Mechanisms of Injury

Overuse

Overuse injuries occur from repeating the same activity over and over. The repeated activity (such as throwing, lifting, or swimming) stresses joints and other tissues and may lead to irritation and inflammation. This is called an overuse injury, even though no obvious injury occurred. 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 of shoulder, 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 or muscle treas.

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.

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 baseball. 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 occassion, 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 boundry 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*¹ without pain and beyond 120* without pain (the humerus has rotated behind the acromion), but abducting 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

Sharing a common sheath with the supraspinatus tendon, 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 differes, 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 imobility 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 dystophy 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 somwhere else causes the pain in the shoulder. Heart problems (like a heart attack), problems with the abdomen (like gall stones), and problems with the lungs (like pleuritis) may cause referred pain. Since many of these problems are very serious, they must be ruled out before other more benign diagnoses are persued.

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 and malformation of bone² 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 meet the muscles, the NMJ³.

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 healthy muscle fibres are still functional. The muscle itself may range from almost full function to complete weakness. In myopathy the ability of the nerve to fire is totally intact.

Neuropathy means disease to the nerves. In this situation the nerves are unable to carry their signal to the muscles. If the muscles get stimulated they will contract, but they are not getting stimulated by the nerves. In these diseases the muscle atrophies. The muscle atrophy, or loss of muscle mass, is a clinical sign of neuropathy. Other signs include:

• 
  Loss of sensation in the affected limb
• decreased reflexes

Myopathy

Causes of myopathy:

• Steroids
• Alcohol
• AZT
• Hypothyroidism
• Duchenne's muscular dystrophy
• Polymyositis
• AIDS
• Mitochondrial disease

Neuropathy

Causes of peripheral neuropathy

• Diabetes
• Alcohol
• Guillain-Barre'
• Trauma
• Hereditary
• Environmental toxins
• Rheumatic (collagen vascular)
• Amyloid
• Paraneoplastic
• Infections
• Systemic diseases
• Tumors

As you can see, the causes spell out the easy to recall and hard to forget acronym: DAG THE RAPIST.

NMJ dysfunction

Myopathy differs from NMJ disorders. Diseases of the NMJ(like myasthenia gravis) are characterized by fatigability. Fatigability means that the muscle quickly weakens as a maneuver is performed multiple times in succession.