Scaphotrapeziotrapezoid Joint Osteoarthritis: Differentiating SOAC Wrist From SLAC Wrist.

Scaphotrapeziotrapezoid (STT) osteoarthritis (OA) is the third most common osteoarthritic joint in the wrist. Recent research challenges prior assumptions about the STT OA relationship with scapholunate dissociation (SLAC) and proposes a new progression model leading to scaphotrapeziotrapezoid osteoarthritis advanced collapse (SOAC) wrist. Normal wrist mechanics enable the scaphoid and proximal carpal row to move synchronously for optimal motion. STT arthrodesis may have adverse effects if midcarpal instability is unrecognized. Understanding outcomes from early clinical studies of STT arthrodesis provides insight into the natural progression of STT OA. Altered wrist mechanics, particularly disruption of synchronous scaphoid and proximal row motion, demonstrate how instability and load redistribution can contribute to progressive arthritic degeneration and advanced collapse. This article emphasizes the importance of identifying and managing midcarpal instability. The purpose of this article is threefold: (1) It reviews the pathophysiology of STT OA and its progression to SOAC, (2) highlights surgical options for different stages, and (3) proposes a modified classification system for early awareness and to guide treatment.