Orthopaedic Sports Medicine Center of Ioannina

Purpose: The purpose of our study was to clarify the events that take place during ACL failure, focusing on the behavior of the ACL as a composition of multiple fibers, during uniaxial tension along the ligament.

Methods: Ten fresh-frozen human cadaveric knee specimens were fixed in an Instron machine and load was applied parallel to the ACL axis. Two cameras were used to detect the failure mode of the ACL and its different groups of fibers. The distinct bundles of fibers were marked in each specimen prior the testing. The macroscopical findings during the experiment were used for comparison with the biomechanical results.

Results: ACL showed a non-monotonic response during testing. The load-elongation curve demonstrated a plateau or a second peak post the initial drop of load. Macroscopically, some fibers were failing initially, whereas the intact fibers had a remaining load potential. In our setting, three different failure patterns were recognized, specifically a midsubstance tear of the AM or the PL bundle with a subsequent failure of the intact bundle or an initial avulsion of the AM attachment, respectively. The analysis of the video frames revealed a direct connection between the failure patterns in the loadelongation curves and the macroscopic sequence of events during ACL failure.

Conclusions: The ACL ligament acts as a multi-fiber construction. In our setting, rupture follows three specific patterns where a complete or partial tear of the fiber bundles is preceding while the remaining intact fiber bundles have a potential load resistance.

Clinical relevance: Our study allows better understanding of the mechanical properties of the ACL. An update on the biomechanics of ACL failure during uniaxial tension post the "double bundle revolution" could provide data helpful for ACL reconstruction.