• Shinjiro Kaneko, Ken Ishii, Kota Watanabe, Takashi Tsuji, Masaya Nakamura, Morio Matsumoto, Yoshiyuki Yato, and Takashi Asazuma.
• Department of Orthopaedic Surgery, National Hospital Organization Murayama Medical Center, Tokyo, Japan. ShinjiroKaneko@gmail.com.
• Eur Spine J. 2018 Jun 1; 27 (6): 1303-1308.

PurposeHinge-like hyper-mobility is occasionally observed at the atlanto-occipital (O-C1) joint. However, it has not been clear if this kind of hinge-like hyper-mobility at the O-C1 joint should be regarded as "pathologic", or referred to as "instability". To solve this issue, we aimed to establish a reliable radiographic assessment method for this specific type of O-C1 instability and figure out the "standard value" for the range of motion (ROM) of the O-C1 joint.MethodsTo figure out the standard range of the O-C1 angle, we acquired magnetic resonance imaging (MRI) sagittal views of the cervical spine for 157 healthy volunteers [average: 37.4 year-old (yo)] without spine diseases, at neutral, maximum flexion and maximum extension positions.ResultsThe average value (AVE) for ROM of O-C1 angle was 9.91°. The standard value for ROM of O-C1 angle was calculated as 0°-21°. There was no statistically significant gender difference. We also found that the older population (≧ 40 yo) significantly had a larger ROM of O-C1 angle (AVE: 11.72°) compared to the younger population (< 40 yo) (AVE: 8.99°).ConclusionsWe consider that hinge-like instability at O-C1 joint, which cannot be assessed by measuring Powers ratio, can be assessed by measuring the range of O-C1 angles using dynamic-MRI. Evaluation of O-C1 instability is important especially when we perform surgical treatment for diseases with upper cervical instability (such as retro-odontoid pseudotumor). We consider that the current study provides important information in such a case.

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