Residency · Residency · Neurosurgery

Occipitocervical and Atlantoaxial Pathology

Overview

The craniovertebral junction comprises the occiput, atlas (C1), and axis (C2), forming one of the most biomechanically complex regions of the spine. Its unique anatomy, which lacks intervertebral discs and relies on specialized articulations, permits approximately 50 percent of total cervical rotation and significant flexion-extension. Pathology affecting this region includes traumatic, degenerative, inflammatory, congenital, and neoplastic conditions. The critical proximity of the vertebral arteries, brainstem, upper cervical spinal cord, and lower cranial nerves makes pathology here particularly dangerous and surgical intervention technically demanding.

Anatomy

Occiput

The foramen magnum has an anteroposterior diameter of approximately 35 millimeters and a transverse diameter of approximately 30 millimeters. The occipital condyles present convex articular surfaces that articulate with the C1 lateral masses. The hypoglossal canal, which transmits cranial nerve XII, passes just above the occipital condyle.

Atlas (C1)

The atlas is a ring-shaped vertebra unique in having no vertebral body and no spinous process. Its lateral masses bear articular surfaces that are concave superiorly for articulation with the occipital condyles and flat to convex inferiorly for articulation with C2. The anterior arch articulates with the dens of C2, and the transverse ligament attaches to its inner tubercles. The posterior arch bears a groove on its superior surface through which the vertebral artery courses medially before entering the foramen magnum. The transverse foramen transmits the vertebral artery. C1 has no pedicles in the traditional sense; the lateral mass serves as the primary fixation point for instrumentation.

Axis (C2)

The axis is distinguished by the odontoid process or dens, which projects superiorly from the body. Embryologically, the dens represents the C1 vertebral body that fused to C2 during development. The transverse ligament is the primary stabilizer of the dens, supplemented by the alar ligaments. The pars interarticularis, the isthmus of bone between the superior and inferior articular surfaces, is the site of hangman's fractures. The vertebral artery takes a lateral course through the C2 transverse foramen, making C2 pedicle and pars screw placement technically demanding. C2 pedicle screws pass through the pars and into the body medial to the vertebral artery.

Ligamentous Structures

The transverse ligament is the primary stabilizer of the atlantoaxial joint, holding the dens against the anterior arch of C1; its rupture allows more than 3 millimeters of atlantodental interval. The alar ligaments connect the tip of the dens to the medial occipital condyles and limit rotation. The tectorial membrane is the cranial continuation of the posterior longitudinal ligament, extending from the posterior body of C2 to the anterior margin of the foramen magnum. The cruciform ligament consists of the transverse ligament plus superior and inferior longitudinal bands. The apical ligament connects the dens tip to the basion and serves as a minor stabilizer.

Atlantodental Interval

The atlantodental interval in normal adults measures less than 3 millimeters and in children less than 5 millimeters due to physiologic ligamentous laxity. An ADI exceeding 3 millimeters in adults suggests transverse ligament incompetence. The Space Available for the Cord equals the canal diameter minus the ADI; when the SAC falls below 14 millimeters, it correlates with myelopathy risk.

Traumatic Conditions

Occipital Condyle Fractures

The Anderson and Montesano classification divides these into three types. Type I is a comminution fracture from axial loading and is stable. Type II represents a basilar skull fracture extending into the condyle and is also stable. Type III is an avulsion fracture caused by the alar ligament and is potentially unstable, with bilateral Type III fractures being highly unstable. These fractures are often missed on plain radiographs; CT is diagnostic. Treatment involves a rigid collar for stable fractures and occipitocervical fusion for unstable Type III injuries.

Atlanto-Occipital Dislocation

Atlanto-occipital dislocation results from disruption of the ligaments between the occiput and C1. Historically considered universally fatal, it is increasingly recognized in survivors due to improved prehospital immobilization and resuscitation. The dislocation is classified as anterior (most common), posterior, or longitudinal (distraction).

Diagnosis requires CT with specific measurements: the basion-dens interval exceeding 12 millimeters is abnormal, and the condyle-C1 interval exceeding 2 millimeters is the most sensitive indicator. MRI demonstrates ligamentous injury. Treatment is occipitocervical fusion. Halo immobilization and traction are dangerous and contraindicated because they can cause further distraction of an already disrupted craniovertebral junction. This injury is associated with pediatric trauma due to the large head size, horizontal occipital condyles, and ligamentous laxity in children.

Atlas Fractures (Jefferson Fracture)

Axial loading causes a burst of the C1 ring with bilateral lateral mass fractures. The classic Jefferson fracture is a four-part fracture with bilateral anterior and posterior arch breaks. The Rule of Spence states that lateral mass overhang exceeding 7 millimeters total on open-mouth odontoid view or CT suggests transverse ligament rupture. MRI is used to directly assess transverse ligament integrity.

Treatment depends on ligament status. With an intact transverse ligament, a rigid cervical collar for 8 to 12 weeks suffices. With a ruptured transverse ligament, C1-C2 fusion or occipitocervical fusion is required. Isolated posterior arch fractures are managed with a cervical collar alone.

Odontoid Fractures

Odontoid fractures are the most common cervical fracture in the elderly, typically resulting from falls. The Anderson and D'Alonzo classification defines three types. Type I is an avulsion of the dens tip and is rare; its presence should prompt evaluation for atlanto-occipital dislocation. Type II is a fracture at the base of the dens and is the most common type, carrying the highest nonunion rate ranging from 15 to 85 percent depending on risk factors. Type III extends into the cancellous body of C2 and heals well with immobilization.

The Grauer modification subdivides Type II fractures into IIA (transverse), IIB (anterior-superior to posterior-inferior obliquity), and IIC (comminuted). Treatment of Type II fractures depends on patient and fracture factors. Non-operative management with a rigid collar or halo is appropriate for patients under 50 with non-displaced fractures and anterior oblique fracture lines. Anterior odontoid screw fixation preserves C1-C2 rotation and is ideal for acute transverse or anterior-oblique Type II fractures with an intact transverse ligament; it is contraindicated for comminuted or posterior-oblique fractures. Posterior C1-C2 fusion is the gold standard for displaced or unstable Type II fractures, elderly patients, comminuted fractures, or failed non-operative management. In patients over 65 to 70, Type II fractures carry high nonunion rates with conservative treatment, and surgical fixation is increasingly recommended despite surgical risk.

Hangman's Fracture

Hangman's fracture is a bilateral fracture of the C2 pars interarticularis resulting from hyperextension and axial loading, also termed traumatic spondylolisthesis of C2. The Levine and Edwards classification guides management. Type I shows less than 3 millimeters of displacement with no angulation, is stable, and is treated with a rigid collar. Type II demonstrates more than 3 millimeters of displacement with angulation and an abnormal C2-C3 disc; treatment is halo vest or surgery. Type IIA is primarily a flexion-distraction injury with significant angulation but minimal translation; traction is contraindicated because it causes further distraction, and posterior fusion is indicated. Type III involves C2-C3 facet dislocation, is highly unstable, and requires surgical fixation.

Paradoxically, the spinal canal is often widened by the fracture pattern because the bilateral pars fractures effectively decompress the canal, making neurological injury uncommon despite the dramatic radiographic appearance.

Atlantoaxial Rotatory Subluxation

This condition is more common in children and can occur after pharyngitis (Grisel syndrome), trauma, or minor upper respiratory infection. The Fielding classification categorizes Types I through IV based on degree of rotatory displacement. Patients present with torticollis in the "cock-robin" position with the head tilted and rotated. Diagnosis requires dynamic CT with the head rotated to each side, demonstrating that C1 rotates with C2 instead of independently. Treatment depends on duration: soft collar for acute presentations less than one week, halo traction for subacute cases, and C1-C2 fusion for chronic cases lasting more than three months or recurrent episodes.

Non-Traumatic Conditions

Rheumatoid Arthritis

Rheumatoid arthritis causes pannus formation and ligamentous laxity leading to atlantoaxial instability with an ADI exceeding 3 millimeters. The condition progresses from atlantoaxial subluxation to cranial settling (basilar invagination) to subaxial subluxation. Surgical indications include myelopathy, ADI greater than 8 to 9 millimeters, posterior atlantodental interval less than 14 millimeters, and brainstem compression. Treatment is C1-C2 fusion using the Harms or Goel technique, or occipitocervical fusion when cranial settling is present.

Basilar Invagination

Basilar invagination occurs when the odontoid migrates superiorly through the foramen magnum. It may be primary, arising from congenital conditions such as os odontoideum, Klippel-Feil syndrome, or Down syndrome, or secondary to rheumatoid arthritis, Paget disease, or osteomalacia. Chamberlain's line, drawn from the posterior hard palate to the opisthion, provides a radiographic benchmark; the odontoid tip projecting more than 3 millimeters above this line is abnormal. The condition causes brainstem and cervicomedullary compression. Treatment options include transoral or endoscopic endonasal odontoidectomy for ventral compression combined with posterior fusion, though posterior-only reduction with distraction-fixation is increasingly favored.

Chiari Malformation Type I

In adults, Chiari I malformation is defined as cerebellar tonsillar herniation greater than 5 millimeters below the foramen magnum. Symptoms include suboccipital headache worsened by Valsalva maneuvers, neck pain, sensory changes, and coordination difficulties. Syringomyelia is associated in 30 to 70 percent of cases. Treatment is posterior fossa decompression consisting of suboccipital craniectomy and C1 laminectomy with or without duraplasty.

Os Odontoideum

Os odontoideum is a separate ossicle above a hypoplastic dens, either congenital or representing an old non-united fracture. It may be orthotopic, in the normal dens position, or dystopic, fused to the clivus or anterior ring of C1. It can cause atlantoaxial instability and myelopathy. Treatment is C1-C2 fusion when unstable or symptomatic.

Surgical Fixation Techniques

Posterior C1-C2 Fusion (Harms/Goel Technique)

This technique uses C1 lateral mass screws connected by rods to C2 pedicle or pars screws and represents the gold standard for atlantoaxial instability, achieving fusion rates exceeding 95 percent. C2 pedicle screws enter at the superolateral quadrant of the C2 pars with a trajectory directed medially and cephalad, carrying a 2 to 4 percent risk of vertebral artery injury. C2 pars screws are shorter and directed more superiorly, used when the pedicle is too narrow. C1 lateral mass screws enter at the junction of the C1 posterior arch and lateral mass with a trajectory directed anteriorly, medially, and slightly cephalad toward the anterior tubercle of C1. Preoperative CT angiography is mandatory to assess vertebral artery anatomy and detect anomalous courses, as a high-riding vertebral artery occurs in up to 20 percent of patients.

Posterior C1-C2 Wiring Techniques

Historical wiring techniques include the Gallie fusion using a single midline sublaminar wire with iliac crest bone graft, which poorly resists flexion, and the Brooks fusion using two bilateral sublaminar wires with bilateral grafts providing better rotational control. These have been largely replaced by screw-rod constructs that offer superior biomechanics and higher fusion rates.

Occipitocervical Fusion

Occipitocervical fusion is indicated when C1 lateral masses are fractured, destroyed, or insufficient; when cranial settling is present; or for atlanto-occipital dislocation. An occiput plate with screws placed into the keel of the occiput at the midline, where bone is thickest, is connected to subaxial lateral mass or pedicle screws. This procedure sacrifices O-C1 motion, approximately 25 degrees of flexion-extension, which carries significant functional impact. The fusion extends to C2 at minimum and may incorporate subaxial levels depending on pathology.

Anterior Odontoid Screw Fixation

A single or double lag screw is placed from the anterior inferior body of C2 through the fracture line into the dens tip, preserving C1-C2 rotation. Indications include acute Type II odontoid fractures with transverse or anterior-oblique fracture lines and an intact transverse ligament. Contraindications include comminuted fractures, posterior-oblique fracture lines, barrel chest limiting surgical trajectory, chronic fractures with established nonunion, and transverse ligament rupture. The technique uses a Smith-Robinson anterior approach with guidewire placement under biplanar fluoroscopy followed by cannulated screw insertion.

Transoral and Endonasal Odontoidectomy

These approaches address irreducible ventral brainstem compression from basilar invagination or rheumatoid pannus. The transoral approach involves mouth retraction, posterior pharyngeal mucosa incision, C1 anterior arch removal, and dens resection. The endoscopic endonasal approach is gaining popularity due to less pharyngeal morbidity. Both must always be combined with posterior fusion because the anterior decompression creates further instability.

<image>Sagittal CT reconstruction of the craniovertebral junction demonstrating a Type II odontoid fracture with posterior displacement of the dens and narrowing of the spinal canal at the C1-C2 level, with the fracture line clearly visible at the base of the odontoid process</image>

<image>Three-dimensional CT reconstruction of the craniovertebral junction showing posterior C1-C2 screw-rod fixation (Harms-Goel technique) with bilateral C1 lateral mass screws and C2 pedicle screws connected by contoured rods, demonstrating the anatomical relationships between the screws, the vertebral arteries in the C1-C2 groove, and the posterior arch of C1</image>

<image>Sagittal T2-weighted MRI of the craniovertebral junction in a patient with rheumatoid arthritis showing pannus formation around the odontoid process, atlantoaxial subluxation with increased atlantodental interval, and compression of the cervicomedullary junction with T2 signal change indicating myelopathy</image>

<image>Axial CT scan at the C2 level demonstrating bilateral pars interarticularis fractures (hangman's fracture) with anterior displacement of the C2 vertebral body relative to the posterior elements, widening the spinal canal at this level</image>

Clinical Pearls

In atlanto-occipital dislocation, traction must never be applied; this can be fatal by causing further distraction of an already disrupted craniovertebral junction. For C2 pedicle screw placement, preoperative CT angiography is always required to assess vertebral artery anatomy; a high-riding vertebral artery or anomalous course may preclude C2 pedicle screws on that side, necessitating pars or translaminar screws instead. Type II odontoid fractures in the elderly are often called the "hip fracture of the spine" because they carry significant morbidity and mortality regardless of treatment strategy; early surgical fixation may reduce mortality compared to halo immobilization in patients over 65. The Rule of Spence, indicating more than 7 millimeters of total C1 lateral mass overhang, is a rough indicator of transverse ligament rupture in Jefferson fractures, but MRI provides more reliable direct assessment of ligament integrity. Hangman's fracture Type IIA is a flexion-distraction injury where traction worsens displacement; recognizing the pattern of angulation without translation is essential to avoiding this dangerous error. Posterior C1-C2 fusion using the Harms/Goel technique has become the gold standard for atlantoaxial instability due to its high fusion rate and avoidance of sublaminar wires. In rheumatoid arthritis, the posterior atlantodental interval less than 14 millimeters is a better predictor of neurological deficit than ADI alone.

References

1. Harms J, Melcher RP. Posterior C1-C2 fusion with polyaxial screw and rod fixation. Spine. 2001;26(22):2467-2471.
2. Anderson LD, D'Alonzo RT. Fractures of the odontoid process of the axis. J Bone Joint Surg Am. 1974;56(8):1663-1674.
3. Levine AM, Edwards CC. The management of traumatic spondylolisthesis of the axis. J Bone Joint Surg Am. 1985;67(2):217-226.
4. Goel A, Laheri V. Plate and screw fixation for atlanto-axial subluxation. Acta Neurochir. 1994;129(1-2):47-53.
5. Patel AJ et al. Management of odontoid fractures in the elderly. Neurosurgery. 2010;66(3 Suppl):A112-A118.

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