This 3D animation from The Human Atlas explains Periarthritis of the Shoulder.

The shoulder comprises bones, ligaments, tendons, and muscles that connect the arm to the torso. The glenohumeral joint, or shoulder joint, is a ball-and-socket type joint. The "ball" is the top, rounded part of the humerus, and the "socket" is the bowl-shaped part of the scapula, called the glenoid, into which the ball fits. This joint allows the arm to move in a circular rotation as well as towards and away from the body.

The joint capsule is a group of soft tissue ligaments that surround the joint. The rotator cuff is a group of four muscles that pull the humerus into the scapula. The rotator cuff muscles stabilize the glenohumeral joint and help with rotation of the arm.

"Frozen shoulder," also called adhesive capsulitis or periarthritis of the shoulder, is a chronic, inflammatory disorder of the shoulder and surrounding soft tissues. This condition is frequently caused by injury, leading to pain and lack of use. As the joint becomes progressively tighter and stiffer, simple movements, such as raising the arm, become difficult. If inflammation occurs within the capsule itself, the shoulder bones are unable to move within the joint. In some instances, the patient may be unable to move the shoulder at all.

The condition rarely appears in people under age 40. An arthrogram may be needed to confirm the diagnosis. Treatment can include non-steroidal anti-inflammatory medications (NSAIDs) and physical therapy. However, it may take up to a year to see improvement. Arthroscopic surgery and shoulder manipulation under anesthesia may be necessary in more severe cases.