Annular pulley stress during digital flexion: A finite element analysis

For this investigation, a computer generated three-dimensional anatomical model of a human digit was developed and biomechanically analyzed using the finite element (FE) method. Borrowed from engineering sciences, the FE method utilizes mathematical solutions to estimate in-vivo biomechanical phenomena that are otherwise impossible to measure. While the present model is appropriate for estimating many biomechanical events, this study used it to simulate baseline stress distributions and strain energy changes in components of the flexor tendon sheath after simulating a muscle contraction force of 117 N (26 pounds) during digital flexion. These biomechanical conditions were then analyzed after single and combined excision of the A1, A2, A3, and A4 pulleys. After mathematical solution of the intact model, low restraining Von Mises stress was isolated primarily at the distal edges of both the A2 and A4 pulleys. For single pulley excisions, loss of the A2 pulley resulted in the greatest overall stress increase and caused the largest change in strain energy. Stress results of excising single pulleys A1, A3, and A4 were similar; however, excision of A4 alone caused large peak strain energy and displacement increases. Combined excision of annular pulleys A2 and A3 resulted in high stress and strain energy increases, but less than the triple excision of A1, A2, and A3. Of the double pulley excisions, loss of A1 and A3 combined (A2, A4 intact) had very little effect on stress and no effect on strain energy, but an increased proximal tendon displacement was noted. The three pulley system of A2, A3, and A4 reduced flexor tendon excursion needs when compared to the A2, A4 intact system. Of all pulley protocols, loss of A2 consistently increased residual stress and contact force redistribution when compared to the intact (control) system. We conclude that on the basis of stress redistribution, contact analysis, and strain energy, the A2 pulley is the most important for retaining the flexor tendons against bowstringing during digital flexion. Further, any combination of pulley loss that includes the A2 results in the greatest biomechanical change from the normal intact system.