By Matthew Law

The joint between the scapula and ribs is known as the costoscapular joint, which allows a lot of movement.

Joints which are very mobile are often not very stable.
There is a cartilagenous lip around the glenoid to help stabilize the joint. The supraspinatus and infraspinatus help too.

The head of the humerus is encircled by the rotator cuff.
It is common to get a tear in the subscapularis muscle or subscapularis tendon. When that happens, the deltoid muscle pulls the head of the humerus up - this is very painful. An impingement would show on the head of the humerus. See rotator cuff disease

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