etiology The atlantoaxial joint is an important "transitional zone" in the cervical spine prone to instability by both degenerative and traumatic processes Pathophysiology adult causes degenerative Down syndrome rheumatoid arthritis os odontoideum traumatic type I odontoid fracture (very rare) atlas fractures transverse ligament injuries pediatric causes degenerative JRA Morquio syndrome lysosomal storage disorder trauma/infection rotatory atlantoaxial subluxation Anatomy Osteology bony articulations C1-2 facet joints Ligaments transverse apical alar ligament complex transverse ligament most important stabilizer apical ligament single midline structure alar ligaments paired parasagittal ligaments Biomechanics the atlantoaxial joint provides ~50% of rotation in the cervical spine this is enabled by the peg (C2) in ring (C1) anatomy Physical Exam Symptoms neck pain neurologic symptoms Physical exam neurologic deficits often appear late in disease process due to capacious nature of spinal canal at the C1 level myelopathic symptoms hyperreflexia (patellar tendon reflex) muscle weakness broad based gait decreased hand dexterity loss of motor milestones bladder problems Imaging Radiographs flexion-extension views atlantodens interval (ADI) measurement distance between odontoid process and the posterior border of the anterior arch of the atlas adult parameters >3.5 mm considered unstable >10 mm indicates surgery in RA other must get preoperative flexion-extension views to clear all high-risk patients for any type of surgery space available for the cord (SAC) = posterior atlantodens interval (PADI) measurement distance from posterior surface of dens to anterior surface of posterior arch of atlas adult parameters in adults with RA, <14 mm is associated with increased risk of neurologic injury and is an indication for surgery open mouth odontoid sum of lateral mass displacement measurement lateral masses are connect by the ring of C1 and can only be displaced relative to each other if: bony fracture (disruption of the ring) the transverse ligament is ruptured transverse ligaments binds them together adult parameters if >8.1 mm, then a transverse ligament rupture is assumed and the injury pattern is considered unstable Treatment Determined by specific condition adult atlantoaxial instability Down syndrome rheumatoid arthritis os odontoideum odontoid fracture atlas fractures transverse ligament injuries pediatric atlantoaxial instability JRA Morquio syndrome rotatory atlantoaxial subluxation