from the mastoid bone laterally, crosses the bottom of the occipital groove. The stylomastoid foramen is situated lateral to the jugular foramen, at the anterior end of the mastoid notch. The styloid process is located anterior and slightly medial to the stylomastoid foramen. B: Superior view of the foramen magnum, occipital condyle, and hypoglossal canal. The occipital condyle projects downward from the lateral margin of the anterior half of the foramen magnum. The articular surface of the occipital condyle is located on the lateral surface of the condyle, rather than on the medial surface that is seen in this view. The intracranial entrance of the hypoglossal canal is located above the condyle. The canal passes anteriorly and laterally above the condyles. 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 glossopharyngeal, vagus, and accessory nerves pass behind the jugular tubercle to reach the intrajugular portion of the jugular foramen, which is located between the petrosal portion of the foramen, which receives the drainage from the inferior petrosal sinus, and the sigmoid portion, which receives the drainage from the sigmoid sinus. 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. C: Inferolateral view. A probe has been passed through the hypoglossal canal, which passes above the occipital condyle. From its intracranial to its extracranial end, it is directed forward, lateral, and slightly upward. The intracranial end of the hypoglossal canal is located approximately 5 mm above the junction of the posterior and middle third of the occipital condyle and approximately 8 mm in front of the posterior edge of the condylar process. The articular surface of the condyle faces downward and laterally. D: Medial aspect of the occipital condyle and supracondylar region. The inner surface of the squamous portion of the occipital bone is grooved by the sulcus of the transverse sinus. 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 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. E: Anterior view of the foramen magnum, occipital condyles, hypoglossal canals, and jugular foramen in a skull sectioned in a coronal plane at the level of the posterior clinoid processes and the floor of the middle fossa just anterior to the foramen spinosum. The left jugular bulb and IJV have been reconstructed in this specimen. A probe inserted into the hypoglossal canal displays its anterior, superior, and lateral direction. The jugular foramen is situated lateral to the hypoglossal canal and occipital condyle. The entrance into the carotid canal is situated immediately in front of the jugular foramen. F: Posterior view of an occipital bone sectioned in a coronal plane located along the posterior margin of the jugular foramen and occipital condyles. The hypoglossal canal courses below the jugular tubercle and above the occipital condyle. The most prominent part of the jugular tubercle is located anterior to the area in which the glossopharyngeal, vagus, and accessory nerves often course in a shallow groove on the upper surface of the tubercle. The jugular process of the occipital bone forms the posterior margin of the jugular foramen. Drilling the jugular process lateral to the condyle exposes the jugular bulb. CN = cranial nerve; Ext. = external; Int. = internal; Occip. = occipital; Post. = posterior; Sup. = superior; V. = vein. 557

4 H. T. Wen, et al. below where the transverse process of the atlas can be palpated through the skin (Fig. 2A). The skin flap was reflected downward and medially to expose the most superficial layer of muscles formed by the sternocleidomastoid and splenius capitis muscles laterally and the trapezius and the semispinalis capitis muscles medially (Figs. 2B and 3A). Muscular Dissection. The sternocleidomastoid muscle passes obliquely downward across the side of the neck from the lateral half of the superior nuchal line and the adjacent surface of the mastoid process, partially covering the splenius capitis muscle to attach to the sternum and the adjacent portion of the clavicle. The trapezius covers the back of the head and neck. It attaches to the medial half of the superior nuchal line, the external occipital protuberance, and the spinous processes of the cervical and thoracic vertebrae, extending downward and partially covering the splenius capitis muscles and semispinalis capitis muscle to converge on the shoulder, where it attaches to the scapula and the lateral third of the clavicle. The trapezius and sternocleidomastoid, in the region of the exposure, are thin muscles. They are continuous superiorly with the galea aponeurotica and the occipitofrontalis muscle, which should not be transected by extending the incision to the depth of the occipital bone. The posterior triangle of the neck is located between the anterior border of the trapezius and the posterior border of the sternocleidomastoid muscle. Dividing the sternocleidomastoid muscle just below, and with preservation of, its upper attachment for closure and reflecting it laterally exposes the upper extension of the splenius capitis, which forms the upper portion of the floor of the posterior triangle of the neck and is attached to the mastoid process of the temporal bone and to the rough surface of the occipital bone just below the lateral third of the superior nuchal line (Figs. 2B and C and 3A and B). It extends inferiorly and medially to insert on the nuchal ligament and spines of the lower cervical and upper three thoracic vertebrae. Detaching the trapezius and splenius capitis muscles, while preserving a cuff of their upper attachments for closure, and reflecting them medially exposes the longissimus capitis muscle, the deep lamina of the deep cervical fascia, and the OA. The longissimus capitis is attached to the posterior margin of the mastoid process, deep to the splenius capitis and sternocleidomastoid muscles, and extends inferiorly and medially to insert on the transverse processes of the upper thoracic vertebrae (Figs. 2D and 3B). Reflecting the longissimus capitis downward exposes the semispinalis capitis and the superior and inferior oblique muscles as well as the transverse process of the atlas, which has a prominent apex palpable through the skin between the mastoid process and mandibular angle (Figs. 2D and E, and 3B and C). The semispinalis capitis arises in the area between the superior and inferior nuchal lines, beginning in the midline at the external occipital crest, and extending laterally to the occipitomastoid suture. It attaches by a series of tendons on the articular processes of C4 6 and on the tips of the transverse processes of C7 T7. The semispinalis capitis is reflected medially to expose the suboccipital triangle, which is limited by three muscles; above and medially by the rectus capitis posterior major: above and laterally by the superior oblique, and below and laterally by the inferior oblique (Figs. 2E H and 3C E). The superior oblique muscle arises from the upper surface of the transverse process of the atlas and attaches to the occipital bone lateral to the semispinalis capitis between the superior and inferior nuchal lines overlapping the insertion of the rectus capitis posterior major. The inferior oblique extends from the spinous process and lamina of the axis to the transverse process of the atlas. The rectus capitis posterior major muscle is attached to the lateral portion of the occipital bone at and below the inferior nuchal line and inserts on the spinous process of the axis. The rectus capitis posterior minor, which is situated medial to, and is partially covered by, the rectus capitis posterior major, arises from the tubercle in the midline on the posterior arch of the atlas and attaches to, and below, the medial portion of the inferior nuchal line (Fig. 3D F). The suboccipital triangle is covered medially by the semispinalis capitis and laterally by the longissimus capitis, and sometimes by the splenius capitis, which overlaps the superior oblique. The triangle deep to these muscles is covered by a layer of dense fibrofatty tissue. The floor in the depth of the triangle is formed by the posterior atlantooccipital membrane and the posterior arch of the atlas (Figs. 2F K and 3D F). The structures in the triangle are the VA and the C-1 nerve, both of which lie in a groove on the upper surface of the lateral portion of the posterior arch of the atlas. The suboccipital triangle is opened by reflecting the rectus capitis posterior major inferiorly and medially, the superior oblique laterally, and the inferior oblique medially. Opening the triangle exposes the portion of the vertebral venous plexus that surrounds the VA, the segment of the VA that passes behind the atlantooccipital joint, and the upper edge of the posterior arch of the atlas (Figs. 2G I and 3C F). Reflecting the superior oblique muscle, as described earlier, exposes the rectus capitis lateralis, a short, flat muscle that is an important landmark in identifying the jugular foramen (Fig. 2H K). It arises from the upper surface of the transverse process of the atlas and attaches above to the rough, lower surface of the jugular process of the occipital bone behind the jugular foramen. The jugular process is a plate of occipital bone extending laterally from the posterior half of the occipital condyle. It is indented in front at the site of the jugular notch, which forms the posterior edge of the jugular foramen (Fig. 1A and B). The rectus capitis lateralis, because it is attached to the jugular process at the posterior edge of the jugular foramen, provides a landmark for estimating the position of the jugular foramen and the facial nerve, which exits the stylomastoid foramen just lateral to the jugular foramen. Vascular Structures. Reflecting the muscles forming the suboccipital triangle, as described earlier, exposes the VA, which is surrounded by a rich venous plexus that must be obliterated and partially removed if the VA is to be exposed or transposed (Fig. 2I). Venous flow in this area empties into two systems: one drained by the internal jugular vein and another draining into the vertebral venous plexus. 6 There are abundant communications between the two systems. The internal jugular vein (IJV) and its tributaries form the most important drainage system in the craniocervical area. The IJV originates at the jugular foramen by the confluence of the sigmoid and inferior petrosal sinuses. The venous plexus surrounding the VA in the sub- 558

5 Transcondylar, supracondylar, and paracondylar approaches FIG. 2. Photographs showing muscular dissection, extradural dissection, and intradural exposure. A: A horseshoe scalp incision with the lateral limb descending farther than the medial limb starts in the midline, ascends, and curves just above the internal occipital protuberance to reach the base of the mastoid process, where it turns downward, lateral to the posterior border of the sternocleidomastoid muscle (interrupted line), crossing the mastoid tip and descending below the level at which the transverse process of the atlas can be palpated through the skin. B: The scalp flap has been reflected inferiorly to expose the most superficial muscular layer formed by the sternocleidomastoid, trapezius, splenius capitis, and semispinalis capitis muscles. The splenius capitis is partially covered by the sternocleidomastoid muscle above and laterally and the trapezius muscle below and medially. The semispinalis capitis muscle is partially covered by the splenius capitis and trapezius muscles. The OA and occipital vein course between the splenius capitis and semispinalis capitis muscles to reach the upper portion of the posterior triangle of the neck, the latter being located between the trapezius and sternocleidomastoid muscles. The asterion, located at the confluence of the lambdoid, parietomastoid, and occipitomastoid sutures, approximates the lower half of the junction of the transverse and sigmoid sinuses. C: The sternocleidomastoid muscle has been reflected laterally to expose the upper part of the splenius capitis. The splenius capitis forms part of the floor of the posterior triangle of the neck and is attached to the mastoid process and the occipital bone below the lateral third of the superior nuchal line. D: The trapezius and splenius capitis muscles have been reflected medially to expose the longissimus capitis, semispinalis capitis, and superior oblique muscles. The OA lies successively on the longissimus capitis, superior oblique, and semispinalis capitis muscles. This artery may also course deep to the longissimus capitis muscle. E: The longissimus capitis muscle has been reflected inferiorly to expose the transverse process of C-1, which serves as a site of attachment for the superior and inferior oblique and levator scapulae muscles. The oblique muscles are covered medially by the semispinalis capitis muscle. The OA passes backward between the posterior belly of the digastric muscle and the superior oblique muscle. The posterior belly of the digastric muscle is attached medial to the mastoid tip along the mastoid notch. F: The semispinalis capitis muscle has been reflected downward to expose the suboccipital triangle, which is limited superiorly and medially by the rectus capitis posterior major, superiorly and laterally by the superior oblique, and inferiorly by the inferior oblique muscles. See p. 574 for definitions of abbreviations. occipital triangle is formed by numerous small channels that empty into the internal vertebral plexuses (between the dura and the vertebrae), which issue from the vertebral canal above the posterior arch of the atlas. This vertebral venous plexus and multiple small veins from the deep muscles communicate with the dense venous plexus, which accompanies the VA into the foramen in the transverse process of the atlas and descends through the transverse foramina of successive cervical vertebrae to end in the vertebral vein, which emerges from the transverse foramen of C-6 and empties into the brachiocephalic vein. The posterior condylar emissary vein, which passes through the posterior condylar canal, forms a communication between the vertebral venous plexus and the sigmoid sinus (Fig. 4A C). The venous plexus of the hypoglossal canal passes along the hypoglossal canal to connect the basilar venous plexus with the marginal sinus, which encircles the foramen magnum. Obliteration of a portion of the venous plexus exposes the upper extradural segment of the VA (Figs. 2H P and 4A C). 559

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7 Transcondylar, supracondylar, and paracondylar approaches The VA has been divided into four parts: the first segment extends from the subclavian artery to the entrance into the lowest transverse foramen, usually that of C-6; the second segment extends from the transverse process of C-6 to the transverse foramen of the atlas; the third segment extends from the transverse foramen of the atlas to the site of entry into the dura; and the fourth segment is the intradural segment (Fig. 2J O). 23,35 The artery above the transverse foramen of the axis veers laterally to reach the transverse foramen of the atlas, which is situated farther lateral than the transverse foramen of the axis. This artery, after ascending through the transverse process of the atlas, is located on the medial side of the rectus capitis lateralis muscle. From here it turns medially behind the lateral mass of the atlas and the atlantooccipital joint and is pressed into the groove on the upper surface of the posterior arch of the atlas, where it courses along the floor of the suboccipital triangle and is covered behind the triangle by the semispinalis capitis muscle. The C-1 nerve courses on the lower surface of the artery, between the artery and the posterior arch of the atlas (Fig. 2L P). After passing medially above the lateral portion of the posterior arch of the atlas, the artery enters the vertebral canal by passing below the lower, arched border of the posterior atlantooccipital membrane, which transforms the sulcus in which the artery courses on the upper edge of the posterior arch of the atlas into an osseofibrous casing that may ossify, transforming it into a complete or incomplete bony canal surrounding the artery (Figs. 2H and 3E and F). A complete bony canal surrounding the artery is found in 7 to 28% of VAs, and an incomplete bony canal is found in 2 to 24%. 9 The third segment of the VA gives rise to muscular branches and the posterior meningeal arteries. The muscular branches arise as the artery exits the transverse foramen and courses around the lateral mass of the atlas to supply the deep muscles and communicate with the occipital and ascending and deep cervical arteries (Fig. 2J). Some of the muscular branches may need to be divided to mobilize and transpose the VA. 2 The posterior meningeal artery arises from the posterior surface of the VA as it passes behind the lateral mass, or above the posterior arch, of the atlas or just before penetrating the dura in the region of the foramen magnum; however, this artery may also have an intradural origin from the VA, in which case it pierces the arachnoid over the cisterna magna to reach the dura (Fig. 2M O). 22 The OA is also exposed as the superficial and deep muscles in the region are reflected (Fig. 2C K). It originates from the posterior wall of the external carotid artery at the level of the angle of the mandible, ascends parallel and medial to the external carotid artery, and lateral to the IJV to reach the area posteromedial to the styloid process, where it changes its course posteriorly and laterally, passing first between the rectus capitis lateralis muscle and the posterior belly of the digastric muscle, and then between the superior oblique and the posterior belly of the digastric, where it courses in the occipital groove medial to the mastoid notch, in which the posterior belly of the digastric muscle arises. After exiting the area between the superior oblique muscle and the posterior belly of the digastric muscle, it courses medially, being related to the longissimus capitis and the semispinalis capitis muscles. If the occipital groove is present, the OA will course under the longissimus capitis muscle; however, if the groove is absent, the artery will course superficially to this muscle (Fig. 2D). It courses medially across the semispinalis capitis, just below the superior nuchal line in the upper portion of the posterior triangle, to pass between the upper attachment of the trapezius and the semispinalis capitis muscles, where it pierces the attachment of the trapezius muscle to the superior nuchal line and ascends in the superficial fascia of the posterior scalp. Osseous Structures. The transverse process of the atlas, an important landmark in these approaches, projects farther lateral than the transverse processes on the adjacent cervical vertebrae, and has an apex that can be felt through the skin in the area between the mastoid process and the angle of the mandible (Fig. 2A). Several muscles important in completing the exposure attach to the transverse process of the atlas (Figs. 2G and 3C and D). The rectus capitis lateralis arises from the anterior portion, and the superior oblique arises from the posterior portion of the upper surface of the transverse process. The inferior oblique muscle inserts on the lateral tip of the transverse process. The levator scapulae, splenius cervicis, and scalenus medius attach to the inferior and lateral surface of the transverse process. The levator scapulae is also attached by tendinous slips to the posterior tubercles of the transverse processes of C2 4. Neural Structures. The neural structures encountered during the muscle dissection arise predominantly from the C-1 and C-2, and to a lesser extent from the C-3 spinal nerves, which are formed by the united dorsal and ventral roots (Figs. 2G J and 3E and F). The neurons of the dorsal roots collect to form ganglia; however, the first cervical dorsal root and associated ganglion may be absent. The C-1, C-2, and C-3 nerves distal to the ganglion divide into dorsal and ventral rami. The dorsal rami divide into medial and lateral branches that supply the skin and mus- FIG. 2 G: Enlarged view of the suboccipital triangle. The OA passes backward between the digastric and superior oblique muscles. The greater occipital nerve, arising from the dorsal ramus of C-2, courses below and then behind the inferior oblique muscle to pierce the semispinalis capitis. The segment of the VA that courses between the transverse foramina of C-1 and C-2 is exposed medial to the levator scapulae muscle. This segment of the artery is directed slightly lateral as it ascends to reach the transverse foramen of C-1, which is lateral to that of C-2. H: The superior oblique muscle has been reflected laterally and the rectus capitis posterior major muscle inferomedially. The floor of the suboccipital triangle is formed by the posterior atlantooccipital membrane and the posterior arch of the atlas. The VA and the dorsal ramus of the C-1 nerve root, which are surrounded by the vertebral venous plexus, course along the upper surface of the posterior arch of the atlas. The inferior oblique muscle runs from the transverse process of C-1 to the spinous process of C-2. During the operative approach, the superior oblique muscle is detached from the occipital bone and reflected laterally, the rectus capitis posterior major muscle is detached from the occipital bone and reflected inferomedially, and the inferior oblique muscle is detached from the transverse process of C-1 and reflected medially. 561

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9 Transcondylar, supracondylar, and paracondylar approaches cles of the posterior region of the neck. The C-1 nerve, termed the suboccipital nerve, leaves the vertebral canal between the occipital bone and atlas and has a dorsal ramus that is larger than the ventral ramus (Fig. 2M O). The dorsal ramus courses between the posterior arch of the atlas and the VA to reach the suboccipital triangle, where it sends branches to the rectus capitis posterior major and minor, superior and inferior oblique, and semispinalis capitis muscles; and occasionally has a cutaneous branch that accompanies the OA to the scalp. The C-1 ventral ramus courses between the posterior arch of the atlas and the VA; it passes forward, lateral to the lateral mass of the atlas and medial to the VA, and supplies the rectus capitis lateralis. The C-2 nerve emerges between the posterior arch of the atlas and the lamina of the axis where the spinal ganglion is located extradurally, medial to the inferior facet of C-1 and the VA. Distal to the ganglion, the nerve divides into a larger dorsal ramus and a smaller ventral ramus (Fig. 2I K). After passing below and supplying the inferior oblique muscle, the dorsal ramus divides into a large medial and a small lateral branch. It is the medial branch that is most intimately related to this operative field and that forms the greater occipital nerve. It ascends obliquely between the inferior oblique and the semispinalis capitis, pierces the latter and the trapezius muscle near their attachments to the occipital bone, and is joined by a filament from the medial branch of C-3. It supplies the semispinalis capitis muscle, ascends with the OA, and supplies the scalp as far forward as the vertex and, occasionally, the back of the ear. The lateral branch sends filaments that innervate the splenius, longissimus, and semispinalis capitis muscles and is often joined by the corresponding branch from the C-3 nerve. The C-2 ventral ramus courses between the vertebral arches and transverse processes of the atlas and axis and behind the VA to leave this operative field. Two branches of the C-2 and C-3 ventral rami the lesser occipital and greater auricular nerves curve around the posterior border and ascend on the sternocleidomastoid muscle to supply the skin behind the ear. Extradural Stage The extradural stage begins with a suboccipital craniectomy or craniotomy, identification of the occipital condyle, and removal of at least one-half of the posterior arch of the atlas and, possibly, the posterior root of the transverse foramen if mobilization of the VA is needed (Fig. 2K O). Two osseous landmarks important in planning the suboccipital craniotomy are the asterion and the inion. The asterion is located at the junction of the occipitomastoid, lambdoid, and parietomastoid sutures (Figs. 2C and 3B). The site of convergence of these three sutures is located along the upper one-third of the groove on the inner table of the skull, marking the course of the transverse and sigmoid sinuses near the point at which the transverse sinus empties into the sigmoid sinus. It provides a useful external landmark for estimating the site of the junction of the transverse and sigmoid sinuses. The inion, the site of the external occipital protuberance, is located an average of 1 cm below the apex of the internal occipital protuberance and the inferior margin of the confluence of the sagittal and transverse sinuses (Fig. 1D). In completing the removal of the posterior arch of the atlas, the tip of the transverse process is preserved, along with the attachment of the superior oblique muscle, which is reflected laterally while preserving the attachment of the rectus capitis lateralis. At this stage, the segment of the VA that extends from the transverse foramen of C-2 to its entrance to the dura is exposed. Removal of the posterior root of the transverse foramen will permit the artery to be displaced downward and medially away from the atlantooccipital joint to expose the occipital condyle (Fig. 2K and L). The occipital condyles project downward along the lateral edges of the anterior half of the foramen magnum (Figs. 1 and 4). The articular surfaces, which are ovoid with the long axis extending in the anteroposterior direction, are located on the lower-lateral margin of the condyles. They face downward and laterally to articulate with the superior facets of the atlas, which face upward and medially. The intracranial end of the hypoglossal canal is located approximately 5 mm above the junction of the posterior and middle one-third of the occipital condyle and appropriately 5 mm below the jugular tubercle (Fig. 1A F). The canal is directed forward and laterally at a 45 angle with the sagittal plane. 30 The extracranial end of the hypoglossal canal is located immediately above the junction of the anterior and middle one-third of the occipital condyle and medial to the jugular foramen. The average length of the longest axis of the condyle is 21 mm (range mm) and the average distance between the posterior edge of the occipital condyle and the posterior border of the intracranial end of the hypoglossal canal is 8.4 mm (range 6 10 FIG. 2. I: The superior and inferior oblique and rectus capitis posterior major and minor muscles have been removed in this dissection to provide this view of the VA and its muscular branches. The vertebral venous plexus, which surrounds the VA in the suboccipital triangle, is formed by numerous small tributaries from the internal vertebral plexuses (between the dura and vertebrae) that issue from the vertebral canal above the posterior arch of the atlas. These tributaries unite with small veins from the deep muscles and form a channel that surrounds the VA in the transverse foramen of the atlas and descends, as a dense plexus around the VA, through the successive transverse foramina of the cervical vertebrae. This plexus ends in the vertebral vein, which emerges from the transverse foramina of C-6 and runs downward to open into the brachiocephalic vein. The OA courses between the rectus capitis lateralis muscle and the posterior belly of the digastric muscle. The levator scapulae muscle attaches to the transverse processes of C1 4 and extends downward to the medial border of the scapula. J: The vertebral venous plexus has been removed to expose the VA, which veers laterally and anteriorly as it ascends from the transverse foramen of C-2 and passes medial to the fibers of the levator scapulae muscle to reach the transverse foramen of C-1 and the area medial to the rectus capitis lateralis muscle. On exiting the transverse foramen of C-1, the VA turns medially behind the superior articular facet of C-1. The rectus capitis lateralis muscle arises from the upper surface of the transverse process of the atlas and extends upward to be inserted on the jugular process of the occipital bone at the posterior margin of the jugular foramen. The suboccipital nerve arises from the dorsal ramus of C-1 and courses between the VA and the upper surface of the posterior arch of the atlas. The posterior meningeal artery arises immediately before the VA penetrates the dura and supplies the posterolateral spinal and posterior fossa dura. 563

10 H. T. Wen, et al. FIG. 2 K: A suboccipital craniectomy has been completed. The occipital condyle, the superior facet of C-1, and the atlantooccipital joint have been exposed by depressing the VA after removing part of the joint capsule. The rectus capitis lateralis muscle is located lateral to the VA. The VA gives rise to several large muscular branches. After exiting the transverse foramen of C-1, the VA courses posterior to the superior facet of C-1 and above the posterior arch of C

11 Transcondylar, supracondylar, and paracondylar approaches FIG. 2. L Q: Photographs obtained in another specimen. L: A suboccipital craniectomy has been completed and the right half of the posterior arch of C-1 has been removed. The posterior root of the transverse foramen of the atlas has been removed while preserving the portion of the tip of the transverse process of the atlas to which the rectus capitis lateralis, levator scapulae, and superior oblique muscles attach. The suboccipital nerve, a branch of the C-1 nerve, passes backward between the VA and the posterior arch of the atlas. The occipital condyle and the posterior condylar emissary vein are exposed. The ventral rami of the C-1 and C-2 nerve roots pass behind the VA. mm). The hypoglossal canal is surrounded by cortical bone. The contents of the hypoglossal canal are the hypoglossal nerve, a meningeal branch of the ascending pharyngeal artery, and the venous plexus of the hypoglossal canal, which communicates between the basilar venous plexus and the marginal sinus, which encircles the foramen magnum (Figs. 2M O, 4D G, and 5D). Posterior to the occipital condyle, a depression the condylar fossa may be pierced by the condylar canal, which transmits the posterior condylar emissary vein, a communication between the vertebral venous plexus and the sigmoid sinus (Figs. 1A and 4A C). The canal is directed slightly upward as it proceeds anteriorly to join the sigmoid sinus at the hooklike turn immediately proximal to the point at which the sinus empties into the jugular bulb. The condylar canal does not communicate with the hypoglossal canal. The jugular process of the occipital bone serves as a bridge between the condylar and squamosal portions of the occipital bone and forms the posterior margin of the jugular foramen (Fig. 1A and B). It extends laterally from the posterior half of the condyle. The jugular process also serves as the site of attachment of the rectus capitis lateralis muscle behind the jugular foramen. The stylomastoid foramen, which transmits the facial nerve, is situated lateral to the jugular foramen at the anterior end of the mastoid notch (Figs. 2R and S, and 4H and I). The styloid process is located anterior to the stylomastoid foramen and anterolateral to the jugular foramen (Fig. 1A and C). After removing the superficial layer of cortical bone covering the occipital condyle, soft cancellous bone will be encountered. Further drilling of the cancellous bone, in and above the posterior one-third of the condyle, exposes the second layer of hard, cortical bone that surrounds the hypoglossal canal (Figs. 2M O and 4D G, I, and K). Subsequent drilling of this cortical bone exposes the venous plexus of the hypoglossal canal. The lateral aspect of the intracranial end of the hypoglossal canal is reached after removal of approximately the posterior one-third of the occipital condyle (8.4 mm of 21 mm) (Figs. 1A and 565

12 H. T. Wen, et al. 566

13 Transcondylar, supracondylar, and paracondylar approaches FIG. 2. O: The hypoglossal canal has been opened and a portion of the venous plexus in the canal has been removed to expose the hypoglossal nerve and the meningeal branch of the ascending pharyngeal artery coursing through the hypoglossal canal. The broken line indicates the site of dural opening. The posterior meningeal artery, which originates from the VA and supplies the dura of the posterior fossa, crosses the dural incision and will be obliterated as the dura is opened. The greater occipital nerve arises from the dorsal ramus of the C-2 nerve. 4G). Further drilling of the occipital condyle can be performed after reaching the lateral aspect of the intracranial end of the hypoglossal canal because the canal is directed anteriorly and laterally, permitting the lateral portion of the posterior two-thirds of the condyle to be removed without entering the hypoglossal canal. The distance between the upper surface of the hypoglossal nerve and the roof of the hypoglossal canal averages 4.4 mm. After exposing the hypoglossal canal above the occipital condyle, the bone of the jugular tubercle that is situated above the hypoglossal canal can be removed to gain additional exposure. 1 4,10,19,24 The jugular tubercle is a rounded prominence located at the junction of the basilar and condylar portions of the occipital bone (Figs. 1B, 2Q, and 4D F). It is situated above the hypoglossal canal and medial to the lower half of the intracranial end of the jugu- FIG. 2. M: Enlarged view. The occipital condyle has been drilled to the depth of the cortical bone surrounding the hypoglossal canal. The change from cancellous to cortical bone indicates that the hypoglossal canal has been reached. The dorsal ramus of the C-1 nerve root, also termed the suboccipital nerve, passes backward between the posterior arch of the atlas and the VA, supplies the muscles bordering the suboccipital triangle, and sends fibers to the rectus capitis posterior minor and semispinalis capitis muscles. N: The hypoglossal canal has been opened to expose the venous plexus within, which surrounds the hypoglossal nerve in the canal and connects the basilar venous plexus with the marginal sinus, which encircles the foramen magnum. 567

14 H. T. Wen, et al. FIG. 2. P: The dura has been opened to expose the intradural course of the nerves entering the jugular foramen and the hypoglossal canal. The dura around the full circumference of the VA has been opened so that the artery can be mobilized at its entrance into the dura. lar foramen. The average distance from the posterior edge of the jugular tubercle (the site of the groove in which the lower cranial nerves course) to the upper border of the hypoglossal canal is 4.5 mm. The glossopharyngeal, vagus, and accessory nerves cross the posterior portion of the jugular tubercle in passing from the brainstem to the jugular foramen, sometimes coursing in a shallow groove on the surface of the tubercle (Figs. 1F, 4D F, and 5B and C). The prominence of the jugular tubercle blocks access to the basal cisterns and clivus anterior to the lower cranial nerves. As the jugular tubercle is removed extradurally, the cranial nerves, which course along the back margin of the tubercle and are intradural, are not visualized. As the drilling proceeds, bone is removed from below the cisternal segment of the accessory and vagus nerves that course above the tubercle just inside the dura. Caution is required in removing the jugular tubercle to avoid damaging the lower cranial nerves, either by direct trauma, by stretching the dura, or by heat generated by the drilling (Fig. 2P and Q). The lateral margin of the jugular tubercle is situated just medial to, and below, the medial edge of the jugular bulb. If a more lateral exposure is needed or if the jugular foramen is to be opened from behind, the jugular process of the occipital bone, which extends laterally from the occipital condyle, can be removed after the rectus capitis lateralis muscle has been detached from its lower surface (Fig. 4F J). Removing the jugular process, which forms the posterior margin of the jugular foramen, will expose the transition between the sigmoid sinus, jugular bulb, and IJV. 17 Care is required to avoid damaging the VA because it passes upward through the transverse process of the atlas and turns medially in the area directly below the jugular process. For an even more lateral exposure, the posterior belly of the digastric muscle can be separated from the mastoid notch to expose the facial nerve, just distal to the stylomastoid foramen (Figs. 2R and S, and 4H and I). A partial mastoidectomy can be performed to expose the mastoid segment of the facial nerve in the facial canal at this stage. Intradural Stage The dural incision begins behind the sigmoid sinus and 568

15 Transcondylar, supracondylar, and paracondylar approaches FIG. 2. Q: The nerves in the cerebellopontine angle have been exposed. The glossopharyngeal, vagus, and accessory nerves exit the brainstem behind the olive. The glossopharyngeal and vagus nerves pass anterior to the choroid plexus, which protrudes from the foramen of Luschka. The glossopharyngeal nerve arises as one root and the vagus nerve arises from the brainstem as several rootlets. The cranial portion of the accessory nerve arises from the brainstem as four or five delicate rootlets that ascend laterally to reach the jugular foramen. The hypoglossal nerve arises anterior to the olive and passes behind the VA to reach the hypoglossal canal. The posterior wall of the hypoglossal canal has been removed to expose the hypoglossal nerve, meningeal branch of the ascending pharyngeal artery, and the venous plexus in the canal. The jugular tubercle is located above the hypoglossal canal and serves as a trochlea around which the glossopharyngeal, vagus, and accessory nerves course in their passage from the brainstem to the jugular foramen. extends behind the VA into the upper cervical area (Fig. 2O and P). The upper extent of the dural opening depends on how much of the cerebellopontine angle is to be exposed. Possible sources of bleeding during dural opening are the marginal sinus, which encircles the foramen magnum, and the posterior meningeal artery, which usually originates from the VA extradurally but may infrequently originate intradurally (in which case it crosses the lateral medullary cistern and pierces the arachnoid to reach the dura). Opening the dura exposes the fourth (intradural) segment of the VA. As the artery pierces the dura, it is encased in a fibrous tunnel that binds the posterior spinal artery, dentate ligament, first cervical nerve, and spinal accessory nerve to the VA (Fig. 2P, Q, U, and V). 25 The posterior spinal artery usually originates extradurally from the superomedial surface or intradurally from the medial surface of the VA (Figs. 2U and V and 5A C). In the subarachnoid space, it courses medially across the back surface of the upper triangular process of the dentate ligament. On reaching the lower medulla, the posterior spinal artery divides into ascending and descending branches. The ascending branch courses through the foramen magnum and supplies the inferior cerebellar peduncle, the gracile and cuneate tubercles, the rootlets of the accessory nerve, and the choroid plexus near the foramen of Magendie. The descending branch passes downward between the dorsal rootlets and the dentate ligament on the posterolateral surface of the spinal cord and supplies the 569

16 H. T. Wen, et al. FIG. 2. R: The VA has been retracted medially. The posterior belly of the digastric muscle has been detached from the mastoid notch to expose the facial nerve below the stylomastoid foramen just anterior to the attachment of the posterior belly of the digastric muscle. The rectus capitis lateralis muscle extends from the transverse process of the atlas to the jugular process of the occipital bone. superficial portion of the dorsal half of the cervical spinal cord. Care should be taken to preserve the posterior spinal artery during dural opening and mobilization of the VA because it may be incorporated into the dural cuff around the VA. The medial border of the dentate ligament, which is attached to the pia mater between the dorsal and ventral rootlets along the length of each side of the spinal cord, presents a series of triangular toothlike processes on each side that are attached at intervals to the dura mater (Figs. 2U and V and 5A C). At the craniocervical junction the dentate ligament is located between the VA and the ventral roots of C-1 anteriorly and the branches of the posterior spinal artery and the spinal accessory nerve posteriorly; in addition, it is often incorporated into the dural cuff around the VA. The most rostral attachment of the dentate ligament is located at the level of the foramen magnum, above which the VA pierces the dura and courses behind the accessory nerve, although the dentate ligament is located anterior to the accessory nerve at lower levels. The second triangular process is attached to the dura below the site at which the VA and the roots of C-1 pierce the dura. Sectioning the upper two triangular processes will increase access anterior to the spinal cord. 2,3 The first cervical nerve courses along the posteroinferior surface of the VA as it pierces the dura. The ventral root is located anterior to the dentate ligament and the dorsal root, which is infrequently present, passes posterior to the dentate ligament. There are frequently communications between the C-1 nerve root and the spinal accessory nerve. The spinal portion of the accessory nerve is formed by a series of rootlets that arise from the cervical portion of the spinal cord midway between the dorsal and ventral rootlets as far caudally as C-5 (Figs. 2T V and 5A C). These accessory rootlets unite to form a trunk that ascends through the foramen magnum between the dentate ligament and the dorsal roots and enters the posterior fossa behind the VA. The accessory nerve has communications with the dorsal roots of the upper cervical nerves. The most common and largest connection is with the dorsal root of the first cervical nerve, but the largest connection can also be with any root down to C-5. 9 The fourth or intradural segment of the VA, after emerging from the fibrous dural tunnel, ascends in front of the rootlets of the hypoglossal nerve to reach the front of the medulla oblongata where it unites near the junction of the pons and medulla with its mate to form the basilar artery (Figs. 2P and Q and 5A D). Before reaching the lower border of the pons, the VA produces the PICA, which courses backward around the lateral surface of the medulla and between the rootlets of glossopharyngeal, vagus, and accessory nerves. The PICA is divided into five segments: 20 the anterior medullary segment courses anterior to the medulla; the lateral medullary segment courses around the lateral sur- 570

17 Transcondylar, supracondylar, and paracondylar approaches FIG. 2. S: The cerebellum has been elevated to expose the nerves in the cerebellopontine angle. A partial mastoidectomy has been performed to expose the mastoid (descending) segment of the facial nerve in the facial canal and the chorda tympani. 571

18 H. T. Wen, et al. FIG. 2 T: The cerebellum has been elevated to expose the facial and vestibulocochlear nerves, the flocculus, and the trigeminal and abducens nerves. The PICA courses between the rootlets of the vagus nerve to reach the area between the cerebellum and the medulla. The jugular tubercle is located anterior to the glossopharyngeal, vagus, and accessory nerves. face of the medulla and extends to the origin of the glossopharyngeal, vagus, and accessory nerves; the tonsillomedullary segment is related to the caudal portion of the tonsil and the posterior portion of the medulla; the telovelotonsillar segment courses between the rostral pole of the tonsil and the tela choroidae and inferior medullary velum, which form the lower half of the roof of the fourth ventricle; and the cortical segment is distributed to the cerebellar surface. The first three segments are exposed in these approaches (Figs. 2P and Q and 5A and B). The glossopharyngeal nerve usually arises as one or, rarely, as two rootlets just caudal to the pontomedullary junction along the posterior margin of the olive and courses anterior to the choroid plexus protruding from the foramen of Luschka (Figs. 2Q and T and 4D H). The vagus nerve arises as a series of as many as seven or more rootlets below the glossopharyngeal nerve along the posterior margin of the olive and courses caudal to the glossopharyngeal nerve to reach the jugular foramen. The glossopharyngeal nerve is separated from the upper rootlets of the vagus nerve by a dural septum as it pierces the dura over the jugular foramen. The cranial portion of the accessory nerve is composed of four or five delicate rootlets that arise from the medulla below the vagal rootlets (Figs. 2P and T and 4D J). It passes superiorly and laterally to pierce the dura covering the jugular foramen. The most rostral medullary rootlets of the accessory nerve join the vagus nerve inside the jugular foramen. The lower medullary rootlets join the spinal portion of the nerve on their way to the jugular foramen. The hypoglossal nerve arises along the front of the inferior olive anterior to the origin of the cranial accessory fibers as a series of rootlets that pass behind the VA as they converge on the dural orifice of the hypoglossal canal (Figs. 2P and Q, 4D J, and 5D). The rootlets usually join to form two bundles (superior and inferior) that have sep- (Text continued on p. 579) 572

19 Transcondylar, supracondylar, and paracondylar approaches FIG. 2. U: View of the craniocervical junction before opening the dural cuff around the VA. The dura has been opened behind the VA. The posterior spinal artery originates from the VA outside the dura, courses medially between the first triangular process of dentate ligament and the accessory nerve, and bifurcates into ascending and descending branches. The dentate ligament is located between the VA and the ventral roots of C-1 anteriorly and the posterior spinal artery and the accessory nerve posteriorly. The upper or first triangular process of the dentate ligament attaches to the dura at the level of the foramen magnum, and the second triangular process is attached to the dura just below the site at which the VA enters the dura. 573

20 H. T. Wen, et al. FIG. 2. V: View of the craniocervical junction after opening the dural cuff around the VA. The dura around the VA has been opened so that the artery can be mobilized to gain access to the front of the brainstem. A. = artery; A.I.C.A. = anterior inferior cerebellar artery; Asc. = ascending; Br. = branch; Ext. = external; Fiss. = fissure; Inf. = inferior; Lat. = lateralis; M. = muscle; N. = nerve; Occip. = occipital; P.I.C.A. = posterior inferior cerebellar artery; Post. = posterior; S.C.A. = superior cerebellar artery; Sup. = superior; Trans. = transverse; V. = vein; Vert. = vertebral. 574

21 Transcondylar, supracondylar, and paracondylar approaches FIG. 3. Photographs displaying stepwise dissection of the muscles in the occipitocervical area. A: Posterior view. The most superficial muscles, the trapezius and sternocleidomastoid, have been preserved on the left; however, on the right the trapezius and sternocleidomastoid muscles have been reflected along with the galea aponeurotica to expose the underlying semispinalis capitis, splenius capitis, and levator scapulae muscles. The trapezius muscle partially covers the splenius capitis and semispinalis capitis and is attached above to the medial third of the superior nuchal line, the external occipital protuberance, and the nuchal ligament and below to the scapula and the adjacent portion of the clavicle. The sternocleidomastoid muscle partially covers the splenius capitis and the levator scapulae muscles and is attached above to the lateral portion of the superior nuchal line and the mastoid process and below to the sternum and adjacent portion of the clavicle. The trapezius and sternocleidomastoid muscles are continuous superiorly with the galea aponeurotica and the occipitofrontalis muscle. The splenius capitis forms part of the floor of the posterior triangle of the neck; it is attached to the mastoid process and to the occipital bone below the lateral third of the superior nuchal line and extends inferiorly and medially to be inserted on the nuchal ligament and the spinous processes of C7 T3. The nuchal ligament, a midline intermuscular septum, extends from the external occipital protuberance to the spinous processes in the cervical region, and serves as part of the attachment for the trapezius and splenius capitis muscles. B: The left sternocleidomastoid and trapezius muscles have been removed to expose the left splenius capitis. The right splenius capitis has been removed to expose the underlying semispinalis and longissimus capitis muscles. The longissimus capitis muscle is attached to the mastoid process and extends inferiorly and medially to attach to the transverse processes of T1 4. The semispinalis capitis is attached to the occipital bone below the superior nuchal line and extends inferiorly to the transverse processes of T1 7. The levator scapulae muscle, which is attached superiorly to the transverse process of C-1, is located anterolateral to the longissimus capitis. C: The left splenius capitis muscle has been removed to expose the left semispinalis capitis muscle. The right semispinalis capitis muscle has been removed to expose the muscles that form the margins of the suboccipital triangle. The superior oblique muscle runs from the upper surface of the transverse process of C-1 to the occipital bone below the superior nuchal line. The inferior oblique muscle runs from the transverse process of C-1 to the spinous process of C-2, and the rectus capitis posterior major muscle extends from the lateral portion of the occipital bone below the inferior nuchal line to the spinous process of C-2. The right levator scapulae muscle has been detached from the transverse process of C-1. At the transverse foramen of the axis, the VA is located medial to the levator scapulae. The IJV, a segment of which has been removed, is located anterior to the transverse process of C-1 and behind the internal carotid artery and vagus nerve. The semispinalis cervicis muscle attaches above to the spinous processes of C2 5 and below to the transverse processes of T1 5. On the right side, the angle and ramus of the mandible and a segment of the IJV have been removed to expose the internal and external carotid arteries. D: Both semispinalis capitis muscles have been reflected laterally to expose the suboccipital triangles on both sides. The vertebral venous plexus has been removed from the suboccipital triangle on the right side to expose the VA. E: The muscles forming the suboccipital triangle on the right side have been removed. The VA ascends anteriorly and laterally from the transverse foramen of C-2 to reach the transverse foramen of C-1. After ascending through the transverse foramen of C-1, the artery turns posteriorly and then medially behind the superior facet of C-1 to reach the upper surface of the posterior arch of C-1. The ganglion of C-2 is located between the posterior arch of C-1 and the lamina of C-2 and medial to the VA. The dorsal ramus of C-2 produces a medial branch that forms the greater occipital nerve. The ventral ramus of C-2 courses posterior to the VA. FIG. 3 continued 575

22 H. T. Wen, et al. FIG. 4. Photographs displaying neurovascular relationships in the transcondylar, supracondylar, or paracondylar exposures. A F: Photographs obtained from the right side. A: A segment of the VA coursing behind the superior articular process of the C-1 has been removed. The articular surface of the occipital condyle faces downward and laterally and the articular surface of the superior facet of C-1 faces upward and medially. The condylar emissary vein passes through the condylar canal and above the hypoglossal canal to connect the vertebral venous plexus with the sigmoid sinus just proximal to the jugular bulb. The jugular process of the occipital bone, which extends laterally from the posterior half of the occipital condyle, forms the posterior edge of the jugular foramen. The rectus capitis lateralis muscle attaches above to the jugular process of the occipital bone behind the jugular foramen and below to the transverse process of C-1. The IJV descends anterior to the rectus capitis lateralis muscle and the transverse process of C-1. The stylomastoid foramen, which transmits the facial nerve and the stylomastoid artery, is located lateral to the jugular foramen. The OA gives rise to the stylomastoid artery, which enters the stylomastoid foramen with the facial nerve, and to a meningeal branch, which passes through the jugular foramen. The hypoglossal canal is directed forward and laterally above the occipital condyle and below the jugular tubercle. B: The cancellous bone within the occipital condyle has been drilled away while preserving the cortical and articular surfaces. The posterior condylar vein, when present, crosses above the occipital condyle and hypoglossal canal. The transition between the sigmoid sinus and the jugular bulb is located lateral to the occipital condyle in front of the jugular process of the occipital bone. The hypoglossal canal, which courses above the occipital condyle and is directed 45 laterally from its intracranial end, has been opened to expose the hypoglossal nerve, which commonly enters the hypoglossal canal as two separated bundles that fuse as they approach the extracranial end of the canal. The hypoglossal canal also transmits a meningeal branch of the ascending pharyngeal artery and the venous plexus of the hypoglossal canal. The posterior one-third of the occipital condyle can be removed without entering the hypoglossal canal. The extracranial end of the hypoglossal canal is located medial to the jugular foramen. See p. 582 for definitions of abbreviations. FIG. 3. F: The muscles forming both suboccipital triangles have been removed. The rectus capitis posterior minor muscle extends from the posterior arch of C-1 to the occipital bone below the inferior nuchal line. The right VA courses through an incomplete bony canal formed by an ossified posterior atlantooccipital membrane on the upper surface of the posterior arch of C-1. The left VA and the vertebral venous plexus are covered by a nonossified posterior atlantooccipital membrane. A. = artery; Atlanto-occip = atlantooccipital; Ext. = external; Inf. = inferior; Int. = interior; M. = muscle; N = nerve; Occip. = occipital; Post. = posterior; Sup. = superior; Trans. = transverse; V. = vein; Vert. = vertebral. 576

23 Transcondylar, supracondylar, and paracondylar approaches FIG. 4. C: The portion of the rectus capitis lateralis muscle that attaches to the jugular process of the occipital bone has been removed to expose the IJV, which descends in front of the muscle and the transverse process of the atlas. The jugular process of the occipital bone, which extends laterally from the occipital condyle and forms the posterior margin of the jugular foramen, has been removed to expose the jugular bulb. The facial nerve and stylomastoid foramen are located lateral to the jugular bulb. An emissary vein from the vertebral venous plexus joins the jugular bulb. FIG. 4. D E: Transcondylar exposure of the hypoglossal canal. D: The right VA has been depressed to expose the atlantooccipital joint. The cancellous bone within the occipital condyle and in the supracondylar area has been removed to expose the cortical bone surrounding the hypoglossal canal. The rectus capitis lateralis muscle has been removed to expose the posterior surface of the IJV. The condylar canal passes above the hypoglossal canal. The posterior portion of the jugular tubercle acts as a trochlea, around which the accessory nerve turns to reach the jugular foramen. Removing the jugular process lateral to the occipital condyle would expose the posterior surface of the jugular bulb. 577

24 H. T. Wen, et al. 578

25 Transcondylar, supracondylar, and paracondylar approaches (continued from p. 572) arate dural orifices, but commonly fuse before they exit the hypoglossal canal. At its origin, the PICA may pass rostral or caudal to, or between, the hypoglossal rootlets. Discussion The basic far-lateral approach without drilling of the occipital condyle may be all that is required to reach some lesions located along the anterolateral margin of the foramen magnum. However, it also provides a route through which the transcondylar, supracondylar, and paracondylar approaches and several modifications of these approaches can be completed. The transcondylar exposures can be separated into various subcategories. One variant, an atlantooccipital transarticular approach, in which the adjacent posterior portions of the occipital condyle and the superior articular facet of C-1 are removed to facilitate completion of a circular dural incision, permits the VA with the surrounding cuff of dura to be mobilized. A more extensive removal of the articular surfaces and condyles can be performed to gain access to extradural lesions situated along the anterior and lateral margins of the foramen magnum. Another subcategory, the occipital transcondylar variant, is directed above the atlantooccipital joint through the occipital condyle and below the hypoglossal canal to access the lower clivus and the area in front of the medulla. The supracondylar approach directed above the occipital condyle can also be varied, depending on the anatomy to be exposed. The supracondylar exposure can be directed above the occipital condyle to the hypoglossal canal or both above and below the hypoglossal canal to the lateral side of the clivus. In the transtubercular variant of the supracondylar approach, the prominence of the jugular tubercle, which blocks access to the area in front of the glossopharyngeal, vagus, and accessory nerves, is removed to increase visualization of the area in front of the brainstem and to expose the origin of a PICA, which arises from the distal portion of the VA near the midline. The paracondylar approach also has several variants. In the transjugular variant, the exposure is directed laterally to the condyle through the jugular process of the occipital bone to the posterior surface of the jugular bulb. The approach can also be extended lateral to the jugular foramen into the posterior aspect of the mastoid to access the mastoid segment of the facial nerve and the stylomastoid foramen. There are muscular landmarks that aid each aspect of these exposures. Many suboccipital operations are completed without requiring identification of each individual muscle. However, identification of the individual muscles is an essential portion of completing the transcondylar, supracondylar, and paracondylar approaches. In the extradural stage of the operation, two significant vascular hazards can be reduced by an understanding of the relationships of the muscles to the VA and the vertebral venous plexus. Neurosurgeons are generally aware of the risk of VA damage in completing the lateral suboccipital exposure, and this risk becomes greater when the exposure is carried laterally and lower for deeper variants of the farlateral exposure. Especially significant in identifying the neural, vascular, and osseous structures involved in these exposures are the three muscles forming the suboccipital triangle: the levator scapulae, the rectus capitis lateralis, and the posterior belly of the digastric muscle. Proper identification of these deep muscles is facilitated by an understanding of the deep muscles relationships to the more superficial landmarks related to the trapezius, sternocleidomastoid, splenius capitis, and longissimus capitis muscles. Identification of the individual muscles is also helpful in exposing and preserving the OA if it is needed for a bypass procedure and in preserving the peripheral branches of the upper cervical nerves. Exposure of the VA as it ascends between the transverse processes of the axis and atlas, and as the artery passes medially around the posterior margin of the atlantooccipital joint, requires special care. The levator scapulae muscle provides an excellent landmark for locating the VA as it ascends between the transverse foramina of the atlas and axis where the artery courses medial to the upper attachments of the muscle. The main risk in this area is related to a tortuous VA that loops posteriorly as it ascends between the transverse processes of the axis and atlas, making it vulnerable to injury when one expects the artery to be passing straight upward from the lower to the upper transverse foramen. The artery is also susceptible to damage as it passes behind the superior articular facet of the atlas. The artery normally hugs the posterior surface of the superior articular facet of the atlas, extending upward only to the level of the atlantooccipital joint. However, if the artery is elongated and tortuous, it can loop upward behind the occipital condyle, even resting against the occipital bone behind the condyle. It also can loop backward and bulge posteriorly between the lips of the suboccipital triangle, where it can be damaged if one expects it to be found in the depth of the suboccipital triangle. Another special hazard in the area is the vertebral venous plexus, which can be a source of air emboli. There also is the risk in obliterating and coagulating the venous plexus that some branches of the VA that arise in an extradural location, or even a hypoplastic VA, might be occluded or divided. The posterior spinal artery and, uncommonly, the PICA may arise extradurally in the region of the part of the vertebral venous plexus, which may need to be partially excised or obliterated to gain access to the VA. The far-lateral approach in which exposure is carried up to, but does not include, the posterior margin of the occipital condyle may be selected for lesions located along the lateral or anterolateral aspect of the foramen magnum. It is frequently necessary to remove a small portion of both the occipital condyle and/or the superior articular facet of FIG. 4. E: The hypoglossal canal has been opened to expose the hypoglossal nerve and the venous plexus of the hypoglossal canal. The apex or highest point of the jugular tubercle is located in front of the vagus nerve. F: The jugular process of the occipital bone, which forms the posterior margin of the jugular foramen, the sigmoid sinus, and the jugular bulb have been removed to provide this exposure of the glossopharyngeal, vagus, and accessory nerves as they pass through the jugular foramen and the hypoglossal nerve as it passes through the hypoglossal canal. The posterior portion of the bone adjoining the atlantooccipital joint has been removed to expose the articular surfaces. Bone has been drilled in a supracondylar location above the hypoglossal canal to remove a portion of the jugular tubercle around which the vagus and accessory nerves curve to reach the jugular foramen. 579

26 H. T. Wen, et al. 580

27 Transcondylar, supracondylar, and paracondylar approaches the atlas, if there is a need to complete a circumferential dural incision around the site at which the artery penetrates the dura so that the artery can be displaced for access to lesions that are located ventral to the artery and in front of the cervicomedullary junction. For lesions requiring a greater anterior and superior exposure, the posterior one-third of the occipital condyle can be removed without entering the hypoglossal canal. It is possible to drill the cancellous bone of the occipital condyle to expose the lateral clivus and hypoglossal canal while preserving some of the cortical bone of the condyle and the articular surface so that the joint is not disrupted (Fig. 4B E). The cortical surface around the hypoglossal canal can be preserved if there is no need to expose the nerve within the canal. Another key aspect of this approach is the condyle drilling, which requires an understanding of the relationship of the hypoglossal canal to the occipital condyle. The maximum extent of the upper portion of the occipital condyle that could be drilled without exposing the hypoglossal canal is the posterior one-third of its long axis. The hypoglossal canal is reached when the posteromedial one-third of the occipital condyle is removed. The change from cancellous to cortical bone observed while drilling the occipital condyle indicates that the outer perimeter of the hypoglossal canal is being reached (Fig. 4D). Opening the cortical bone around the hypoglossal canal exposes the venous plexus of the hypoglossal canal that surrounds the nerve. The occipital condyle sometimes can be covered by a hypertrophic superior articular facet of C-1 that protrudes into the foramen magnum, making it easy to overlook the upper medial portion of the occipital condyle that should be drilled. In exposing lesions located along the anterior portion of the cervical cord, the inferior portion of the occipital condyle and the superior facet of C-1 can be removed after retracting the VA inferiorly and medially. In drilling the upper posterior portion of the condyle, the posterior condylar vein may be a source of bleeding that does not indicate that the hypoglossal canal has been entered. After exposing the hypoglossal canal, the jugular tubercle, which is located just above and anterior to the canal, is identified. The drilling can be extended to a supracondylar location above the hypoglossal canal for removal of all or part of the jugular tubercle so that the dura covering the tubercle can be pushed forward to gain access to the front of the medulla and the pontomedullary junction. Removal of the jugular tubercle may yield better visualization of the intradural segment of the VA and the origin of the PICA, especially if the PICA originates from the upper portion of the VA. The supracondylar approach, in which the jugular tubercle is removed and the hypoglossal canal is exposed or opened, provides a route for reaching extradural lesions located in the lower lateral portion of the clivus in front of the hypoglossal canal. The extradural removal of the jugular tubercle should be performed with caution, because of the risk of injuring the glossopharyngeal, vagus, and accessory nerves, which hug and often course in a shallow groove as they cross the tubercle. The paracondylar exposure that accesses the posterior margin of the jugular foramen and the jugular bulb can be completed without drilling the occipital condyle. An excellent landmark for identifying the jugular process is the rectus capitis lateralis muscle, which extends upward from the transverse process of the atlas to attach to the jugular process just behind the jugular bulb. The muscle is located medial to the site at which the OA enters the retromastoid area by passing between the rectus capitis lateralis and the posterior belly of the digastric muscle. The jugular foramen and jugular bulb are accessed by drilling the jugular process at the posterior margin of the foramen. Drilling lateral to the jugular bulb from this posterior exposure risks damaging the facial nerve in the facial canal at and just above the stylomastoid foramen. The posterior belly of the digastric muscle, which attaches along the digastric groove just posterior to the stylomastoid foramen, provides a useful landmark for identifying the facial nerve. A limited or more extensive mastoidectomy may be completed, depending on the length of the mastoid segment of the facial nerve to be exposed and the extent to which the bone on the lateral aspect of the jugular bulb must be removed. A wider exposure of the jugular foramen is achieved by a retrolabyrinthine transtemporal approach, in which a more extensive mastoidectomy is completed and the mastoid and the tympanic segments of the facial nerve are exposed so that the facial nerve can be transposed forward to provide access to both the lateral and posterior margins of the jugular foramen (Fig. 4I). 5,26 The retrolabyrinthine exposure also provides access to the dura located in front of the sigmoid sinus, thus permitting the dura to be opened in a presigmoid location. The exposure of the lateral aspect of the jugular foramen can be extended farther forward by completing a translabyrinthine exposure in which the semicircular canals and vestibule are removed to gain access to the internal acoustic meatus for a more generous and, possibly, posterior transposition of the facial nerve. Several controversies surround the positioning of the patient and the type of the skin incision. The modified park-bench position that we use 2,3,19,25 offers the main advantage of avoiding air embolism. The sitting position, recommended by others, 4,9,24 permits a wider angle of view that is associated with a less distended venous plexus, but the rich net of veins around the cervical muscles and the VA offers the risk of air embolism. The risk of air em- FIG. 4. G J: Photographs obtained from the left side. G: A left suboccipital craniectomy has been completed and the dura opened. The cerebellum has been elevated to expose the vestibulocochlear nerve entering the internal acoustic meatus and the glossopharyngeal, vagus, and accessory nerves entering the jugular foramen. The bone posterior to the intracranial end of the hypoglossal canal has been removed to expose the nerve entering the canal. The accessory nerve ascends behind the VA and passes across the posterior surface of the jugular tubercle. The medial portion of the jugular bulb has also been exposed. The posterior condylar vein passes above the condyle and the hypoglossal canal to empty into the terminal portion of the sigmoid sinus. A bridging vein passes from the lateral aspect of the medulla to the jugular bulb. H: The rectus capitis lateralis muscle and the posterior belly of the digastric bone have been detached and the jugular process of the occipital bone has been removed to expose the jugular bulb in the jugular foramen. A mastoidectomy has been completed to expose the mastoid segment of the facial nerve and the stylomastoid foramen, which are located lateral to the jugular foramen and the jugular bulb. 581

28 H. T. Wen, et al. FIG. 4. I: The jugular bulb and an adjoining segment of the IJV have been removed to expose the glossopharyngeal, vagus, and accessory nerves as they pass through the jugular foramen and descend behind the internal carotid artery. The cortical bone lining the hypoglossal canal has been removed to expose the hypoglossal nerve and the venous plexus of the hypoglossal canal. The hypoglossal nerve joins the nerves exiting the jugular foramen to descend in the upper portion of the carotid sheath. The mastoid segment of the facial nerve has been exposed and transposed forward. The bony capsule of the semicircular canals is in the upper portion of the exposure. J: Enlarged view of the nerves entering the hypoglossal canal and jugular foramen in the supracondylar and paracondylar areas. A. = artery; A.I.C.A. = anterior inferior cerebellar artery; Horiz. = horizontal; Inf. = inferior; Int. = interior; Lat. = lateral; M. = muscle; Mid. = middle; N. = nerve; Post. = posterior; Semicirc. = semicircular; Sup. = superior; Trans. = transverse; V. = vein. 582

29 Transcondylar, supracondylar, and paracondylar approaches FIG. 5. Photographs of cadaveric specimens. A: Posterior view of the craniocervical junction following removal of the cerebellum and the right half of the medulla. The VAs cross the upper surface of the posterior arch of C-1, turn upward and forward to penetrate the dura, and pass in front of the upper attachment of the dentate ligaments, spinal portion of the accessory nerves, and the hypoglossal nerves to reach the front of the medulla. They give rise to the posterior spinal arteries immediately after penetrating the dura. The anterior spinal artery arises from the VAs in front of the medulla. The dentate ligaments have their rostral attachment to the dura at the level of the foramen magnum. The triangular processes of the dentate ligaments attach to the dura below the site at which the VAs enter the dura. B: The medulla has been removed to expose the anterior surface of the posterior fossa and the junction of the VAs and basilar artery. The hypoglossal nerves enter the dura approximately 5 mm above the occipital condyles. The jugular tubercles are rounded prominences located above the hypoglossal canals and medial to the jugular foramina. The glossopharyngeal, vagus, and accessory nerves cross the jugular tubercles on their way to the jugular foramina. The segment of the dentate ligaments located below the foramen magnum are situated in front of the accessory nerves; however, the upper attachments of the dentate ligaments to the edge of the foramen magnum are located behind the accessory nerves and the VAs. bolism becomes greater if the condylar emissary vein, hypoglossal venous plexus, sigmoid sinus, jugular bulb, and IJV are to be exposed. A straight scalp incision has been recommended 15,19,24 as being easier to open and close. However, the thick cervical muscular mass and need for extensive retraction create a deep surgical field. An inverted horseshoe incision with the longer limb located at the midline, with all the cervical and suboccipital muscles reflected laterally, has also been recommended. 28,31 The inverted horseshoe incision, with the longer limb located laterally and with most of the cervical and suboccipital muscles reflected medially, as advocated by de Oliveira, was used in this study 2,9 because it moves the bulk of the muscular mass away from the direction of the surgical approach. Anatomically the horseshoe skin incision and muscular dissection, layer by 583

30 H. T. Wen, et al. FIG. 5. C: Anterior surface of the posterior fossa following removal of the intradural segment of the VAs and basilar artery. The skull base in front of the brainstem forms a deep trough that may be easier to reach after removal of some of the lateral bone margin of the trough formed by the occipital condyle and supracondylar bone including all or part of the jugular tubercle. D: Left-sided anterior view of a skull section coronally at the level of the right occipital condyle and the hypoglossal canal. The superior facet of C-1 and the anterior wall of the jugular foramen, internal acoustic meatus, and hypoglossal canal have been removed. The rootlets of the hypoglossal nerve originate between the medullary pyramid and the inferior olive and course behind the VA. The hypoglossal rootlets usually join to form two rootlets (superior and inferior) that enter the hypoglossal canal as separate bundles, but commonly fuse before exiting the hypoglossal canal. The nerve, after exiting the canal, courses in the interval between the ICA and the IJV and posterior to the glossopharyngeal and vagus nerves. The ventral root of C-1 penetrates the dura and hugs the lower margin of the VA as it crosses the posterior arch of the atlas. The glossopharyngeal, vagus, and accessory nerves penetrate the dura on the medial side of the jugular bulb. The PICA passes backward between the upper and lower hypoglossal rootlets. A. = artery; A.I.C.A. = anterior inferior cerebellar artery; Ant. = anterior; Br. = branch; Int. = internal; N. = nerve; Post. = posterior; Sup. = superior. layer with the muscles reflected medially, allow additional space for the lateral approach and offer the best preservation of muscular landmarks such as the suboccipital triangle and levator scapulae for locating the VA and the rectus capitis lateralis for locating the posterior portion of the jugular bulb. However, care should be taken to preserve the blood supply to these muscles to avoid flap dehiscence. 584