Elbow Medial Ulnar Collateral Ligament (UCL) Injury (Tommy John)

• Demographics: young, competitive overhead athletes — adolescents through professionals. Baseball pitchers are the prototype; pitch velocity and volume are the dominant risk factors.
• A rising incidence, most steeply in adolescents, attributed to early single-sport specialisation, year-round throwing, showcase culture and chasing velocity — which is why prevention (pitch counts, rest, mechanics) is now examinable, not just treatment.
• Risk factors: high pitch counts and innings, pitching while fatigued, year-round play, higher ball velocity, breaking pitches at a young age, and poor kinetic-chain mechanics that shift load onto the medial elbow.
• Why examiners care: it cleanly tests applied anatomy, a specific clinical test, MR interpretation, and a graded operative/non-operative decision in one athletic-population case.

The medial (ulnar) collateral ligament complex has anterior, posterior and transverse bundles. The anterior bundle — from the medial epicondyle of the humerus to the sublime tubercle of the ulna — is the primary restraint to valgus and the clinically important structure. During the overhead throwing motion, especially the late-cocking and early-acceleration phases, the elbow experiences enormous valgus torque; the anterior bundle is loaded close to its tensile limit with every pitch. Repetitive valgus overload therefore causes attritional microtearing and, ultimately, failure — a chronic partial tear or an acute complete rupture.

• Repetitive submaximal valgus load produces attritional microtearing of the anterior bundle faster than it can heal — a chronic degenerative process, not usually a single traumatic event.
• The deep (undersurface) fibres tear first, producing the partial undersurface tear that shows the "T sign" on MR arthrography, before progressing to full-thickness failure.
• Valgus instability shifts load — medial tension overload (UCL, flexor-pronator, ulnar nerve traction), lateral compression (radiocapitellar → capitellar OCD in the young), and posteromedial shear (valgus extension overload → posteromedial olecranon osteophytes). This triad explains the associated conditions.
• In the skeletally immature, the weak link is the medial epicondyle apophysis rather than the ligament — repetitive valgus traction causes "little league elbow" (medial epicondyle apophysitis/avulsion) instead of a mid-substance UCL tear.

MRI, often with MR arthrography, is the imaging standard. A partial undersurface tear classically shows the 'T sign' — contrast tracking between the distal UCL and the sublime tubercle. Dynamic ultrasound demonstrates valgus laxity (medial gapping under stress) and is excellent for serial assessment, but interpret laxity against the contralateral elbow, since asymptomatic throwers develop adaptive UCL thickening and some baseline laxity. Plain radiographs may show traction spurs, loose bodies or an avulsed fragment, and stress views can quantify medial opening.

• Anterior bundle (medial epicondyle → sublime tubercle) = primary valgus restraint and the structure injured.
• Mechanism: repetitive valgus overload (late-cocking/early-acceleration), classically baseball pitchers.
• Moving valgus stress test (pain through the 70–120° arc) is the most useful test — and assess the ulnar nerve.
• MR arthrogram 'T sign' = partial undersurface tear; dynamic US shows valgus laxity.
• Partial → non-op (kinetic-chain rehab ± PRP); complete/failed → reconstruction (palmaris/gracilis graft); repair ± internal brace for select acute avulsions.
• RTP ~80–90%; rehab ~9–12 months; prevention via pitch counts and mechanics.