Replication of the Range of Native Anterior Cruciate Ligament Fiber Length Change Behavior Achieved by Different Grafts Measurement Using Computer-Assisted Navigation

Background: The native anterior cruciate ligament (ACL) does not behave as a simple bundle of fibers with constant tension but as a continuum of ligament fibers with differential length change during knee flexion/extension. Computer-assisted navigation can be used to assess length change in different fibers within the native ACL and to evaluate how different reconstruction grafts replicate the range of native ligament fiber length change behavior. Hypothesis: Anterior cruciate ligament reconstruction graft size and configuration (single-vs double-bundle) are deciding factors as to how much of the native ACL fiber length change behavior is replicated. Study Design: Controlled laboratory study. Methods: The fiber length change behavior of the entire native ACL was assessed by measuring the length change pattern of representative anteromedial (AM) and posterolateral (PL) bundle fibers (1 at the center and 4 at the periphery of each bundle). The tibial and femoral ACL attachment areas in 5 fresh-frozen cadaveric knees were digitized, and the length change of each representative fiber was recorded during knee flexion/extension using an image-free, optical navigation system. Subsequently, single-bundle ACL reconstructions of different diameters (6, 9, and 12 mm) positioned at the center of the overall native femoral and tibial attachment sites were modeled to assess how much of the range of ligament fiber length change of the native ligament was captured. This was compared with a double-bundle graft using 6-mm-diameter AM and PL grafts positioned at the centers of the femoral and tibial attachment sites of each separate bundle. Results: The 6-, 9-, and 12-mm single-bundle grafts simulated 32%, 51%, and 66% of the ligament fiber length change behavior of the native ACL, respectively. The length change patterns in these grafts were similar to the central fibers of the native ACL: the PL fibers of the AM bundle and AM fibers of the PL bundle. However, even a 12-mm graft did not represent the most AM and PL native fibers. The 6- mm AM and PL bundle grafts (equivalent in cross-sectional area to a 9- mm single-bundle graft) simulated 71% of the native ACL and better captured the extremes of the range of native ligament fiber length change. Conclusion: Increasing single-bundle graft size appears to capture more of the range of native ACL fiber length change. However, for a similar graft cross-sectional area, a 2-bundle graft simulates the length change behavior of the native ligament more precisely and thus may better emulate the synergistic actions of anisometric and isometric fibers of the native ligament in restraining knee laxity throughout the range of flexion.