Types of Elbow Instability

Elbow stabilizers and instability patterns — LUCL posterolateral instability (top) and posteromedial instability (bottom). Miller's Review of Orthopaedics, 9th Ed.

Overview

Classification by Direction

1. Posterolateral Rotatory Instability (PLRI) — Most Common

• Cause: Incompetence of the lateral ulnar collateral ligament (LUCL), which originates on the posterior lateral epicondyle and inserts on the crista supinatoris
• History: Often follows a previous elbow dislocation treated non-operatively
• Mechanism: The ulna rotates externally on the trochlea, causing posterior radial head subluxation
• Clinical test: Lateral pivot-shift test — reproduces instability with supination + axial compression + valgus loading as elbow is brought from full extension to ~40° flexion; radial head reduces with a palpable clunk as triceps becomes taut
• Note: Patient apprehension is common; examination under general anaesthesia or intra-articular local anaesthetic may be needed
• Imaging: MRI demonstrates LUCL pathology in ~50% of cases; elbow is more stable in forearm pronation
• Treatment: Reconstruction of the LUCL with tendon autograft for chronic instability

2. Varus Posteromedial Rotatory Instability (PMRI)

• Cause: Fracture of the anteromedial coronoid process (combined with LCL injury and posterior band of MCL disruption)
• Description: Increasingly recognized but poorly understood; the coronoid fracture leads to varus posteromedial rotatory instability if untreated
• Treatment: ORIF of coronoid with a buttress plate + assessment of MCL insertion

3. Valgus Instability

• Cause: Acute elbow trauma OR chronic repetitive loading with attenuation of the medial collateral ligament (MCL)
• Population: Classic in overhead-throwing athletes (baseball pitchers — "Tommy John" injury; javelin throwers)
• Biomechanics: Late cocking and acceleration phases of throwing place maximum stress on the MCL; FCU is the primary dynamic stabilizer, FDS secondary
• Clinical note: Surrounding muscle stabilizers can mask valgus instability in up to 50% of cases
• Imaging: MRI evaluates MCL integrity and flexor-pronator mass
• Associated pathology: Ulnar neuritis / cubital tunnel syndrome
• Treatment:
  • Acute: Primary ligamentous repair or suture anchor reattachment (especially partial tears or avulsions with a "pop")
  • Chronic: MCL reconstruction with tendon autograft (Tommy John surgery) — 75–80% return to sport at same level or better at 1 year
• Sequelae of untreated MCL insufficiency: Capitellar wear, posteromedial impingement, olecranon osteophyte formation, loose bodies → valgus overload syndrome

Classification by Complexity (Elbow Dislocation Model — O'Driscoll's Circle of Horii)

Complex (Fracture-Dislocation) Patterns

• Terrible Triad: Elbow dislocation + LCL injury + radial head fracture + coronoid fracture — the most unstable pattern
• Monteggia fracture-dislocation: Proximal-third ulnar fracture + radial head subluxation/dislocation; persistent proximal radioulnar instability may require annular ligament reconstruction

Summary Table