The scapula (shoulder blade) is a strange-looking triangular bone that's slightly curved. It houses a socket into which the head of the humerus fits to form the shoulder joint. When at rest, the scapula must sit at the correct height on the trunk as well as the correct distance from the spine. This sets the stage for arm movement. When the arm moves, the scapula must lift upward or elevate, rotate away from the spine, slide away from the spine (abduct), and finally tilt backward (posterior tilting). It must do this to help the arm perform whatever task it's trying to do. If the shoulder blade doesn't help the arm, then excessive stress is placed on the shoulder joint because it bears more than its share of the load. In almost every shoulder patient I see, at least a few of these functions aren't working so well.
Elevation, rotation, and abduction take place as the shoulder blade slides around on the trunk. In contrast, posterior tilting involves the scapula scooping away from the trunk.
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