The effect of increasing distraction on digital motion after external fixation of the wrist.

Purpose Although several factors have been related to the development of digital stiffness after external fixation of distal radius fractures, distraction can be considered one of the most important. This study determined a threshold for distraction during external fixation of distal radius fractures and documented the effect of distraction of the wrist on metacarpophalangeal (MCP) joint motion. Methods Eight fresh-frozen cadaveric forearms were mounted on a testing frame and the flexor digitorum superficialis (FDS) tendon of each finger was pulled individually until a 90° flexion at the MCP joint was generated. A load cell was used to record the force applied on the tendons. Tendon displacement was calculated by using a linear potentiometer. Sequential distraction across the wrist was applied through a distraction outrigger on the external fixator and was measured both on the external fixator in millimeters and on fluoroscopic images. Finger motion was captured by using a 2-dimensional motion tracking system. The experiment was repeated at different levels of wrist distraction (increments of 2 mm to a maximum of 14 mm). Results There was a significant difference in the effect of distraction on the individual fingers. For the index finger the load required for the FDS to produce a 90° flexion at the MCP joint was increased linearly between 0 and 8 mm of wrist distraction. For the other fingers a threshold of distraction was found. A significant increase in the load was noticed after 5 mm of wrist distraction. Conclusions More than 5 mm of wrist distraction increases the load required for the FDS to generate MCP joint flexion for the middle, ring, and small fingers. For the index finger, however, as much as 2 mm of wrist distraction significantly increases the load required for flexion at the MCP joint.