Atlas (C1) and Axis (C2)¶
Definition¶
The atlas (C1) and axis (C2) are the first two cervical vertebrae, forming the craniovertebral junction along with the occiput. They are structurally unique compared to all other vertebrae and are responsible for approximately 50% of cervical rotation (C1–C2) and 50% of cervical flexion-extension (occiput–C1). Their complex anatomy and critical biomechanical role make this region particularly important in trauma imaging.
Anatomy¶
Atlas (C1)¶
The atlas is a ring-shaped vertebra with no vertebral body and no spinous process:
- Anterior arch — short curved bar with an anterior tubercle (midline) and a posterior articular facet for the dens
- Posterior arch — larger semicircular structure with a small posterior tubercle; contains the groove for the vertebral artery on its superior surface
- Lateral masses — the thickest, most substantial portions; bear the weight of the skull
- Superior articular facets — concave, kidney-shaped; articulate with the occipital condyles (atlanto-occipital joints — primarily flexion/extension and lateral bending)
- Inferior articular facets — flat to slightly convex; articulate with C2 superior facets (atlantoaxial joints — primarily rotation)
- Transverse processes — unusually long and stout; contain transverse foramina for the vertebral arteries
Axis (C2)¶
The axis is defined by the odontoid process (dens), a tooth-like projection extending superiorly from the body:
- Dens (odontoid process) — projects superiorly to articulate with the anterior arch of C1; held in position by the transverse ligament posteriorly
- Body — larger than C1; the dens is embryologically the "body" of C1 that fused to C2 during development
- Superior articular facets — large, convex, and laterally positioned; articulate with C1 inferior facets
- Pars interarticularis — the region between the superior and inferior articular facets, a common fracture site (Hangman fracture)
- Spinous process — large and bifid
Ligamentous Stabilizers¶
The C1–C2 complex relies heavily on ligaments for stability:
| Ligament | Function |
|---|---|
| Transverse ligament | Most important stabilizer of the atlantoaxial joint; holds dens against anterior arch of C1 |
| Alar ligaments | Paired; connect the dens to the occipital condyles; limit rotation |
| Apical ligament | Connects the tip of the dens to the anterior foramen magnum |
| Tectorial membrane | Continuation of the posterior longitudinal ligament; covers the dens and transverse ligament posteriorly |
| Cruciate ligament | Composite of transverse ligament plus superior and inferior longitudinal bands |
Clinical Pearl
The transverse ligament is the most critical stabilizer at C1–C2. Rupture results in atlantoaxial instability with an atlantodental interval (the distance between the posterior surface of the anterior arch of C1 and the anterior surface of the odontoid process of C2) > 3 mm in adults. This is assessed on lateral flexion radiographs or sagittal CT/MRI. In rheumatoid arthritis, pannus formation can erode the transverse ligament, causing progressive instability.
Imaging Findings¶
Radiography¶
- Open-mouth (odontoid) view — evaluates dens integrity, C1 lateral mass symmetry, and lateral overhang
- Lateral view — measures the atlantodental interval (ADI) and posterior atlanto-dens interval (PADI)
| Measurement | Normal | Significance |
|---|---|---|
| ADI (adult) | ≤ 3 mm | > 3 mm suggests transverse ligament insufficiency |
| ADI (child) | ≤ 5 mm | Wider due to ligamentous laxity |
| PADI | ≥ 14 mm | < 14 mm associated with neurological deficit |
| Lateral mass overhang (combined) | ≤ 6.9 mm | > 6.9 mm = Jefferson fracture rule of Spence |
CT¶
CT with thin-section axial images and multiplanar reformats is the imaging standard for C1–C2 trauma:
- Fracture detection (dens fractures, C1 ring fractures, Hangman fractures)
- Assessment of articular alignment
- Measurement of lateral mass overhang
MRI¶
MRI is essential for evaluating ligamentous integrity and spinal cord compression:
- Transverse ligament — best seen on axial T2 or proton density images; appears as a dark band posterior to the dens
- Dens — T1: intermediate signal; T2: intermediate signal
- Cord compression — sagittal T2 demonstrates compression and myelopathy signal (intramedullary T2 hyperintensity within the spinal cord, representing edema, gliosis, demyelination, or myelomalacia resulting from cord injury)
Differential Diagnosis¶
| Finding | Consider |
|---|---|
| Dens fracture | Odontoid fracture (Types I, II, III) |
| C1 ring widening | Jefferson fracture (burst) |
| Widened ADI | Transverse ligament rupture, RA, os odontoideum |
| Dens aplasia/hypoplasia | Os odontoideum, congenital anomaly |
Key Points¶
- C1 is a ring without a body; C2 features the dens (odontoid process)
- The transverse ligament is the primary stabilizer at C1–C2
- ADI > 3 mm in adults suggests transverse ligament insufficiency
- The C1–C2 complex provides ~50% of cervical rotation
- CT is the primary modality for fracture evaluation; MRI for ligament and cord assessment
References¶
- Karwacki GM, Schneider JF. Normal ossification patterns of atlas and axis: a CT study. AJNR Am J Neuroradiol. 2012;33(10):1882-7. doi:10.3174/ajnr.A3105.
- Smoker WR. Craniovertebral junction: normal anatomy, craniometry, and congenital anomalies. Radiographics. 1994;14(2):255-77. doi:10.1148/radiographics.14.2.8190952.
- Riascos R, Bonfante E, Cotes C, Guirguis M, Hakimelahi R, West C. Imaging of atlanto-occipital and atlantoaxial traumatic injuries: what the radiologist needs to know. Radiographics. 2015;35(7):2121-34. doi:10.1148/rg.2015150035.
- Lustrin ES, Karakas SP, Ortiz AO, Cinnamon J, Castillo M, Vaheesan K, et al. Pediatric cervical spine: normal anatomy, variants, and trauma. Radiographics. 2003;23(3):539-60. doi:10.1148/rg.233025121.
- Morimoto LR, Kase DT, Esmanhotto PG, Maciel MA, Augusto ACL, Catricala PF, et al. Imaging assessment of nontraumatic pathologic conditions at the craniovertebral junction: a comprehensive review. Radiographics. 2024;44(5):e230137. doi:10.1148/rg.230137.
- Waxenbaum JA, Reddy V, Margetis K. Anatomy, back, cervical vertebrae. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026.
- Forbes Kaprive J, Das JM. Anatomy, head and neck: atlantoaxial joint. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023.
- Gaillard F, Jones J, et al. Atlas (C1). Radiopaedia.org. Available from: https://radiopaedia.org/articles/atlas-c1
- Gaillard F, Jones J, et al. Axis (C2). Radiopaedia.org. Available from: https://radiopaedia.org/articles/axis-c2