The ulnar collateral ligament (UCL) of the elbow has an anterior bundle (the primary stabiliser against valgus stress, from the medial epicondyle to the coronoid medial sublime tubercle), a posterior bundle (thickening of the posterior capsule, from medial epicondyle to olecranon), and a transverse oblique band (from olecranon to coronoid, deepens the sigmoid notch). The anterior bundle is the critical structure in overhead throwing athletes.
The anterior bundle is the primary constraint against valgus at 30-120 degrees of elbow flexion. The posterior bundle augments valgus resistance at greater than 90 degrees. Together they maintain elbow medial stability during the extreme valgus moment of overhead throwing.
UCL insufficiency from repetitive microtrauma in overhead athletes (baseball pitchers, javelin throwers, tennis players) produces medial elbow pain at late cocking and acceleration phases with progressive valgus instability. The valgus stress test and milking manoeuvre reproduce symptoms. MRI arthrogram demonstrates partial or complete anterior bundle tears at the medial epicondyle origin (most common) or mid-substance. Ulnar collateral ligament reconstruction (Tommy John surgery) with palmaris longus or gracilis graft has a 83-92% return-to-sport rate in baseball pitchers.
Repetitive valgus stress during the late cocking phase of overhead throwing produces anterior bundle UCL microtears at the medial epicondyle attachment, progressing to a partial tear producing medial elbow pain and declining velocity; MRI arthrogram detects T-sign (contrast extending from the ulnar attachment into the ligament substance) indicating partial undersurface tear amenable to non-operative treatment or UCL repair in younger athletes.
