Shoulder Anatomy and Function
The shoulder is a ball-and-socket joint like many joints in the human body but differs in several important ways. The unique features of shoulder anatomy not only make the joint highly functional but also susceptible to injury. The ball of the shoulder is on the upper humerus or arm bone and the socket is on the scapula or shoulder blade. The shoulder is the most mobile joint in the human body(1) and is the foundation for the ability to position the hand around us. To permit this high amount of motion the shoulder is not constrained by a full cup. To the contrary, the glenoid socket is a shallow dish with a depression similar to a golf tee.
The glenoid socket is made deeper by a bumper of cartilage (a “seal”) that wraps around the periphery of it called the labrum. The shoulder bones are held together by leathery tissue called ligaments that hold the bones together. The joint moves by a concerted action of muscles called the rotator cuff and deltoid. Furthermore, shoulder mobility is further expanded by the movement of the shoulder blade on the chest, which effectively is a way our bodies move the glenoid socket to more opportunistic locations depending on our demands for function of our arm at the time of activity. There are 17 muscles that connect to the shoulder blade alone, making it the shoulder blade one of the most unique bones in the body.
The shoulder joint can be damaged by a variety of causes, and the causes are generally grouped into traumatic and atraumatic. The traumatic injuries include dislocation or instability, where the humeral ball forcefully comes out of the socket. When this happens, either the socket can fracture or the labrum tears (called a bony or a soft bankart injury, respectively). When these dislocation events occur, there can be fractures of the humeral ball as well, called a hill-sachs lesion. Sometimes falls or injuries that forcefully strain the tissue of the rotator cuff causes it to rupture, as with a fall from a ladder or down the stairs. This is called a traumatic rotator cuff tear.
There can also be atraumatic injuries and problems with these tissues. When the ligaments connecting the ball and socket are loose, the condition is called laxity, and that can lead to pain and difficulty with activities. Sometimes the opposite problem can occur, where the ligaments contract and become inflamed, a condition known as a frozen shoulder. Another very common problem with the muscles and tendons is when the rotator cuff starts to become partially or completely torn, whether a result of the tendons wearing out or rubbing on a bone spur. Rotator cuff tears become very painful and when they start to enlarge can compromise the strength of the shoulder for activity. When this gets particularly worse, the majority of the tendons detach, and the powerful deltoid muscle pulls the ball out of the socket, a condtion known as “escape”(2).
Like any joint in the body, the lining surfaces of the shoulder can wear out, leading to arthritis. Arthritis is a completely separate condition than a rotator cuff or tendon disorder. The analogy is to an automobile, where the moving parts in the joint is like the wheels of the car and the muscles and tendons are like the engine. Both can wear out and lead to pain. When the cartilage lining surfaces become substantially worn out, the shoulder gets stiff and more painful.
There are many other conditions that lead the shoulder to stop functioning and become painful. If you have been struggling with shoulder pain or have suffered a recent injury, it is important to be evaluated by Dr. Obermeyer so you can know your diagnosis, what the treatment options are, and what the advantages and disadvantages of those options are.
- Quillen DM, Wuchner M, Hatch RL. Acute shoulder injuries. Am Fam Physician. 2004 Nov 15;70(10):1947-54. PMID: 15571061.
- Neer CS 2nd, Craig EV, Fukuda H. Cuff-tear arthropathy. J Bone Joint Surg Am. 1983 Dec;65(9):1232-44. PMID: 6654936.
At a Glance
Dr. Thomas Obermeyer
- 15+ years of training and experience treating complex shoulder and sports medicine conditions
- Expert subspecialized and board-certified orthopedic care
- Award-winning outstanding patient satisfaction scores
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