Causes / typical injury mechanism: Medial ulnar collateral ligament (UCL) injury of the elbow and resultant valgus instability was first described in the literature in 1946 as seen in javelin throwers. Since then, the injury has become increasingly associated with throwing athletes, with baseball pitchers most commonly affected.

The mechanism of injury is most commonly microtrauma due to repetitive valgus stress on the elbow during the late cocking and early acceleration phases of throwing. Less common mechanisms include acute trauma (elbow dislocation) and iatrogenic causes during unrelated elbow surgery.

Classic history and presentation: Traditionally, this injury has been associated with adult athletes, but recent literature has shown an increasing prevalence of these injuries in the younger athlete. Despite this, UCL injury is relatively uncommon in skeletally immature patients, who tend to suffer a related condition termed "Little League elbow." This injury has been identified in other sports besides baseball and javelin throwing, including gymnastics, softball, football, tennis, and wrestling.

Patients will typically endorse medial (ulnar)-sided elbow pain during overhead throwing. Acute injuries may be associated with an audible pop. Chronic injuries are more common overall and may present with pain, along with decreased velocity and accuracy while pitching. Ulnar nerve paresthesias may be present with acute and chronic injuries.

Prevalence:

• Age – Typically 13-26 years.
• Sex / gender – Males are more commonly affected.

Risk factors: Risk factors include high pitch counts, pitching over 100 innings per year, high velocity pitching (85+ mph), improper pitching mechanics, throwing breaking pitches (curveballs, sliders) at ages younger than 13 years, pitching on consecutive days, year-round pitching, and pitching for multiple teams.

Pathophysiology: The UCL is comprised of 3 structures: the anterior bundle, the posterior bundle, and the oblique bundle (transverse ligament). The anterior bundle is the strongest and most important stabilizer to elbow valgus stability, and it is therefore most commonly implicated in these injuries.