10/27/2025
There are two ways of measuring the clivoaxial angle (CXA) for CCI on MRI. These are both well-documented and used by radiologists reviewing images. The first image demonstrates the CXA using the soft tissue as markers for the angle, which measures 124.7º.
The second image demonstrates the CXA using the bony prominences as markers, and the angle measures 138.4º. This is a huge difference, which can mean the difference of having 'concerning' instability and 'pathologic instability'.
A "normal" CXA is between 145º - 155º, and a reduction of about 10º is expected with flexion, and increase of 10º with extension.
From our research, the most widely accepted measurement uses the soft tissues as markers. When compared to CT scan images using bones as landmarks for the CXA, the measurements are within an acceptable range (+/- 2º). Additionally, upright/open MRI has more shadowing, which may have a wider variability of bone density. Therefore, we have adopted the technique of using soft tissue when measuring the CXA.
And yes, this patient has CCI based on the CXA. We perform additional measurements, including Harris' measurement, Grabb-Oaks measurement, and basin-dens interval (BDI).