Axial Superior Facet Slope Influences Atlantoaxial Displacement Direction in Os Odontoideum

Overview

This study identifies the axial superior facet slope (ASFS) as a key anatomical determinant of anterior versus posterior atlantoaxial displacement (AAD) in patients with os odontoideum (OO). It also reveals compensatory adjustments in the atlantooccipital joint and subaxial cervical spine associated with AAD direction.

Background

Atlantoaxial displacement (AAD) can result from trauma, inflammation, genetics, and notably os odontoideum (OO), which often leads to anterior displacement (AD) more frequently than posterior displacement (PD). The axial superior facet (ASF) serves as the sliding surface during AAD, but its role in displacement direction has been underexplored. Understanding ASF characteristics, including the axial superior facet endplate angle (ASFEA) and axial superior facet slope (ASFS), may clarify displacement mechanisms and guide treatment. Additionally, compensatory changes in the atlantooccipital joint and subaxial cervical spine following AAD have not been well characterized.

Data Highlights

Key Findings

• The axial superior facet slope (ASFS) is significantly associated with the direction of atlantoaxial displacement in OO patients: positive ASFS correlates with anterior displacement (AD), negative ASFS with posterior displacement (PD).
• 79.2% of OO patients exhibited positive ASFS values indicating an anteroinferior to posterosuperior inclination of the ASF, favoring anterior sliding.
• Atlantooccipital joint and subaxial cervical spine demonstrate compensatory sagittal adjustments in response to AAD, similar to compensations seen in spondylolisthesis.
• Measurement of ASFS using supine cervical CT and standing lateral radiographs is reliable with excellent intra- and interobserver consistency.
• Control patients without OO showed varied cervical sagittal curve types, providing a comparative baseline for ASF morphology and slope.

Clinical Implications

Assessment of ASFS can aid clinicians in predicting the direction of atlantoaxial displacement in OO patients, which is critical for surgical planning and prognosis. Recognizing compensatory changes in the atlantooccipital joint and subaxial cervical spine may influence postoperative management and rehabilitation strategies. Reliable measurement techniques for ASFS facilitate its integration into routine radiographic evaluation.

Conclusion

The axial superior facet slope is a pivotal anatomical factor determining the direction of atlantoaxial displacement in os odontoideum and is associated with compensatory sagittal adjustments in adjacent cervical segments. These insights enhance understanding of AAD pathomechanics and support tailored clinical interventions.

References

1. Wang et al. 2023 -- The Role of Axial Superior Facet Slope in Anterior or Posterior Atlantoaxial Displacement Associated with Os Odontoideum