gms | German Medical Science

Objectives/Interrogation: The distal radioulnar joint (DRUJ) is essential for forearm rotation. Stability is provided by the articulation of the ulnar head with the sigmoid notch and the supporting soft tissue stabilizers (TFCC and distal oblique band of the interosseous ligament or DIOL). In some cases of DRUJ dysfunction, an ulnar head replacement is necessary to address pathology in the bony structures. In lieu of using a constrained DRUJ prosthesis, surgeons often rely on reconstructing soft tissues to provide DRUJ stability. However, this is difficult and stability is frequently poorly restored. We hypothesize that by incorporating an offset on the ulnar head implant that can be dialed into the rotational position to where it is most beneficial, tension will be reintroduced to the DIOL and additional stability will be provided.

Methods: Cadaveric forearms were used to measure the relative displacement of the ulna during simulated push pull (piano key sign) testing of the wrist. The ulna was fixed to a rigid beam and the hand was pinned through the 2nd-5th metacarpals allowing the radius to be rotated and fixed in supination, neutral, and pronation. The initial location of the radius was measured under a 5lbF preload. Gradual loads were applied up to 20lbF and the resultant displacement was measured. The TFCC was then excised to create DRUJ instability. The magnitude and direction of instability was measured by comparing radial displacement under load to that of the intact specimen. The ulnar head was then replaced with an ulnar head arthroplasty of the same size as the native head. Measurements were then repeated using a standard ulnar head (no offset) and repeated with a 2mm offset ulnar head. Specimens were tested in pronation, neutral, and supination and with the direction of the offset dialed into full pronation, neutral and full supination.

Results and Conclusions: The addition of a 2 mm offset decreased displacement of radius relative to the ulna in all positions but most effectively in the direction in which the offset was dialed. In conclusion, a directed 2 mm ulnar head offset may improve stability of the DRUJ joint after ulnar head replacement.