Kinematics of the Distal Radioulnar Joint: A Dynamic CT Analysis
Authors: Li Euphemia , Bain Gregory I. , MacLean Simon B. M. 
Published: January 2025

Abstract
Background The kinematics of the distal radioulnar joint (DRUJ) are complex and not fully understood. Much current knowledge is based on cadaveric and static imaging studies. Management of DRUJ pathology will remain a challenge until the anatomy and kinematics at the normal DRUJ are defined. The aim of this study was to report kinematic findings at the DRUJ and their relationships in normal wrists using static and dynamic computed tomography (CT).
Materials and Methods This was a retrospective cohort study of patients who underwent dynamic CT scanning of one or both wrists at Tauranga Hospital between March 2018 and July 2022, utilizing a previously described protocol. Sigmoid notch and ulnar head dimensions in the axial and coronal planes were measured on static imaging, with sigmoid notch morphologies described utilizing the Tolat classifications. The degree of translation of the ulnar head with the wrist in full pronation, neutral rotation, and full supination were measured relative to a perpendicular line drawn across the midpoint of the sigmoid notch. The ulnar variance and sigmoid notch gap on coronal views were measured with the wrist in radial and ulnar deviation and with a relaxed and clenched fist.
Results A total of 38 wrists were studied in 24 patients, consisting of 15 females and 9 males with a mean age of 38.0 years (range 15–61). There was no statistically significant difference between ulnar variance or translation of the ulna relative to the midpoint of the sigmoid notch in each position when compared for morphology types in coronal or axial planes. The mean radius of curvature of the ulnar head was 8.3 degrees, with the mean radius of the sigmoid notch measuring 17.1 degrees (p < 0.001), at a mean ratio of 0.51. The ulnar head was most dorsal to the midpoint of the sigmoid notch in full pronation (mean −2.8 mm dorsal), and translated volarly as the wrist moved toward neutral (mean −1.1 mm dorsal) and to full supination (mean −0.6 mm dorsal) (p < 0.001), but remained dorsal to the sigmoid notch midpoint in each position. The mean sigmoid notch gap was 1.3 mm relaxed, which increased to 2.0 mm in a clenched fist (p = 0.001). Mean ulnar variance measured −1.0 mm relaxed, and −0.5 mm in a clenched fist (p = 0.014). The mean sigmoid notch gap on radial deviation was 2.0 mm, and 1.4 mm on ulnar deviation (p = 0.012).
Conclusion The ulnar head remains dorsal relative to the midpoint of the sigmoid notch throughout the range of forearm rotation but becomes relatively more volar as the wrist rotates from full pronation to a position of full supination. The morphology of the notch was not associated with any parameter measured in our study. We have shown that ulnar variance is dynamic in response to deviation and loading. We have presented new data that loading and ulnar deviation increase the gap at the distal sigmoid notch between the radius and ulna. Our findings have implications in the surgical management of pathologies at the joint, particularly in reconstructive procedures for DRUJ instability. Further study is indicated to investigate the relationship between morphology and kinematics at the DRUJ.