Impact-induced Sagittal Plane Tibial Acceleration As A Mechanism For Acl Injury

Abstract

For single-limb landings in which ACL injury is common, the large intersegmental accelerations arising at impact must be restrained by the internal joint structures and the pre-existing muscle-tendon loading across the joint. If an ineffective overarching neuromuscular strategy prevails, however, these accelerations may be large enough to injure the passive restraint mechanism. PURPOSE: To demonstrate that impact-induced peak anterior tibial accelerations during a simulated single leg landing are proportional to peak ACL strain. Further, to show that this relation is directly affected by posterior tibial slope. METHODS: Twelve female cadaveric specimens (65.3 ± 10.5 yrs) were subjected to five (2*BW) loading trials using the Withrow (2008) apparatus that successfully simulates 3-D knee loading during unipedal landings. Each limb was initially fixed at 15° knee flexion while quadriceps (180N), hamstring (140N) and gastrocnemius (140N) muscle forces statically pre-loaded the joint. For each trial, 3-D tibial and femoral forces and moments, 3-D knee joint kinematics and ACL strain were synchronously recorded. Tibiofemoral joint displacement data were subsequently filtered at 50 Hz, from which, anterior tibial accelerations were quantified over the first 200 ms post-impact. The tibial slope of each specimen was quantified from lateral radiographs. Peak anterior tibial acceleration (pTA) and ACL strain (pAS), and posterior tibial slope (PTS) and PTA were then submitted to respective linear regressions to test for association. RESULTS: A strong positive correlation (r = 0.75) was observed between mean pTA (7938.1 ± 2009.5 mm.s-2) and pAS (4.22 ± 1.53 %), which occurred 66 ± 7 ms and 66 ± 4 ms post-impact, respectively. A strong positive correlation (r = 0.80) was also found between mean PTS (6.53 ± 1.52°) and pTA. CONCLUSIONS: Impact-induced anterior tibial accelerations are directly associated with resultant ACL strain. Considering the large accelerations possible during landings, therefore, ACL injury via such a mechanism seems plausible. The potential for injury via this mechanism is also directly influenced by posterior tibial slope. Screening for high-risk acceleration profiles and additional investigation into 3-D knee structural and mechanical interactions during landings appears warranted.