Anatomy of the Shoulder

The shoulder is an amazing complex of bones, ligaments, muscles and tendons. The bones of the shoulder include the scapula, or “shoulder blade”, the clavicle, or “collarbone”, and the humerus, or “arm bone”.

The scapula is an anchoring bone that forms the foundation and attachment point for the muscles of the rotator cuff. The scapula widens at the shoulder joint to form the “socket” of the shoulder. This socket is called the glenoid. The glenoid surface is shallow and small, creating more of a “saucer” than a socket. The rotator cuff muscles coalesce to form the tendons which surround the ball of the humerus. These tendons essentially form a “mobile socket” that constantly contracts to keep the ball of the humeral head centered in the glenoid with movement of the shoulder. This structure is unique to the shoulder and is the reason why the shoulder has the most range of motion of any joint in the human body. Lifting the arm requires rotation of the humeral head on the glenoid, which is accomplished by contraction of the rotator cuff muscles. At the same time, the scapula rotates along the chest wall and strong supporting muscles of the shoulder add power to the motion. To keep the shoulder stable through this motion, there is an inner lining, or capsule, that attaches the humeral head to the glenoid. This lining is normally loose when the shoulder is at rest, but tightens when the arm is taken to the extremes of motion.

The anatomy of the shoulder is essential to provide its amazing range of motion. Without its complex structure, simple motion, such as reaching for something high on a shelf or reaching behind the back, would not be possible. The shoulder positions the arm in space which allows for everyday activities, such as writing or eating. Along with fine, precise motions the shoulder muscles also allow for complicated tasks, such as a tennis serve or swinging a golf club.

When one or several anatomic structures of the shoulder are injured, a multitude of shoulder and upper extremity problems can occur. The different types of injuries or problems that can occur in the shoulder are fractures, separations, dislocations, adhesive capsulitis, rotator cuff tears, and arthritis.

The most common bones fractured are the clavicle, or collarbone, and proximal humerus, the upper part of the arm bone. Most of these fractures can be treated conservatively with an arm sling, but some require surgery in order to provide the best chance of achieving normal function and limited pain. It is important to see a shoulder specialist soon after an injury in order to discuss treatment options.

Another type of injury to the shoulder is a “separation,” which occurs at the junction of the clavicle and the acromion process of the scapula called the acromioclavicular joint (ACJ). An injury here can cause the ligaments of this joint to tear, making the clavicle appear to “pop up” as the rest of the arm droops away from it. Usually, minor separations can be treated conservatively and do well without surgery. More severe injuries can be treated by reconstructing the torn ligaments and stabilizing the joint surgically. 

Shoulder instability refers to dislocation or subluxation of the humeral head from the socket. This most common occurs anteriorly with the ball of the humerus dislocating in front and below the socket. A less common dislocation is the posterior type, where the ball dislocates to the back of the socket. Sometimes the shoulder may be very loose and unstable in several directions. These patients tend to be younger and are hyperflexible, commonly seen in young female dancers or gymnasts. Occasionally dislocations may be associated with rotator cuff tears in patients over the age of 40. If patients complain of chronic instability or are found to have a rotator cuff tear, surgery may be required to repair the torn capsule attachment to the glenoid, the supporting labrum cartilage, and any torn rotator cuff tendons.

On the other end of the spectrum is adhesive capsulitis, or “frozen shoulder”. This is a common condition that occurs when the capsule of the shoulder becomes tight and relatively thick, usually for unknown reasons, but is more common in diabetics and patients with thyroid problems. Usually conservative treatment with a stretching program, light pain medication and anti-inflammatories, and occasional cortisone injections can help speed up improvement with this condition. Rarely, surgery is required to “manipulate” the shoulder’s motion back to normal or to surgically release the tight tissue arthroscopically.

Another common problem with shoulders is rotator cuff disease. The rotator cuff is the critical set of muscles and tendons that are required to power the shoulder. Rotator cuff disease can range from simple irritation, or tendinitis, to small and intermediate sized repairable tears, to extensive tears that sometimes cannot be fixed. One of the two biceps tendons in the shoulder, the “long head”, is located close to the most common location of rotator cuff tears, the supraspinatus insertion. As a result, sometimes the long head of the biceps tendon is involved with the rotator cuff tear and requires treatment. 

Finally, certain shoulders may develop joint arthritis. When the word “arthritis” is mentioned, most people think of pain in a joint from “getting old”. To an orthopaedic surgeon, arthritis is a very specific term. It refers to loss of cartilage. Joints, including shoulders, require moving surfaces that glide without friction. It is the cartilage and the joint fluid within joints that give that smooth motion. When a joint becomes arthritic, this cartilage is lost causing “bone on bone” grinding and pain. This usually progresses over time and can cause significant pain, stiffness, and disability. Joint replacements of the hip and knee are well known treatments for arthritic pain of those joints. Modern shoulder replacement has been shown to be just as effective as hip and knee replacement.

Understanding the anatomy of the shoulder can help you understand why your shoulder is not functioning the way it should. It is easy to misdiagnose the condition, so you should be evaluated by a knowledgeable professional.

To schedule an appointment with one of our specialty-trained shoulder doctors at Western Orthopaedics, please call .