Pathophysiology

Impingement

Rotator cuff impingement occurs as the rotator cuff tendons are pinched as they pass through the subacromial space formed between the acromion, coracoacromial arch, and the AC joint (above) and the glenohumeral joint (below). Impingement may be external or internal.  

External impingement occurs when the structures above the rotator cuff are abnormally shaped, pinching the rotator cuff tendons. The acromion may be “beaked, curved, or hooked.” These abnormalities result from degenerative changes or may be present from birth. External impingement may also be caused by inadequate muscular stabilization of the scapula.  

Some throwing athletes may experience internal impingement. In this case, the rotator cuff tendons are pinched from below by the upper portion of the glenoid fossa. This occurs in the cocking phase of throwing, when the shoulder is in extension, abduction, and external rotation.

Rotator Cuff Tendonitis and Tears

Inflammation of the rotator cuff is called rotator cuff tendonitis and usually results from impingement, as described above. There is normally a frictionless interface between the rotator cuff and coracoacromial arch in the subacromial space. Overhead movements result in gliding of the rotator cuff’s bursa against the acromial undersurface, corocoacromial ligament, and acromioclavicular joint. Any process that a) interferes with cuff mechanics or b) compromises the normal outlet to the coracoacromial arch can lead to inflammation of the rotator cuff tendons.  

Most degenerative rotator cuff tears are located in the supraspinatus muscle. This muscle, wedged between the humeral head and acromion, is subject to multiple forces, and has little blood supply at the site of attachment. With age, these factors can cause increased degeneration that can ultimately result in a tear. Most cuff tears are located in the tendinous part of the cuff, rather than in the muscle bellies. The effect of the tear depends on the size of the defect. For example, one study showed that a tear involving 1/3 to 2/3 of the supraspinatus reduced muscle force by 5% at most.  

A full thickness tear of the supraspinatus can cause increased tension on the intact fibers of the rotator cuff, and can extend the tear. Trauma accelerates this process. Even without trauma, many would argue that progression of these tears is slow but inevitable. The torn muscles atrophy, and their bulk is replaced by fat and scar tissue.  

Individuals who rely on manual wheelchairs for mobility are at risk for developing rotator cuff tendonitis and tears. This is because of the heavy, repetitive activity associated with propelling the wheelchair.

Instability

Instability refers to abnormal mobility of the humeral head with respect to the glenoid fossa. If the humeral head is partially displaced, it is subluxed. If the humeral head is no longer in the socket, it is dislocated. The glenohumeral joint is the most commonly dislocated joint in the body. The humeral head usually dislocates anteriorly (or to the front of the glenohumeral joint). The joint is at greatest risk for dislocation when it is raised and rotated outward. As stated above, the AIGHL is taut in this position and holds the humeral head in place. Often, a first-time dislocation is associated with trauma and disruption of the stabilizing ligaments. Detachment of the anterior inferior, or the front and lower, labrum is called a Bankart lesion. If there is a fracture of the inferior glenoid rim, it is called a Bankart fracture. Disruption of the stabilizing ligaments and/or labrum can make the joint susceptible to further instability.  

A Hill Sachs lesion is a defect in the posterolateral (or back and outside) aspect of the humeral head. This may occur when a dislocated humeral head is put back into place, or reduced. The humeral head can abut the front of the glenoid rim causing a small fracture. This finding might inform the investigating physician that the joint has been unstable previously.  

Anterior dislocation puts the axillary nerve at risk for injury. This nerve is a branch of the brachial plexus that runs along subscapularis and ultimately serves the deltoid and teres minor muscles and sensation to the outer arm.

Subluxation refers to the incomplete dislocation of the humeral head from the glenoid fossa. Stroke survivors with one-sided weakness (or hemiparesis) often suffer from pain associated with subluxation. This is because the muscles that usually work to hold the humeral head in place are weak. Gravity pulls the humeral head down and out of the fossa.

Glenoid Labrum Injuries

The labrum may become torn without obvious instability. Tears of the labrum are divided into SLAP lesions and non SLAP lesions. SLAP is an acronym for Superior Labrum Anterior Posterior. Anterior and posterior mean “to the front of” and “to the back of” respectively and in this case are in reference to the location of the biceps tendon. Examples of non SLAP tears include Bankart lesions (as stated previously), degenerative tears, flap tears, and vertical tears. Glenoid labrum lesions are most often seen in overhead throwing athletes, swimmers, and tennis players. The repetitive motion in these activities pulls on the biceps tendon, which is found in the superior labrum. Eventually the biceps tendon anchor may be torn.

Osteoarthritis 

As in other joints of the body, osteoarthritis may occur in the shoulder joint. Osteoarthritis may affect the glenohumeral or acromioclavicular joints. The disease affects that cartilage of a joint, leading to progressive damage to the articular surface. As the space between bones narrows, there is alteration of the bone underlying the damaged cartilage, and bony growths called osteophytes form. These findings may be seen on x-ray. Osteoarthritis is considered a normal part of aging. The cause of the degeneration seen is currently unknown.  

Acromioclavicular (AC) Joint Injuries

In addition to osteoarthritis, the AC joint may be injured when there is a fall onto the point of the shoulder, a so-called “shoulder separation.”  AC joints are graded by severity, Types I-V. In a Type I injury, the joint capsule is sprained. In Type II injuries, the AC ligaments are torn with a sprain of the coracoclavicular ligaments. In Type III and V injuries, the coracoclavicular ligaments are torn and a deformity may be felt on palpation. Type V injuries are distinguished from Type III by a greater degree of displacement and further soft tissue injury. In Type IV injuries, the clavicle projects backward; in Type VI injuries, the clavicle projects downward. Types IV-VI injuries are rarer than Types I-III.

Adhesive Capsulitis

Also known as frozen shoulder, adhesive capsulitis results when there is inflammation of the glenohumeral joint and its surrounding capsule. As a result, there is limited range of motion in the shoulder. Sometimes, adhesive capsulitis follows an acute injury to the shoulder followed by prolonged immobility due to pain. Other times, the cause is unclear. Frozen shoulder is often seen in diabetics.

Soft Tissue Injury

The shoulder joint is surrounded by many structures that can cause pain. Because of their close proximity to the joint, pain that seems to originate in the shoulder may actually be due to some nearby structures.  

A very common cause of shoulder pain is subacromial bursitis. A bursa is a fluid-filled sac that acts as a cushion. There is such a cushion on the supraspinatus tendon, under the acromion. Because this bursa lies in an area subject to stress and impingement, the bursa can become inflamed and painful. Another common example is biceps tendonitis. As stated above, the long head of the biceps originates in the superior lip of the glenoid, from which it passes down the humerus to attach at the forearm. This tendon is subject to overuse injury, especially in athletes who perform weightlifting activities such as bench presses and dips.⁵ Trigger points are also common causes of pain in the soft tissues surrounding the shoulder. Trigger points are tender, focal areas of spasm in muscle tissue. Trigger points in the trapezius, levator scapulae, rhomboids, and other muscles that surround the shoulder complex can cause pain that is experienced in the shoulder region.

Shoulder Pain from Outside the Shoulder

Referred pain is a common phenomenon in which injury to a structure in one location is experienced as pain in another location. There are several structures that “refer” pain to the shoulder. Cardiac pain, as in an acute coronary syndrome or heart attack, may be experienced in the shoulder or between the shoulder blades. For this reason, acute onset of shoulder pain without clear provocation is a reason to seek urgent medical attention, especially if accompanied by shortness of breath, dizziness, or nausea. Pneumonia and peptic ulcer disease may also cause shoulder pain. 

A very common source of referred pain to the shoulder is the cervical spine. The spine is a complex column of discs and bones. Osteoarthritis at articulations of the spine, herniated discs, or stenosis can cause pain that radiates toward the shoulder, even without the presence of neck pain. This is because the nerves branching off from the spinal cord in the neck travel to the shoulder, down the arm, and to the hands. Similarly, disease in the upper thoracic spine may also be experienced in the shoulder.