Quadrilateral Space Syndrome

In order for muscles to work properly, the nerves that innervate (supply) them must also function appropriately. If the nerves are somehow damaged or compressed somewhere along their course, it can affect their ability to conduct signals to the muscles. When this occurs, nerve function decreases and the targeted muscles experience weakness and atrophy (shrinking).

On the posterior (back) side of the shoulder, several muscles form the borders of a space called the Quadrilateral Space: the teres minor above, the teres major below, the triceps muscle to one side, and the humerus (arm) bone to the other side. The axillary nerve and the posterior circumflex humeral artery run through this space. When the arm is in certain positions, such as in a throwing motion, that space is closed down and the axillary nerve can be compressed. It is also possible for fibrous tissue to form and compress the nerve. The axillary nerve innervates the deltoid muscle, which is the muscle that covers the dome of the shoulder and plays a large role in lifting the arm.