Assessment of the “functional length” of the three bundles of the anterior cruciate ligament

The objectives of the present study are to compare “functional length” between the three bundles of the anterior cruciate ligament (ACL) from extension to deep flexion and to perform a sensitivity study on the patterns of “functional length” change due to deviations in insertion site selection. Nine knees of healthy volunteers were examined using a horizontal open magnetic resonance apparatus with the knee at 0°–150° of flexion and created three-dimensional (3D) virtual models of the knee. The femoral and tibial attachment sites of the three ACL bundles were determined, and the distance between the attachment sites was automatically calculated as the “functional length” of each bundle in each position. After changing the attachment sites, computer simulation was performed to investigate in vivo “functional length” changes for the three bundles of the ACL in normal knee kinematics from extension to deep flexion. Three bundles statistically significantly changed in “functional length” during flexion/extension, while they were longest at full extension, and decreased with increasing knee flexion to 100°. They were smallest at flexion of 100°, with increase lengths with flexion deeper than 100°. Deviation of 6 mm of insertion site beyond attachment area caused significant alteration in the pattern of “functional length” change of each bundle, while deviation of 3 mm within attachment area caused no significant difference. The “functional length” of the three ACL bundles was not isometric, not even the antero-medial bundle. The “functional length” might be of major importance in terms of the site of the ACL insertion. The “functional length” of multiple bundles of the ACL and the pattern of their changes are useful for not only graft choice and preparation but also knee angle and initial tension at graft fixation during multiple-bundle ACL reconstruction surgery.