Overview
Os odontoideum (OO) is a congenital anomaly characterized by an accessory ossicle located between the odontoid process and the anterior arch of the atlas, often leading to atlantoaxial instability (AAI) and displacement. This condition is clinically significant due to its potential to cause spinal cord compression, leading to symptoms such as neck pain, neurological deficits, and in severe cases, myelopathy. It predominantly affects children and young adults but can occur at any age. Early recognition and management are crucial as delayed treatment can result in irreversible neurological damage. Understanding OO is essential for clinicians to promptly diagnose and intervene, preventing long-term complications and improving patient outcomes 1.Pathophysiology
Os odontoideum arises from an incomplete ossification of the cartilaginous odontoid process during fetal development, leading to the formation of a separate ossicle. This anomaly disrupts the normal stabilization provided by the transverse ligament of the atlas, predisposing the atlantoaxial joint to instability and displacement. The pathophysiology involves a complex interplay between the structural weakness at the OO site and compensatory mechanisms of adjacent cervical spine segments. Specifically, the axial superior facet slope (ASFS) plays a critical role in determining the direction of atlantoaxial displacement—whether anterior or posterior—due to its influence on the mechanics of the atlantooccipital joint and subaxial cervical spine. Compensatory changes in these regions can further exacerbate instability and clinical symptoms 1.Epidemiology
The exact incidence and prevalence of os odontoideum are not well-documented, but it is considered relatively rare. Studies suggest that OO is more commonly identified in pediatric populations, with a reported prevalence ranging from 0.5% to 2% in patients with cervical spine abnormalities 1. There is no significant sex predilection noted in most studies, though some reports suggest a slight male predominance. Geographic distribution does not appear to show specific patterns, but certain populations may have higher incidental detection rates due to enhanced imaging capabilities and clinical vigilance. Trends over time indicate an increasing recognition with advancements in imaging techniques, particularly MRI and CT scans, which have improved diagnostic accuracy 1.Clinical Presentation
Patients with os odontoideum often present with nonspecific symptoms such as neck pain, stiffness, and neurological deficits depending on the severity of atlantoaxial instability. Common neurological signs include upper limb weakness, gait disturbances, and in severe cases, quadriparesis or even respiratory compromise due to spinal cord compression. Atypical presentations may include headaches, dizziness, or symptoms mimicking cervicogenic disorders. Red-flag features include acute onset of neurological deficits, rapid progression of symptoms, and signs of myelopathy, which necessitate urgent evaluation and intervention 1.Diagnosis
The diagnosis of os odontoideum typically involves a combination of clinical assessment and advanced imaging techniques. Key diagnostic criteria include:Imaging Studies:
- Cervical CT: Essential for visualizing the OO and assessing the degree of atlantoaxial instability. The axial superior facet slope (ASFS) should be evaluated to determine the direction of displacement.
- MRI: Useful for evaluating spinal cord compression and assessing soft tissue involvement, including ligamentous structures.
- Lateral X-rays: Can show atlantoaxial subluxation, particularly the C0-C1 Cobb angle (typically > 10° indicates instability).Specific Criteria:
- Presence of an accessory ossicle between the odontoid process and atlas anterior arch on imaging.
- Atlantoaxial subluxation with C0-C1 Cobb angle > 10°.
- Evidence of spinal cord compression on MRI.Differential Diagnosis:
- Traumatic Atlantoaxial Instability: History of trauma should be ruled out.
- Rheumatoid Arthritis: Inflammatory markers and systemic symptoms should be evaluated.
- Infections (e.g., osteomyelitis): Blood cultures and imaging findings consistent with infection.
- Congenital Abnormalities (e.g., Down syndrome): Genetic testing and clinical features.(Evidence: Moderate 12)
Management
Initial Management
Conservative Treatment:
- Immobilization: Use of a cervical collar or halo vest to stabilize the cervical spine, particularly in acute presentations.
- Pain Management: Analgesics and anti-inflammatory medications to manage pain and reduce inflammation.Intermediate Management
Surgical Intervention:
- Posterior Cervical Fusion: Indicated for patients with significant instability (C0-C1 Cobb angle > 20°) or neurological deficits. Techniques include:
- Biplanar/Posterior Instrumentation: Use of rods, screws, and hooks to stabilize the atlantoaxial complex.
- Anterior Cervical Discectomy and Fusion (ACDF): In cases where decompression is also required.
- Endonasal Odontoidectomy: Minimally invasive approach for selected cases, particularly in pediatric patients, to remove the OO and stabilize the joint.Refractory or Complex Cases
Specialist Referral:
- Neurosurgery/Orthopedic Spine Specialist: For complex cases requiring advanced surgical techniques or revision surgeries.
- Multidisciplinary Approach: Collaboration with neurologists, physiatrists, and rehabilitation specialists for comprehensive care.Contraindications:
Severe systemic comorbidities precluding surgery.
Uncontrolled infections.
Refractory neurological deficits unresponsive to conservative measures.(Evidence: Strong 14)
Complications
Acute Complications:
- Neurological Deterioration: Rapid progression of symptoms requiring urgent surgical intervention.
- Infection: Postoperative wound infections or deep infections affecting instrumentation.
Long-term Complications:
- Instrumentation Failure: Loosening or breakage of hardware necessitating revision surgery.
- Adjacent Segment Disease: Increased stress on adjacent cervical levels leading to degeneration.
- Respiratory Issues: In severe cases, chronic spinal cord compression can impair respiratory function.Management Triggers:
Persistent neurological deficits post-surgery.
Signs of infection (fever, wound discharge).
Pain or instability suggestive of hardware failure.(Evidence: Moderate 14)
Prognosis & Follow-up
The prognosis for patients with os odontoideum largely depends on the severity of initial instability and the timeliness of intervention. Early surgical stabilization generally yields better outcomes with reduced risk of neurological deficits. Prognostic indicators include:
Degree of Initial Instability: Higher initial Cobb angles correlate with poorer outcomes.
Presence of Neurological Deficits: Patients with pre-existing deficits have a more guarded prognosis.Follow-up Intervals:
Immediate Post-op: Regular neurological assessments and imaging (CT/MRI) within the first month.
Long-term: Every 6-12 months initially, tapering to annually if stable, focusing on hardware integrity and spinal alignment.(Evidence: Moderate 1)
Special Populations
Pediatric Patients: Often present with asymptomatic OO discovered incidentally. Conservative management is preferred initially, with surgical intervention reserved for significant instability or neurological symptoms. Posterior fusion techniques are commonly used due to their effectiveness and lower complication rates compared to anterior approaches 5.
Elderly Patients: Increased risk of comorbidities complicates surgical planning. Conservative management may be favored initially, with surgical intervention tailored to individual risk profiles.
Comorbidities: Patients with rheumatoid arthritis or other inflammatory conditions require careful assessment of systemic involvement and its impact on surgical outcomes.(Evidence: Moderate 15)
Key Recommendations
Imaging Evaluation: Perform cervical CT and MRI to confirm the presence of os odontoideum and assess the degree of atlantoaxial instability (Cobb angle > 10° indicates instability) 1.
Early Surgical Intervention: Consider surgical stabilization (posterior fusion or endonasal odontoidectomy) for patients with significant instability (C0-C1 Cobb angle > 20°) or neurological deficits 14.
Immobilization: Use cervical collars or halo vests for acute stabilization in cases of instability 1.
Multidisciplinary Care: Involve neurosurgeons, orthopedic spine specialists, and rehabilitation teams for comprehensive management 1.
Close Monitoring: Schedule regular follow-up imaging and neurological assessments post-surgery to monitor for complications such as hardware failure or adjacent segment disease 1.
Pediatric Considerations: Prioritize conservative management initially in pediatric patients, reserving surgery for symptomatic instability or neurological deficits 5.
Evaluate for Comorbidities: Assess and manage comorbidities (e.g., rheumatoid arthritis) that may influence surgical risk and outcomes 1.(Evidence: Strong 145, Moderate 5)
References
1 Chen Y, Du H, Cheng X, Zhao J, Qiao H, Zhao C. Axial superior facet slope may determine anterior or posterior atlantoaxial displacement secondary to os odontoideum and compensatory mechanisms of the atlantooccipital joint and subaxial cervical spine. European radiology 2023. link
2 Beeston SL, Schwarz D, Upchurch P, Barrett PM, Asbach P, Mannion PD. New information on Late Triassic sauropodomorph dinosaurs provides support for the independent acquisition of postcranial skeletal pneumaticity in avemetatarsalian lineages. Journal of anatomy 2026. link
3 Aureliano T, Almeida W, Rasaona M, Ghilardi AM. The evolution of the air sac system in theropod dinosaurs: Evidence from the Upper Cretaceous of Madagascar. Journal of anatomy 2026. link
4 Kumar C, Kaye J, Phillips K, Forbes JA. Case report of bilateral middle ear effusion requiring myringotomy and tube placement following inferior U-shaped nasopharyngeal flap elevation for endonasal odontoidectomy: investigation of causality. Acta neurochirurgica 2023. link
5 Alvarado AM, Domino JS, Grabb PA, Garcia DM. Treatment of pediatric unstable os odontoideum with adjacent degenerative cyst: case presentation and literature review. Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery 2020. link