The most common overuse syndrome is related to excessive wrist extension and commonly referred to as “tennis elbow,” but it is actually more common in non-tennis players. It is also commonly referred to as lateral epicondylitis, but this is usually a misnomer because, in general, microscopic evaluation of the tendons does not show signs of inflammation, but rather angiofibroblastic degeneration and collagen disarray. Light microscopy reveals both an excess of fibroblasts and blood vessels that are consistent with neovessels or angiogenesis. 
The tendons are relatively hypovascular proximal to the tendon insertion. This hypovascularity may predispose the tendon to hypoxic tendon degeneration and has been implicated in the etiology of tendinopathies.  Most typically, the primary pathology is tendinosis of the extensor carpi radialis brevis (ECRB) tendon 1-2 cm distal to its attachment on the lateral epicondyle. [2, 3]
The annual incidence is 1-3% of the U.S. population. Men and women are equally affected. Typically, lateral epicondylitis affects individuals greater than age 40 years. There is usually a history of repetitive activity aggravating the extensor tendons of the forearm. Repetitive, eccentric motion of the wrist extensor muscles may increase risk of injury. Individuals with a current or prior history of tobacco use were also noted to be at increased risk.
A study by Sanders et al reported the age- and sex-adjusted annual incidence of lateral elbow tendinosis decreased from 4.5 per 1000 people in 2000 to 2.4 per 1000 in 2012. The recurrence rate within 2 years was 8.5% and the proportion of surgically treated cases within 2 years of diagnosis tripled from 1.1% during the 2000-2002 time period to 3.2% after 2009. The study also added that about 1 in 10 patients with persistent symptoms at 6 months required surgery. 
Herquelot et al conducted a study that aimed to estimate the association between repeated measures of occupational risk factors and the incidence of lateral epicondylitis in a large working population. The study highlights the importance of temporal dimensions for occupational risk factors on the incidence of lateral epicondylitis. The authors conclude that further research should evaluate the risk associated with the duration and repetition of occupational exposure on the incidence of lateral epicondylitis. 
The area of maximal tenderness is usually an area just distal to the origin of the extensor muscles of the forearm at the lateral epicondyle. Most typically, the ECRB is involved, but others may include the extensor digitorum and extensor carpi ulnaris.
The radial nerve splits into the superficial radial and posterior interosseus nerve (PIN) at the radiocapitellar joint. The PIN may become entrapped by pericapsular structures, causing radial tunnel syndrome.
Tennis is the most common sport to cause lateral epicondylitis, but the condition can also be seen in those who play squash and badminton. Symptoms can occur after an improper backhand hitting technique, which can occur when the athlete attempts to increase power by increasing forearm force rather than relying on core, rotator cuff, and scapular power. This results in snapping the wrist with supination and irritation of the extensor tendons. Symptoms can also occur with improper technique when an athlete does not get his or her feet into position and hits the ball late or when the athlete strikes the ball with a bent "leading" elbow. The power of the hit is again generated from the forearm instead of the core. This condition has also been observed more frequently in novice players when compared with more experienced players. This may be secondary to the ability of experienced players to decrease the impact forces from the racquet to the wrist.
In comparing one-handed versus two-handed backhand stroke, electromyography (EMG) results demonstrated reduced amplitude with a two-handed backhand versus a one-handed backhand stroke. 
Other causes of extensor tendinopathy in tennis are using new racquet, using a racquet that is strung too tightly, or using a racquet that is too heavy, as well as hitting wet or heavy balls or hitting into the wind. It is unclear if a grip that is sized too small or too large contributes to the development of lateral epicondylitis. However a recent, very small study by Rossi et al revealed that there may be an optimal grip size to reduce grip forces as well as reduce extensor tendon loading during a tennis stroke.  In addition, string vibration dampeners have not been shown to decrease the incidence of lateral epicondylitis. 
Industrial athletes have certain occupational and leisure activities that lead to overuse injuries of the forearm wrist extensors, causing pain at the lateral epicondyle. These include carpenters, bricklayers, seamstresses and tailors, politicians (excessive handshaking), and musicians (eg, pianists, drummers). Such injuries can also be seen in individuals who perform a lot of computer work, a lot of typing, and a lot of mouse work for their occupations.
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