A grade 4 AC separation occurs when the clavicle is severely displaced posteriorly. It is defined as “significant” posterior displacement. The grade separation definition does not have any quantifiable distance as it is determined simply by the impression of the clinician. The standard x-ray appearance is indistinguishable from a grade 3 AC separation as the direction of movement is in the plane of the x-ray. Notice on the first 3D image it appears as a standard grade 3 AC separation, but when viewed from two other perspectives it is more obvious that there is a severe displacement of the clavicle in the posterior direction. The two videos show this with even greater clarity. Because a grade 4 AC Joint separation is determined in a different plane as other grades and there is no objective quantification, there is considerable overlap in diagnosis. This is very important as a grade 4 shoulder separation is usually considered to be a surgical indication and a grade 3 is more “controversial”.
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