The Role of the Critical Coronoid Angle in Simple Elbow Dislocation: A Computed Tomography-Based Index to Stratify Elbow Dislocation Risk

<b>Introduction:</b> Elbow primary stability is guaranteed by the anatomical congruency between the humeral trochlea and the greater sigmoid notch (GSN). Elbow dislocation typically occurs in a semi-extended position, but computed tomography (CT) scans are typically acquired at 90° of elbow flexion, which may misleadingly suggest that the apex of the coronoid aligns with the trochlear center of rotation. This study aims to evaluate the anatomical features of the coronoid and GSN in a dislocated versus non-dislocated group, demonstrating that a more prominent coronoid process is more commonly observed in elbows without dislocation compared to those with dislocation. <b>Materials and Methods:</b> A total of 50 CT scans, equally divided between dislocated elbows and non-dislocated elbows, were analyzed, and the critical coronoid angle (CCA) was measured on a specific slice of the CT scan (level of evidence III). The CCA was calculated from two lines that arise in the center of the GSN, with the first one crossing the coronoid tip and the second parallel to the posterior olecranon cortex. <b>Results:</b> A significant difference in the CCA (<i>p</i> < 0.001) between the two groups was highlighted. In particular, it was found that 14/25 patients from the dislocated elbow group had a CCA below or equal to 27°, and all the non-dislocated subjects had a CCA ≥ 27°. These preliminary results suggest that a CCA ≤ 27° could be a threshold for requiring further imaging of soft tissues or closer follow-up. This may result from either a hypoplastic coronoid process or a decreased concavity of the GSN. Based on the CCA values, a logistic regression model (DAM model) was proposed to associate a coefficient of protection to the CCA, the angle of flexion during dislocation (FdD), and a parameter X, which is a factor that encompasses the contribution of soft tissues. <b>Conclusions:</b> A low CCA is statistically more frequent in dislocated elbows versus non-dislocated ones, creating a specific anatomical condition. The CCA should be carefully evaluated by elbow surgeons to guide patient-specific treatment. The DAM model can permit the stratification of patients eligible for further diagnostic analysis.