The pulley system of the thumb: anatomic and biomechanical study.

To examine the precise conformation of the annular and oblique pulleys of the thumb flexor sheath, anatomic dissections were undertaken on 14 hands. In all specimens a distinct pulley was found between the A1 and oblique pulleys. This is named the variable annular pulley or Av pulley. There appear to be 3 discrete forms of this pulley designated type I to III. The biomechanical study was done on 8 limbs by using linear strain transduction techniques. The analysis showed that the strain in the oblique pulley was greater in extension than in flexion of the thumb. This statement remains true even after division of either the A1 or Av pulley and after section of both pulleys. The oblique pulley does not prevent bowstringing of the flexor pollicis longus when A1 and Av pulleys have been sectioned. These studies challenge current concepts of both the anatomy and mechanics of the thumb pulley system with implications for clinical procedures such as trigger thumb release and pulley reconstruction.