Mechanisms of ACL Injuries

Mechanisms of noncontact ACL injuries are still not clearly understood. It is hypothesized that ACL can be injured by direct stretch during tibial internal rotation, and for knees with certain 3-D shapes of the intercondylar notch, ACL can be injured by impingement against the lateral notch wall during tibial external rotation/abduction in flexed knees. In general, ACL could also impinge the notch roof at hyperextension. Mechanisms of noncontact ACL injuries during various 3-D tobiofemoral movements were evaluated in cadaver knee specimens using multiple methods, including direct measurement of the ACL strain and its impingement against the intercondylar notch using ultra-microminiature (UM-DVRT) strain sensor and paper-thin force sensor, respectively, arthroscopy with the impingement measured dynamically by a Tekscan pressure sensor, and fluoroscopy.

Modeling of ACL Impingement

The femur and tibia are digitized using a laser scanner. A combination of rotary and planar scans performed by the laser scanner generated an accurate 3-D model of the bines represented as a point cloud. A 3-D geometric reconstruction of the knee specimen is generated based on the digitized data points. A 3-D ACL impingement model can then be implemented using data from an individual cadaveric knee with representative ACL impingement, which is loaded passively to induce ACL impingement with the impingement force and 6-DOF tibiofemoral kinematics measured. Thedigitized intercodylar notch surfaces are surface-fitted using bicubic splines with continuity up to the second derivative. The model detects ACL impingement during tibiofemoral movement and uses a "crawling" algorithm to determine the deformed geometry of the impinging ACL.

Compensatory Mechanisms for ACL Injury and New Rehabilitation Protocols

Compensation for ACL injuries have been studied from several aspects: 1) EMG signals from multiple muscles during locomotion are analyzed, which shows that ACL injured patients tend to have stronger lateral hamstring and weaker medial hamstring contraction during certain phases of locomotion (Hypothesis: to externally rotate the tibia on the femur to unload the ACL and avoid unstable knee positions). Six degrees-of-freedom knee kinematics during locomotion are obtained and compared between injured and uninjured populations, which corroborates the above EMG finding. Furthermore, individual hamstrings are activated selectively through electrical stimulation and the resulting six-axis forces/moments show that the medial and lateral hamstrings generate significant internal and external rotation torque, respectively. Finally, it has been found that chronic ACL injured patients developed significantly higher strength ratio of external rotation over internal rotation, indicating the gradually developed compensation. Along the same line of thinking, we are developing a new rehabilitation protocol for ACL injured patients to selectively and differentially strengthen those muscles which help protect and unload the ACL

Fig. 1. The six-degrees of freedom goniometer used to measure the tibial movement relative to the femur in three-dimensional space. The femoral and tibial segments of the goniometer were strapped to the thigh and leg, respectively. Six precision potentiometers with bearings were used to measure the six-degrees of freedom movement. A suction cup was aligned with and pressed against the lateral epicondyle.

ACL Strain During "Locomotion"

Need for the compensatory mechanism depends on whether the ACL is loaded during free-speed walking. Fresh-frozen cadaver knees were used to evaluate the ACL strain during simulated free-speed walking. The cadaver knees was moved following the average knee flexion pattern of the 30 normal subjects using a computer-controlled servomotor, and the ACL strain was measured with a differential variable reluctance transducer (MicroStrain, Burlington, Vermont). The "free-speed walking" was repeated with the tibia placed at different axial rotation and anteroposterior translation positions. 

ACL strain during simulated "free-speed walking" using a cadaver model. Top plot: Knee flexion as a function of stride %; Bottom plot: ACL strain during simulated "free-speed walking", averaged over 48 strides. From top to bottom, the five curves represent the ACL strain with the tibia positioned at internal rotation (7º) plus anterior translation (10 mm), internal rotation (7º), neutral, external rotation (7º), and external rotation (7º) plus posterior translation (10 mm), respectively.

The results showed that the ACL strain varied with knee flexion systematically and the ACL was loaded considerably during "free-speed walking", especially at full knee extension. Furthermore, ACL strain increased substantially with tibial internal rotation and anterior translation and decreased markedly with tibia external rotation and posterior translation. Considering that the tibia was positioned more anteriorly during locomotion in ACL-deficient knees, it made it more likely that the ACL of the patients would be loaded (or getting into the "ACL-loading positions") in locomotion, which made the compensatory mechanism based on tibial external rotation useful.