Structural predictors of mechanical instability after posterolateral elbow dislocation: a magnetic resonance imaging-based threshold model

Determining which posterolateral elbow dislocations require surgery is challenging as major ligament tears often occur regardless of clinical stability. This study aimed to identify specific magnetic resonance imaging patterns that represent the functional threshold for persistent mechanical instability. We retrospectively reviewed magnetic resonance imagings of 186 elbows following acute posterolateral dislocation. The unstable group (n = 24, 12.9%) showed gross mechanical instability or redislocation. We evaluated single-structure effects and a "Core-4" model comprising the flexor-pronator mass (FPM), posterior medial collateral ligament, common extensor origin, and posterolateral ligament (PLL), representing the principal pillars of stability. Complete tears of the lateral collateral ligament complex and anterior bundle of the medial collateral ligament were highly prevalent in both stable and unstable groups, offering low specificity. In contrast, the concurrent disruption of the FPM and PLL, termed the "Double Hit" pattern, was the most powerful predictor, yielding a specificity of 98.8% and positive predictive value of 91.7%. The Core-4 model showed excellent diagnostic accuracy (area under the curve: 0.979). While Score 2 elbows showed low instability (4.8%), Score 3 elbows exhibited a divergence: those with the Double Hit pattern had a 75% instability rate, while those without it remained stable (0%). All Score 4 elbows were unstable. Mechanical instability following posterolateral elbow dislocation is a threshold-driven phenomenon resulting from the cumulative failure of critical stabilizers. The Double Hit pattern (concurrent failure of the FPM and PLL) represents the critical functional threshold for mechanical failure. By identifying high-risk subsets (Score 4 and Double Hit Score 3) with high specificity, the Core-4 model provides a definitive objective criterion for early surgical intervention while supporting functional rehabilitation for most stable dislocations.