Effect of ACL reconstruction tunnels on stress in the distal femur.

Purpose: This study examined the change in femoral stress caused by graft tunnels drilled for anterior cruciate ligament (ACL) reconstruction. Using a computational model, the number, geometry and position of the graft tunnels exits were varied to determine the effect on bone stress.

Methods: A finite element model of the distal femur was developed from a CT scan of a cadaveric knee. To assess the model, the strain calculated computationally was compared to experimentally measured strains in eleven unpaired human cadaver femurs. Using the computational model, the number, geometry and position of the graft tunnel exits were varied to determine the effect on bone stress based on the stress concentration factor: the ratio of bone stress with tunnels to intact bone stress.

Results: The results indicated that the second tunnel in double-bundle ACL reconstruction results in approximately a 20 % increase in the maximum femoral stress as compared to single-bundle reconstruction. The highest stresses occur at the tunnel exits. The position of the tunnel exits effects femoral stress with the stress increasing slightly (AM SCR from 0.7 to 1 and PL SCR from 1.2 to 1.3) when the AM tunnel exit is moved anteriorly and having greater increases as the posterior lateral (PL) tunnel exit is moved laterally (PL SCR from 1.2 to 1.7) or posteriorly (PL SCR from 1.2 to 2).

Conclusion: In anatomical ACL reconstruction, the tunnel entrances are dictated by anatomy; however, there can be variations in tunnel exit positions. Consideration should be given when positioning tunnel exits on the effect on stress in the femur. Moving the PL tunnel exit laterally or posteriorly increases in the stress at the PL tunnel exit.