Distal Ulnar Centering and Eccentricity (DUCE) grading: assessing the degree of instability in the distal radioulnar joint

Distal radioulnar joint (DRUJ) instability is challenging to diagnose. The Distal Ulnar Centering and Eccentricity (DUCE) method was introduced as an anatomy-based assessment of DRUJ articulation to reduce rotation-related variability of existing metrics. This study evaluated its responsiveness to increasing instability, developing a point-based grading system. Twelve cadaveric forearms were scanned in stable condition and underwent stepwise induction of instability (TFCC lesion, Galeazzi, Essex-Lopresti). Cone-beam CT scans were obtained in pronation and supination at each stage of instability. DUCE parameters Centering (distance from ulnar head centre C to a notch-derived axis), and Eccentricity (distance from C to deepest notch point) were measured. Responsiveness of DUCE parameters to instability was analysed using a mixed-effects model and compared to radioulnar ratio. Thresholds for DUCE Grading were derived via linear regression. Diagnostic accuracy was assessed using ROC analysis, Youden’s Index, and weighted kappa. RR showed no significant change with increasing instability, whereas DUCE parameters increased progressively (P Centering = 0.005, P Eccentricity <0.001). The DUCE Grading showed good discrimination between stable and unstable joints (AUC 0.78, 95% CI 0.69 to 0.88), with 76.0% overall accuracy (95% CI 66.6 to 83.5) and moderate agreement with the true instability grade (quadratic weighted κ = 0.42). In this cadaveric proof-of-concept study, DUCE Grading provided an anatomy-referenced quantification of DRUJ instability and allowed preliminary severity stratification. As the thresholds were derived and internally validated within a single cohort, they require prospective external validation before the system can support clinical decision-making.