The human shoulder is the most mobile joint in the body.  This mobility provides the upper extremity with tremendous range of motion such as adduction, abduction, flexion, extension, internal rotation, external rotation, and 360° circumduction in the sagittal plane. Furthermore, the shoulder allows for scapular protraction, retraction, elevation, and depression. This wide range of motion also makes the shoulder joint unstable.  This instability is compensated for by rotator cuff muscles, tendons, ligaments, and the glenoid labrum. [2, 3, 4, 5, 6]
An image depicting shoulder anatomy can be seen below.
The shoulder or pectoral girdle is composed of the bones that connect the upper extremity to the axial skeleton. Two bones comprise the shoulder girdle. These are the clavicle and scapula.
The scapula is a triangular-shaped bone that functions mainly as a site for muscle attachment. Four rotator cuff muscles that act on the shoulder take their origin from the scapula. These are the supraspinatus, infraspinatus, teres minor, and subscapularis (see the following image).
Additionally, the trapezius, serratus anterior, rhomboids, and levator scapulae insert on the scapula and are responsible for scapular mobility and stability. The scapula is freely moveable, because it is suspended by these muscles. The scapula has 4 processes, the spine, the acromion, the coracoid, and the glenoid. The glenoid cavity (or, alternatively, the glenoid fossa) is set on the expanded aspect of the lateral angle of the scapula. The glenoid cavity is an irregularly shaped oval and has been compared to an inverted comma shape. It articulates with the head of the humerus, forming the glenohumeral joint, which serves as the main joint of the shoulder.
The clavicle is an S-shaped bone that forms the anterior portion of the shoulder girdle that keeps the arm away from the trunk, allowing it to move freely. The clavicle has 2 articulations, the sternoclavicular joint and the acromioclavicular joint. The sternoclavicular joint is formed by the medial aspect of the clavicle articulating with the manubrium of the sternum. This is the only skeletal connection between the axial skeleton and the upper extremity. Furthermore, the clavicle provides protection for the subclavian artery, subclavian vein, and brachial plexus posteriorly and inferiorly. See the images below.
The proximal articular surface of the humerus is termed the humeral head. The humeral head articulates against the shallow glenoid cavity. Only 25% of the humeral head surface makes contact with the glenoid cavity.  The glenoid labrum, a fibrocartilaginous ring attached to the outer rim of the glenoid cavity, provides additional depth and stability.
The sternoclavicular joint is the sole connection between the axial skeleton and the upper extremity. The sternoclavicular joint allows 30-35 º of upward elevation, 35 º of anteroposterior movement, and 44-50 º of rotation about the long axis of the clavicle. 
The acromioclavicular (AC) joint is the only articulation between the clavicle and scapula. It is formed by the distal clavicle articulating with the acromion of the scapula. Little motion exists in this joint. The AC joint is an encapsulated diarthrodial joint held together by its joint capsule and the coracoacromial ligaments: the trapezoid and conoid ligaments.
The glenohumeral joint is the main articulation of the shoulder joint. It is the multiaxial ball-and-socket synovial joint formed by the articular surfaces of the glenoid cavity and the head of the humerus. The glenoid cavity depth is increased by a rim of fibrocartilage that surrounds it. This rim of fibrocartilage is the glenoid labrum.
The glenoid labrum is a ring composed of mostly dense fibrous tissue. The average depth of the glenoid cavity is 2.5 mm, but the labrum serves to increase this depth. Although the labrum increases the depth and volume of the glenoid cavity, it does not seem to increase the stability of the glenohumeral joint. 
The conoid and trapezoid ligaments comprise the coracoclavicular ligaments (CCLs) (see the image below). They function to maintain the articulation of the clavicle with the coracoid process of the scapula. Studies have concluded that the coracoclavicular ligaments are the primary restraint to superior and posterior clavicular dislocation. 
Three glenohumeral ligaments exist: (1) the superior glenohumeral ligament (SGHL), (2) the middle glenohumeral ligament (MGHL), and (3) the inferior glenohumeral ligament (IGHL). The SGHL has a variable origin and inserts on the humerus near the lesser tubercle; this ligament resists inferior translation of the humeral head in the adducted shoulder. The MGHL originates from the labrum and inserts on the humerus medial to the lesser tubercle; this ligament resists inferior translation in the adducted and externally rotated shoulder. The IGHL originates from the labrum and the adjacent glenoid neck, inserts on the anatomic neck of the humerus, and resists humeral head anterior and posterior translation. Furthermore, the IGHL is the primary restraint to inferior dislocation in the abducted shoulder.
The coracohumeral ligament (CHL) originates on the base and lateral border of the coracoid process of the scapula and inserts on the greater tubercle. The biomechanical function of this ligament is not fully understood; however, it appears to have suspensory function of the humeral head.
The supraspinatus, infraspinatus, teres minor, and subscapularis muscles comprise the rotator cuff (see the following image) (see Table 1, below). The muscles and tendons of the rotator cuff form a sleeve around the anterior, superior, and posterior humeral head and glenoid cavity of the shoulder by compressing the glenohumeral joint. In addition to stabilization, the rotator cuff provides the shoulder with tremendous mobility.
Table 1. Origins, Insertions, Actions, and Nerve Supplies of the Rotator Cuff Muscles (Open Table in a new window)
|Supraspinatus||Supraspinous fossa||Greater tubercle of humerus||Abduction of the arm to approximately 30 º||Suprascapular nerve|
|Infraspinatus||Infraspinous fossa||Greater tubercle of humerus||External (lateral)rotation of the arm||Suprascapular nerve|
|Teres minor||Upper 2/3 of the lateral border of the scapula||Greater tubercle of humerus||External (lateral) rotation of the arm||Axillary nerve|
|Subscapularis||Subscapular fossa on the anterior surface of the scapula||Lesser tubercle of humerus||Internal (medial) rotation of the arm||Upper and lower subscapular nerves|
The subacromial bursa lies on the superior aspect of the supraspinatus tendon (see the images below). The bursa acts to cushion and reduce friction during motion between the overlying bone of the acromion and the soft rotator cuff muscles below. It often extends laterally to be continuous with the subdeltoid bursa.
Connective tissues and muscles of the shoulder
The shoulder complex is composed of many different tissue types, and it is the connective tissue that provides the supportive framework for the shoulder's many functions. The different types of connective tissues in the shoulder are bone, articular cartilage, ligaments, joint capsules, and bursa (see Gross Anatomy).
Labrum, clavicle, and scapular notch variability
Several minor anatomic variations exist in the attachment sites, size, and histologic composition of the labrum. These variations are not considered pathologic.
Variations in the shape of the clavicle are considered normal and are not usually pathologic. These variations may range from an almost straight bone to one with exaggerated curves. Another variation of the clavicle that is present in 6-10% of the population is termed the canalis nervi supraclavicularis.  In this variation, a foramen forms through the clavicle, and the medial supraclavicular nerve passes through this accessory osseous canal.
The scapular notch varies in size and shape. The notch is bridged by the superior transverse scapular ligament. This ligament ossifies in 10% of patients, producing a bony foramen for the suprascapular nerve. 
Acromion morphology variability
Bigliani et al separated acromions into 3 categories based on their shape and their correlation with rotator cuff tears (see the image below), as follows:
Type I: Flat undersurface of the acromion (This type has the lowest risk for impingement syndrome.)
Type II: Curved undersurface of the acromion
Type III: Hooked undersurface of the acromion (This type has the highest correlation with subacromial pathology.)Acromion types.
Sprengel deformity is a congenital deformity; this is actually a composite of deformities caused by an undescended, hypoplastic scapula. People with Sprengel deformity have limited range of motion in abduction at the shoulder.  Such deformities include cosmetic concerns (eg, a lump in the back and the appearance of a short neck). Also, 47% of people with Sprengel deformity develop scoliosis, and 29% develop Klippel-Feil syndrome. 
Acute Shoulder Injury
The clavicle plays a significant role in shoulder stability, strength, and range of motion. However, clavicle fractures are very common and account for 5% of all fractures in adults. Clavicle fractures can be categorized into the following 3 groups, as classified by Allman  :
Group 1: A fracture in the middle of the clavicle; the most common clavicle fracture
Group 2: Fracture on the lateral one third of the clavicle; osteoarthritis often develops after a group 2 fracture if the fracture involves the acromioclavicular (AC) joint
Group 3: Fracture on the medial one third of the clavicle; the rarest from of clavicle fracture
Proximal humerus fracture
The anatomic neck of the humerus lies at the junction of the humeral head and the tubercles. Fractures of the anatomic neck of the humeral head are quite rare and have a poor prognosis, because the fracture usually disrupts blood supply to the humeral head. The surgical neck of the humerus is distal to the tubercles. Fractures of the surgical neck are more common and have a better prognosis.
The glenohumeral joint is the major articulation of the shoulder joint. Dislocation of the glenohumeral joint occurs when the humeral head is moved out of contact with the glenoid cavity. Almost 85% of shoulder dislocations are anterior dislocations.  An anterior dislocation is likely to occur when the arm is abducted, extended, and externally rotated. Posterior dislocation of the glenohumeral joint is rare but is more likely to occur when the arm is adducted and internally (medially) rotated. Violent muscle contractions during a seizure or electrocution may also produce a posterior glenohumeral dislocation.
Acromioclavicular joint sprain or dislocation (shoulder separation)
The AC joint is frequently injured in athletes. The injury commonly occurs when direct force is applied to the acromion with the arm adducted. The force causes the acromion to suddenly move inferiorly, which first strains or tears the AC ligaments and may subsequently strain or tear the coracoclavicular ligaments as well.
Rotator cuff tear
Rotator cuff tears are common injuries; such a diagnosis indicates one or more of the rotator cuff tendons have torn. The injury may be result of chronic impingement and tendonitis that has progressed, or it may refer to an acute injury such as a fall or direct trauma. People with a rotator cuff tear may experience pain and weakness in their shoulder.
Inflammation of the bursa is relatively rare but may occur.
People that participate in repetitive overhead activities such as swimming or throwing a ball have an increased risk of labral tear. A labral tear may be asymptomatic or manifest as shoulder instability, pain, or crepitus.
Glenohumeral osteoarthritis is a slowly progressive arthropathy that is caused by the loss or destruction of articular cartilage. This is usually a condition that develops as people age and their articular cartilage wears down. However, it can also be due to trauma such as a humeral head fracture, shoulder dislocation, or rotator cuff tendon tears.
Primary adhesive capsulitis causes a painful and stiff shoulder usually without a known inciting event. The stiff glenohumeral joint is most likely a result of chronic inflammation and fibrosis. Adhesive capsulitis has 3 phases, and each phase typically lasts 4-6 weeks, with wide variability. The 3 phases are as follows:
- "Freezing phase": Spontaneous pain and stiffness in the shoulder
- "Frozen phase": Increased stiffness and stable or decreased pain
- "Thawing phase": Increased range of motion and decreased pain