New perspectives on ACL injury: On the role of repetitive sub‐maximal knee loading in causing ACL fatigue failure

Abstract

In this paper, we review a series of studies that we initiated to examine mechanisms of anterior cruciate ligament (ACL) injury in the hope that these injuries, and their sequelae, can be better prevented. First, using the earliest in vitro model of a simulated single-leg jump landing or pivot cut with realistic knee loading rates and trans-knee muscle forces, we identified the worst-case dynamic knee loading that causes the greatest peak ACL strain: Combined knee compression, flexion, and internal tibial rotation. We also identified morphologic factors that help explain individual susceptibility to ACL injury. Second, using the above knee loading, we introduced a possible paradigm shift in ACL research by demonstrating that the human ACL can fail by a sudden rupture in response to repeated sub-maximal knee loading. If that load is repeated often enough over a short time interval, the failure tended to occur proximally, as observed clinically. Third, we emphasize the value of a physical exam of the hip by demonstrating how limited internal axial rotation at the hip both increases the susceptibility to ACL injury in professional athletes, and also increases peak ACL strain during simulated pivot landings, thereby further increasing the risk of ACL fatigue failure. When training at-risk athletes, particularly females with their smaller ACL cross-sections, rationing the number and intensity of worst-case knee loading cycles, such that ligament degradation is within the ACL's ability to remodel, should decrease the risk for ACL rupture due to ligament fatigue failure. Keywords: anterior cruciate ligament; fatigue failure; muscle; tibial rotation; repetitive loading DIFFERENT APPROACHES FOR STUDYING ACL INJURY MECHANISMSSuccessful injury prevention efforts usually require a detailed understanding of the mechanism(s) of injury. The fact that anterior cruciate ligament (ACL) injuries continue to occur at a high rate in young athletes means that those mechanisms have remained elusive. Many reports of ACL injury describe a noncontact landing from a jump or a pivot maneuver while changing direction.1,2 The direction of movement and body and lower extremity positions have naturally then become the focus of analyzing injury mechanisms. However, while in vivo experiments might seem preferable, we have been reticent to ask athletes to perform the very maneuvers known to cause these injuries for obvious ethical reasons. Adding to our reticence is the fact that all in vivo field studies involve the use of surface markers or sensors that cannot accurately record the underlying skeletal kinematics.3 While animal studies are an alternative option, the external validity of transferring findings from quadrupeds to humans can be problematic. In silico approaches are another alternative but they require physiological kinematic data. That leaves in vitro studies with cadaver limbs as offering potential for exploring ACL injury mechanisms, as long as the rate and type of loading, the presence of muscle forces, and dynamic load...

Morphology of Mouse Anterior Cruciate Ligament‐Complex Changes Following Exercise During Pubertal Growth

“…Not only did A/J‐exercise mice show a narrower distal femoral morphology than their controls but they also showed significantly less steep posterior medial and lateral tibial plateaus. Therefore, although A/J‐exercised mice did not show a larger ACL Ell.CSA, a less steep posterior tibial slope in A/J‐control mice may allow the knee joint of mice that ran to withstand greater physiological loading about the knee and perhaps protect the ACL from increased peak strains by reducing anterior tibial translation compared with the less active controls. Moreover, the broader femoral ACL insertion and significantly deeper penetration of CF within the anteromedial portion of the tibial enthesis in A/J mice that ran may allow the ACL to dissipate applied strains more effectively, while better resisting anterior tibial load during knee extension, compared with their less active controls.…”

“…It is widely reported that certain anatomical features of the knee joint are associated with increased ACL‐injury risk in amateur and professional athletes . Of particular concern is the fact that adolescent females are 1.6 times more likely to suffer an ACL injury per athletic exposure compared with adolescent males .…”

“…Of particular concern is the fact that adolescent females are 1.6 times more likely to suffer an ACL injury per athletic exposure compared with adolescent males . The morphological traits attributed to this disparity are a narrow intercondylar femoral notch, a narrow ACL, and a steep posterior tibial plateau . These traits are widely considered to be non‐modifiable .…”

Muscle contributions to tibiofemoral shear forces and valgus and rotational joint moments during single leg drop landing

“…Rupture may be a consequence of a single catastrophic load or the consequence of repetitive cyclic loading leading to microdamage and thus fatigue failure. 6 Irrespective of the circumstance, the relevant mechanical loads at the knee that are considered most likely to cause damage to the ACL are anterior shear forces, valgus moments, and internal rotation moments, especially when these loads occur simultaneously. 7,8 It is therefore important to understand how these mechanical loads are developed during key injurious maneuvers, such as change of direction or single-leg landing tasks.…”

2016—The year in review at the JOR

“…In the last year and a half, we began highlighting the work of the Society's most prestigious award, the Kappa Delta Awards and the Orthopaedic Research and Education's Clinical Research Award . We have initiated a monthly review article that will provide the latest synthesis of studies on a particular area.…”