Movement dysfunction following repetitive hand opening and closing: anatomical analysis in Owl monkeys

Repetitive strain injuries are thought to result from biomechanical stress and tissue microtrauma. The purpose of this study was to investigate the presence of local inflammation, scarring or anatomical variations of the flexor tendons and the median and ulnar nerves in four Owl monkeys behaviorally trained at a repetitive motor or sensory task. Three monkeys were trained to repetitively open and close a handpiece. The two monkeys that used rapid, stereotypical hand squeezing developed a task-specific movement dysfunction (one in 5 weeks and one in 24 weeks). The third monkey used a variable shoulder-pulling strategy and did not develop movement problems. The fourth monkey served as a control subject for the repetitive motor movements, trained on a repetitive sensory task, and did not develop a task-specific movement dysfunction. On dissection and histologic staining, there were no signs of active inflammation in the median nerve, the ulnar nerve, or the flexor tendons in any of the monkeys. However, the monkey that developed movement problems after 5 weeks of repetitive hand squeezing had an anatomical restriction of the flexor profundus tendon on the fourth digit of the trained side and the third digit of the untrained side. Reorganization of the representation of the hand on the contralateral somatosensory cortex (area 3b) was noted in the two monkeys that developed motor control problems and in the monkey performing the repetitive sensory task. These findings suggest that repetitive, stereotypical motor behaviors can lead to motor control problems without local signs of tendon or nerve inflammation. Preexisting anatomical restrictions may modify the time course for the development of movement dysfunction under conditions of stressful repetition. This animal model may simulate clinical focal hand dystonia (or occupational hand cramps) which can develop in human subjects who perform prolonged, repetitive, stereotypical movements.