The shoulder is structurally and functionally complex as it is one of the most freely moveable areas in the human body due to the articulation at the glenohumeral joint. It contains the shoulder girdle, which connects the upper limb to the axial skeleton via the sternoclavicular joint. The high range of motion of the shoulder comes at the expense of decreased stability of the joint, and it is prone to dislocation and injury.
The shoulder girdle is composed of the clavicle and the scapula, which articulates with the proximal humerus of the upper limb. Four joints are present in the shoulder: the sternoclavicular (SC), acromioclavicular (AC), and scapulothoracic joints, and glenohumeral joint.
The sternoclavicular joint is a synovial saddle joint and is the only joint that connects the upper limb to the axial skeleton. It connects the clavicle and the manubrium of the sternum and gets stabilization from by the costoclavicular ligament. The acromioclavicular joint is a plane synovial joint that connects the acromion of the scapula to the clavicle. It receives stabilization from the coracoacromial, coracoclavicular, and super and inferior acromioclavicular ligaments. The scapulothoracic joint is not a true joint, but rather the articulation of the scapula gliding over the posterior thoracic cage.
The glenohumeral joint is a highly moveable ball-and-socket joint that is stabilized by the rotator cuff muscles that attach to the joint capsule, as well as the tendons of the biceps and triceps brachii. The humeral head articulates with the glenoid fossa of the scapula. It is a shallow articulation, as the fossa accommodates less than 1/3 of the humeral head. The labrum, a fibrocartilaginous ring that is a continuation of the biceps brachii tendon, attaches to the outer rim of the glenoid fossa and provides additional depth and stability securing the humeral head. A small number of fluid-filled sacs known as bursae surround the capsule and aid in mobility. These are the subacromial, subdeltoid, subscapular, and subcoracoid bursae.
The major movements at the glenohumeral joint are:
All elements of the human body arise from the three primary germ layers in the young embryo: the ectoderm, endoderm, and mesoderm. Cartilage, bone (and marrow), muscles and ligaments, and connective tissue all arise from the mesoderm, which lies between the ectoderm and endoderm.
The axillary artery is the major blood vessel in the shoulder, with many of its branches supplying the area. These branches include the superior thoracic artery, thoracoacromial artery, lateral thoracic artery, subscapular artery, anterior humeral circumflex artery, and posterior humeral circumflex artery. Before becoming the axillary artery after passing beyond the lateral edge of the first rib, the subclavian artery also includes branches that supply the area of the shoulder. The thyrocervical trunk off of the subclavian artery adds the suprascapular artery and the transverse cervical artery. The dorsal scapular artery most often branches off of the subclavian, but may sometimes branch off the transverse cervical artery.
See "Muscles" section for innervations.
The intrinsic muscles of the shoulder connect the scapula and/or clavicle to the humerus. These include
Supraspinatus (Rotator Cuff)
Infraspinatus (Rotator Cuff)
Teres minor (Rotator Cuff)
Subscapularis (Rotator Cuff)
Other muscles that affect movement at the shoulder joint include:
The most common variants are related to the anterosuperior aspect of the shoulder joint, as well as variable development and shape of the bones within the shoulder girdle.
As it contains the most mobile joint in the body, the shoulder is very susceptible to injury. Surgical interventions may be required to repair or replace bones, joints, or tendons. Techniques used include arthroscopy, total arthroplasty, and shaving down bone in cases of impingement.
Shoulder pain affects approximately 18 million Americans a year, most of which are a result of rotator cuff tears. Tears can occur from a mix of trauma, overuse, or age-related degeneration and can be asymptomatic or cause severe pain and decreased mobility. Smoking, hypercholesterolemia, and family history have been shown to predispose to tears. Even with small full-thickness tears, conservative, non-surgical treatment is the first line and may be effective. When it is not, or with larger full-thickness tears, surgical repair is a reliable fix. Rotator cuff tendinitis/impingement presents with pain during overhead activities and results from the tendon of the supraspinatus muscle being pinched down by the acromion in most cases, which can cause inflammation around the tendon and in the fluid-filled bursae surrounding it.
The price of being the most mobile joint in the body is that the glenohumeral joint lacks stability and is subject to dislocation. Anterior dislocations are the most common as they make up 97% of all dislocations. The typical cause by a blow to an abducted, externally rotated and extended extremity. Anterior dislocation may damage the axillary nerve, causing paralysis of the deltoid and decreased cutaneous sensation over the shoulder, as well as ligament tears and fractures. Patients usually recover functionality of the axillary nerve with reduction of the humeral head back into the glenoid fossa. Posterior dislocations are less frequent but are associated with seizures. There is more risk of rotator cuff and ligament tears with posterior dislocations than anterior. Inferior dislocations are very rare and are the result of hyperabduction. They have the highest incidence of axillary nerve and artery damage.
Adhesive capsulitis 
Adhesive capsulitis occurs in 2 to 5% of the population, with most patients being over the age of 55. The thinking is that inflammation in the area of the shoulder capsule causes initial pain as well as capsular fibrosis and adhesions that lead to a decreased range of motion in all planes. Treatment is conservative, with most cases resolving spontaneously. Surgical intervention is reserved for refractory cases and involves releasing the frozen capsule.
Like other joints with extensive use, the shoulder joint is susceptible to wear and tear degeneration of the articular cartilage within the joint. Age, female gender, obesity, anatomical factors, muscle weakness, and joint injury are predisposing factors to the development of osteoarthritis. The bone-on-bone friction causes moderate to severe pain in patients. Treatment is usually conservative, with NSAIDS being the first-line choice. Refractory osteoarthritis may need intra-articular corticosteroid injections to decrease the inflammation. Surgical intervention in the form of arthroplasty is reserved for severe cases in which pharmacotherapy is not relieving symptoms.