Table of Contents
Spine
CERVICAL SPINE
Discuss the anatomy of the atlas (C1 cervical vertebra) including the associated joints and ligaments (30 marks)
PARTS
- Anterior arch
- Paired lateral masses with their superior and inferior articular facets
- Bilateral large transverse processes with transverse foramen for vertebral arteries
- Posterior arch
- (No body – replaced by dens of C2)
- Two parts of vertebral canal: anterior 1/3 = dens, posterior 2/3 spinal cord
Anterior arch
- anterior tubercle: sits on the anterior aspect of the anterior arch and is the site of attachment of the anterior longitudinal ligament
- posterior facet for the dens: sits on the posterior aspect of the anterior arch
- upper border: attachment of the anterior atlanto-occipital membrane and lateral parts of the anterior longitudinal ligament
- lower border: attachment of the anterior atlanto-axial membrane and lateral parts of the anterior longitudinal ligament
Posterior arch
- 3/5th of circumference of the ring
- posterior tubercle: sits posteriorly to the posterior arch, is a rudimentary spinous process and attachment site for the ligamentum nuchae
- superior surface: contains paired grooves for the C1 nerve and vertebral artery, sits just posterior to the lateral mass
- superior border: attachment for the posterior atlanto-occipital membrane
- inferior border: attachment for the ligamentum flava
Lateral masses
- paired, ovoid
- superior articular facet: kidney-shaped, concave and articulates with the occipital bone
- inferior articular facet: circular, with a flat or slightly concave surface articulating with the lateral atlanto-axial joint
- medial surface: marked by vascular foramina and a tubercle for the attachment of the transverse ligament
Transverse processes
- longer than all of the transverse processes of the cervical vertebrae except C7
- typically covered by costal lamella
- transverse foramina: contains the vertebral arteries
- anterior tubercle: sometimes present on the anterior aspect of the transverse process
ACTION
- Atlantooccipital joint: 50% cervical flexion/extension and limited lateral motion
- Atlantoaxial joint: 50% cervical rotation
JOINTS
Atlantooccipital Joint (atlas & occipital)
- Inferior articular facet of occipital condyle: Oval, convex surface, projects laterally
- Superior articular facet of C1: Oval, concave anteroposteriorly, projects medially
- kidney shaped concave facets to cup occipital condyles
- allows nutation (anterior posterior flexion)
- capsule is thin and loose
Median atlantoaxial joints = 2 joint cavities – anterior and posterior (atlas & dens of C2)
- Pivot type joint between dens with ring formed by anterior arch with transverse ligament of C1
- 2x Synovial cavities between transverse ligament/odontoid and atlas/odontoid articulations
- Anterior median atlantoaxial joint – posterior surface of anterior arch concave, almost circular facet for dens
- no capsule
Lateral atlantoaxial joints (atlas & axis (C1 to C2))
- Inferior articular facet of C1: Concave, projects medially in coronal plane
- Superior articular facet of C2: Convex surface, projects laterally
- allows nutation (nodding)
*Tip: C1 superior and inferior articulations always concave + projects medially!
LIGAMENTS
From anterior to posterior: anterior atlantooccipital membrane —> odontoid ligament (apical & alar ligament) —> Cruciate ligament (transverse + craniocaudal) —> Tectorial membrane —> Posterior Atlantooccipital membrane —> Ligamentum nuchae
- Anterior atlantooccipital membrane
- Connects anterior arch of C1 with anterior outer margin of foramen magnum.
- Continues inferiorly (from median cord) as anterior longitudinal ligament
- Odontoid ligaments
- Apical ligament: Small fibrous band extending from dens tip to basion
- Alar ligaments: Thick, horizontally directed ligaments extending from lateral surface of dens tip to anteromedial occipital condyles
- Cruciate ligament
- Transverse ligament: Strong horizontal component between lateral masses of C1, passes behind dens
- Craniocaudal component: Fibrous band running from transverse ligament superiorly to foramen magnum and inferiorly to C2
- Tectorial membrane:
- Continuation of posterior longitudinal ligament; attaches to anterior rim of foramen magnum (posterior clivus)
- Posterior atlantooccipital membrane
- Posterior arch C1 to posterior margin of foramen magnum
- Deficit laterally to allow C1 nerve passage and vertebral artery enters on superior surface of C1 (variant – if lateral opening calcifies “arcuate foramen”)
- Innervated by C1
- Ligamentum nuchae
- Attaches to the posterior tubercle which is a rudimentary spinal process
- Attaches to spinous process of cervical vertebrae and occiput
- Limits forward lesion
MUSCULAR ATTACHMENTS to C1
- Anterior:
- Longus colli – superior oblique parts attaches to anterior surface of C1 body (anterior)
- Rectus capitus anterior – attaches to anterior surface of the lateral mass of C1 (anterior)
- Lateral:
- Rectus capitus laterus – attached to transverse process of C1 superiorly – up to skull (lateral)
- levator scapulae – tip of transverse process of C1 (lateral)
- Posterior:
- Rectus capitus posterior minor – attaches to posterior tubercle of C1 arch (Posterior)
- splenius cervicis – transverse process of C1 (posterior)
- Obliquus capitus superior – attached to transverse process of C1 (posterior)
- Obliquus capitus inferior – attached to apex transverse process of C1 (posterior)
VARIANTS
- unfused arch of atlas : anterior, posterior, both
- central part of the posterior arch can be absent and replaced by fibrous tissue
- arcuate foramen: calcification of the posterior atlanto-occipital membrane
- superior articular facets of the lateral masses can be divided into two parts, with the anterior part being larger and the posterior smaller
- fusion (with the occiput – partially or fully)
- accessory ossicle of the anterior arch of the atlas
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