LABORATORY INVESTIGATION J Neurosurg Spine 29:18–27, 2018 The subatlantic triangle: gateway to early localization of the atlantoaxial vertebral artery Ali Tayebi Meybodi, MD, Sirin Gandhi, MD, Mark C. Preul, MD, and Michael T. Lawton, MD Department of Neurological Surgery, Barrow Neurological Institute, Phoenix, Arizona OBJECTIVE Exposure of the vertebral artery (VA) between C-1 and C-2 vertebrae (atlantoaxial VA) may be necessary in a variety of pathologies of the craniovertebral junction. Current methods to expose this segment of the VA entail sharp dissection of muscles close to the internal jugular vein and the spinal accessory nerve. The present study assesses the technique of exposing the atlantoaxial VA through a newly defined muscular triangle at the craniovertebral junction. METHODS Five cadaveric heads were prepared for surgical simulation in prone position, turned 30°–45° toward the side of exposure. The atlantoaxial VA was exposed through the subatlantic triangle after reflecting the sternocleidomas- toid and splenius capitis muscles inferiorly. The subatlantic triangle was formed by 3 groups of muscles: 1) the levator scapulae and splenius cervicis muscles inferiorly and laterally, 2) the longissimus capitis muscle inferiorly and medially, and 3) the inferior oblique capitis superiorly. The lengths of the VA exposed through the triangle before and after unroof- ing the C-2 transverse foramen were measured. RESULTS The subatlantic triangle consistently provided access to the whole length of atlantoaxial VA. The average length of the VA exposed via the subatlantic triangle was 19.5 mm. This average increased to 31.5 mm after the VA was released at the C-2 transverse foramen. CONCLUSIONS The subatlantic triangle provides a simple and straightforward pathway to expose the atlantoaxial VA. The proposed method may be useful during posterior approaches to the craniovertebral junction should early exposure and control of the atlantoaxial VA become necessary. https://thejns.org/doi/abs/10.3171/2017.11.SPINE171068 KEYWORDS suboccipital triangle; dissecting aneurysm; far-lateral approach; extreme lateral infrajugular transcondylar approach; cerebral revascularization; surgical technique ESPITE various surgical approaches designed to ac- needed for reconstructive procedures on the VA, includ- cess its different segments, surgical exposure of ing those involving a bypass between the external carotid the extracranial vertebral artery (VA) is challeng- artery (ECA) and the VA.9,10,15,27 This is usually achieved Ding. Embedded in a rich venous plexus, as the VA ascends using lateral approaches through the anterior triangle of through the transverse foramina of the cervical vertebrae, the neck4,7,12,14,20,21,29,32,45,46 or through an inferior extension it is progressively protected by multiple layers of muscles. of the far-lateral approach;8,30,44 however, these approaches This anatomical complexity reaches its zenith at the cra- require early identification of the C-1 transverse process niocervical junction (occiput–C2 region), where the mean- and detachment of the deep muscles of the cranioverte- dering course of the VA through the C-1 and C-2 foramina bral junction in close vicinity of the VA.1,4,14,29,46 Such de- is covered by multiple intersecting muscles.30 tachment usually entails sharp dissection maneuvers that Exposure of the upper extracranial segment of the might cause VA injury. Furthermore, when the C-1 trans- VA between C-1 and C-2 (atlantoaxial VA) may become verse process is not available as a landmark (e.g., due to tu- necessary during surgical treatment of a variety of pa- mor invasion), no other clear landmark exists to efficiently thologies including neoplastic, vascular, congenital, and control the atlantoaxial VA. degenerative lesions.3,17,26 Such an exposure may also be We performed cadaveric surgical explorations to find ABBREVIATIONS ECA = external carotid artery; IJV = internal jugular vein; SCM = sternocleidomastoid muscle; VA = vertebral artery. SUBMITTED September 29, 2017. ACCEPTED November 7, 2017. INCLUDE WHEN CITING Published online April 27, 2018; DOI: 10.3171/2017.11.SPINE171068. 18 J Neurosurg Spine Volume 29 • July 2018 ©AANS 2018, except where prohibited by US copyright law Unauthenticated | Downloaded 10/04/21 12:43 AM UTC A. Tayebi Meybodi et al. an anatomical gateway to expose the atlantoaxial VA the subatlantic triangle in all specimens. The VA was sur- through a posterior craniovertebral approach. We identi- rounded by a venous plexus, and the anterior ramus of fied an anatomical triangle at the craniovertebral junc- the C-2 nerve crossed the posterior aspect of the VA in tion, the subatlantic triangle, the exposure of which leads all specimens (Figs. 2 and 3). The average length of the to early and efficient localization of the atlantoaxial VA. exposed VA was 19.5 ± 3.1 mm (range 17–25 mm) be- This study defines the boundaries and contents of this tri- fore and 31.5 ± 2.6 mm (range 28–35 mm) after unroofing angle and the surgical steps to expose it. the C-2 transverse foramen. The transverse processes of C-1 and C-2 were always palpable within the boundaries Methods of the subatlantic triangle. The anterior ramus of the C-2 spinal nerve crossed the posterior aspect of the atlanto- Five cadaveric heads, embalmed in an alcohol-based axial VA in all specimens. The posterior ramus of the C-2 customized solution, were injected with colored silicone and prepared for surgical simulation. Of the 5 specimens, nerve could also be found in the medial wall of the tri- 4 were Caucasian female specimens and 1 was a Cauca- angle coursing posteriorly (Figs. 2 and 3); however, the sian male specimen (mean age 69.7 years, range 37–88 spinal accessory nerve did not cross the triangle in any years). None of the specimens had any known intracra- of the specimens. When dissection was continued ante- nial or craniocervical junction pathology. A preliminary rior to the levator scapulae muscle (i.e., anterolateral to dissection was performed on 1 craniovertebral junction to the subatlantic triangle), the spinal accessory nerve was assess the anatomical location of the atlantoaxial segment found to emerge under the posterior belly of the digastric, of the VA and its relationship with the adjacent anatomi- coursing inferiorly between the internal jugular vein (IJV) cal structures. Based on the findings of this preliminary and the levator scapulae to reach the undersurface of the dissection, the remaining heads (a total of 9 sides) under- SCM (Fig. 4). The levator scapulae and splenius cervicis went surgical simulations to expose the atlantoaxial VA muscles formed a muscular cushion between the atlanto- through the subatlantic triangle as follows. axial VA and the IJV in all specimens (Fig. 4). Other structures consistently found within the subat- Exposure of the Subatlantic Triangle lantic triangle included the C-2 pars interarticularis abut- ting the anteromedial aspect of the atlantoaxial VA and Each head was placed in a 3-pin Mayfield clamp in a the C2–3 facet joint located at the inferior half of the tri- prone position, with the vertex flexed toward the floor and angle (Figs. 2 and 4). turned 30°–45° toward the side of dissection, such that the mastoid process was the highest point in the field. This would mimic the positioning for a far-lateral approach. An Discussion L-shaped skin incision was started at the anterior border Utilization of the subatlantic triangle consistently leads of the sternocleidomastoid muscle (SCM), 5 cm below the to exposure of the whole length of the atlantoaxial VA mastoid tip, and extended superiorly along the anterior (Fig. 5). Our results show that the subatlantic triangle is an SCM border to the level of the external auditory meatus, efficient corridor to gain control over the atlantoaxial VA and then turned medially to reach the inion. The skin flap with minimal use of sharp dissection and without expos- was reflected inferiorly and medially to expose the SCM ing adjacent critical neurovascular structures, including (Fig. 1A). Leaving a 5-mm cuff, the SCM was sharply de- the IJV and the spinal accessory nerve. tached from its insertion to the mastoid process to expose the underlying splenius capitis muscle (Fig. 1B), which Indications and Current Existing Approaches for was similarly detached from its insertion to the superior Exposure of the Atlantoaxial VA nuchal line. This maneuver exposed the underlying semi- In 1830, Louis Joseph Sanson reported the first surgi- spinalis capitis medially, and longissimus capitis later- cal exposure of a gunshot-induced aneurysm of the atlan- ally (Fig. 1C). The fat pad lying lateral to the longissimus toa xia l VA. 31 He wrote, “The vertebral artery cannot be capitis was then removed to expose the levator scapulae ligated…nor compressed…The wounds of this vessel are and splenius cervicis muscles (Fig. 1D). These muscles at- beyond the resources of art.” More than 180 years later, tached to the posterior aspect of the C-1 and C-2 transverse although much more confident, the cerebrovascular sur- processes, which were palpable in the surgical field. The geon may still hesitate to surgically access the atlantoaxial subatlantic triangle was then identified between 3 groups VA, as evidenced by the increasing number of endovas- of muscles: 1) the levator scapulae and splenius cervicis cular procedures to treat the dissecting aneurysms of this muscles inferiorly and laterally, 2) the longissimus capitis artery. 6,24,33,34 However, surgical exposure of the atlanto- muscle inferiorly and medially, and 3) the inferior oblique axial VA may be necessary in a variety of craniovertebral capitis superiorly. Next, the atlantoaxial VA was found in- junction pathologies, including traumatic, neoplastic, vas- side the subatlantic triangle (Figs. 1D and 2). The impor- 1,2,9,10,12,14,21,27,43 tant anatomical relationships within and in the vicinity of cular, congenital, and degenerative lesions.