The subatlantic triangle: gateway to early localization of the atlantoaxial vertebral artery

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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 sternocleidomastoid 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 unroofing 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.

ABBREVIATIONS ECA = external carotid artery; IJV = internal jugular vein; SCM = sternocleidomastoid muscle; VA = vertebral artery.

Article Information

Correspondence Michael T. Lawton: Barrow Neurological Institute, Phoenix, AZ. michael.lawton@barrowbrainandspine.com.

INCLUDE WHEN CITING Published online April 27, 2018; DOI: 10.3171/2017.11.SPINE171068.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Stepwise surgical simulation to expose the left atlantoaxial VA through the subatlantic triangle. A: After positioning the head for a far-lateral approach, an L-shaped skin incision is started 5 cm below the mastoid tip at the anterior border of the sternocleidomastoid muscle. Reflection of the skin flap exposes the sternocleidomastoid muscle. B: The sternocleidomastoid is detached from its insertion and reflected inferomedially to expose the underlying splenius capitis muscle. C: Similarly, the splenius capitis muscle is detached to expose the semispinalis capitis, medially, and the longissimus capitis, laterally. Note the fat pad lying lateral to the longissimus muscle, which covers the underlying subatlantic triangle (asterisk). D: Following removal of the fat pad lateral to the longissimus muscle, the subatlantic triangle is exposed with the atlantoaxial VA. The subatlantic triangle is limited by the inferior oblique capitis muscle superiorly, the levator scapulae muscle inferolaterally, and the longissimus capitis muscle inferomedially. C1TP = C-1 transverse process; IOM = inferior oblique muscle; LCM = longissimus capitis muscle; LSM = levator scapulae muscle; OA = occipital artery; SeCM = semispinalis capitis muscle; SpCM = splenius capitis muscle. Figure is available in color online only.

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    Dissection of the left craniovertebral junction to show the boundaries and contents of the subatlantic triangle. A: The subatlantic triangle (purple) is formed anterolateral to the longissimus capitis muscle. It is formed between 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. B: The longissimus capitis muscle has been retracted medially to widen the triangle. Please note the C-2 pars interarticularis (asterisk) and the C2–3 facet joint exposed within the triangle. The C-2 ganglion is not located in the subatlantic triangle. However, the posterior and anterior rami of the C-2 spinal nerve are inside the triangle. The anterior C-2 ramus crosses the posterior aspect of the atlantoaxial VA. The vertebral artery may have muscular and/or radicular branches originating at this segment. C: Retracting the levator scapulae and the splenius cervicis muscles widens the subatlantic triangle (highlighted). Note: The lesser occipital nerve has been dissected and saved during dissection. Br. = branch; C1TP = C-1 transverse process; C2 ant. ramus = C-2 anterior ramus; C2 post. ramus = C-2 posterior ramus; GON = greater occipital nerve; IOM = inferior oblique muscle; LCM = longissimus capitis muscle; LON = lesser occipital nerve; LSM = levator scapulae muscle; OA = occipital artery; Proc. = process; SeCM = semispinalis capitis muscle; SpCvM = splenius cervicis muscle. Figure is available in color online only.

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    Dissection of the left craniovertebral junction and exposure of the course of the atlantoaxial and suboccipital segments of the VA. The C-2 ganglion is located just posterior to the C2–3 facet joint. Moving laterally the C-2 nerve divides into the anterior and posterior rami. The posterior ramus turns posteriorly while the anterior ramus embraces the atlantoaxial VA. The atlantoaxial VA may have a muscular (asterisk) and/or radicular branch (arrowhead) at this level. It may also give rise to the anterior meningeal artery which supplies the anterior dura of the craniovertebral junction. Note that the cranial aspect of the specimen is at the bottom of the figure –consult the green compass). AMA = anterior meningeal artery; C1TP = C-1 transverse process; C2 ant. ramus = C-2 anterior ramus; C2 post. ramus = C-2 posterior ramus; DGM = digastric muscle; I = inferior; L = lateral; M = medial; OA = occipital artery; RCLM = rectus capitis lateralis muscle; S = superior; XI = accessory nerve. Figure is available in color online only.

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    Posterolateral dissection of the right side of a cadaveric neck showing the exposure of the atlantoaxial VA and adjacent structures within the subatlantic triangle. Please note that the accessory nerve does not cross the subatlantic triangle and is related to the anterior aspect of the levator scapulae muscle. Also, the subatlantic triangle exposes the VA without the need to expose the internal jugular vein. A muscular cushion formed by the levator scapulae and splenius cervicis muscles separates the internal jugular vein and the atlantoaxial VA. The blue asterisk marks the rectus capitis lateralis muscle. C2TP = C-2 transverse process; DGM = digastric muscle; IOM = inferior oblique muscle; LCM = longissimus capitis muscle; LON = lesser occipital nerve; LSM = levator scapulae muscle; SOM = superior oblique muscle; SpCM = splenius capitis muscle; XI = accessory nerve. Figure is available in color online only.

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    Neuronavigation system used to demonstrate and verify the exposure of the atlantoaxial VA via the subatlantic triangle. The right inferior panel shows the navigation pointer pointing at the atlantoaxial vertebral artery. Figure is available in color online only.

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    Cadaveric surgical simulation showing the anterolateral approach to the atlantoaxial VA. A: The carotid sheath containing the internal jugular vein is exposed in the anterior triangle of the neck medial to the sternocleidomastoid muscle. The accessory nerve is exposed lateral to the internal jugular vein. The C-1 transverse process is palpated at the superior point of exposure. B: The internal jugular vein is retracted medially and the accessory nerve is also retracted away from the surgical field. The plane between the sternocleidomastoid and the internal jugular vein is developed. This exposes the levator scapulae and splenius cervicis muscles attached to the posterior aspect of the C-1 transverse process. The atlantoaxial VA could be found medial (deep) to the levator scapulae (elevated and marked with a yellow star). The levator scapulae muscle is routinely detached from C-1 transverse process to widen the VA exposure (not shown). Note the proximity of the sympathetic trunk and the vagus nerve during exposure of the VA via a lateral approach. C1TP = C-1 transverse process; IJV = internal jugular vein; IOM = inferior oblique muscle; LSM = levator scapulae muscle; SCM = sternocleidomastoid muscle; SpCvM = splenius cervicis muscle; Symp. = sympathetic; X = vagus nerve; XI = accessory nerve; XII = hypoglossal nerve. Figure is available in color online only.

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    Exposure of the ECA through the posterior triangle of the neck. The ECA is exposed through a plane developed between the parotid gland and the posterior belly of the digastric muscle. When the VA is exposed through the subatlantic triangle, a VA-ECA anastomosis can be completed by simultaneous exposure of the ECA in the operative field. Proc. = process. Figure is available in color online only.

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