The Atlanto-Occipital & Atlanto-Axial Joints.
Related Ligaments & Syndesmoses.

The joints between the occipital bone and two upper cervical vertebrae (so-called craniocervical junction¹) are reasonable to combine under one topic, as the skull, atlas, and axis normally function as a composite unit². Due to the lack of continuous attachment to the rest of the spine through the intervertebral disc, the first cervical vertebra becomes the highly mobile skeletal element subordinate to the influence of other parts of the spine, i.e., skull from above and axis – from below. The experimental data back this assumption – the motion of the atlas may be opposite to the neck flexion and extension. These "paradoxical" motions are caused by individual variations in the eccentricity of compression loads exerted on the lateral masses of the atlas³. Moreover, some syndesmoses, like alar ligaments and tectorial membrane, directly connect the C2 with the occipital bone, bypassing C1.

¹ Cone R, et al. The creaniocervical junction. Radiographics, 1981, 1(2):1–37.

² Mercer S, Bogduk N. Joints of the cervical vertebral column. J Orthop Sports Phys Ther, 2001, 31(4):174-82.

³ Van Mameren H, et al. Cervical spine motion in the sagittal plane (I). Range of motion of actially performed movements, an X-ray cinematographic study. Eur J Morphol, 1990, 28(1):165–173.

Atlanto-occipital and atlanto-axial joints. Anterior view — Atlanto-occipital and atlanto-axial joints.
Anterior view.

Atlanto-occipital and atlanto-axial joints. Anterior oblique view — Atlanto-occipital and atlanto-axial joints.
Oblique anterior view.

Click an image to dislocate the occipital bone and atlas from their natural position and see articular facets involved in these joints. Note that between the occipital bone and atlas, there is only one joint on each side. In contrast, between C1 and C2, there are two types of joints – the symmetrical lateral atlanto-axial joints and unpaired median atlanto-axial joints. The lateral and median atlanto-axial joint motions occur simultaneously (i.e., these joints are compound^*).

^* Compound joint. (n.d.) Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition, 2003.

Capsules of the atlanto-occipital and lateral atlanto-axial joint — The **capsular ligaments** of the atlanto-occipital and the lateral atlanto-axial joint. Anterior and lateral view to the occipital bone and C1–C3. The image demonstrates the capsular ligaments of all illustrated joints.
The atlanto-occipital and the lateral atlanto-axial joint capsules are very elastic, as they have to adapt to the large amplitude motions. However, recent studies have demonstrated the significant contribution of these capsules to the stability of the spine in the craniocervical junction region^1,2.

¹Phuntsok R, et al. The occipitoatlantal capsular ligaments are the primary stabilizers of the occipitoatlantal joint in the craniocervical junction: a finite element analysis. J Neurosurg Spine, 2019, 15:1-9.

²Phuntsok R, et al. The atlantoaxial capsular ligaments and transverse ligament are the primary stabilizers of the atlantoaxial joint in the craniocervical junction: a finite element analysis. J Neurosurg Spine, 2019, 31(4):1-7.

The superficial ligaments and membranes of the craniocervical junction: the anterior atlanto-occipital membrane & anterior atlanto-axial membrane — The **anterior atlanto-occipital** & **anterior atlanto-axial membranes**. C0–C3 complex with syndesmoses. Anterior and lateral view.

The anterior atlanto-occipital and atlanto-axial membranes are thin and elastic structures that connect the most ventral parts of the C0–C2 complex. Laterally these ligaments fuse with the capsules of the atlanto-occipital and lateral atlanto-axial joint, respectively. The anterior atlanto-occipital membrane is reinforsed by the more superfial anterior^* and lateral atlanto-occipital ligaments. The anterior atlanto-axial ligament could be considered the cranial extension of the anterior longitudinal ligament, whereas the lateral one – the homolog of the intertransverse joints.

^* Kikuta S, et al. Superficial anterior atlanto-occipital ligament: anatomy of a forgotten structure with relevance to craniocervical stability. J Craniovertebr Junction Spine, 2019, 10(1):42–45.

The list of anatomical terms also contains the posterior atlanto-occipital and atlanto-axial membranes², depicted in many anatomical atlases but lacking in this webpage. The reason for that is the recent surgical^3,4, and histological⁵ studies explicitly focused on the morphology of this region. These studies did not reveal the structures that would correspond to the classical description of these posterior membranes. Instead of the ligamentous membrane connecting bony structures, they describe the fascial connection between the deep neck muscles (mostly m. rectus capitis posterior major & minor) and the dural sac – the so-called myodural bridge⁶ – the structure involved in controlled deformation of the dural sac during neck motions^4,5, and potentially having pathophysiological and therapeutic implications^7,8.

¹ Kikuta S, et al. Superficial anterior atlanto-occipital ligament: anatomy of a forgotten structure with relevance to craniocervical stability. J Craniovertebr Junction Spine, 2019, 10(1):42–45.

² https://ta2viewer.openanatomy.org: membrana atlantooccipitalis posterior & membrana atlantoaxialis posterior

³ Pimenta N, et al. Posterior atlanto-occipital and atlanto-axial area and its surgical interest. Arq Neuropsiquiatr, 2014, 72(10):788–792.

⁴ Scali F, et al. Anatomical connection between the rectus capitis posterior major and the dura mater. Spine, 2011, 36(25):E1612–E1614.

⁵ Venne G, et al. Rectus capitis posterior minor: histological and biomechanical links to the spinal dura mater. Spine, 2017, 42(8):E466–E473.

⁶ Hack G, et al. Anatomic relation between the rectus capitis posterior minor muscle and the dura mater. Spine, 1995, 20(23):2484–2486.

⁷ Palomeque-Del-Cerro L, et al. A systematic review of the soft-tissue connections between neck muscles and dura mater: the myodural bridge. Spine, 2017, 42(1):49–54.

⁸ Alix M, Bates D. A proposed etiology of cervicogenic headache: the neurophysiologic basis and anatomic relationship between the dura mater and the rectus posterior capitis minor muscle. J Manipulative Physiol Ther, 1999, 22(8):534–539.

The cruciate, or cruciform ligament of atlas + the alar ligaments — *Lig. cruciforme atlantis & ligg. alaria*. Upper dorsal view. Posterior part of the occipital bone is removed.
The image demonstrates the two strongest¹, and therefore biomechanically essential ligaments of the craniocervical junction. The first is the symmetrical **alar ligament**, connecting an odontoid process of C2 with the medial condylar surface of the occipital bone (C0)². The second is the **transverse ligament of the atlas**, locking the dens axis against the anterior arch of C1. Together with the thin longitudinal fascicles, the transverse ligament forms the cruciform ligament of the atlas. Click an image to remove the cruciform ligament and see the dorsal surface of the dens axis covered with the cartilage. The area of the transverse ligament gliding over the odontoid process is also covered with the fibrocartilage³. In essence, the transverse ligament permits the rotation, while the alar ligaments prevent an excessive rotation of atlas⁴.

¹ White A. & Panjabi M. Clinical Biomechanics of the Spine. Lippincott Williams & Wilkins, 1990; Table 1–3, p.22

² Osmotherly P, et al. Revisiting the clinical anatomy of the alar ligament. Eur Spine J, 2013, 22:60–64.

³ Menezes A, Traynelis V. Anatomy and biomechanics of normal craniovertebral junction (a) and biomechanics of stabilization (b). Childs Nerv Syst, 2008, 24(10:1091–1100

⁴ Cattrysse E, et al. 3D morphometry of the transverse and alar ligaments in the occipito-atlanto-axial complex: an in vitro analysis. Clin Anat, 2007, 20(8):892–898.

The video clip demonstrating the function of the transverse ligament of the atlas during the C1 axial rotation.

The transverse ligament of atlas is arguably the most important ligament in the body¹. The tear of this ligament induces a significant alteration of the kinematics and load distribution within the C0–C2 complex, making virtually any head motion potentially fatal.².

¹ Offiah C, Day E. The craniocervical junction: embryology, anatomy, biomechanics and imaging in blunt trauma. Insights Imaging, 2017, 8(1):29–47.

² Mesfar W, Moglo K. Effect of the transverse ligament rupture on the biomechanics of the cervical spine under a compressive loading. Clin Biomech, 2013, 28(8):846–852.

Transverse ligament of atlas and alar ligaments in situ. Oblique posterior view — *Lig. cruciforme atlantis & ligg. alaria*. Postero-lateral view. The dorsal arch of the atlas (C1) is removed.
Please note the *lig. apicis dentis* – a short ligament connecting the tip of the odontoid process with the anterior margin of the *foramen magnum*. The ligament is of minor biomechanical importance. Still, it is worth mentioning as it is the derivate of the notochord¹ – the axial structure with the central role in embryogenesis. To note – other structures – derivates of notochord are the pulpous nuclei of intervertebral discs².

¹ Tubbs R, et al. The apical ligament: anatomy and functional significance. J Neurosurg, 2000, 92(2 suppl):197–200.

² Lawson L, Harfe B. Notochord to nucleus pulposus transition. Curr Osteoporos Rep, 2015, 13(5):336–341.

The ligaments of the craniocervical junction. View through the foramen magnum. The topography of membrana tectoria — ***Membrana Tectoria***. The top view through the *foramen magnum*. Click the image to see the cranio-cervical junctiton ligaments covered by this membrane.
*Membrana tectoria* is the broad ligamentous structure that *de facto* is the cranial extension of the posterior longitudinal ligament. Membrane is attached to the body of C2 and extends up to the basal part of the occipital bone – semicircularly around the ventral half of the foramen magnum^*.

^* Osmotherly P, Rivett D. Reconsidering the tectorial membrane: a morphological study. J Craniovertebr Junction Spine, 2020, 11(3):180–185.

Ligaments not included in the Anatomical Terminology list

Surprisingly, despite the high clinical significance of the craniocervical junction, many ligaments routinely identified here are not included in the official Anatomical Terminology list. Therefore you may not find them in the anatomical atlases that strictly follow Terminology, like Sobotta¹ or Feneis². Another consequence is the unnecessary inconsistency in the terms used to describe them. The current one and all the following illustrations in this webpage are dedicated to these ligaments.

‣ Accessory atlanto-axial ligament (possible synonyms: Y-ligament³, accessory part of the tectorial membrane⁴, atlantal-axial-occipital ligament⁵) – the symmetrical ligament located close to the lateral border of the tectorial membrane and at least partially covered with this membrane. The ligament most commonly connects the axis with both – atlas and occipital bone^3,6.
‣ Transverse occipital ligament is the small ligament connecting both occipital condyles posterosuperior to the alar ligaments. The reported prevalence of this ligament varies significantly⁷, although, most likely, it is above 50%⁸.
‣ Barkow's ligament is the thin horizontal band connecting both occipital condyles, but, in contrast with the transverse occipital ligament, running in front of the alar ligaments and odontoid process of C2⁹.