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Document Title: DeFrate-AJSM-Aug06.shtml
Article Title: The 6 Degrees of Freedom Kinematics of the Knee After Anterior Cruciate Ligament Deficiency -- An In Vivo Imaging Analysis
Authors: Louis E. DeFrate, ScD, Ramprasad Papannagari, MS, Thomas J. Gill, MD, Jeremy M. Moses, MD, Neil P. Pathare and Guoan Li, PhD.
Publication: American Journal of Sports Medicine, Baltimore, Maryland
Date: August 2006
Volume 34, pages 1240-1246
Keywords: biomechanics, knee motion, kinematics, kinetics, anterior cruciate ligament (ACL), degrees of freedom, MRI imaging, X-ray imaging.
(Reference-denoting numbers appear in the same font and point size as the document text. As with all Knee Library documents, this article is provided in full-text form, complete with all figures and tables.)
Comments: This study, done on single-ACL-torn people using fluoroscopy (i.e. X-ray video; note that this technique results in a high X-ray dose because it uses the same X-ray source as is used for static imaging, but the shutter remains open, thus exposing the imaged area to the equivalent of 50 still X-ray shots every second!) and MRI (which is completely harmless), shows how the biomechanics of the ACL-deficient knee varies from the natural knee. The six degrees of freedom (i.e. 3 translations and 3 rotations) are universal to any field involving motion. They are anterior-posterior (front-to-back translation), medial-lateral (side-to-side translation), superior-inferior (up-down translations, also known as proximal-distal), internal-external rotation (i.e. rotation in a transverse plane, i.e. any plane parallel to the person's belt line), varus-valgus rotation (i.e. what you get if you force the knee inwards or outwards in such a way as to tear the MCL or LCL, respectively; this is a rotational motion in a coronal/frontal plane, which is any plane parallel to the bed if you are lying on your back), and saggital-plane rotation (i.e. the normal flexion and extension of the knee). (In aviation, these 6 degrees of freedom are known as x, y, z, and roll, pitch, and yaw.) The authors conclude that the ACL plays a role not only in anterior-posterior stability, but in medial-lateral stability as well (and in addition to contributing to extension limitation and constraining inwards tibial rotation).
ABSTRACT
Background: Previous studies of knee joint function after anterior cruciate ligament deficiency have focused on measuring anterior-posterior translation and internal-external rotation. Few studies have measured the effects of anterior cruciate ligament deficiency on 6 degrees of freedom knee kinematics in vivo.
Objective: To measure the 6 degrees of freedom knee kinematics of patients with anterior cruciate ligament deficiency.
Study Design: Controlled laboratory study.
Methods: The knee joint kinematics of 8 patients with unilateral anterior cruciate ligament rupture was measured during a quasi-static lunge. Kinematics was measured from full extension to 90° of flexion using imaging and 3-dimensional modeling techniques. The healthy, contralateral knee of each patient served as a control.
Results: Anterior cruciate ligament deficiency caused a statistically significant anterior shift (approximately 3 mm) and internal rotation of the tibia (approximately 2°) at low flexion angles. However, ligament deficiency also caused a medial translation of the tibia (approximately 1 mm) between 15° and 90° of flexion.
Conclusion: The medial shift of the tibia after anterior cruciate ligament deficiency might alter contact stress distributions in the tibiofemoral cartilage near the medial tibial spine. These findings correlate with the observation that osteoarthritis in patients with anterior cruciate ligament injuries is likely to occur in this region.
Clinical Relevance: The data from this study suggest that future anterior cruciate ligament reconstruction techniques should reproduce not only anterior stability but also medial-lateral stability.
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