Dr. Zeeshan Sardar, MD, MSc, F.R.C.S.C
Co-Chief of Spinal Deformity Surgery • Director, Quality & Patient Safety • Medical Director, Spine Unit
Och Spine Hospital at NewYork-Presbyterian / Columbia University • New York, NY
The junction between the first and second cervical vertebrae — C1 (the atlas) and C2 (the axis) — is the most mechanically complex region of the spine. It is responsible for approximately 50 percent of the head’s rotational movement, and it sits directly adjacent to the brainstem and the upper spinal cord. When the structural integrity of this junction is compromised — by disease, injury, or congenital anomaly — the consequences can range from chronic neck pain and occipital headache to catastrophic spinal cord injury from a seemingly minor movement or fall.
C1–C2 instability is not common, but it is serious, and it is frequently underdiagnosed. Patients are sometimes managed for years for occipital headaches, neck pain, or vague upper extremity symptoms before the underlying instability is recognized. This page is intended to help patients, families, and referring clinicians understand when C1–C2 instability should be considered, how it is evaluated, and what surgical treatment involves.
THE ANATOMY OF C1–C2 AND WHY IT IS UNIQUELY VULNERABLE
C1 (the atlas) is a ring-shaped bone without a vertebral body. It sits at the top of the cervical spine, balanced on C2 (the axis), which has a bony projection called the odontoid process (or dens) that extends upward through the ring of C1. The head effectively rotates around this peg. The entire construct is held together not by bony interlocking — as in the subaxial cervical spine — but almost entirely by ligaments: the transverse atlantal ligament, the alar ligaments, the apical ligament, and the tectorial membrane.
This ligamentous dependence is what makes C1–C2 uniquely vulnerable. Any condition that damages, stretches, erodes, or severs these ligaments — or that directly damages the odontoid process itself — can produce abnormal, potentially dangerous motion between C1 and C2. When C1 slides forward on C2 with neck flexion (anterior atlantoaxial subluxation), the space available for the spinal cord in the C1 ring is dramatically reduced, and cord compression can occur. When the odontoid process migrates upward toward the foramen magnum (as in cranial settling), the brainstem itself may be compressed.
CAUSES OF C1–C2 INSTABILITY
Rheumatoid Arthritis
The most common cause of C1–C2 instability in adults. Synovial inflammation erodes the transverse ligament and the odontoid process itself, allowing progressive anterior subluxation of C1 on C2, and can permit the odontoid to migrate upward toward the foramen magnum (cranial settling or basilar invagination). The degree of instability correlates with disease duration and prior disease control. A full discussion of rheumatoid arthritis and the spine, including management principles specific to that condition, is available on the dedicated page.
Os Odontoideum
Os odontoideum is a condition in which the odontoid process fails to fuse to the body of C2, existing instead as a separate ossicle (a small free-floating bone). Whether congenital or the result of an unrecognized childhood injury, it leaves the atlantoaxial joint without its central stabilizing post, resulting in potentially severe C1–C2 instability. Os odontoideum is frequently discovered incidentally on imaging obtained for unrelated reasons — or it may be the source of longstanding, previously unexplained neck pain and neurological symptoms. It is one of the most surgically important incidental findings in the cervical spine.
Odontoid Fractures
Fractures of the odontoid process — particularly Type II fractures at the base of the dens — are among the most common cervical fractures in elderly patients following low-energy falls. Many heal with external immobilization; some require surgical stabilization, either with direct odontoid screw fixation (when the fracture geometry allows) or C1–C2 posterior fusion when it does not. Inadequately treated odontoid fractures that progress to nonunion represent a source of chronic C1–C2 instability that may present months or years after the original injury.
Down Syndrome
Atlantoaxial instability occurs in approximately 10 to 20 percent of individuals with Down syndrome, due to ligamentous laxity affecting the transverse atlantal ligament. Most remain asymptomatic and can be managed with periodic imaging surveillance. Surgery is reserved for symptomatic patients — those with myelopathy, progressive neurological findings, or imaging measurements that place the cord at high risk.
Traumatic Ligamentous Disruption
High-energy trauma — motor vehicle collisions, falls from height, sports injuries — can disrupt the transverse atlantal ligament or avulse its bony attachment without producing a visible fracture, resulting in purely ligamentous C1–C2 instability. These injuries require careful MRI evaluation to identify the ligamentous disruption; plain films and CT may appear reassuringly normal in a patient with significant instability.
Inflammatory and Other Systemic Conditions
Ankylosing spondylitis, psoriatic arthritis, calcium pyrophosphate deposition disease (crowned dens syndrome), and other inflammatory conditions can involve the atlantoaxial complex, producing instability, erosion, or pseudotumoral pannus formation. Each carries its own specific perioperative considerations, particularly regarding immunosuppressive medication management and bone quality.
SYMPTOMS AND WHEN TO SEEK EVALUATION
C1–C2 instability can present across a wide spectrum, from asymptomatic imaging findings to acute neurological emergency. The most common presentations are:
- Occipital headache — pain at the back and top of the skull, often radiating to the retroorbital region; caused by compression or irritation of the C2 nerve root (the greater occipital nerve), a characteristic symptom of C1–C2 pathology
- Neck pain with flexion — pain or a sense of instability or mechanical clunking with forward neck flexion; in severe instability some patients learn to hold their head in slight extension to reduce cord compression
- Myelopathic symptoms — hand clumsiness or weakness, gait instability, balance problems, or Lhermitte’s sign (electric shock sensation with neck flexion); indicates spinal cord compression and requires urgent evaluation
- Brainstem or lower cranial nerve symptoms — difficulty swallowing, hoarseness, double vision, facial numbness; suggests the odontoid has migrated toward or into the foramen magnum (cranial settling) and is compressing the brainstem
- Incidental imaging finding — os odontoideum or odontoid fracture nonunion discovered on imaging obtained for another reason; requires evaluation even without symptoms given the risk of sudden catastrophic injury
DIAGNOSIS AND IMAGING
Flexion-Extension X-Rays
Dynamic radiographs in maximal (comfortable) flexion and extension are the essential first step in quantifying C1–C2 instability. The atlanto-dens interval (ADI) — the distance between the posterior surface of the anterior C1 arch and the anterior surface of the odontoid — is measured in both positions. In adults, an ADI greater than 3 mm is abnormal; greater than 9 to 10 mm indicates severe instability with significant neurological risk. The space available for the cord (SAC) — the distance from the posterior odontoid to the posterior C1 ring — is equally important; a SAC below 13 mm correlates with increased risk of neurological injury.
MRI
MRI is essential for evaluating the degree of cord compression, assessing for cord signal change (T2 signal change within the cord indicates established myelopathy), characterizing the soft tissue components of the instability (pannus, ligamentous disruption, os odontoideum morphology), and evaluating the brainstem when cranial settling is suspected. An MRI that shows cord signal change indicates established neurological injury and generally argues for surgical intervention without delay.
CT Scan
CT provides the bony detail essential for surgical planning: pedicle and lateral mass dimensions at C1 and C2, the morphology of the odontoid or os odontoideum, and the relationship of the vertebral arteries to the planned screw trajectories. CT angiography may be added when the vertebral arteries are known or suspected to be in an anomalous position.
SURGICAL TREATMENT
Posterior C1–C2 Fusion (Harms Technique)
For the large majority of C1–C2 instability cases — regardless of cause — posterior C1–C2 fusion is the standard surgical treatment. The Harms technique, which uses polyaxial screws placed into the C1 lateral masses and C2 pedicles connected by rods, has become the preferred method at high-volume centers for several reasons: it provides rigid three-column fixation, it does not require the odontoid to be intact (unlike transarticular screw techniques), it accommodates reducible and irreducible subluxation, and it has a high published fusion rate with well-established safety in experienced hands.
The procedure is performed prone under general anesthesia. After careful positioning — with fluoroscopic or CT-based confirmation that the C1–C2 alignment is acceptable before fixation is applied — the C1 lateral masses and C2 pedicles are exposed, screws are placed under direct visualization and navigation guidance, rods connect the construct, and bone graft is placed to achieve solid bony fusion across the atlantoaxial joint. Robotic navigation supports screw trajectory planning in this area, where the vertebral arteries and spinal cord provide almost no margin for error. Continuous intraoperative neuromonitoring is non-negotiable throughout the procedure.
Irreducible Instability and Anterior Decompression
For patients with irreducible C1–C2 instability — where the subluxation or cranial settling cannot be corrected by traction or positioning and the odontoid is causing direct anterior cord or brainstem compression — additional anterior decompression may be required before or alongside the posterior fusion. Transoral odontoidectomy (removal of the odontoid through the mouth) or endoscopic transnasal odontoidectomy are the approaches most commonly used. These procedures are performed in centers with specific craniocervical surgery expertise and require close collaboration with neurotology and anesthesia teams experienced in airway management at the skull base.
Occipitocervical Fusion
When instability extends beyond the atlantoaxial joint to include the craniocervical junction itself — as in severe cranial settling, occipitocervical instability from trauma, or patients with conditions affecting the occipitoatlantal joint — the fusion construct must be extended to incorporate the occiput. Occipitocervical fusion from the skull base to C2 (or further into the subaxial spine when there is coexisting subaxial instability) eliminates motion at both the craniocervical junction and the atlantoaxial joint. Planning for craniocervical alignment in these cases is critical — the angle at which the head is fixed to the neck permanently affects the patient’s ability to look forward comfortably, to swallow, and to lie flat.
When Non-Surgical Management Is Appropriate
Not every patient with C1–C2 instability requires surgery. Asymptomatic atlantoaxial instability with preserved cord space — such as the mild instability seen in some patients with Down syndrome — is appropriately managed with periodic imaging surveillance, activity counseling (particularly regarding contact sports and activities involving sudden neck loading), and close monitoring for any new neurological symptoms. The threshold for surgical intervention in asymptomatic patients depends on the ADI, the SAC, the specific cause, and the patient’s overall health and lifestyle.
WHAT MAKES THIS SURGERY DEMANDING
C1–C2 fusion is performed in an anatomical zone where the margin for error is genuinely small. The C2 pedicles are immediately adjacent to the vertebral arteries; the C1 lateral mass screws must navigate around the C2 nerve root and the venous plexus overlying the lateral mass; the proximity to the spinal cord and brainstem means that any change in neurological monitoring during the procedure requires an immediate response. These are not theoretical concerns — they are the daily reality of operating in this region, and they are why volume and experience matter.
Surgeons who perform C1–C2 fusion regularly — within a high-volume upper cervical or complex cervical surgery practice — are intimately familiar with the anatomical variants that must be anticipated and the intraoperative decision-making that complex cases require. Surgeons who encounter this procedure occasionally are not working from the same foundation, even if technically competent in the subaxial cervical spine.
At NewYork-Presbyterian / Columbia University, C1–C2 and upper cervical surgery is performed within a practice that includes complex cervical reconstruction, rheumatoid cervical spine disease, spinal deformity involving the craniocervical junction, and revision upper cervical surgery. The infrastructure supporting these cases — robotic navigation, continuous four-limb neuromonitoring, experienced craniocervical anesthesia teams, and a dedicated intensive care unit — is present and available as standard.
FREQUENTLY ASKED QUESTIONS
How will losing rotation at C1–C2 affect my daily life?
C1–C2 is responsible for approximately 50 percent of normal neck rotation. After fusion, this rotation is eliminated and cannot be recovered. In practice, most patients adapt well over time — the subaxial cervical spine (C2 through C7) continues to provide meaningful rotation, and the brain compensates for the loss. Activities like driving require greater head-shoulder movement to check blind spots. Most patients report that while the limitation is real, it is substantially less disabling than the instability it replaces.
What is os odontoideum and how serious is it?
Os odontoideum is a condition in which the odontoid process of C2 exists as a separate, free-floating bone rather than fusing to the C2 body as it normally does. It leaves the atlantoaxial joint without its central stabilizing post, creating potentially severe instability. When discovered, it always requires expert evaluation to assess the degree of instability and neurological risk, even in the absence of symptoms, because a sudden fall or impact can produce catastrophic cord injury in a patient who was neurologically intact moments before.
I have been having occipital headaches for years. Could this be C1–C2?
C2 nerve root compression or irritation from C1–C2 pathology is a recognized cause of occipital neuralgia — pain at the back of the skull, behind the eyes, and along the scalp. If occipital headaches have not responded to standard treatment, and particularly if they coexist with any risk factor for atlantoaxial instability (rheumatoid arthritis, prior neck injury, Down syndrome, or known congenital anomaly), C1–C2 pathology deserves evaluation with flexion-extension X-rays and MRI before attribution to a more benign cause.
Is surgery the only option?
For patients with significant myelopathy, imaging-documented severe instability (ADI greater than 9 mm or SAC below 13 mm), or cord signal change on MRI, surgery is generally the appropriate course — the risk of delaying surgery exceeds the risk of proceeding in most of these situations. For patients with milder instability and no neurological deficit, observation with careful monitoring is a reasonable option, depending on the underlying cause, degree of instability, and patient-specific factors.
How long is the recovery after C1–C2 fusion?
Hospital stay is typically 2 to 4 days. A cervical collar is worn for 6 to 12 weeks to protect the fusion during the early healing period. Most patients return to light activity within 4 to 6 weeks and to full activity once fusion is confirmed on imaging, typically at 3 to 6 months. The rotational limitation at the neck is immediately present and permanent; adaptation to it occurs gradually over the weeks and months following surgery.
Should I get a second opinion before C1–C2 fusion?
Yes. C1–C2 fusion is an uncommon procedure with permanent consequences for neck mobility, performed in a high-risk anatomical zone. A second opinion from a surgeon with specific, regular experience in upper cervical surgery is appropriate before committing to any course of action — whether the recommendation is surgery or continued observation.
RELATED CONDITIONS & PROCEDURES
- Rheumatoid Arthritis & the Spine — the most common cause of C1–C2 instability in adults; atlantoaxial instability, cranial settling, and subaxial subluxation
- Cervical Myelopathy — spinal cord compression causing progressive neurological deficits; C1–C2 instability is an important and underrecognized cause
- Posterior Cervical Laminectomy and Fusion — when C1–C2 instability coexists with subaxial cervical instability, the fusion is extended to include all affected levels
- Robotic Spine Surgery — precision navigation is essential in C1–C2 surgery where vertebral arteries and the spinal cord leave almost no margin for error
- Intraoperative Neuromonitoring — continuous four-limb monitoring throughout all upper cervical procedures at this practice
- Spinal Disorders in Achondroplasia — the small foramen magnum and narrow cervicomedullary junction in achondroplasia creates related upper cervical compression that may require surgical management
- Second Opinion for Complex Spine Surgery — upper cervical surgery is among the procedures most appropriate for specialist second opinion review
About Dr. Zeeshan Sardar
Dr. Sardar is Co-Chief of Spinal Deformity Surgery at NewYork-Presbyterian / Columbia University and evaluates patients with C1–C2 instability from rheumatoid arthritis, trauma, congenital anomalies, and other causes. To schedule a consultation or request a second opinion, call 212-932-5187 or visit the contact page. Telemedicine consultations available in NY, NJ, CT, FL, PA, MO, CA, and TX.
This page is for educational purposes only and does not constitute individualized medical advice. Please consult a qualified spine specialist to discuss your specific condition.
Reviewed by Dr. Zeeshan Sardar, MD, MSc, F.R.C.S.C — Last reviewed: July 2026.