Properly described as a complex, the shoulder consists of a system of three interconnecting bones, several ligaments and groups of muscles. Due to its complexity, the shoulder enjoys several unique features:

• The range of motion (ROM) of the shoulder is the greatest of all the joints in the body.
• The shoulder joint is very shallow, and therefore not very stable.
• Ligaments hold every other joint in the body, but the shoulder is held together by the capsule and the small rotator cuff muscles.

It is impossible to discuss damage to the shoulder without using the names of bones, muscles and nerves, and it is very difficult to describe the function and location of the anatomy without using some technical language. The following lays out basic anatomy and its common terms before progressing to the discussion of diseases and their treatments.

Skeletal Structures

The shoulder complex is intricately connected with the spine. The spine consists of different type of vertebrae. From top to bottom they are: cervical (there are eight), thoracic (12), lumbar (five), sacral (five, but they're fused together) and coccygeal (four are fused together). For the shoulder only the cervical and the first thoracic vertebrae play a role. When they are mentioned they are labeled as such: C5-C6. This indicates the 5th and 6th cervical vertebrae.

The shoulder consists of three bones: the clavicle (collar bone), the humerus (the long bone that runs from the shoulder to the elbow) and the scapula (shoulder blade). The humerus forms a joint with the glenoid fossa (part of the scapula), the clavicle joins with the acromion of the scapula (forming the AC joint) and the coracoid process of the scapula floats over the entire structure.

The clavicle acts as a strut keeping the shoulder and arm braced away from the thorax, also forming a canal between the neck and the arm. The clavicle sits above this canal, protecting the vessels running to and from the arm.

The scapula is built like a big triangle, made from smaller triangles. The spine of the scapula divides the scapula into top and bottom parts.

1. The infraspinous fossa - the lower triangle of the scapula provides attachment sites for the large muscles.
2. The spine of the scapula - the divider of the scapula, it runs from the medial (side of the bone towards the centre line of the body) edge to the lateral (side of the bone towards the outside line of the body) edge. At the lateral edge it forms the acromion. The acromion is an arch that turns toward the front of the body. The clavicle attaches to the acromion at the top of the shoulder. This structure can be felt by following the collarbone out to where it enters the shoulder and forms the A-C(acromion/clavicular) joint.
3. The glenoid fossa - this is found on the front side on the lateral edge of the spine of the scapula. The glenoid forms the flat part (facing laterally) that provides the rigid joint for the head of the humerus. This is where the arm joins the rest of the body.
4. The coracoid process - this beak-like projection is found above the glenoid from the lateral edge of the upper triangle. The arm rotates below the coracoid and several ligaments attach here.

Despite the apparent singularity of the humerus, knowledge of its structure is essential in understanding the working of the shoulder. The four structures of importance for the shoulder are:

1. The head - This is the top of the upper arm. It forms a ball-like structure. The head of the humerus fits into the shallow glenoid fossa.
2. The inter-tubercular groove of the humerus - is the groove between the greater and lesser tubercles.
3. The greater tubercle - A tubercle is an elevated rough spot on bone to which muscles attach. Found at the lateral margin of the humerus, the greater tubercle is the point of attachment of some of the rotator cuff muscles.
4. The lesser tubercle - is found at the front of the humerus. This anchors one of the muscles of the rotator cuff as well as forming a border for the inter-tubercular groove.

Musculature

The pectoralis muscles (major and minor) run from the front of the chest to the shoulder. The pectoralis major is the largest muscle of the chest. It helps to bring the arm toward the body on the side and in front of the body from the sides. It attaches to the clavicle and to the inter-tubercular groove from the sternum; acts as an adductor¹; and performs internal rotation.

The pectoralis minor sits underneath the pectoralis major and is covered by the clavipectoral fascia.

The subclavius runs immediately below the clavicle; it acts to lower the clavicle.

The serratus anterior protracts the scapula and pulls it forward. Lying medial to the axilla (underarm or armpit²), these muscles originate from the first eight ribs. The axilla is defined by the rib cage and serratus anterior medially, the long head of biceps laterally, and by the anterior and posterior axillary folds³. The anterior fold consists of the pectoralis major and minor, while the posterior fold is made by the teres major and latissimus dorsi. The subscapularis and scapula form the posterior wall of the axilla nearer the top of the shoulder joint.

The Posterior Muscles

• The trapezius is the biggest muscle of the neck. This muscle runs from the neck and spine out to the shoulder. This large muscle moves the shoulder blade up and down, as well as pointing the shoulder blade toward the back⁴.
• The latissimus dorsi are the large muscles along the body under the armpit. This muscle extends, adducts, and internally rotates the arm. It also raises the body toward the arm while climbing.
• The levator scapulae runs from the C1-C4 cervical vertebrae to the top of the scapula. This muscle elevates the scapula and rotates is, pointing the glenoid fossa down.
• The deltoid is the biggest muscle in the shoulder. This is the muscle that runs over the shoulder joint down to the arm. Parts of the muscle come from the back, the top and the front of the shoulder and run to the top of the outer part of the arm. The deltoid has many functions: it flexes, extends, medially rotates, laterally rotates, and adducts the arm.
• The teres major runs from the bottom of the scapula to the inter-tubercular groove. This muscle abducts⁵ and medially rotates the arm.

The rotator cuff muscles are commonly referred to as the SITS muscles. As a group these muscles keep the head of the humerus in the glenoid fossa.

1. The supraspinatus attaches from the scapula to the greater tubercle and initiates and assists the deltoid muscle in abduction.
2. The infraspinatus muscle runs from the scapula to the greater tubercle and laterally rotates the arm.
3. The teres minor attaches from the scapula to the greater tubercle and serves the same function as the infraspinatus.
4. The subscapularis attaches from the scapula to the lesser tubercle of the humerus. This muscle medially rotates the arm and adducts it.

Mechanisms of Injury

Because of the intricacy, the shoulder carries the risk of unique injury. Despite the variety and age of patients and the range of injuries that may occur, shoulder problems can be grouped into several general categories. For patients who have been receiving some prior treatment, shoulder complaints often arise from overuse. For these patients injuries might occur secondary to wheelchair use, or to compensate for a functional loss such as amputation or stroke. Many patients who have already suffered one injury are prone to a second. For patients with no prior problems, injuries may include:

Overuse

Overuse injuries occur from repeating the same activity over and over. The repeated activity (such as throwing, lifting, swimming and manual labour of repetitive nature) stresses joints and other tissues and may lead to irritation and inflammation. Examples of overuse injuries include bursitis, tendinitis and muscle strains. A frozen shoulder (adhesive capsilitis) involves stiffness and decreased shoulder movement and may follow an overuse injury.

Trauma

A trauma is any sudden injury. With the shoulder, common injuries include broken bones, dislocation, and acute muscle tears. Many of these injuries occur when the head of the humerus slides out of the glenoid fossa, stretching the capsule and the SITS muscles. The sliding can result in dislocation, which can damage the axillary nerve leading to weakness of the deltoid and teres minor.

The brachial plexus are the nerve fibers which run from the spine through the neck and down the arm. The network provides feeling and movement for the shoulder, arm and hand. Brachial plexus injuries, for example Erb's Palsy - often presents with arm straight and wrist fully bent; weak biceps and triceps; and the shoulder joint may be unstable. Nerves re-grow at the approximate rate of one mm per day (one inch per month).

Chronic damage

The bony structures of the shoulder undergo remolding as time passes. That is, the bones are constantly being broken down and remade. During this process changes in the typical layout of the bone may occur. Newly formed irregular bone that irritates the structures around it or compresses nerves can be a source of pain and other disability. Bone spurs, pitting of the bone and fusion of two bones are examples of this type of injury.

Dislocation

Dislocation of joints is common in young children as they are still developing. Many present to GPs and Emergency Departments (accompanied by fretting parents) from the ages of 13 months to three years with dislocated elbows and shoulders. It's quite a common injury and most probably caused by jerking the child away from dangerous situations like traffic (better a dislocated shoulder than a dead child).

Common Causes of Shoulder Pain

Rotator cuff tear

A torn rotator cuff commonly causes shoulder pain in adults. This is often the result of overuse in high torque activities, like throwing a cricket ball (although repeated use of the arm for throwing any handy objects would do the same thing). When the tendon of the supraspinatus muscle tears completely the shoulder cannot abduct or forward flex, and cannot be held in that position once passively placed there. This change distinguishes between a completely torn tendon and a partial tear. In fact, the clinical test for torn rotator cuff is called the 'Drop Arm Test'. Diagnosis of rotator cuff tear can be made clinically on occasion, but MRI and arthrographic studies are typically used to aid in uncertain situations or to evaluate the extent of the disease. Since these tests are performed even if the diagnosis is certain clinically, they are routinely ordered by physicians in a rotator cuff work-up.

Impingement Syndrome AKA Entrapment Syndrome

As the arm moves overhead from the side, the greater tuberosity along the upper and outer edge of the humerus grinds beneath the acromion of the shoulder blade. This grinding impinges upon nerves that runs through this space.

The acromion forms the upper boundary for the shoulder. In this condition the muscles that cover the greater tuberosity undergo deterioration from rubbing against the acromion. This is sometimes called rotator cuff tendinitis or subacromial bursitis.

Tendinitis

In acute tendinitis the arm can lift overhead from the side to about 60°⁶ without pain and beyond 120° without pain (the humerus has rotated behind the acromion); but adducting between 60-120° causes pain. The painful arc is due to the rubbing of an irritated tendon against a rigid acromion. When the arm is rested, the pain typically subsides as the inflammation decreases.

Bursitis

As the subacromial bursa, which lies beneath the supraspinatus tendon, has a common sheath, bursitis often mimics tendinitis.
In fact, in both injuries the same motion causes the irritation. The difference between the two diseases is a difference of origin. The source of the pain differs, but all other features are the same.

Adhesive Bursitis AKA 'Frozen Shoulder'

Inflammation of tissues in the glenohumeral joint can result in adhesive capsulitis/adhesive bursitis. The soft tissue on either the humerus side or the glenoid side become sticky. These adhesions cause pain and restriction of motion in all directions. The immobility is severe enough that the shoulder cannot be moved passively.

In patients with a frozen shoulder, shoulder-hand syndrome, a possible manifestation of reflex sympathetic dystrophy must be ruled out. Reflex sympathetic dystrophy (RSD or algoneurodystrophy) can be fatal if unrecognized and is common in patients with damage to the central nervous system. Although rare, the severity of the disorder warrants immediate investigation.

Referred pain

Referred pain means that damage somewhere else causes pain in the shoulder. Heart problems (like a heart attack), problems with the abdomen (like gall stones), and problems with the lungs (like pleuritis, an inflammation of the lining of the lungs that causes pain when you take a breath) may cause referred pain. Since many of these problems are very serious, they must be ruled out before other more benign diagnoses are pursued.

Degenerative Joint Disease

Bony changes in the shoulder can cause irritation in many different ways. The most common cause is osteoarthritis. Rheumatoid arthritis, congenital diseases of the bones (like Marfan Syndrome) and malformation of bone⁷ may also lead to shoulder pain. These diseases often shrink the window beneath the acromion crushing structures that lie beneath.

Causes of Shoulder Weakness

The causes of muscle weakness are not limited to the shoulder. The diseases listed below may affect other muscle groups. Their involvement in shoulder weakness might coincide with involvement of other muscle groups.

Shoulder weakness results from injury to either the muscle, the nervous system or where the nerve meets the muscles: the Neuromuscular Junction (NMJ).

Myopathy

Myopathy is disease of the muscle. Strangely, if the disease harms the muscle itself then the muscle does not atrophy, or lose muscle mass. In this situation not all muscle fibers are injured, the ones that are healthy can still flex. The ability of the damaged muscle fibers to contract diminishes. The muscle itself may range from almost full function to complete weakness. In myopathy the ability of the nerve to fire is totally intact.

Causes of myopathy: Steroids; Alcohol; AZT; Hypothyroidism; Duchenne's muscular dystrophy; Polymyositis; AIDS; Mitochondrial disease.

Neuropathy

Neuropathy means disease of the nervous system. In this situation the nerves are unable to carry their signal to the muscles. If the muscles get stimulated they will contract, but with neuropathy they are not being stimulated by the nerves. The muscle atrophy, (loss of muscle mass), is a clinical sign of neuropathy. Other signs include loss of sensation in the affected limb and/or decreased reflexes.

Causes of peripheral neuropathy: Diabetes; Alcohol; Guillain-Barre Syndrome; Trauma; Hereditary; Environmental toxins; Rheumatic (collagen vascular); Amyloid; Paraneoplastic; Infections; Systemic diseases (eg lupus); Seizures; and Tumors. Patients who have suffered oxygen deprivation may continue to live in a persistent vegetative state (PVS) but their muscles will atrophy from lack of use.

Neuromuscular Junction dysfunction

Diseases of the NMJ (like myasthenia gravis) are characterized by fatigability, which means that the muscle quickly weakens as a manoeuvre is performed multiple times in succession.

Therapy

Depending on the reason for the disability, patients will be treated by either neurologists or physiotherapists. Frozen shoulder is treated with corticosteroid injections into the shoulder joint.

Surgery is possible in some cases, affording the patient some pain relief.

New treatments are being introduced as technological advancements are made, and scientists are hoping to eventually rebuild a shoulder joint which will last the patient's lifetime. Still in its early stages is the interactive 'robot therapist' which helps patients to re-learn movements after paralysis.

Patients recovering from a stroke can find their physical abilities impaired. By improving movement in the shoulder and upper arm, patients can do simple things like use a computer mouse. When you have lost the ability to move your arm, even small improvements are a success and an important first step towards regaining mobility.