The anatomy of the radial side of the thumb: static restraints in preventing subluxation and rotation after injury.

This study sought to define the anatomic variations of the radial side of the thumb metacarpophalangeal joint and to assess the contributions of the radial structures to stability. In Part I, the radial side of the thumb metacarpophalangeal joint was studied in 31 cadavers, showing that the extensor pollicis brevis insertion was variable. In Part II, 13 additional hands were studied to evaluate the static restraints to rotation and subluxation, showing that variability in the amount of volar subluxation and pronation was possible in the intact thumb. This study showed that the radial collateral ligament and extensor pollicis brevis capsule collectively contribute to stability against rotation and volar subluxation. We found roentgenograms with a pure volar displacement force to be more helpful than pronation stress roentgenograms in showing metacarpophalangeal instability after injury. This instability may require surgery to repair the injured structures and restore the stability to varus stress and volar subluxation.