Clinical Scorecard: The Role of Axial Superior Facet Slope in Anterior or Posterior Atlantoaxial Displacement Associated with Os Odontoideum and Compensatory Adjustments in the Atlantooccipital Joint and Subaxial Cervical Spine

At a Glance

Key Highlights

• Os Odontoideum is a significant cause of atlantoaxial displacement, predominantly anterior displacement.
• Axial superior facet slope (ASFS), derived from cervical CT and X-ray measurements, correlates with the direction of AAD.
• AAD triggers compensatory sagittal balance adjustments between the atlantooccipital joint and subaxial cervical spine.

Guideline-Based Recommendations

Diagnosis

• Use supine cervical CT to measure axial superior facet endplate angle (ASFEA) and cervical sagittal parameters.
• Calculate ASFS as the sum of ASFEA and C2 slope (C2S) from standing cervical lateral radiographs.
• Identify displacement direction by ASFS value: positive ASFS indicates anterior displacement tendency; negative ASFS indicates posterior displacement tendency.

Management

• Consider ASFS and compensatory cervical sagittal balance when planning surgical intervention for OO with AAD.
• Exclude patients with trauma, rheumatoid arthritis, infection, tumor, or prior cervical surgery to isolate OO-related AAD.

Monitoring & Follow-up

• Perform repeated ASFS measurements with high intraobserver and interobserver reliability to monitor displacement progression.
• Use standing cervical lateral radiographs and supine CT scans for ongoing assessment of sagittal balance and facet morphology.

Risks

• Higher morbidity associated with anterior displacement compared to posterior displacement in OO-related AAD.
• Potential for cervical sagittal imbalance if compensatory mechanisms fail or are insufficient.

Patient & Prescribing Data

Patients undergoing surgery for upper cervical myelopathy due to OO with AAD

Radiographic assessment of ASFS can guide surgical decision-making and predict displacement direction and compensatory spinal adjustments.

Clinical Best Practices

• Measure ASFEA and C2S accurately using standardized CT and X-ray protocols to calculate ASFS.
• Use ASFS as a key parameter to determine displacement direction and plan individualized treatment.
• Monitor compensatory changes in the atlantooccipital joint and subaxial cervical spine to maintain cervical sagittal balance.
• Exclude confounding conditions such as trauma or inflammatory diseases when diagnosing OO-related AAD.

References

• 1. Os Odontoideum as a cause of atlantoaxial displacement
• 2-4. Regulation of lateral atlantoaxial joints in displacement
• 5. Spondylolisthesis and vertebral displacement mechanics
• 6. Orientation of upper sacral endplate in upright position
• 7. Cervical-global sagittal balance regulation
• 8-9. Use of C7 slope to replace T1 slope in cervical sagittal measurements
• 10. Intraclass correlation coefficient for reliability analysis
• 11. Cervical sagittal curve types classification