The anterior cruciate ligament (ACL) is one of the major stabilizers of the knee joint. Each year, as many as 250,000 people (approximately one in 1,200) in the United States alone rupture their ACL. Among a younger population (age 15 to 45 years), the incidence of ACL injury is even higher, elevated to one in 1,750. The annual financial cost for ACL reconstruction is estimated to be well over one billion dollars. About 70 percent of these injuries are through non-contact mechanisms, involving such maneuvers as sudden deceleration, cutting, and pivoting. Further, females have a risk factor for ACL injuries that is two to eight times greater than males while playing the same sports. Such high incidence in females, coupled with a significant rise in the number of females participating in sports, has led many to consider the phenomenon as an epidemic.
This research initiative represents a collaborative effort between the Institute and the Musculoskeletal Research Center (MSRC) at the University of Pittsburgh (Dr. Savio L-Y Woo). Extensive work has been performed to gain better understanding of the contribution of the ACL to joint stability, as well as to quantify the in situ (in place) force in the ACL. A significant portion of our findings have already been translated into clinical practice, as improved ACL reconstruction procedures have been developed based on sound biomechanical principles. In this research endeavor, we strive to build on the experience of these two institutes and study ACL function in vivo.
By combining the newly developed technology at both research centers, we will be able to advance our knowledge on the forces experienced by the ACL in vivo (as it happens). Thus, our overall objective is to obtain better quantitative data of gender-specific function of the ACL that would lead to the understanding of the causes and mechanisms of the higher rates of non-contact ACL injuries in females. To do this, we will quantify the in situ force and force distribution in the ACL during activities and examine potential differences between genders. In addition, we will evaluate how preventive training in female athletes can reduce the risk of ACL injury by lowering the force in this ligament. Knowledge of the mechanisms of ACL injuries will aid in the designing of effective prevention training programs to reduce non-contact ACL injuries, as well as designing gender- and/or patient-specific surgical treatment and postoperative rehabilitation protocols. This, in turn, will lead to improved patient outcomes by minimizing the risk of re-injury, as well as preventing the eventual development of osteoarthritis following ACL reconstruction.
To achieve our objective, we will utilize our high-speed, dual-plane fluoroscopy system that has been developed to collect accurate tibiofemoral-joint kinematics in vivo from healthy volunteers landing from a jump. Then, the subject-specific knee kinematics will be reproduced on matched human cadaveric knees using a high payload robotic/universal force-moment sensor (UFS) testing system at the University of Pittsburgh to determine the in situ force and force distribution in the ACL. The six degrees-of-freedom (6-DOF) in vivo knee kinematics will also be used as input data to subject-specific finite element models with a complex constitutive model of the ACL to determine the stress and strain distributions in the ACL during the same dynamic jump landing motion.
The dual-plane fluoroscopy system consists of two commercially available BV Pulsera c-arms (Philips Medical Systems, Best, Holland), which were modified under appropriate FDA guidelines and Colorado State Radiation Safety Regulations. The image intensifiers were removed from their c-arm configuration and mounted on a custom gantry that allows for variable Source-to-Image-Distance (SID; generator to image plane distance) of 1.0-2.0 meters, as well as variable beam-angle configurations between the two fluoroscopy systems to allow for the ability to optimize viewing volume, movement freedom and technique factors.
With this system, knee motions will be captured with a knee-to-image-intensifier distance of 25-40 cm and exposure duration of 1.0 s. In order to capture the high speed nature of the landing motion, two coupled, high-speed, high-resolution (1024 x 1024) digital cameras with frame rates of 1,000 frames per second were interfaced with the two image intensifiers of the fluoroscopy systems using a custom design interface.
We then proceeded to validate the approach for a complex motion on human cadaveric knees. To this end, the kinematics of a cadaveric knee were recorded on the biplanar fluoroscopic system at the Institute using both bead markers and bone tracking. The knee, along with its recorded kinematics, was then shipped to the MSRC in Pittsburgh to be replayed on our high payload Robotic/UFS testing system. The kinematics show that the knee was flexed from 15 to 108 degrees and internally rotated by about 17 degrees as it was flexed. As is characteristic with knee motion, the tibia translated in the anterior-posterior direction with flexion, as part of the femoral-roll-back mechanism. Using the proposed methodology, the in-situ force in the ACL was measured during the replay of this motion. The forces in the ACL ranged from a minimum of 8 N towards extension and increased to a maximum of approximately 24 N in flexion.