1) Description of Measurement

CCI measures the joint space between the occipital condyles and the lateral masses of C1. It is currently considered the most reliable CT-based metric for diagnosing occipitoatlantal dissociation.

2) Instructions to Measure

• Use coronal CT reconstructions.

• Measure the distance between each occipital condyle and the corresponding C1 superior articular surface.

• Record the maximum value or the mean, depending on institutional protocol.

3) Normal vs. Pathologic Ranges

• Normal: ≤2 mm

• 2 mm widening is diagnostic of occipital-C1 dissociation, even in the absence of gross displacement.

• In children, a "medial occipital condyle notch" is common (peak ages 2-4) and a normal developmental irregularity. It should not be included in the CCI measurement, as it would falsely increase the value.

• Normal: The joint space is typically < 1.5 mm on CT.   

• Pathologic (Adults):

• > 2.0 mm: This is the widely accepted cutoff for diagnosing AOD in adults.   

• > 1.5 mm: Some studies advocate this stricter cutoff to increase sensitivity, 

4) Important References

Smith P, Linscott LL, Vadivelu S, Zhang B, Leach JL. Normal Development and Measurements of the Occipital Condyle-C1 Interval in Children and Young Adults. AJNR Am J Neuroradiol. 2016;37(5):952-957. doi:10.3174/ajnr.A4543

Corcoran B, Linscott LL, Leach JL, Vadivelu S. Application of Normative Occipital Condyle-C1 Interval Measurements to Detect Atlanto-Occipital Injury in Children. AJNR Am J Neuroradiol. 2016;37(5):958-962. doi:10.3174/ajnr.A4641

Bono CM, Vaccaro AR, Fehlings M, et al. Measurement techniques for upper cervical spine injuries: consensus statement of the spine trauma study group. Spine. 2007;32:593-600.

Joaquim AF, Schroeder GD, Vaccaro AR. Traumatic Atlanto-Occipital Dislocation-A Comprehensive Analysis of All Case Series Found in the Spinal Trauma Literature. Int J Spine Surg. 2021;15(4):724-739. doi:10.14444/8095

5) Other info....

In pediatric groups, the CCI shows 100% sensitivity and specificity when used correctly, unlike the Rule of 12 (BDI/BAI), which has a sensitivity of 31-50%. This is because the BDI depends on dens (ossification) and the BAI on the basion (shape), whereas the CCI directly measures the injury site.