Long-term study of chronic scapho-lunate instability treated by scapho-trapezio-trapezoid arthrodesis.

The effect of scapho-trapezio-trapezoid arthrodesis on wrist kinematics was studied in 25 patients with chronic static scapho-lunate instability and in 16 patients with dynamic instability, with follow-up ranging from 24 to 101 months (average, 56 months). Postoperative planar and cineradiographic examination in patients returning to heavy labor reveal an absence of carpal shift-influence of the scaphoid proximal pole on the lunate-triquetral unit in ulnar deviation. Scapholunate diastasis present before operation persists in ulnar deviation as the STT fusion mass, capitate, and hamate rotate with the hand into ulnar deviation; the lunate-triquetral unit is not physiologically "pulled" radially into the lunate fossa of the radius. Triquetro-hamate mechanics remain normal as active engagement along the helicoidal triquetro-hamate interface initiates proximal row dorsiflexion. Clinical postarthrodesis wrist motion is a combination of intercarpal and radiocarpal mechanics, with energy in the flexion and extension arc dissipated through the scapho-lunate interface. The paucity of arthritic changes after up to 101 months after surgery is based on the kinematic changes presented in these data.