Anatomy of Atlas and Axis Vertebrae

Atlas (C1)

• Dens has 2 facets laterally and 1 medially with an anterior arch, stabilized by the transverse ligament to control anterior displacement and allow nodding
• Provides a cartilaginous surface, along with the alar ligament, to limit flexion and extension, enabling right rotation to occur with left lateral facet sliding anteriorly and right lateral facet sliding posteriorly

Axis (C2-C7)

• Vertebrae are 50% wider than they are deep
• Transverse process contains a foramen for the vertebral artery, vein, and plexus, and a groove for the spinal nerve
• Facet orientation is approximately 45 degrees in the transverse plane with loose capsules, allowing for motion in all planes and more rotation and lateral flexion than other regions

Clinical Pearls

• The cervical IVD develops distinctly differently from the lumbar disk
• The orientation of the zygapophyseal joint planes can be thought of as passing through a patient's nose
• Uncovertebral joints guide cervical flexion and extension, reduce side bending of the cervical spine, prevent posterior translation of neighboring vertebrae, and reinforce the posterolateral aspect of the IVD
• Degeneration or degradation of the IVD can lead to repeated contact between the bony surfaces of the uncovertebral joint, resulting in hypertrophic changes in the bone

Ligaments

• Long (continuous): posterior longitudinal ligament and anterior longitudinal ligament
• Short (segmental): interspinous, intertransversarius, ligamentum flavum, and interlaminar
• Important for maintaining head position during locomotion and stability of the vertebral column

Atlanto-Occipital Joint

• Total range of flexion and extension is 20 degrees
• Important for maintenance of stability of the vertebral column

Atlanto-Axial Joint

• 50% of cervical rotation occurs at this level
• A loose fibrous capsule encloses the joints at the articular margins

Blood Supply

• Vertebral artery arises from the subclavian artery and ascends via transverse foramina of C6-C1
• Supplies the ipsilateral upper cervical spinal cord

Muscles

• Lateral and posterior cervical spine muscles: upper trapezius, SCM, scalenes, splenius capitis and cervicis
• Anterior cervical spine muscles: semispinalis capitis, deep muscles
• Clinical pearls:
  • Levator scapulae imbalance can cause contralateral suboccipital muscle spasms and headaches
  • Longus colli and longus capitis play an important role in cervical stabilization
  • Referred symptoms from cervical spine can occur in the upper extremities, thoracic spine, scapula, and occasionally the upper chest

Test your knowledge on the anatomy of the Atlas (C1) and Axis (C2) vertebrae, including the dens, transverse ligament, facets, and their role in neck rotation and stability.