Effect of tibial tunnel diameter on the outcome of ACL reconstruction

The number of variables that influence the success of an anterior cruciate ligament (ACL) reconstruction is so high that an in-depth analysis of the problem can only be carried out through numerical tools. Once the diameter of the substitute plasty and the interference screw has been chosen for a given patient, one of the main concerns of the surgeon is to find the most suitable diameter of the tibial tunnel for its fixation. In this work, a finite element model was developed in order to simulate both the reconstruction and the subsequent rehabilitation of the ACL at its tibial insertion. For the simulation, the chosen tendon and screw diameters were 4 mm and 7 mm, respectively, while diameters of 7, 8, 9, and 10 mm were tested for the tibial tunnel. The parameters of the behavior models of the different materials (screw, bone and tendon) were obtained through experimental tests. The results obtained show that, as the diameter of the tunnel decreases, the compressive stress over the plasty will increase (theoretical objective of the fixation), but the deformation induced on the trabecular bone also increases, which can trigger its failure. For this reason, the maximum values of the interferential pressure must be limited to those strictly necessary to ensure that the reconstruction is properly done, that is, that it prevents the tendon from slipping in the tunnel during the rehabilitation process. The simulation of the rehabilitation process was done by pulling the already fixed tendon in the femoral direction in order to extract it. It was obtained that the most suitable diameter of the tibial tunnel for the chosen plasty-screw assembly is 8 mm, since it provides a suitable subjection without high values of deformation in trabecular bone, that is, no damage in this part of the bone.