No abstract

The femoral enthesis of the human anterior cruciate ligament (ACL) is known to be more susceptible to injury than the tibial enthesis. To determine whether anatomic differences might help explain this difference, we quantified the microscopic appearance of both entheses in 15 unembalmed knee specimens using light microscopy, toluidine blue stain and image analysis. The amount of calcified fibrocartilage and uncalcified fibrocartilage, and the ligament entheseal attachment angle were then compared between the femoral and tibial entheses via linear mixed-effects models. The results showed marked differences in anatomy between the two entheses. The femoral enthesis exhibited a 3.9-fold more acute ligament attachment angle than the tibial enthesis (p < 0.001), a 43% greater calcified fibrocartilage tissue area (p < 0.001), and a 226% greater uncalcified fibrocartilage depth (p < 0.001), with the latter differences being particularly pronounced in the central region. We conclude that the ACL femoral enthesis has more fibrocartilage and a more acute ligament attachment angle than the tibial enthesis, which provides insight into why it is more vulnerable to failure.

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...

This investigation compared the amplitude and the timing of the muscle activity of the lower limb, as well as the three-dimensional kinematics of the hip, knee and ankle joints, of male and female elite soccer players performing an unanticipated cutting manoeuvre. These data were recorded for 15 female and 15 male participants for five successful cutting manoeuvres. For this manoeuvre to be performed in an unanticipated manner, the participants were instructed to execute one of three tasks, which were signalled to them with a target board composed of three different coloured lights. Female participants performed the cutting manoeuvre with greater lateral gastrocnemius activity in comparison with the male participants. It was also observed that they contracted their vastus lateralis to a greater extent than their vastus medialis, whereas the men adopted the opposite strategy. These neuromuscular control strategies adopted by the female athletes may elucidate the reasons for which women struck the ground with a more abducted knee during the cutting task. Given that this knee position places greater strain on the anterior cruciate ligament, a restoration of the medial/lateral activation balance of the lower limb muscles may reduce one's risk of injury.

Decreased range of internal femoral rotation results in greater ACL strain and may therefore increase the susceptibility to ACL rupture with athletic cutting and pivoting activities. Screening for a limited range of hip internal rotation should therefore become a component of not only ACL injury prevention programs but also evaluation protocols for those with ACL injuries and/or reconstructions.