A Three-Dimensional Analysis of The Sigmoid Notch
Evan D. Collins, M.D., Faranak Vossoughi, M.D., Sabir Ismaily, BS

Fractures of the distal radius are among the most common injuries of the upper extremity, although treatment has focused on restoration of the radiocarpal joint - and late sequelae may persist. X-ray seems to underestimate sigmoid notch involvement following distal radius fractures. Currently no classification system for disruption patterns of the sigmoid notch of the radius associated with distal radius fractures exists. This study quantifies the anatomy of the sigmoid notch and identifies the landmarks of the articular surface and the proximal boundaries of the Distal Radioulnar Joint (DRUJ) capsule.

Materials & Methods:
Fifteen freshly frozen, unpaired mid-brachium hand specimens were used. CT Scans were taken, followed by dissection and reconstruction of the distal radius. The sigmoid notch surface is divided into two surfaces - the articulating, an average of 79.78 mm2 (69.13%); and the non-articulating, an average of 35.84 mm2 (30.87%). The Anterior Posterior (AP) and Proximal Distal ((PD) widths of the articulating surface and the PD width of the sigmoid notch were reviewed, along with the radius of curvature, version angle and depth of the sigmoid notch.

The inclination of the sigmoid notch towards the radial axis was visible at 1.64 degrees with ranges between - 14.46 degrees to +11.34 degrees. The AP width of the distal radius was 12.9 mm, the PD width of the articulating surface was 5.1 mm and the PD width of the sigmoid notch was 8.1 mm. The radius of curvature of the sigmoid notch was 14.6 mm, and the version angle was 9.8 degrees. The maximum depth of the sigmoid notch was 1.67 mm.

Our measurements of the AP width and radius of curvature of the sigmoid notch were compatible to previous studies. We defined the version angle and maximum depth of the sigmoid notch. We defined the proximal border of the DRUJ and the articular surface. The study showed that the sigmoid notch is flatter than previously believed. Only the distal 69% of its surface is covered by cartilage. On average, the sigmoid notch has about 9 degrees of retroversion and its average inclination is almost parallel to the anatomical axis of the radius. Clinical implications exist for evaluation of the DRUJ involvement in distal radius fractures or degenerative diseases - and in the future development and evaluation of hemiarthroplasty replacement of the distal radius.