Morphologic Association of the RUAL Classification of Distal Radius Fractures With Ulnar Styloid Base Fractures: A Retrospective Case Series Based on Three-Dimensional Fracture-Line Mapping

Abstract Objective To develop the RUAL classification based on three-dimensional fracture-line mapping, investigate the association between distinct distal radius fracture morphologies and ulnar styloid base fractures, and assess the potential value of these morphologic patterns as imaging clues to injury of the ulnar-sided osteoligamentous complex. Methods We retrospectively included 233 patients with distal radius fractures accompanied by ulnar styloid fractures who were treated at Beijing Jishuitan Hospital between January and December 2023. Preoperative CT data were used for three-dimensional reconstruction, virtual reduction, and fracture-line superimposition. RUAL classification was performed according to the involvement patterns of the sigmoid-notch rim, the scapholunate ridge, and the articular surface. The primary outcome was the distribution of ulnar styloid tip versus base fractures. Categorical variables were analyzed using the Pearson chi-square test; the Fisher–Freeman–Halton exact test was used when expected cell counts were < 5, and age was compared using one-way analysis of variance. In an exploratory analysis, types II-A, II-C, and III were combined as a morphologic high-risk group, whereas types I and II-B were combined as a low/intermediate-risk group. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated. In addition, 80 imaging studies were randomly selected and independently interpreted twice, 4 weeks apart, by two experienced musculoskeletal attending physicians who had undergone standardized training. Interobserver and intraobserver reliability were assessed using unweighted Cohen’s kappa with 95% CIs. Results RUAL type I was identified in 100 patients (42.9%), type II in 115 (49.4%), and type III in 18 (7.7%). Among type II fractures, 16 were type II-A, 87 were type II-B, and 12 were type II-C. The distribution of ulnar styloid fracture location did not differ significantly across the six subtypes (χ²=5.071, P = 0.407) or among the three major RUAL types (χ²=0.533, P = 0.766). After exploratory pooling, ulnar styloid base fractures were present in 26 of 46 patients (56.5%) in the high-risk group and 74 of 187 patients (39.6%) in the low/intermediate-risk group. This difference was statistically significant (χ²=4.329, P = 0.037), with an OR of 1.99 for base fracture in the high-risk group relative to the low/intermediate-risk group (95% CI, 1.03–3.81; Fisher’s exact test, P = 0.046). For reliability, interobserver κ values were 0.631 (95% CI, 0.496–0.765) and 0.570 (95% CI, 0.424–0.717) in the two reading rounds, respectively; intraobserver κ values were 0.909 (95% CI, 0.831–0.986) and 0.826 (95% CI, 0.726–0.927) for the two observers, respectively (all P < 0.001). Conclusion Overall comparisons across the three major RUAL types and six subtypes did not demonstrate a significant difference in ulnar styloid fracture location. However, exploratory risk grouping suggested that volar or combined volar–dorsal sigmoid-notch rim disruption and a central sagittal split were associated with enrichment of ulnar styloid base fractures. These morphologic patterns may therefore serve as imaging warning signs prompting further assessment of the triangular fibrocartilage complex (TFCC) and distal radioulnar joint (DRUJ) stability, but they should not be used to directly diagnose a deep TFCC tear or to determine whether the ulnar styloid should be fixed.