Ultrasound-Detected Anconeus Epitrochlearis and Cubital Tunnel Syndrome Signs: A Clustering-Adjusted Cross-Sectional Study

<title>Abstract</title> <p>Purpose. The anconeus epitrochlearis (AE) is an accessory muscle variably implicated in cubital tunnel syndrome (CuTS). This study used ultrasound to determine AE prevalence and bilaterality and evaluate its clustering-adjusted association with positive clinical signs of CuTS. Methods. Fifty-two participants (104 elbows) underwent bilateral elbow ultrasound and standardized clinical assessment, including Tinel’s sign and the elbow flexion test. Elbow-level data were analyzed using Generalized Estimating Equations (GEE) to account for two-elbow clustering. Effect sizes, inter-test agreement (Cohen’s κ), and a dose–response gradient (Spearman correlation) were also assessed. Results. AE was identified in 32/104 elbows (30.8%); 26/52 participants (50.0%) had at least one AE-positive elbow, 6 (11.5%) bilaterally. After clustering adjustment, AE presence was independently associated with a positive Tinel’s sign (OR = 5.46; p = 0.009), elbow flexion test (OR = 3.52; p = 0.016), and symptomatic status, defined as at least one positive test (OR = 3.72; p = 0.014). AE prevalence rose monotonically with the number of positive tests, from 21.4% to 63.6% (Spearman r = 0.307, p = 0.002). AE presence showed 50.0% sensitivity and 78.6% specificity for symptomatic status (AUC 0.643); agreement between the two provocation tests was fair (κ = 0.38). Conclusion. Ultrasound reliably identifies the AE muscle, whose presence is associated with significantly higher rates of positive ulnar-nerve provocation signs in a graded, dose-dependent pattern. Whether this subclinical association, in a young and largely asymptomatic cohort, extends to clinically diagnosed CuTS requires confirmation in symptomatic populations.</p>