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The shoulder doesn’t always get the recognition it deserves. As the only major joint that can rotate a full 360°, the shoulder is the most mobile and flexible joint in the body, and this flexibility allows you do things like throw a baseball, reach for faraway objects, drive a car, hoist a child above your head, and complete countless other complex movements. This wide range of motion, however, also makes the shoulder one of the most common locations for pain.

Shoulder pain ranks only behind back pain and knee pain as third most common site for musculoskeletal pain in the body. Assessing its prevalence is difficult because the definition of shoulder pain is not clear cut, but some studies have found the annual prevalence to be as high as 47% and the lifetime prevalence to be as high as 67%. There are many conditions that can lead to shoulder pain and disability, which can arise from a variety of causes. To help you better understand what can cause shoulder pain and how it might feel, we’d like to provide you with answers to some of the most frequently asked questions about shoulder anatomy and common painful conditions.

Q: Is the shoulder a single joint?

A: Although the shoulder is often referred to as one joint, it technically consists of four joints, with the acromioclavicular and glenohumeral joints being most important for movement. The acromioclavicular joint is a gliding joint where a part of the shoulder blade (scapula) called the acromion and the collarbone (clavicle) meet, and it allows forces to be transmitted from the arm to the clavicle. The glenohumeral joint is what most people think of when visualizing the shoulder, and it’s responsible for the shoulder’s extremely wide range of motion. It is a ball-and-socket joint consisting of the head of the upper arm bone (humerus) as the ball and the glenoid—a shallow cuplike part of the scapula—as the socket.

Q: What other structures make up the shoulder?

A: Connecting the bones and muscles of the shoulder are several ligaments, tendons, plus several other important structures, including the following:

• Rotator cuff: a group of four muscles that run from the humerus to the scapula; the tendons of these muscles form a “cuff” around the head of the humerus, and all the muscles work together to allow movement and stabilize the shoulder


• Deltoid: the largest and strongest muscle of the shoulder, which provides the strength to lift the arm


• Bursa: a fluid-filled sac that acts as a cushion between tendons and other structures of the shoulder


• Labrum: a ring of cartilage surrounding the glenoid that creates a deeper socket for the ball to stabilize the joint


• Joint capsule: a fibrous sheath the encloses the structures of the shoulder joint

Q: What’s the difference between rotator cuff tendinitis, shoulder impingement, and a rotator cuff tear?

A: Any of these structures can be damaged in an acute or overuse injury, but most shoulder conditions—about 85%—involve the rotator cuff. Of these, rotator cuff tendinitis, shoulder impingement, and rotator cuff tears are most common.

• Rotator cuff tendinitis (shoulder tendinitis): the most common cause of shoulder pain, this condition results from irritation or inflammation of any of the rotator cuff tendons occurring gradually over time; the main symptoms are pain and swelling in the front of the shoulder and side of the arm, usually while raising or lowering the arm


• Shoulder impingement syndrome: a condition in which the bursa or any rotator tendons are trapped (or impinged) by the humerus and the acromion, which is usually due to an outgrowth of bone (bone spur); symptoms include shoulder pain and weakness, and difficulty reaching up behind the back


• Rotator cuff tear: a tear results when one of the rotator cuff tendons detaches from the bone, either partially or completely; these injuries can occur either traumatically due to a single incident, or gradually over time, which is usually the case in older patients; the most common symptom is pain that is most noticeable when lying on the shoulder or lifting or lowering the arm

Q: What’s a SLAP tear?

A: A SLAP tear, which stands for superior labrum, anterior to posterior, is a common injury to the labrum. More specifically, the top (superior) part of the labrum is torn from front (anterior) to back (posterior). SLAP tears can result from a single incident, such as falling on an outstretched arm or shoulder, or from regularly doing lots of overhead activities. Sports like baseball and tennis, and professions that involve lifting heavy objects can all increase the likelihood of a SLAP tear. Typical symptoms include a sensation of locking, popping, or catching, pain with many movements of the shoulder, especially lifting heavy objects overhead, and reduced shoulder strength and range of motion.

Q: Which other shoulder diagnoses are common?

A: Here are four other common shoulder conditions:

• Shoulder bursitis: a bursa is a fluid-filled sac that acts as a cushion to prevent structures from rubbing against each other; the subacromial bursa in the shoulder is the largest bursa in the body, and when it becomes inflamed—often from regularly performing too many overhead activities—the result is shoulder bursitis; the most common symptom is pain at the top, front, and outside of the shoulder that gets worse with sleeping and overhead activity



• Frozen shoulder: a condition that occurs when scar tissue forms within the shoulder capsule, another structure that helps to keep the shoulder stable; this causes the shoulder capsule to thicken and tighten around the shoulder joint, which means there is less room for the shoulder to move normally, eventually causing it to “freeze;” symptoms include pain and stiffness that makes it difficult or impossible to move the
  shoulder



• Shoulder dislocation: an injury in which the humerus pops out of the glenoid; this is typically due to a forceful motion, and the dislocation can be either partial or complete; symptoms include pain, swelling, and difficulty moving the shoulder



• Calcific tendinitis: a condition in which small deposits of calcium form within the tendons of the rotator cuff; calcific tendinitis is most often seen in individuals between the ages of 30–60 years, and the reasons it occurs are not entirely understood; in most cases it does not cause symptoms, but can lead to severe pain if the calcium deposits get bigger or become inflamed

Q: Am I at risk for frozen shoulder?

A: Frozen shoulder affects up to 5% of the population, but it’s not completely clear why it develops. There are, however, certain factors that may increase one’s risk for getting it, such as not moving the shoulder for a long period of time—due to a recent injury, surgery, or pain—being between ages of 40–60, female, or having arthritis, diabetes, or cardiovascular disease. Therefore, your risk could be higher if you fit into any of these categories, but predicting whether you will get frozen shoulder is difficult.

In our next post, we’ll provide some simple strategies you can follow to reduce your risk for all causes of shoulder pain.