Effect of A3 pulley and adjacent sheath integrity on tendon excursion and bowstringing

The effect of the A3 pulley and adjacent sheath integrity on tendon function at the proximal interphalangeal (PIP) joint was investigated in 21 fingers in 7 fresh-frozen cadaver hands. Excursions of the flexor digitorum profundus (FDP) tendons were measured when the tendons were pulled to produce PIP joint flexion of 110 degrees from a resting position of 0 degrees. Excursions of the FDP tendons in 10 fingers were tested within the intact sheath and after incision of the A3 pulley, of the A3 pulley with its proximal sheath up to the distal border of the A2 pulley, and of the sheath between the A2 and A4 pulleys. Eleven fingers were tested after incision of the A3 pulley, of the A3 pulley and its distal sheath up to the A4 pulley, and of the sheath from the A3 to A4 pulleys. Excursions of the FDP tendons increased to 103% +/- 3% after incision of the A3 pulley, 110% +/- 4% after incision of the A3 pulley and its proximal sheath, and 107% +/- 6% after incision of the A3 pulley and its distal sheath. Excursions increased to 116% +/- 6% after incision of the sheath from the A3 to A4 pulleys and to 119% +/- 3% after incision of the sheath between the A2 and A4 pulleys. Tendon bowstringing was 0.3 mm after incision of the A3 pulley, 0.6 mm after incision of the A3 pulley with its distal sheath, 0.8 mm after incision of the pulley with its proximal sheath, 1.4 mm after incision of the sheath from the A3 to A4 pulleys, and 1.6 mm after incision of the sheath between the A2 and A4 pulleys. The results suggest that the sheath adjacent to the A3 pulley plays an important role in restraining tendon bowstringing at the PIP joint, whereas the A3 pulley alone is of little importance. This study elucidates the role of individual parts of the sheath around the PIP joint in maintaining tendon function and may guide decisions regarding the area and length of the sheath feasible for surgical release or requiring repair in the treatment of tendon lacerations.