Acromioclavicular (AC) joint
The AC joint is functionally triaxial. The scapula moves around each of the three acromioclavicular axes.
- The scapula wings around the AC joint's VERTICAL axis
- The scapula tips around the AC joint's LATERAL axis
- The scapula rotates upward or downward around the AC joint's A-P axis
Mobility at the AC joint allows the scapula to move in three dimensions so that it follows the contours of the ribcage. After all, the scapula must move on the ribcage, and the ribcage is not flat!