Surgical Exposure of the Vertebral Artery

Bernard George, MD, Alexandre Blanquet, MD, and Oscar Alves, MD

Precise surgical technique permits exposure of any part of the vertebral artery (VA) without problems. The lateral approach is the most reliable technique and can be applied at any level. At the third segment level, another option is the posterolateral approach. Generally, the field between the sterne-mastoid mus- cle and the internal jugular vein is opened. Then the prevertebral muscles are divided to expose the transverse processes. At the V2 and V3 segments, it is important to work out of the periosteal sheath surrounding the VA and its venous plexus. This article describes the particular technical points for each VA segment and potential complications. The main causes of morbidity are the leakage of lymph in the first segment, Homer's syndrome in the second segment, and accessory nerve (CN XI) pain or palsy in the third segment. Also discussed are various extensions of the lateral approach from the VA exposure at each level. Copyright �9 2001 by W.B. Saunders Company

O ne surgical approach--the lateral approach-- can be used to expose any part of the VA, at any level, and with one

patient position. The route passes between the sternocleido- mastoid muscle (SM) laterally and the internal jugular vein (IJV) medially (Fig 1). Except above C3 where the accessory nerve (XI cranial nerve) must be controlled, no structure crosses the surgical field in the exposure of the transverse pro- cess (Fig 2). The route follows a natural plane that can be opened simply by coagulating and dividing the vascular pedicles of the SM.

The patient's position varies little regardless of which VA segment must be exposed. The patient is placed in the supine position with a cushion under the shoulders and the head extended and rotated slightly toward the contralateral side. For the V1 and V2 segments, head rotation is slight (10-15 degrees), while for the V3 segment the extent of rotation varies according to the location of the lesion. For a posterior lesion, the head is rotated 40 degrees to project the posterior arch of the atlas into the middle of the field. For an anterior lesion, the head is rotated only 10 degrees, just enough to move the angle of the jaw out of the surgical field.

Exposure of the V1 Segment

The skin incision follows the lower part of the medial edge of the SM; it is occasionally extended by curving it along the clavicle (Fig 3). 1-s

From the Department of Neurosurgery, H6pital Lariboisiere, Paris, France.

Address reprint requests to Bernard George, MD, Department of Neurosurgery, H6pital Lariboisiere, 2, rue Ambroise Par& 75010 Paris, France, E-mail: bernard.george@lrb.ap-hop-paris.fr.

Copyright �9 2001 by W.B. Saunders Company 1092-440X/01/0404-0003535.00/0 doi:l 0.1053/otns.2001.30169

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Fig 1. Drawing of the general principle of the surgical ap- proach to the VA (V) passing between the sternocleidomas- told muscle (S) and the internal jugular vein (JI). The acces- sory nerve (XI) is the only element crossing the field at the level of C2-C3, Cl = internal carotid artery. CE = external carotid artery.

There are 2 ways to expose the V1 segment. The first begins by exposing the subclavian artery; the other begins by exposing the C6 transverse process and then progresses inferiorly.

The first exposure requires expertise in vascular surgery be- cause exposing the subclavian artery is often difficult and the vessel could be torn during the dissection> As neurosurgeons, we favor the second method in which the field is opened be- tween the SM and the IJV (Fig 4). The omohyoid muscle is divided, and the fat layer covering the prevertebral muscles is retracted laterally. Care must be taken at the V1 segment to identify 2 elements: the inferior thyroid artery and the lym- phatic vessels. The inferior thyroid artery runs horizontally in this fat layer at the level of C6-C7. It can be ligated and divided. The lymphatic vessels, including the thoracic duct on the left side, are inside the fat layer. They must be carefully identified and ligated. Minor leakage of lymph with no apparent vessel damage can be controlled by packing the area with Surgicel stuck with fibrin glue. After these elements are identified and ligated, the prevertebral aponeurosis is exposed at the level of the transverse process of C6 (Fig 5 and 6B). Beneath this ape- neurosis, the sympathetic chain must be identified. Then the aponeurosis is incised laterally or medially along the sympa- thetic chain so the sympathetic chain can be retracted accord- ingly medially or laterally (Fig 6C). The aponeurosis is re- tracted with one or .two stitches and rolled around the

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Fig 2. Exposure of the whole length of the VA on the right side through a lateral approach passing between the sternoclei- domastoid muscle (S) and the internal jugular vein (I). The numbers 2 to 6 indicate the corresponding level of the transverse processes from C2 to C6. Note that the only element crossing the field at the C2-C3 level is the accessory nerve (N).

sympathetic chain to protect it during retraction. The C6 trans- verse process is easily identified as the lowest transverse process that can be palpated. The C5 transverse process is palpated just above C6 at the same depth. Below the C6 transverse process, no transverse process can be felt because the C7 transverse process is located more deeply. Its location can be confirmed by fluoroscopy.

After the sympathetic chain is retracted, the longus colli muscle is cut along the transverse process of C6, and the peri- osteum of the transverse process is elevated with a smooth spatula. The anterior aspect of the VA can then be seen. It is accompanied by 2 veins, the anterior and posterior vertebral veins, which may be ligated or coagulated. Exposure of the VA can then progress inferiorly as required. At this level, the VA is in a free anatomical space with no direct contact with the nerve roots that cross the posterior aspect of the VA. The VA courses parallel to the vertebral bodies but at a distance of 5 to 10 mm.

In some circumstances, especially for patients with short necks, the SM may be detached from the sternum and the clavicle and folded superiorly and laterally, permitting the sur- gical field to be enlarged and allowing a deeper exposure.

Exposure of the V2 Segment

The skin incision for exposing the V2 segment follows the medial edge of the middle portion of the SM (Fig 3). 2'4'6-1~

The exposure is begun by opening the field between the medial aspect of the SM and the IJV. The fat layer deep to these structures is retracted laterally (Fig 6A). The prevertebral apo- neurosis is exposed and the sympathetic chain below it is iden- tified (Fig 6B). The aponeurosis is incised along the sympa- thetic chain and usually retracted laterally to preserve the connections with the cervical nerve roots (rami communican-

Fig 3. Scheme of head position and skin incision. The dotted lines separate the 3 levels of exposure of the V1, V2, and V3 segments of the VA (I, II, III).

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Fig 4. Surgical view of the exposure of the Vl segment of the VA (after removal of a neurinoma). The C6 (6) and C7 (7) transverse processes and the C7 nerve root (N) are visible. Reprinted with permission from The Vertebral Artery. Pathology and Surgery. George B, Laurian C. Springer-Verlag Wien, New York, NY 1987 p 258.

tes). The longus colli muscle is cut along the lateral aspect of the vertebral bodies and the transverse processes (Figs 6C and 6D). The surgeon must verify that the VA does not enter the transverse canal at an abnormal level above C6; in this case, the VA courses anterior to the transverse processes, just behind the longus colli muscle. After the longus colli muscle is cut, the periosteum of the transverse processes is elevated with a smooth spatula. The periosteum inside the transverse foramen is then split from the bone. In the space between bone and periosteum, a small rongeur or a small Kerrison rongeur (2 to 3 mm) is pushed and the anterior part of the foramen is opened (Fig 6E). Still working extraperiosteally, the VA is exposed

inside the foramen with the venous plexus kept inside the periosteal sheath. The VA is then exposed between two trans- verse foramina with resection of the small intertransverse mus- cles (Fig 6E).

The V2 segment runs along the vertebral bodies, in close contact with them. In fact, the medial side of the transverse foramen is formed by the inferior part of the lateral aspect of the vertebral bodies. The VA is also close to the cervical nerve roots, which cross its posterior aspect obliquely from medial to lateral and from superior to inferior (Fig 7). The nerve roots emerge from behind the upper transverse process, cross the VA in the space between two transverse processes, and then run along the

Fig 5. Schematic drawing of the relevant anatomical structures in the exposure of the V2 segment of the VA. CO = Iongus colli muscle, CA = Iongus capitis muscle, T = sympathetic trunk, P = transverse process, V = VA. S = scalenus muscle, N = cervical nerve root, I = intertransverse muscle.

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Fig 6. Surgical exposure of the V2 segment of the VA (left side). (A) Opening the field between the internal jugular vein (IJ) and the sternocleidomastoid muscle (S). Note the fat sheath in the depth. (B) The fat sheath (F) is retracted laterally. The aponeurOsis of the Iongus colli muscle (C) has been cut lateral to the sympathetic t runk (T). The blade retracts the internal jugular vein (IJ). (C) The sympathetic t runk (T) is retracted medially with the internal jugular vein (IJ). The Iongus colli muscle (C) is cut along the lateral aspect of the vertebral body and along the C5 transverse process (P). This incision begins at the lateral edge of the anterior longitudinal l igament (L). (D) The Iongus colli muscle (C) has been resected to reveal the C5 and C6 transverse processes (5 and 6). The intertransverse muscle (I) still covers the VA. The C5 transverse foramen (5) has been opened showing the vertebral artery (star). The C4 and C6 transverse processes (4, 6) are visible. Note the anterior longitudinal l igament (L) under the blade retracting the internal jugular vein. The C4-C5 and C5-C6 disc spaces have been opened (arrow). C = Iongus colli muscle. M = sternocleidomastoid muscle.

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Fig 7. Operative view (right side) of the C5 and C6 cervical nerve roots (N) crossing the posterior aspect of the vertebral artery (star) after removal of an hemangioma. The blade retracts the internal jugular vein (IJ). The self-retaining retractor retracts the sternocleidomastoid muscle.

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Fig 8. (A) Cadaveric exposure of the C3 to Cl portion of the VA (left side). The C3 and C2 transverse foramina have been opened. The numbers 3 and 2 indicate the VA at the level of the C3 and C2 transverse foramina. The spatula points to the Cl -C2 joint. Note the accessory nerve (N) crossing the field. The periosteal sheath of the VA has been opened between Cl and C2 but is intact between C3 and C2. (B) Corresponding MR image showing the course of the VA from C3 to Cl. The numbers 1, 2, and 3 indicate the Cl, C2, and C3 transverse processes. Note the internal jugular vein (white asterisk) and the Cl-C2 joint (arrow). Reprinted with permission from Management of the Vertebral Artery in Surgery of the Skull Base. Donald P J (ed). Lippincott-Raven, Philadelphia, PA 1998, pp 533-553.

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tip of the lower transverse process. The sheath of the nerve roots is adherent to the periosteal sheath of the VA but can be split from it using a smooth spatula. To expose the cervical nerve roots lateral to the VA, the longus capitis muscle must be CUt.

Exposure of the C3 to C2 Portion of the V 2 Segment

The VA exposure from the C6 to C3 level raises no particular difficulty because the VA runs vertically along the spine from one transverse process to the next. However, between C3 and C2 the VA changes its course and so needs particular attention (Fig 8A and B). After exiting the foramen of C3, the VA runs vertically for 5 to 6 mm. It bends sharply on the side of the C2 vertebral body, a few millimeters below the base of the C2 transverse process. Then the VA runs horizontally toward the C2 transverse foramen, which is more laterally located than the other transverse foramina because it is longer and oblique infe- riorly and laterally instead of horizontally like the other trans- verse processes (Fig 8B).

The surgical exposure of this portion of the VA starts on the C3 transverse process and progresses upward along the lateral aspect of the C3 and C2 vertebral bodies until the bend in the VA is reached. At this point the base of the C2 transverse process is also exposed. The C2 transverse process can be ex- posed subperiosteally from its base to its tip and the C2 trans- verse foramen can be opened subperiosteally, if needed.

Exposure of the V3 Segment

Two surgical routes are available to approach the V 3 segment: the anterolateral and the posterolateral approaches. 4,11,12,13,14

The Anterolateral Approach

The skin incision follows the medial edge of the rostral portion of the SM to the mastoid process and is extended along the mastoid process and the superior occipital line (Figs 3, 9A and 10A).2,11-17

The exposure is begun by detaching the SM from the mastoid process and opening the field between the SM and the IJV. The superior limit of the surgical field is the mastoid process and the attachment of the digastric muscle on a groove of the mastoid process (Figs 9B and 10B).

The fat layer filling the space between the SM and the IJV is separated from the deep cervical muscles after the accessory nerve (XI cranial nerve) is identified at its junction with the SM. The nerve is exposed along its course from the SM to the jugular foramen. The fat layer is rolled around the nerve to protect it during retraction. The nerve is retracted superiorly when the exposure is between C3 and C1 and inferiorly when the expo- sure is between C2 and the foramen magnum (Fig 10C and D).

Beneath the fat layer, the small muscles attached on the C1

transverse process are visible. The tip of the C1 transverse process can be palpated 15 mm in front and below the tip of the mastoid process. The muscles are divided flush to the trans- verse process of the atlas exposing the VA segment at C1-C2, the posterior arch of the atlas, and the VA segment above C1 (Figs 9C and 10E). These 2 VA segments are almost parallel to each other with only the posterior arch of the atlas interposed between them. The C1-C2 segment is crossed perpendicularly by the anterior branch of the second cervical nerve root, which is a good landmark in the exposure of the VA at this level (Fig IOF).

Next the tip of the C2 transverse process is identified. The process is exposed, progressing from its tip toward its base. Still working subperiosteally, the foramen is controlled and opened as required.

Similarly the C1 transverse process and the posterior arch of the atlas are exposed. The C1 transverse process is exception- ally large. It is about 10 m m long from the C1 foramen to the tip of the C1 transverse process. Most of this bone must be re- moved with a large rongeur before the foramen is opened with a small Kerrison rongeur (Figs 9D and 10G). Before any trans- verse foramen can be opened, the periosteum must be split from the bone. At C1, the periosteum is elevated from the posterior arch of the atlas and not from the transverse process.

Control of the C1-C2 segment presents no difficulty; the C2 root may be divided without causing further neurological def- icit because its main branch has already been cut subcutane- ously.

Control of the segment above C1 may raise some problems. The VA periosteal sheath is covered by the occipitoatlantal membrane, which must be cut. It must be incised carefully at the level of the superior aspect of the VA. This level is difficult to identify because there are no particular landmarks. Cutting the membrane is even more difficult when it is calcified or ossified. At this level, the VA venous plexus is connected with the condylar vein. The connection and sometimes the condylar vein are coagulated and divided. The condylar vein does not present a problem. On the VA side, it can be coagulated with bipolar coagulation. On the condylar side, a piece of Surgicel mixed with bone wax can be packed into the bony canal. In contrast, the inferior aspect of the VA is easily controlled. The periosteum of the posterior arch of atlas is elevated from its inferior edge toward the superior edge to reach the VA groove.

The tiny C 1 nerve root is visible emerging from the vicinity of the inferior aspect of the VA. It has a limited and only sensory function and does not need to be respected. The VA can be mobilized from its groove to reach the lateral mass of the atlas. The medial end of this groove is often easily recognized because it corresponds to an increase in the height of the posterior arch of the atlas.

The whole length of the V3 segment can be mobilized and transposed including if necessary opening the C2 transverse

Fig 9. Schematic drawing of the anterolateral approach to the V3 segment of the VA (right side). (A) Skin incision. (B) The sternocleidomastoid muscle (SM) has been detached from the mastoid process. The field between the internal jugular vein (IJ) and the SM has been opened. In the depth, the accessory nerve (N) can be seen inside the fat sheath. The Cl transverse process (Cl) can be seen 15 mm before and under the tip of the mastoid process. (C) The fat sheath (F) is rolled around the accessory nerve and retracted medially and inferiorly. The small muscles attached to the tip of the Cl transverse process (Cl) have been divided, exposing the C l -C2 and the above Cl portions of the VA (black stars). (D) The Cl transverse process (Cl) has been opened and the VA (black stars) is ready for transposition (same legends as in Fig 9.C). PF = posterior fossa. Reprinted with permission from Management of the Vertebral Artery in Surgery of the Skull Base. Donald P J (ed). Lippincott-Raven, Philadelphia, PA, 1998, pp 533-553.

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foramen (Fig 10H). Before the VA is mobilized from the trans- verse process of the atlas, the periosteum must be elevated from the bone all around the VA. The most lateral part of the poste- rior arch of the atlas, in the concavity of the VA loop, must be resected to avoid tearing the periosteal sheath when pulling out the VA. After transposing the VA, the lateral wall of the cranio- cervical junction comes onto view including the vertebral body of C2, the C1-C2 joint, the lateral mass of the atlas, the C0-C1 joint, the occipital condyle, and the jugular tubercle.

Posterolateral Approach (Fig 11A, B, and C) 2,11-15,1s

The posterolateral approach is the lateral extension of the stan- dard posterior midline approach. As in the posterior midline approach, the patient may be placed in the prone, lateral, or sitting position. Regardless of the position, it is best to keep the head straight to avoid changing the anatomical relations of the VA to neighbouring structures.

The skin incision may be oblique from the midline at C4 to the top of the auricle, or vertical paramedian between the mid- line and the mastoid process, or vertical on the midline from the level of C4 to the occipital protuberance then curved laterally along the superior occipital line to the mastoid process. We favor the sitting position to minimize bleeding and the midline incision curved laterally because it starts from the standard midline exposure with which every surgeon is familiar.

The lower part of the posterior fossa, the posterior arch of the atlas, and Usually the lamina of C2 are exposed primarily on one side but overtaking the midline. The periosteum of the poste- rior arch of the atlas is elevated medially to laterally and inferi- orly to superiorly (Fig 12A, B, C, and D). Much like the expo- sure through the anterolateral approach, the VA groove is reached. The end of the groove is indicated by a step in the posterior arch of the atlas. The C1 root may be identified run- ning under the VA in the groove. The superior aspect of the VA is exposed by cutting the occipitoatlantal membrane. The VA is followed along its groove and from the end of the groove to the dura mater. On the other end, the groove is followed up to the transverse foramen. The distance from the midline to the trans- verse foramen is long (22-25 mm) (Fig 12B), about twice the distance as that between the midline and the end of the VA groove.

Below the posterior arch, the space between C1 and C2 is opened with the C2 nerve root running horizontally from the lateral side of the dura to the C1-C2 segment of the VA. This segment can be exposed by extending the exposure a little more

laterally. However, it is challenging to open the C1 or the C2 transverse foramen through the posterolateral approach.

The posterolateral approach can be done on both sides simul- taneously. To do so, a T-shape incision is performed with the usual vertical part and a horizontal incision from one mastoid process to the other.

The posterolateral approach can also be combined with the anterolateral approach. The patient is placed in the lateral po- sition. The incision follows the medial edge of the SM, the mastoid process, the occipital line up to the occipital protuberance, and then the posterior midline down to the C4 spinous process.

Extensions from the VA Exposure

V1 and V2 Segment

After controlling the VA, the lateral approach may be extended medially and laterally. A medial extension exposes the anterior aspect of the vertebral bodies and the anterior longitudinal ligament. Therefore, lesions involving the cervical spine can be followed from the transverse canal to the opposite lateral side.

A particular technique known as oblique corpectomy can also be performed. This technique permits enlargement of the intervertebral foramen, permitting access to lesions located in the posterior part of the vertebral body and exposing the ante- rior aspect of the dural sac. The posterolateral corner of one or several vertebral bodies is drilled out. Drilling begins from the lateral aspect(s) of the vertebral body(ies) just above the VA exposed in the transverse canal. The VA is left in place, but its anterior and medial aspects are exposed by unroofing the de- sired number of transverse foramina. At first, the bone medial to the VA is preserved to protect the VA during the drilling. The drilling is directed obliquely toward the posterior longitudinal ligament and can be extended as far as the contralateral pos- terolateral corner and pedicle. Finally, the bone medial to the VA is resected, if necessary. This piece of bone corresponds to the pedicle. The oblique corpectomy technique minimizes bone drilling and avoids bone grafting or osteosynthesis in most cases. The main indications are spondylotic myelopathy or radiculopathy or both; small bone tumors; or hourglass tumors, especially neurinomas.

Laterally, the approach to the VA may be enlarged toward the cervical nerve roots and the brachial plexus. For this extension, the longus capitis muscle must be divided. The tips of the

Fig 10. (A) Positioning and skin incision for a lateral approach on the left side. Note that the auricle has been folded anteriorly and held in place with sutures, (B) Operative view. Opening of the field between the internal jugular vein (black circle) and the sternocleidomastoid muscle (M) detached from the mastoid process (not visible). The accessory nerve (N) is identified inside the fat sheath (F) separated from the small muscles attached to the C l transverse process (1). Note the digastric muscle (D). Operative view. The fat sheath (F) has been rolled around the accessory nerve showing the C1 transverse process (1) and the small muscles attached to it. (D) Cadaveric view corresponding to Fig. 10.C. Black circle = internal jugular vein. 1 = C l transverse process. (E) Operative view. The small muscles attached to the C l transverse process 1 have been divided showing C l - C 2 (star) and the portions of the vertebral artery above Cl (asterisk). (F) Cadaveric view of the V3 segment (see Fig 10G). The Cl transverse process has been resected and the C l transverse foramen unroofed exposing the loop of the vertebral artery. Notice that the periostaal sheath has been opened. The forceps pulls the VA from the atlas groove (G); note the end of the groove with a step in the posterior arch of atlas. The C2 nerve root (R) has been divided. 1 = C l - C 2 VA part; 2 = VA at the level of the C l transverse foramen; 3 = above Cl VA part. The white star indicates the C l - C 2 joint. (G) Operative view after resection of the C l transverse process and opening of the C l transverse foramen (see Fig 10F). The loop of the VA is clearly visible. The blade is positioned on the anterior arch of atlas protecting the accessory nerve (N) and the internal jugular vein. The C2 nerve root (R) crosses the C l - C 2 part of the VA. 2 = VA at the level of the C l transverse foramen. 3 = Above Cl VA part. (H) The VA (star) is being transposed out of the Cl transverse foramen. The inside of the foramen is clearly visible (1). The blade protects the accessory nerve and the internal jugular vein. MA = mastoid process; IJ = internal jugular vein.

SURGICAL EXPOSURE OF THE VERTEBRAL ARTERY 1 9 1

Fig 11. Schematic drawing of the posterolateral approach. (A) Skin incision. (B) Exposure of the occipital bone, Cl posterior arch (1) and lamina of C2 (2). The subperiosteal exposure of the posterior arch of the atlas shows the VA in the groove of the atlas. (C) The occipital bone has been removed showing the posterior fossa. The posterior arch of the atlas (1) has been resected including part of the groove showing the part of the VA above Cl running toward the dura mater at the foramen magnum level. Reprinted with permis- sion from Management of the Vertebral Artery in Surgery of the Skull Base. Donald PJ (ed). Lippincott-Raven, Philadel- phia, PA, 1998, pp 533-553.

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Fig 12. (A) Operative view (left side). Exposure of the occipital bone (O), posterior arch of atlas (Cl) and lamina of C2 (C2). The VA (star) is visible in the groove of atlas with the end of this groove clearly indicated by a step (arrow) in the posterior arch of atlas. R = C2 nerve root. Notice the midline with the spinous process of C2 (open circle). (B) Cadaveric view (right side) similar to Fig. 12.A. The spatula is inside the groove of atlas (G) elevating the VA inside its periosteal sheath at the level of the Cl transverse foramen. Notice the long distance from the midline (open circle) to the Cl transverse foramen. The arrow indicates the lateral edge of the dural sac. (C) Operative view (left side). The occipital bone has been removed showing the posterior fossa dura (P) with the end of the sigmoid sinus (S). The posterior arch of atlas has been resected exposing the above Cl part of the VA (star) running toward the dura mater (arrow) at the foramen magnum level. R = C2 nerve root. Notice the spinous process of C2 (open circle), which indicates midline. (D) Cadaveric view (right side) similar to Fig. 12.C with the VA (star), C2 nerve root (R), and junction of the VA with the dura mater at the foramen magnum level (arrow) and the spinous process of C2 indicating the midline (open circle).

transverse processes are exposed. The cervical nerve roots cross the posterior aspect of the VA obliquely, medially to laterally and superiorly to inferiorly. They then run along the tip of the transverse processes between the digits of the scalenus muscle. The cervical nerve roots must not be confused with the tendons of the scalenus muscle, which look similar and run in the same oblique direction. For wider exposure of the nerve roots, the tendons that attach the scalenus muscle to the tip of the trans- verse processes are cut. Laterally, the cervical nerve roots con- nect to each other to form the primary trunks of the brachial plexus. Behind the nerve roots are the joints, which may be reached by working between 2 nerve roots.

V3 segment

Anterolateral Approach. As in the V2 segment, the antero- lateral approach may be extended medially toward the vertebral body of C2 and to the anterior arch of the atlas. At the V3 segment, however, it is safer and more convenient to mobilize the VA from the C1 and/or the C2 transverse foramen. Because the C1-C2 joint is located more anteriorly than the lower cer-

vical joints, it can be found medial to the C1-C2 portion of the VA. Drilling the C1-C2 joint and the lower part of the lateral mass of the atlas allows the odontoid process to be reached and if necessary resected. Medial to the portion of the VA above C1 is the C0-C1 joint (occipital condyle-lateral mass of the atlas). Drilling this joint leads to the tip of the odontoid and the tip of the clivus. Consequently, at this level, any drilling of bone medial to the VA can compromise the stability of the spine.

With VA transposition and following the C2 nerve root, the dura of the anterior part of the craniocervical junction can be exposed with minimal drilling of the posterior part of the C1-C2 joint and the lateral mass of the atlas. For the work toward the anterior part of the crani0cervical junction, the head must not be rotated more than 15 degrees toward the opposite side. Greater rotation causes the anterior arch of the atlas to rotate out of the surgical field.

Laterally and posteriorly, the posterior portion of the cranio- cervical junction is easily exposed to at least the midline. The head is rotated 30 degrees to the opposite side to bring the posterior fossa and the posterior arch of the atlas into the

SURGICAL EXPOSURE OF THE VERTEBRAL ARTERY 1 9 3

middle of the field. Exposure of the C2 lamina is more difficult because head rotation does not influence its position.

Superiorly, the jugular foramen may be opened on its inferior and posterior aspect by the juxtacondylar approach. This ap- proach accesses the posterior fossa from a lateral direction. The mastoid process is resected as much as necessary to expose the sigmoid sinus. The VA is exposed at C1-C2 and above. Then the transverse process of atlas is resected and the C1 foramen is opened. It is seldom necessary to mobilize the VA from the C1 foramen. The occipital condyle and the jugular tubercle are now visible superior to the VA. The jugular tubercle is the piece of bone that covers the junction between the sigmoid sinus and the internal .jugular vein (i.e., the jugular bulb). The jugular tubercle is resected using a drill or Kerrison rongeur pushed between the venous wall and the bone. This technique reaches tumoral processes projecting into the jugular foramen without drilling the petrous bone. A partial mastoidectomy is sufficient. The IX, X and XI cranial nerves can be followed along their course in the neck, jugular foramen, and posterior fossa. To follow the nerves into the posterior fossa, the dura is opened perpendicular to the end of the sigmoid sinus and the incision is extended 3 to 4 cm.

Posterolateral Approach. The posterolateral approach ac- cesses the VA above C1. Superiorly the posterior fossa and inferiorly the posterior arch of the atlas and the lamina of C2 are easily exposed. The posterior fossa can be opened and the posterior arch of the atlas can be resected on the midline and laterally. The lateral limits are the C0-C1 and C1-C2 joints. These joints must be respected as much as possible if they are intact. They may be drilled medially to one-third their width without compromising the stability of the craniocervical junc- tion.

Opening the dura leads to the foramen magnum region. To take the maximum advantage of the bone resection, the dura should be folded laterally after being incised perpendicular to the vertical median or paramedian incision. This horizontal incision is directed toward the VA at the point that it pierces the dura.

The VA invaginates the periosteal sheath and the dura about 4 mm, forming a double sheath that surrounds the VA at this level. Moreover, these 2 sheaths adhere to the VA wall. Dissect- ing the VA free from these sheaths is therefore difficult. In many cases, it is better to cut the dura at some distance (2 to 3 mm) around the VA. Intradurally, the VA passes under the first arch of the denticulate ligament and runs obliquely around the me- dulla oblongata. The lower cranial nerves (CN XII to IX) cross

its posterior aspect. The main indications for the intradural extension of the posterolateral approach are foramen magnum tumors, especially meningiomas.

References 1. Feldman A J: Surgical approach to disease of the first portion of the

vertebral artery. In Vertebro-basilar arterial occlusive disease. Med- ical and surgical management. Raven-Press. New York, NY 231-239, 1984

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3. Lusk MD, Kline DG, Garcia CA: Tumors of the brachial plexus. Neurosurgery 21:439-453, 1987

4. Shumacker HB, Campbell RL, Heimburger RF: Operative treatment of vertebral arteriovenous fistulas. J Trauma 6:3-19, 1966

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7. Raynor RB: Anterior or posterior approach to the cervical spine: An anatomical and radiographic evaluation and comparison. Neurosur- gery 12:7-13, 1983

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10. Verbiest H. La chirurgie ant6rieure et laterale du rachis cervical. Neurochirurgie 16:212, 1973, (suppl)

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14. George B: Management of the vertebral artery in skull base surgery. In Surgery of the Skull Base, Paul J. Donald (ed), Lippincott-Raven, Philadelphia, PA 545-553, 1998

15. AI-Mefty O, Borba LA, Aoki N, et al: The transcondylar approach to extradural nonneoplastic lesions of the craniovertebral junction. J Neurosurg 84:1-6, 1996

16. Henry AK: Extensile exposure. Baltimore, Williams & Wilkins, 2nd ed, 1957

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18. Bertalanffy H, Seeger W: The dorsolateral, suboccipital, transcondy- lar approach to the lower clivus and anterior portion of the cranio- cervical junction. Neurosurgery 29:815-821, 1991

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