Distal Oblique Bundle and Membranous Thickening: Morphology and Integration with the Triangular Fibrocartilage Complex

<b>Background:</b> The distal oblique bundle (DOB) of the interosseous membrane (IOM) has been recognized as an important stabilizer of the distal radioulnar joint (DRUJ). However, its prevalence, morphology, and distal attachments-particularly its relationship to the articular disc and the extensor carpi ulnaris (ECU) tendon sheath-remain inconsistently described. Clarifying these anatomical details is essential for understanding DRUJ stability and guiding surgical reconstruction. <b>Methods:</b> The distal IOM was examined in 48 specimens from 24 embalmed Korean cadavers. In 46 dissected specimens, the presence, morphology, and attachment sites of distal interosseous structures were documented, and attachment levels were measured. In 38 specimens, attachment to the articular disc was assessed. In addition, serial transverse sections from one cadaver were analyzed to confirm three-dimensional relationships. <b>Results:</b> Two morphological patterns were identified: a distinct DOB (21/46, 45.7%) and, when absent, a membranous thickening of the distal IOM (25/46, 54.3%). The mean attachment level was 39.1 ± 9.7 mm for the DOB and 25.4 ± 4.8 mm for the membranous thickening. Both structures assumed an oblique orientation, fanning palmarly toward the capsule and articular disc and dorsally toward the ECU tendon sheath and dorsal septum. In 26 of 38 specimens (68.4%), these structures attached to the proximal palmar portion of the articular disc. Serial transverse sections confirmed this oblique configuration, linking palmar and dorsal stabilizers of the DRUJ. <b>Conclusions:</b> The distal IOM consistently forms specialized structures-either a DOB or a membranous thickening-that integrate with the triangular fibrocartilage complex. By bridging palmar and dorsal stabilizers, these structures contribute to joint congruency and load transfer during forearm rotation. A refined anatomical understanding of these patterns provides clinically relevant insights for surgical preservation or reconstruction, with the potential to improve outcomes in patients with chronic DRUJ instability.