Atlanto-occipital Joint

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Median sagittal section through the occipital bone and first three cervical vertebrae
Atlanto-occipital Joint
Synonym	C0-C1 joint
Primary Type	Synovial Joint
Secondary Type	Condyloid Joint
Bones	C1 (Atlas), Occiput
Ligaments	Two articular capsules, posterior atlanto-occipital membrane, anterior atlanto-occipital membrane
Muscles	Flexion by longi capitis and recti capitis anteriores; extension by recti capitis posteriores major and minor, the obliquus capitis superior, the semispinalis capitis, splenius capitis, sternocleidomastoid, and upper fibres of the trapezius.
Innervation	C1
Vasculature	Anastomosis between the deep cervical, occipital and vertebral arteries.
ROM	3.5 flexion, 21 extension, 10.9 lateral flexion each way, 6.7 rotation each way
Volume
Conditions	Jefferson Fracture, Atlanto-occipital Dislocation

The atlas (C1) isn't really a true cervical vertebra both anatomically or functionally. It is more akin to an occipital vertebra as it resembles the occipital bone more than a cervical vertebra. Functionally it is also more closely linked to movement with the head than with the cervical spine.

Overview

	C0-1	C1-2	C2-7
Anatomy	1. Superior facets of C0 (Atlas): 28° in sagittal and transverse planes 2. No disc	1.  C1 has convex facet joint surface (allow C1 facet to slide in AP direction over C2) 2. No disc 3.  Transverse lig : prevent dense posterior dislocation 4.  Alar lig: limiting rotation	Anatomy structure grossly similar. 1. 2x uncovertebral joints (joints of Luschka) at each segments. They are not true synovial joints (clefts at the lateral edges of IVD between uncinated process inferiorly, and the edge of the vertebral body above) 2. 1 disc between each vertebra body. 3. 2 facet joints: at 45° inclination to horizontal plan (pure rotation, side bending is limited)
Biomechanics	Flexion/extension (nodding): 15° Side bending: 5° Rotation 5°	Rotation: 45° both directions. (Rotation in full flexion is C1-2 rotation)	Side bending and rotation coupled -> uncovertebral joints at the convexity open out ->displacement of the disc towards the convexity. pure side bending is limited Flexion/extension combined with rotation and translation. Flexion: IVD compressed anteriorly; superior vertebral body slides anteriorly Extension: superior vertebral body slides posteriorly, IVD compressed posteriorly. Anterior fibres of AF are stretched.

Structure

Atlas superior view

The atlas has two lateral masses which is the most important bony structural feature.

Superiorly they present superior articular processes and articulate with the occipital condyles, called the atlanto-occipital joint. These are synovial joints and are the only direct connection between the skull and the atlas (C1).

Inferiorly the lateral masses have inferior articular processes that articulate with the axis (C2), forming the atlanto-axial joint.

Through the atlanto-axial joint the load from the head is transmitted to the rest of the cervical spine.

The atlas also has anterior and posterior arches. The primary purpose of these is to hold the two lateral masses in place.

Function

The atlanto-occipital joint provides a small amount of flexion and extension. Axial rotation is not possible due to the depth of their sockets. With axial rotation of the cervical spine, the atlas behaves passively sitting between the skull and the axis (C2)

Innervation

The C1 spinal nerve is unique. It does not have a typical dorsal root ganglion. The ganglion cells are located in the rootlets of the spinal accessory nerve. The dorsal ramus of C1 is found with the posterior suboccipital muscles. In some individuals there is a cutaneous branch.

The ventral ramus of C1 runs past the posterior arch of the atlas (C1), behind the superior articular process. It innervates the atlanto-occipital joint and then enters the cervical plexus.