Ulnar styloid abduction angle: a novel radiological predictor of Palmer Type IB tears of the triangular fibrocartilage complex.

Background: Deep ulnar fossa tears of the triangular fibrocartilage complex (TFCC) frequently lead to instability of the radioulnar joint. Although positive Ulnar Variance (UV) is a recognised longitudinal bony factor associated with TFCC injury, many tears still occur in patients with neutral or negative UV; the influence of the morphological orientation of the ulnar styloid process in the coronal plane on ligament susceptibility remains unclear. Objective: To assess whether the ulnar styloid abduction angle (USAA) is an independent bony predictor of Palmer Type IB TFCC tears, and to investigate its non-linear dose-response relationship and clinical predictive value. Study design: Retrospective case-control study; evidence grade: Level 3. Methods: This study included 362 patients who met strict radiographic quality control criteria in true posteroanterior (PA) views, comprising 110 cases of Palmer Type IB tears confirmed by arthroscopy and 252 control cases with intact TFCC confirmed by 3.0 T MRI and clinical follow-up. Missing values were handled using the Multiple Imputation by Chained Equations (MICE) method, and 1:1 propensity score matching (PSM) was performed to balance baseline characteristics. Multivariate binary logistic regression was applied to identify independent predictors. A restricted cubic spline (RCS) model was used to assess non-linear associations, and the predictive robustness and clinical net benefit of the model were evaluated via Bootstrap resampling for internal validation and decision curve analysis (DCA). Results: The USAA was significantly greater in the case group than in the control group (21.0° ± 4.0° vs. 14.8° ± 3.8°, Conclusion: An increased USAA is an independent and highly significant two-dimensional radiographic predictor of Palmer Type IB TFCC tears, exhibiting a non-linear dose-response relationship with the risk of injury. This intrinsic morphological variation in the coronal plane may reshape the biomechanical tolerance threshold of the deep fibres, with a pathogenic mechanism independent of classic axial ulnar variance. Clinical implications: USAA can be measured cost-effectively and reliably on standard wrist radiographs. A USAA ≥ 17.5° may serve as an objective screening radiographic parameter for first-line clinical screening, aiding in the optimisation of early risk stratification and clinical triage pathways for patients with wrist ulnar pain.