The shoulder joint is called the glenohumeral joint. This is a ball-and-socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula (image). This joint has the largest range of motion of any joint in the body. However, this freedom of movement is due to the lack of structural support and thus the enhanced mobility is offset by a loss of stability.

The large range of motions at the shoulder joint is provided by the articulation of the large, rounded humeral head with the small and shallow glenoid cavity, which is only about one third of the size of the humeral head. The socket formed by the glenoid cavity is deepened slightly by a small lip of fibrocartilage called the glenoid labrum, which extends around the outer margin of the cavity. The articular capsule that surrounds the glenohumeral joint is relatively thin and loose to allow for large motions of the upper limb. Some structural support for the joint is provided by thickenings of the articular capsule wall that form weak intrinsic ligaments. These include the coracohumeral ligament, running from the coracoid process of the scapula to the anterior humerus, and three ligaments, each called a glenohumeral ligament, located on the anterior side of the articular capsule. These ligaments help to strengthen the superior and anterior capsule walls.

However, the primary support for the shoulder joint is provided by muscles crossing the joint, particularly the four rotator cuff muscles. These muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) arise from the scapula and attach to the greater or lesser tubercles of the humerus. As these muscles cross the shoulder joint, their tendons encircle the head of the humerus and become fused to the anterior, superior, and posterior walls of the articular capsule. The thickening of the capsule formed by the fusion of these four muscle tendons is called the rotator cuff. Two bursae, the subacromial bursa and the subscapular bursa, help to prevent friction between the rotator cuff muscle tendons and the scapula as these tendons cross the glenohumeral joint. In addition to their individual actions of moving the upper limb, the rotator cuff muscles also serve to hold the head of the humerus in position within the glenoid cavity. By constantly adjusting their strength of contraction to resist forces acting on the shoulder, these muscles serve as “dynamic ligaments” and thus provide the primary structural support for the glenohumeral joint.

Injuries to the shoulder joint are common. Repetitive use of the upper limb, particularly in abduction such as during throwing, swimming, or racquet sports, may lead to acute or chronic inflammation of the bursa or muscle tendons, a tear of the glenoid labrum, or degeneration or tears of the rotator cuff. Because the humeral head is strongly supported by muscles and ligaments around its anterior, superior, and posterior aspects, most dislocations of the humerus occur in an inferior direction. This can occur when force is applied to the humerus when the upper limb is fully abducted, as when diving to catch a baseball and landing on your hand or elbow. Inflammatory responses to any shoulder injury can lead to the formation of scar tissue between the articular capsule and surrounding structures, thus reducing shoulder mobility, a condition called adhesive capsulitis (“frozen shoulder”).

WHAT ARE THE STRUCTURES OF THE SHOULDER AND HOW DOES IT FUNCTION?

To better understand shoulder problems and how they occur, it helps to begin with an explanation of the shoulder's structure and how it functions.

The shoulder joint is composed of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) (see illustration). Two joints facilitate shoulder movement. The acromioclavicular (ah-KRO-me-o-klah-VIK-u-lahr or AC) joint is located between the acromion (ah-KRO-me-on, the part of the scapula that forms the highest point of the shoulder) and the clavicle. The glenohumeral joint, commonly called the shoulder joint, is a ball-and-socket-type joint that helps move the shoulder forward and backward and allows the arm to rotate in a circular fashion or hinge out and up away from the body. (The "ball," or humerus, is the top, rounded portion of the upper arm bone; the "socket," or glenoid, is a dish-shaped part of the outer edge of the scapula into which the ball fits.) The capsule is a soft tissue envelope that encircles the glenohumeral joint. It is lined by a thin, smooth synovial membrane.

STRUCTURE OF THE SHOULDER

• Tendons, which are tough cords of tissue that attach the shoulder muscles to bone.
• Ligaments, which attach the shoulder bones

In contrast to the hip joint, which more closely approximates a true ball-and-socket joint, the shoulder joint can be compared to a golf ball and tee, in which the ball can easily slip off the flat tee. Because the bones provide little inherent stability to the shoulder joint, it is highly dependent on surrounding soft tissues such as capsule ligaments and the muscles surrounding the rotator cuff to hold the ball in place. Whereas the hip joint is inherently quite stable because of the encircling bony anatomy, it also is relatively immobile. The shoulder, on the other hand, is relatively unstable but highly mobile, allowing an individual to place the hand in numerous positions. It is, in fact, one of the most mobile joints in the human body.

The bones of the shoulder are held in place by muscles, tendons, and ligaments. Tendons are tough cords of tissue that attach the shoulder muscles to bone and assist the muscles in moving the shoulder. Ligaments attach shoulder bones to each other, providing stability. For example, the front of the joint capsule is anchored by three glenohumeral ligaments. The rotator cuff is a structure composed of tendons that work along with associated muscles to hold the ball at the top of the humerus in the glenoid socket and provide mobility and strength to the shoulder joint. Two filmy sac-like structures called bursae permit smooth gliding between bones, muscles, and tendons. They cushion and protect the rotator cuff from the bony arch of the acromion.

The rotator cuff is the name given to the group of four muscles that are largely responsible for the ability to rotate the arm. Three of the four rotator cuff muscles are deep to the deltoid and trapezius muscles and cannot be seen unless those muscles are first removed and one is on the anterior side of the scapula bone and cannot be seen from the surface.

On the anterior side of scapula bone is a single muscle, the subscapularis. It is triangular in shape and covers the entire bone. Its origin is along the fossa that makes up most of the “wing” of the scapula and it inserts on the lesser tubercle of the humerus bone. The subscapularis muscle is shown in Figure 9-1.

Figure 9-1. The subscapularis muscle of the rotator cuff, in red, anterior view. (Image Authored by: Was a bee)

On the posterior side of the scapula bone are the other three muscles of the rotator cuff. All three insert on the greater tubercle of the humerus, allowing them, in combination with the subscapularis, to control rotation of the arm. The supraspinatus muscle is above the spine of the scapula. The infraspinatus muscle is below the spine of the scapula. The relatively thin teres minor muscle is the most inferior of the rotator cuff muscles. The three posteriorly-positioned muscles of the rotator cuff are shown in Figure 9-2.

Figure 9-2. The muscles of the rotator cuff and arm, posterior view. (Image by Authored by: OpenStax College)

The teres major muscle has its origin on the scapula, like the rotator cuff muscles, but is not involved in rotating the arm. It inserts lower on the humerus than the rotator cuff muscles and is involved in adducting the arm (bringing it closer to the midline of the body.)

Rotator cuff is a strong connective tissue structure formed by the fusion of four rotator cuff muscle tendons to the articular capsule of the shoulder joint; surrounds and supports superior, anterior, lateral, and posterior sides of the humeral head

Shoulder Joint

The shoulder joint is called the glenohumeral joint. This is a ball-and-socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula (Figure). This joint has the largest range of motion of any joint in the body. However, this freedom of movement is due to the lack of structural support and thus the enhanced mobility is offset by a loss of stability.

Glenohumeral Joint

The glenohumeral (shoulder) joint is a ball-and-socket joint that provides the widest range of motions. It has a loose articular capsule and is supported by ligaments and the rotator cuff muscles.

The large range of motions at the shoulder joint is provided by the articulation of the large, rounded humeral head with the small and shallow glenoid cavity, which is only about one third of the size of the humeral head. The socket formed by the glenoid cavity is deepened slightly by a small lip of fibrocartilage called the glenoid labrum, which extends around the outer margin of the cavity. The articular capsule that surrounds the glenohumeral joint is relatively thin and loose to allow for large motions of the upper limb. Some structural support for the joint is provided by thickenings of the articular capsule wall that form weak intrinsic ligaments. These include the coracohumeral ligament, running from the coracoid process of the scapula to the anterior humerus, and three ligaments, each called a glenohumeral ligament, located on the anterior side of the articular capsule. These ligaments help to strengthen the superior and anterior capsule walls.

However, the primary support for the shoulder joint is provided by muscles crossing the joint, particularly the four rotator cuff muscles. These muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) arise from the scapula and attach to the greater or lesser tubercles of the humerus. As these muscles cross the shoulder joint, their tendons encircle the head of the humerus and become fused to the anterior, superior, and posterior walls of the articular capsule. The thickening of the capsule formed by the fusion of these four muscle tendons is called the rotator cuff. Two bursae, the subacromial bursa and the subscapular bursa, help to prevent friction between the rotator cuff muscle tendons and the scapula as these tendons cross the glenohumeral joint. In addition to their individual actions of moving the upper limb, the rotator cuff muscles also serve to hold the head of the humerus in position within the glenoid cavity. By constantly adjusting their strength of contraction to resist forces acting on the shoulder, these muscles serve as "dynamic ligaments" and thus provide the primary structural support for the glenohumeral joint.

Injuries to the shoulder joint are common. Repetitive use of the upper limb, particularly in abduction such as during throwing, swimming, or racquet sports, may lead to acute or chronic inflammation of the bursa or muscle tendons, a tear of the glenoid labrum, or degeneration or tears of the rotator cuff. Because the humeral head is strongly supported by muscles and ligaments around its anterior, superior, and posterior aspects, most dislocations of the humerus occur in an inferior direction. This can occur when force is applied to the humerus when the upper limb is fully abducted, as when diving to catch a baseball and landing on your hand or elbow. Inflammatory responses to any shoulder injury can lead to the formation of scar tissue between the articular capsule and surrounding structures, thus reducing shoulder mobility, a condition called adhesive capsulitis ("frozen shoulder").

Prognosis

A recent meta-analysis published in 2019 out of the United Kingdom evaluating the treatment outcomes (patient-reported outcomes) demonstrated both surgical and non-surgical treatment groups had the largest improvement at 12 months. Then improvements leveled out. There was no clear advantage of surgery over non-surgical treatments.

An algorithm was developed to help guide patient selection where tears categorize into three groups. Group 1: early operative repair. One indication would be patients with an acute event and imaging corroborating the history. Especially with a subscapularis tear with or without biceps tendon instability. In those patients with pain and weakness before the injury would likely be an acute on chronic tear scenario and early intervention can be considered when imaging shows minimal muscle atrophy or degenerative changes should undergo early surgical interventions. Patients younger than 62 to 65 with small and medium-sized tears with minimal to no atrophy on imaging would be another group to consider early operative management. Group 2: In patients with painful partial or full-thickness tears who do not have an acute onset of pain, even if tendons are potentially repairable, they should undergo non-surgical treatment initially. In these cases, rehabilitation has shown consistent benefit in improving function and outcomes. Group 3: should undergo maximal non-surgical treatment. This group would be those who are tendons are unlikely to heal. Included in this group are those over 70 years old, those with chronic full-thickness tears with significant tendon retraction, advanced degenerative changes in the muscles, any signs of proximal humeral migration.

Complications

The most likely complication would be retearing of the cuff repair — this is minimizable with proper patient selection. Postoperative complications, in addition to those generally seen, would be adhesive capsulitis, inability to regain motion, or cuff strength.

Deterrence and Patient Education

No studies examine prevention strategies. Theoretically, proper cuff function can help decrease the risk of degenerative tears. In an observational study, rotator cuff tears were seen more predominately in those with specific postures. Also, another study looking at radiographically measured subacromial space found reduced acromiohumeral space correlated with hyperkyphotic posture.

Enhancing Healthcare Team Outcomes

Rotator cuff disease is among the most common tendon injuries seen and generally affects the older population. These patients tend to have more chronic conditions and take more medications. Most patients will do well with conservative measures, but open communication with their primary care physician, physical therapist, and sports medicine physician will optimize outcomes.

When the decision is made to proceed to surgery, patients may need clearance to undergo anesthesia; this occasionally requires permission from their primary care doctor or other specialists like cardiology, pulmonary, nephrology if they have renal failure or are undergoing dialysis. The team will need to address any chronic metabolic conditions and should look to optimize blood glucose levels, A1C, as well as optimizing any blood coagulation.

After the surgical procedure, clear communication between the operative and post-operative teams of physicians and nurses (including nurses with specialty training in orthopedics) to ensure the continued safety of the patient as they recover is critical. Discussions about medications and medical conditions should be open and frank, so the entire team ensures an appropriate handoff. The nursing staff should perform post-operative monitoring, assist with patient and family education, and coordinate appropriate follow-up.

As the patient proceeds with recovery and goes through the (typically long) rehabilitation process, open communication between the surgeon and physical therapist will optimize the patient's recovery. Physical therapists also often have involvement in conservatively managed cases or instances attempting conservative management in an attempt to preclude surgery. The therapist must report back to the treating clinician with their findings regarding progress or lack thereof.

Managing rotator cuff tears requires an interprofessional team approach, including the clinicians, specialists, orthopedic and surgical nurses, and physical therapists.

Shoulder girdle is the set of bones, consisting of the scapula and clavicle, which attaches each upper limb to the axial skeleton

The appendicular skeleton includes all of the limb bones, plus the bones that unite each limb with the axial skeleton (Figure 8.2). The bones that attach each upper limb to the axial skeleton form the pectoral girdle (shoulder girdle). This consists of two bones, the scapula and clavicle (Figure 8.3). The clavicle (collarbone) is an S-shaped bone located on the anterior side of the shoulder. It is attached on its medial end to the sternum of the thoracic cage, which is part of the axial skeleton. The lateral end of the clavicle articulates (joins) with the scapula just above the shoulder joint. You can easily palpate, or feel with your fingers, the entire length of your clavicle.

Figure 8.2 Axial and Appendicular Skeletons The axial skeleton forms the central axis of the body and consists of the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of the pectoral and pelvic girdles, the limb bones, and the bones of the hands and feet.

Pectoral Girdle

The pectoral girdle consists of the clavicle and the scapula, which serve to attach the upper limb to the sternum of the axial skeleton.

The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It is supported by the clavicle and articulates with the humerus (arm bone) to form the shoulder joint. The scapula is a flat, triangular-shaped bone with a prominent ridge running across its posterior surface. This ridge extends out laterally, where it forms the bony tip of the shoulder and joins with the lateral end of the clavicle. By following along the clavicle, you can palpate out to the bony tip of the shoulder, and from there, you can move back across your posterior shoulder to follow the ridge of the scapula. Move your shoulder around and feel how the clavicle and scapula move together as a unit. Both of these bones serve as important attachment sites for muscles that aid with movements of the shoulder and arm.

The right and left pectoral girdles are not joined to each other, allowing each to operate independently. In addition, the clavicle of each pectoral girdle is anchored to the axial skeleton by a single, highly mobile joint. This allows for the extensive mobility of the entire pectoral girdle, which in turn enhances movements of the shoulder and upper limb.

Clavicle

A bone on the ventral side of the shoulder girdle, which in humans is commonly called the collar bone.

Scapula

Also called the shoulder blade, it is a flat triangular bone, a pair of which form the back part of the shoulder girdle.

Introduction

The glenohumeral joint is structurally a ball-and-socket joint and functionally is considered a diarthrodial, multiaxial, joint. The glenohumeral articulation involves the humeral head with the glenoid cavity of the scapula, and it represents the major articulation of the shoulder girdle. The latter also includes minor articulations of the sternoclavicular (SC), acromioclavicular (AC), and scapulothoracic joints. The glenohumeral joint ranks as the most mobile joint of the human body. The static and dynamic stabilizing structures allow for extreme degrees of motion in multiple planes of the body that predisposes the joint to instability events.

Structure and Function

The glenohumeral joint is a ball and socket joint that includes a complex, dynamic, articulation between the glenoid of the scapula and the proximal humerus. Specifically, it is the head of the humerus that contacts the glenoid cavity (or fossa) of the scapula. The articulating surfaces of both have a lining of articular cartilage. The glenoid cavity is a shallow osseous element that is structurally deepened by a fibrocartilagenous rim, the glenoid labrum,  that spans the osseous periphery of the vault. The labrum is continuous with the tendon of the biceps brachii at its superior aspect.

Due to the loose joint capsule, and the relative size of the humeral head compared to the shallow glenoid fossa (4:1 ratio in surface area), it is one of the most mobile joints in the human body. This increased mobility contributes to it being the most commonly dislocated joint.

The glenohumeral joint is enclosed by a joint capsule that encapsulates the structures of the joint in a fibrous sheath. Structurally the joint capsule wraps around the anatomic neck of the humerus to the rim of the glenoid fossa.  While the joint capsule itself is a contiguous supportive structure surrounding the articulating elements, the capsulolabral complexes include important characteristic thickened bands that constitute the glenohumeral ligaments. First described in 1829, the glenohumeral ligaments do not act as traditional ligaments that carry a pure tensile force along their length, but rather, the glenohumeral ligaments become taut at varying positions of abduction and humeral rotation. A synovial membrane forms the lining of the inner surface of the joint capsule. This membrane produces synovial fluid to reduce friction between the articular surfaces.

In addition to the synovial fluid reducing friction within the joint, there are multiple synovial bursae present as well. These bursae functionally act as a cushion between joint structures, such as tendons. The most clinically significant are the subacromial and subscapular bursae. There are numerous, including:

• Subacromial/subdeltoid bursa - This structure lies between the deltoid muscle and joint capsule in the superolateral aspect of the joint. It is superficial to the supraspinatus tendon. This bursa reduces friction underneath the deltoid muscle, allowing an increased range of motion. This bursa, excluding anatomic variants, does not usually communicate with the shoulder joint itself.

• Subcoracoid bursa -  This bursa is between the coracoid process and the subscapularis.

• Subscapular bursa - is located between the tendon of the subscapularis muscle and the capsule. It functions to reduces frictional damage to the subscapularis muscle during movement of the glenohumeral joint, particularly during internal rotation.  

Static stabilizing structures include the osseous articular anatomy and joint congruity, the glenoid labrum, the glenohumeral ligaments, joint capsule, and negative intraarticular pressure:

• Glenohumeral ligaments- Composed of a superior, middle, and inferior ligament, these three ligaments combine to form the glenohumeral joint capsule connecting the glenoid fossa to the humerus. Due to their location, they protect the shoulder and prevent it from dislocating anteriorly — this group of ligaments functions as the primary stabilizers of the joint.

• Coracoclavicular ligament – This ligament is composed of the conoid and trapezoid ligaments and spans from the coracoid process to the clavicle. It functions to maintain the position of the clavicle in conjunction with the acromioclavicular ligament. Strong forces can rupture these ligaments during acromioclavicular joint injuries.

• Coracohumeral ligament – This ligament supports the superior aspect of the joint capsule. It is a dense fibrous structure connecting the base of the coracoid process to the greater and lesser tuberosities. At its origin, the ligament is thin and broad, measuring about 2 cm in diameter at the base of the coracoid. Laterally, the CHL separates into two distinct bands that envelope the Long Head Biceps tendon at the proximal extent of the bicipital groove.

Dynamic stabilizing structures include the Long head biceps tendon, rotator cuff muscles, the rotator interval, and the periscapular muscles.

Soft tissue pulley system and Long head of the biceps tendon (LHBT) 

• The subscapularis has superficial and deep fibers that envelope the bicipital groove, creating the “roof” and “floor,” respectively. These fibers also coalesce with those from the supraspinatus and superior glenohumeral ligament/coracohumeral ligament complex.  These structures attach intimately at the lesser tuberosity to create the proximal and medial aspect of the pulley system, with soft tissue extensions serving to further envelope the LHBT in the bicipital groove. Once the LHBT exits the groove, it takes a 30- to 40-degree turn as it heads toward the supraglenoid tubercle and glenoid labrum. Thus, the proximal soft tissue elements of the groove are especially critical for the overall stability of the entire biceps complex.

The glenohumeral joint possesses the capability of allowing an extreme range of motion in multiple planes.

• Flexion – Defined as bringing the upper limb anterior in the sagittal plane. The usual range of motion is 180 degrees. The main flexors of the shoulder are the anterior deltoid, coracobrachialis, and pectoralis major. Biceps brachii also weakly assists in this action.

• Extension—Defined as bringing the upper limb posterior in a sagittal plane. The normal range of motion is 45 to 60 degrees. The main extensors of the shoulder are the posterior deltoid, latissimus dorsi, and teres major.

• Internal rotation—Defined as rotation toward the midline along a vertical axis. The normal range of motion is 70 to 90 degrees. The internal rotation muscles are the subscapularis, pectoralis major, latissimus dorsi, teres major, and the anterior aspect of the deltoid.

• External rotation - Defined as rotation away from the midline along a vertical axis. The normal range of motion is 90 degrees. Primarily infraspinatus and teres minor are responsible for the motion.

• Adduction – Defined as bringing the upper limb towards the midline in the coronal plane. Pectoralis major, latissimus dorsi, and teres major are the muscles primarily responsible for shoulder adduction.

• Abduction - Defined as bringing the upper limb away from the midline in the coronal plane. The normal range of motion is 150 degrees. Due to the ability to differentiate several pathologies by the range of motion of the glenohumeral joint in this plane of motion, it is essential to understand how different muscles contribute to this action.

I. The supraspinatus is responsible for the first 0 to 15 degrees of abduction

II. The middle fibers of the deltoid are responsible for approximately 15 to 90 degrees of abduction following

III. Scapular rotation due to the actions of the trapezius and serratus anterior allow for abduction beyond 90 degrees 

Embryology

The development of the skeletal shoulder consists of both forms of ossification processes. The clavicle undergoes intramembranous ossification, which is the direct laying down of bone into the mesenchyme. The rest of the bony structures of the shoulder form by endochondral ossification.  The mesoderm germ layer forms nearly all of the connective tissues of the musculoskeletal system, including the glenohumeral joint. Musculoskeletal and limb abnormalities, due to both environmental and genetic contributions, are one of the largest groups of congenital abnormalities.

Blood Supply and Lymphatics

The glenohumeral joint receives vascular supply via the posterior and anterior circumflex humeral arteries, both of which are branches of the axillary artery. The predominant arterial blood supply to the humeral head is via the posterior humeral circumflex artery. The arcuate artery is the extension/continuation of the ascending branch of the anterior humeral circumflex. It enters the bicipital groove and supplies most of the humeral head. A branch of the thyrocervical trunk, the subscapular arteries, and its branches, also contribute to the blood supply of the shoulder.

The majority of the lymph nodes in the upper extremity are located within the axilla. These can be divided based on location into five main groups: pectoral, subscapular, humeral, central, and apical. Efferent vessels coming from the apical axillary nodes travel through the cervico-axillary canal and then converge to form the subclavian lymphatic trunk. This trunk will either continue to enter the right venous angle or drain directly into the thoracic duct on the right and left, respectively. Removal and analysis of axillary lymph nodes is often an essential tool in the staging of breast cancers. The interruption of lymphatic drainage from the upper limb can, however, result in lymphoedema, a condition where accumulated lymph in the subcutaneous tissue leads to painful swelling of the upper limb.

Nerves

Innervation of the glenohumeral joint is a function of the suprascapular, lateral pectoral, and axillary nerves. All of the nerves supplying the glenohumeral joint originate from the brachial plexus, which is a network of nerves formed by the ventral rami of the lower four cervical nerves and the first thoracic nerve (C5, C6, C7, C8, and T1). The anatomy of the axillary nerve is critical as it is close to the glenohumeral joint. The axillary nerve arises from the posterior cord of the brachial plexus, courses along the subscapularis to its inferior edge, and then passes closely along the inferior glenohumeral joint capsule. It then courses posterior to the humerus, wraps around the surgical neck of the humerus with the posterior circumflex artery, running in the deep deltoid fascia.

Muscles

The four muscles that constitute the rotator cuff are the supraspinatus, infraspinatus, subscapularis, and teres minor.  The primary biomechanical role of the rotator cuff is stabilizing the glenohumeral joint by compressing the humeral head against the glenoid. The rotator cuff muscles thus act as dynamic stabilizers of the glenohumeral joint. In addition to the rotator cuff, the LHBT has a controversial contribution and overall role in glenohumeral joint stability. The current consensus agreement is that the stabilizing role of the LHBT in regards to the glenohumeral joint becomes more important and/or relevant in the setting of rotator cuff dysfunction.

The supraspinatus primarily abducts the shoulder and is responsible for the initial 15 degrees of abduction. The infraspinatus and teres minor aid in external rotation of the shoulder. The subscapularis muscle aids in internal rotation of the shoulder. The supraspinatus and infraspinatus muscles are innervated by the suprascapular nerve. The teres minor is innervated by the axillary nerve and subscapularis by the subscapular nerve.

Clinical Significance

Shoulder dislocations

The glenohumeral joint is by far the most commonly dislocated joint of the body, accounting for up to 45% of dislocations. Anterior dislocation accounts for 96% of cases and is often the result of a force directed to the shoulder joint while the arm is in abduction and external rotation. Posterior dislocation is the second most common direction of dislocation, accounting for 2% to 4% of cases. Posterior dislocations are usually due to indirect mechanisms such as electric shock or convulsions, causing contraction of the relatively stronger internal rotators of the shoulder (latissimus dorsi, pectoralis major, and subscapularis muscles).

Glenohumeral joint dislocation has an incidence of approximately 17 per 100,000 a year, with a peak incidence among males in the 21 to 30 year age range and females in the 61 to 80 year age range. Treatment typically involves analgesics and closed reduction, with some patients requiring subsequent surgical correction, especially those with concurrent soft tissue injuries resulting in recurrent shoulder dislocations. The axillary nerve courses within close proximity of the glenohumeral joint and wrap around the neck of the humerus, and can incur damage during dislocation or subsequent attempts at reduction of the dislocated joint. Injury to the axillary nerves causes a loss of sensation over the lateral shoulder and paralysis of the deltoid. Hill-Sachs lesions (impaction fracture of the posterolateral humeral head against anteroinferior glenoid) and Bankart lesions (detachment of anteroinferior labrum with or without an avulsion fracture) can also occur following anterior dislocation. The recurrence rate of glenohumeral joint dislocation is approximately 50% on average; however, there is a significant increase in the risk of reoccurrence with a younger age of initial dislocation.

Adhesive capsulitis

Adhesive capsulitis is a disorder of unclear cause in which the glenohumeral capsule becomes inflamed and stiff, significantly restricting motion and can cause chronic pain. The pain is usually constant, worse at night. Incidence is approximately 3% of the general population, with some controversy over this number due to concerns of over-diagnosis of this disorder. It occurs more commonly in women aged 40 to 70 years of age. Risk factors for adhesive capsulitis include diabetes mellitus, connective tissue diseases, thyroid disease, and heart disease. It has associations as an adverse effect of specific highly active antiretroviral therapies as well. Management consists of physical and occupational therapy, medication (analgesics/NSAIDs), intraarticular steroid injection, and, in rare cases, surgery. Manipulation under general anesthesia may also be an option, which disrupts the scar tissue and adhesions within the joint. Over time, most people regain approximately 90% of their shoulder range of motion.

Rotator cuff injuries  

The tendons of the rotator cuff are often under heavy strain as they function in stabilizing the glenohumeral joint. Therefore, the pathology of these rotator cuff tendons is relatively common. Rotator cuff conditions are the most common source of shoulder pain for primary care office visits. A wide spectrum of pathology exists, including sub-acromial bursitis, rotator cuff tendinitis, shoulder impingement, and rotator cuff tears. Patients with impingements/subacromial bursitis will often complain of pain with overhead activity. Most patients will have a resolution of their symptoms with properly designed and performed physical therapy programs. If patients do not improve after a period of conservative management, however, evaluation by an orthopedic surgeon and/or sports medicine specialist is often necessary. Rotator cuff tendon tears can be chronic due to tendon degeneration or shoulder impingement, or it can be acute following an injury. It could be partial thickness tear or full-thickness tear.