Microsurgical Anatomy of the Foramen Magnum

39 Microsurgical Anatomy of the Foramen Magnum

Alvaro Campero, Juan F. Villalonga, Tomás Ries Centeno, Juan Manuel Revuelta Barbero, Amparo Sáenz, and Carolina Martins

Abstract

The access to the foramen magnum (FM) represents one of the most challenging procedures for a skull base surgeon because of the relevant neurovascular elements present in this region. A complete knowledge of its anatomy is required to perform the different approaches safely. We present a detailed description of the FM anatomy. In addition, useful surgical tips for the extradural and intradural approach stages are provided.

Keywords: foramen magnum, craniocervical junction, anatomy, microscope, skull base

39.1 Brief Historical Review

A detailed understanding of the foramen magnum (FM) anatomy has been one of the reasons for a wide and deep study because of the important and complex neurovascular elements located inside it. The study and description of the FM have been made mainly for the different surgical approaches proposed.

Between the late 1980s and mid-1990s, diverse lateral approaches to the FM have been described.1 ^, ² ^, ³ ^, ⁴ ^, ⁵ ^, ⁶ ^, ⁷ ^, ⁸ ^, ⁹ The main objective of those approaches was to reach the distal vertebral artery (VA) segments, the anterolateral brainstem surface, and, with increasing difficulty, the most medial-anterior clivus portion.¹ ^, ² ^, ³ ^, ⁴ ^, ⁶ ^, ⁷ The first of those approaches was the popular far lateral, described by Heros.⁴ It included a wide resection of the occipital bone laterally, with direction to the sigmoid sinus.⁴ As an extension of the far lateral, Perneczky described the posterolateral approach to the FM, including the drilling of the jugular tubercle.⁶ Afterwards, different far-lateral variants, in relation with the degree of occipital condyle or posterior C1 arc resection, have been described with the premise of avoiding the atlanto-occipital instability.¹ ^, ³ ^, ⁵ ^, ⁷ ^, ⁸ ^, ⁹

During the last decades, important advances in the evolution of the endoscopic endonasal surgery have been made as a consequence of findings of increasingly deeper knowledge of the microsurgical anatomy,¹⁰ ^, ¹¹ ^, ¹² such as the improvement in imaging studies and neuronavigation, as well as in surgical instrumentation and reconstructive techniques of the skull base,¹³ ^, ¹⁴ which have allowed to expand the surgical indications of the procedure. The inferior third of the clivus and the anterior portion of the FM can be reached by an endoscopic endonasal transclival approach,¹⁵ ^, ¹⁶ mainly to the midline segment of the FM and with more limitation to the areas located lateral, dorsal, or craniocaudal to the occipital condyle.¹⁵

39.2 Introduction

The FM is described as an oval orifice that communicates the skull base with the cervical spine.¹⁷ In topographic and surgical anatomy, the FM is a complex region, which is limited by: (a) anteriorly: between the inferior one-third of the clivus and the upper edge of the body of C2; (b) laterally: between the upper border of the jugular tubercles and the upper edge of the C2 laminae; and (c) posteriorly: between a line that runs 1 cm above the posterior border of the FM and the upper border of the spinous process of C2¹⁸(Fig. 39.1).

Fig. 39.1Surgical anatomy of the foramen magnum (FM) region. (a) FM region limits; sagittal section of a cadaveric specimen. (b) Inferior view of the FM. (c) Same image with amplification on the FM. (d) Vascular and nervous elements of the FM region. INL, inferior nuchal line; PICA, posterior inferior cerebellar artery; SNL, superior nuchal line.

This region is the seat of the diverse intradural (e.g., meningiomas and neurinomas) and extradural (e.g., chordomas) pathologies.19

A prerequisite to treat the tumors of the FM is to have perfect anatomical knowledge of the region.

Our purpose in this chapter is to present a systematized study of the anatomy of the FM region and to describe the different chirurgical tips that we consider relevant.

39.3 Surgical Anatomy of the FM Region

To our knowledge, the best method to understand the surgical anatomy of the FM is systematizing its study following the order of the different layers, as we find them in surgery (e.g., suboccipital approach the middle line or far-lateral approach).

39.4 Extradural Stage

39.4.1 Nuchal Muscles and V3 Segment

Nuchal Muscles, Plane by Plane

There are four muscular planes in the nuchal region, which from the surface to depth are: (1) sternocleidomastoid and trapezius muscles; (2) splenius capitis muscle; (3) semispinalis capitis and longissimus capitis muscles; and (4) rectus capitis posterior minor, rectus capitis posterior major, inferior oblique, and superior oblique muscles. The muscles forming the first three planes have their main insertion at the superior nuchal line (SNL), whereas the muscles in the deepest plane (rectus and obliques) have their main insertion at the level of the inferior nuchal line (INL). The rectus capitis posterior minor muscle inserts superiorly on the central part of the INL and below on the C1 spinous process. The rectus capitis posterior major muscle inserts superiorly on the central part of the INL and inferiorly on the C2 spinous process. The superior oblique muscle inserts superiorly on the lateral part of the INL and inferiorly on the C1 transverse process. Finally, the inferior oblique muscle inserts medially on the C2 spinous process and laterally on the C1 transverse process¹⁷ ^, ¹⁸ (Fig. 39.2).

Fig. 39.2Cadaveric step-by-step dissection of the suboccipital muscles. (a) The first layer of muscles are trapezius and sternocleidomastoid; both have their main occipital insertion on the SNL. (b) The second layer of muscles is the splenius capitis; its main occipital insertion is the SNL. (c) The muscles of the third layer are semispinalis capitis and longissimus capitis; both have their main occipital insertion on the SNL. (d) The muscles of the fourth layer are rectus capitis posterior minor, rectus capitis posterior major, superior oblique, and inferior oblique. The V3 segment of the VA, in the depth of the suboccipital triangle (SOT), was exposed. Inf. O, inferior oblique; INL, inferior nuchal line; LC, longissimus cervicis; RC Mj., rectus capitis major; RC Mn., rectus capitis minor; SC, splenius capitis; SCM, sternocleidomastoid; SNL, superior nuchal line; SSC, semispinalis capitis; Sup. O, superior oblique; T, trapezius; VA, vertebral artery.

Suboccipital Triangle

The suboccipital triangle (SOT) is formed by three of the four muscles of the deepest plane: medially by the rectus capitis posterior major, laterally by the superior oblique muscle, and inferiorly by the inferior oblique muscle. The V3 segment of the VA is located deep in the SOT, accompanied by the C1 nerve. The VA is located above the channel formed in the upper surface of the atlas, and a venous plexus surround it²⁰ ^, ²¹ ( Fig. 39.2d).

Surgical Tips

How to avoid damaging the VA? For this, we think that it is important to know three things: (1) the anatomy of the VA (2); the disposition of the venous plexus which surrounds them; and (3) the application of the method of the “nuchal lines.”

39.5 Anatomy of the VA

Both VAs are the first and largest branches of the subclavian artery; they have an ascending direction, usually entering through the transverse foramen of the sixth cervical vertebra. When they pass through the transverse foramen of C1, they take a medial direction, passing behind the joint between the occipital condyle and the C1 joint facet. Then, they cross the dura to become intradural, going forward and up to join with each other and form the basilar artery. Each VA is divided into four portions: (a) V1: from its origin in the subclavian artery to the entrance into the transverse foramen of C6; (b) V2: from the transverse foramen of C6 to the transverse foramen of C1; (c) V3: from the transverse foramen of C1 to its entry into the dura mater; and (d) V4: from the moment it passes through the dura until it joins with the contralateral VA.4 ^, ²² ^, ²³

The V3 segment is the most important to remember in the posterior approach to FM region; in its path searching for the dura mater, it runs in a channel located at the lateral part of the posterior arch of C1. During this route, the VA has located deep to the SOT mentioned above. It is important to mention that at this level the VA is surrounded by a venous plexus that can bleed and lead to inadvertent VA coagulation (Fig. 39.3).

Fig. 39.3Cadaveric dissection simulating a far-lateral approach. (a) A horseshoe-shaped incision was demarcated, with the base directed downward. (b) The incision was initiated in the upper branch of the horseshoe, going deeper to the osseous plane. Subperiosteally, the flap was descended until the SNL was reached. (c) Subperiosteal dissection continued downward until the INL was reached. (d) From the INL, rectus and oblique muscles are left attached to the skull base. The suboccipital triangle (SOT) was exposed as well as the V3 segment of the VA. (e) Muscles of the SOT were dissected; the dotted lines marks the craniotomy area. (f) Dura mater and VA were exposed. Inf. O, inferior oblique; INL, inferior nuchal line; RC Mn., rectus capitis minor; RC My., rectus capitis major; SNL, superior nuchal line; Sup. O, superior oblique; VA: vertebral artery.

39.5.1 Vertebral Venous Plexus

The venous plexus surrounding the VA in the SOT 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. Obliteration of a portion of the venous plexus exposes the upper extradural segment of the VA¹⁷ (Fig. 39.3).

39.5.2 Nuchal Lines Method

The patient is placed in a semi-sitting position, and horseshoe-shaped incision is performed. Then the four-step “nuchal lines” method is used to identify V3:

●Step one (from the upper part of the incision until the SNL): We begin our incision at the upper branch of the horseshoe, going deeper to reach the osseous plane. Subperiosteally, we begin to descend the flap until we reach the SNL (the first stop). At this level, it is going to be difficult to continue the subperiosteal dissection because of the muscle insertions of the first three superficial planes; therefore, it is necessary to perform sharper maneuvers to liberate the SNL muscle insertions. The aim is to leave the first three muscle planes attached to the flap, thus preserving their vascularization and vitality.

●Step two (from the SNL until the INL): After passing the SNL, subperiosteal dissection continues, and the separation of the flap from the osseous plane is easier; thus, we continue down to the INL (the second stop). At this level, the separation of the soft tissue flap and the osseous plane becomes difficult again as the insertions of the muscles of the fourth plane appear. At this moment, the fourth muscle plane is left attached to the skull base.

●Step three (from the INL until C2): We continue our dissection from the INL downward, separating the first three muscles planes (attached to the flap) from the fourth muscle plane (attached to the skull base) until we achieve full exposure of C1 and C2. It is important to respect the muscle fibers of the rectus and oblique muscles to identify the SOT correctly.

●Step four (VA exposure): When the SOT attached to the skull base has been identified, it is time to search for the VA in the fatty tissue; the microscope is useful at this stage. The V3 segment is surrounded by a venous plexus, which will become even more evident if venous returned is diminish. Because of that, it is important to have a correct positioning of the head and neck during surgery. Once the VA has been exposed, the rectus capitis posterior minor and major muscles are detached together with the superior oblique muscle to fully expose the VA, the FM, the articulation between the occipital condyle and C1, and the retrocondylar fossa 20 (Fig. 39.3).

39.6 Bone Structure and Articulations

39.6.1 Occipital Bone

The occipital bone involves the FM completely. It has a squamous part which is the posterior limit of the FM, its interior is concave, and it articulates in the superior margin with both parietal bones in the lambdoid suture. In its inferior and lateral part, it relates to the pars mastoid of both temporal bones through the occipital-mastoid suture. In its external surface on the midline, we can observe a prominence called external occipital protuberance or inion which is medial. The torcula or confluence of sinuses is generally less than 1 cm above the inion, in the internal surface of the bone. We also find laterally the SNL and more caudally the INL. A vertical crest descends from the inion to the midline of the posterior margin of the FM, called the occipital crest.¹⁷ ^, ¹⁸

The internal surface of the squamous portion is concave, and it is divided into four asymmetric quadrants by the occipital crest and the prominence of the transverse sinus in the bone. In the two superior quadrants, we find the occipital poles and in the inferior ones the cerebellum.

The internal occipital crest bifurcates when it reaches the FM and forms a small pit occupied in part by the vermis; hence, it acquires its name the vermian pit.¹⁷

The clivus is a rectangle of bone located in front of the FM with a projection from bottom to top and from back to front, with an angulation of approximately 40 degrees/50 degrees and is articulated in front with the posterior portion of the sphenoid body¹⁷ ( Fig. 39.1b, c).

39.6.2 Occipital Condyles

On both sides in the anterior half of the FM, the occipital condyles can be observed, which are articulated with C1. They have an oval shape, and in their medial sector, some prominences can be observed, where the odontoid alar ligament is inserted. Both condyles are in intimate relation with the hypoglossal canal that has an anterior and lateral direction. Behind and slightly lateral to both condyles, we find the condylar fossa, where, in some cases, we find an orifice for an emissary vein that connects the vertebral venous plexus with the sigmoid sinus⁷ ^, ²⁴ ^, ²⁵ ( Fig. 39.1b, c).

39.6.3 Atlanto-occipital Joints

The atlas and the occipital bone are united by the articular capsules surrounding the atlanto-occipital joint and by the anterior and posterior atlanto-occipital membranes.¹⁷

39.6.4 Atlanto-axial Joints

The articulation of the atlas and axis comprises four synovial joints: two median ones on the front and back of the dens, and paired lateral ones between the opposing articular facets on the lateral masses of the atlas and axis. The atlas and axis are united by the cruciform ligament, the anterior and posterior longitudinal ligaments, and the articular capsules surrounding the joints between the opposing articular facets on the lateral masses.¹⁷

Surgical Tips

The type of craniotomy used, the need to drill the occipital condyle, and the removal of the posterior arch of C1 and/or C2 depend on the size and type of lesion. However, to apply any of these far-lateral approach variants, good exposure of the V3 segment is necessary.

The relationship of the craniospinal instability and the degree of condyle resection is controversial. Some authors observed instability when the condyle resection was bigger than 50%.²⁶ ^, ²⁷ To avoid this, we suggest to stop the condyle resection just behind the hypoglossal canal. This can be appropriately managed with the use of neuronavigation during drilling (Fig. 39.4 and Fig. 39.5).