Far-Lateral Approach and Extensions
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.
Lesions 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 occipital 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 (x3–40) 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.
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.
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 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).
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.
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 suboccipital 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).
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.
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.
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 muscles 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.
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 18–24 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 6–10mm). 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 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 jugular 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.
The dural incision begins behind the sigmoid sinus andextends 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 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 surface 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 separate 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.
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 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 use2,3,19,25 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 embolism 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 recommended15,19,24 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 study2,9 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 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.
The authors wish to thank Margaret Barry for her expertise in medical illustration and Ron L. Smith, the director of the microanatomy laboratory, for his technical support and assistance.
Contributors: Hung T. Wen, MD, Albert L. Rhoton, Jr, MD, Toshiro Katsuta, MD, and Evandro de Oliveira, MD
This article was originally published here: Wen HT, Rhoton AL Jr, Katsuta T, De Oliveira E. Microsurgical anatomy of the transcondylar, supracondylar, and paracondylar extensions of the far-lateral approach. J Neurosurg 1997;87:555–585, doi.org/10.3171/jns.1997.87.4.0555, and is included through an exclusive partnership with the Journal of Neurosurgery and its parent company, the American Association of Neurological Surgeons (AANS). The AANS retains full copyright. The appearance of this material here does not imply open access or free use by any other party.
The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton, Jr, MD.
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