An in Vitro Biomechanical Evaluation of Anterior-Posterior Motion of the Knee

We tested the anterior-posterior motion of nine normal cadaver knees in zero to 90 degrees of flexion using a specially designed apparatus. This ap paratus applied a dynamic anterior-posterior force to each knee and measured the resulting tibial displace ment, rotation, and torque. In the intact knee, an an terior force produced an internal tibial torque and in ternal tibial rotation, while a posterior force produced an external torque and external rotation. Anteriorposterior displacement increased by 30 per cent when the tibia was allowed to rotate freely about its neutral rotation position. Isolated section of the anterior cruciate ligament produced more than double the amount of anterior displacement without affecting posterior displacement. Isolated section of the posterior cruciate ligament pro duced almost triple the amount of posterior displace ment without affecting anterior displacement. After cutting either the anterior or the posterior cruciate lig ament, the resulting internal or external secondary tib ial rotation disappeared. It appears, therefore, that the anterior and posterior cruciate ligaments are the pri mary restraints to motion in the anterior and posterior directions as well as the causes of internal and external tibial rotation during anterior and posterior motion. CLINICALRELEVANCE: Clinical evaluation of the knee requires the measurement of relative femoraltibial motion resulting from an applied force. Cadaver experiments are a useful means by which quantitative * Supported in part by the Jessie Ball duPont Religious, Charitable and Educational Fund, and by the Clark Foundation. t Department of Orthopaedic Surgery, University of Tokyo, To kyo, Japan. t The Hospital for Special Surgery, 535 East 70th Street, New York, N.Y. 10021. Please address reprint requests to Dr. Torzilli. measurements of the motion can be made. Through the selective cutting of various ligament structures, alterations in the motion of the normal knee can be studied and the appropriate characteristics can be defined. In this investigation, we found that in knees with insufficiency of the cruciate ligaments the greatest in creases in anterior and posterior displacement oc curred at 30 and 75 degrees of flexion, respectively. Concomitantly a drastic loss of tibial rotation also oc curred, suggesting that this characteristic must not be ignored when evaluating stability of the knee. Presently most clinical techniques for assessing the anteroposterior or rotational instability of the knee rely on subjective observations and the "feeling" of the examin ing physician. In vivo quantitative data from mechanical devices do not always correspond to the observations of the physician. Thus, it is necessary to correlate in vivo ob jective measurements with the surgical findings and to utilize in vitro tests to determine the methods and sig nificant parameters to be used for diagnostic purposes. In addition, in vitro biomechanical tests can be used to de termine the roles of various structures in knee stability. The objective of this study was to use an in vitro knee-stiffness apparatus to evaluate motion of the knee using an anterior-posterior force. The following mea surements were performed. 1. Anterior-posterior displacement of the knee result ing from applying an anterior or posterior force while allowing tibial rotation and while preventing tibial rota tion. 2. Internal and external rotation of the tibia resulting from applying an anterior or posterior tibial force with the tibia allowed to rotate. THE JOURNAL OF BONE AND JOINT SURGERY BIOMECHANICAL EVALUATION OF ANTERIOR-POSTERIOR MOTION OF THE KNEE 259 3. The internal and external tibial torque generated by applying an anterior or posterior tibial force while pre venting tibial rotation. 4. The changes in these measurements after section of the anterior or posterior cruciate ligament, or both.