TTU Home COE Home Mechanical Engineering Orthopaedic Biomechanics Laboratory Projects In-Vitro Simulation of Non-contact ACL Injury During Jump-Landing and Pivot-Shift

In-vitro Simulation of Non-contact ACL Injury During Jump-landing and Pivot-shift

Researchers

Ryan E. Breighner Dr. Taek Hyun Jang

Background

The knee is the largest and most complicated joint in the human body. It is in fact two joints acting in concert; the femoro-patellar joint and the femoro-tibial joint. The patello-femoral joint provides a mechanism by which the quadriceps femoris muscle acts to extend the knee. The tibio-femoral joint is the joint which bears body weight as the femur articulates with the tibia.

In order to ensure the stability of the knee, ligaments are necessary to connect the femur to the tibia. The four ligaments of the knee are the lateral colateral ligament (LCL), which attaches the femur to the fibula near the tibio-fibular joint; the medial colateral ligament (MCL), which attaches the femur to the tibia; the posterior cruciate ligament (PCL) and the anterior cruciate ligament (ACL), both of which attach the femur to the tibial. The primary focus of this and many other studies is the latter of these four ligaments, the ACL. The ACL is so frequently studied because it is the most frequently injured ligament in the knee and its rupture is  repaired through costly surgery and extensive rehabilitation.

In order to prevent ACL injuries, it is necessary to understand how they occur. It is estimated that nearly 70% of all ACL injuries are the result of non-contact injury. This means that no forces other than muscle forces or ground reaction forces are involved to cause the injury. These non-contact injuries occur frequently in what are often refered to as "cutting motions," or when landing from a jump. Both of these motions involve a sudden change in the body's direction or rapid deceleration, resulting in considerable forces being applied to the ankle, knee, and hip joints. When these forces generate stresses in excess of the ultimate strengths of the tissues involved, injuries occur.

Simulation

The primary aim of this series of studies is to examine the effects of different muscle forces acting on the knee during impact and torsional loading conditions. In-vitro simulation is necessary to study these loading conditions as destructive tests are not possible with subjects.

To evaluate different loading conditions and to test various injury mechanisms, a jump-landing simulator has been built. The simulator can apply dynamic muscle forces through the use of motor-driven linear actuators. At present, the simulator can be configured to apply quadriceps and hamstring muscle forces. The proper lines of action for these muscles are achieved through the use of pulleys and the load is applied via cables attached to the linear actuators.

To simulate jump-landing, the simulator utilizes a linear bearing-guided drop weight.  The weight is dropped, striking a lever which then pushes up on the "ankle" of the machine. The mechanical "ankle" allows for free plantar/dorsi-flexion of the ankle as well as variable varus/valgus to accomodate different subjects and scenarios. The ankle also allows for the evaluation of the influence of diffent varus/valgus angles on knee mechanics under various loading conditions. The simulator also includes a pivot joint to simulate the hip, which can be constrained or left free to rotate in the saggital plane, depending on what conditions are being evaluated.


To quantitatively evaluate the effects of different loading models, the knee is instrumented. Strains in the ACL and Patellar tendon are measured using differential variable reluctance transducers (DVRTs). The DVRTs that we are using are so small that they can be installed on the ACL without interupting the motion of the knee. The muscle forces applied are measured using two load cells placed between the cables and linear actuators. Impact forces are measured both distal and proximal of the knee joint using piezoelectronic impact transducers.

In the future, additional muscle forces and a more realistic hip joint will be implimented to simulate the response of the knee even more accurately. In addition to these improvements, the simulator will also be updated to include a means of applying torque to the knee, either through the femur or tibia, to simulate internal or external tibial rotation.