Measuring knee biomechanics in six degrees of freedom with acceptable accuracy has been proven to be technically challenging. At our bioengineering laboratory, we have employed both an in vitro robotic testing system and an in vivo combined dual fluoroscopic and magnetic resonance imaging technique to analyze the impact of anterior cruciate ligament rupture on the knee joint.
When measuring the tibiofemoral kinematics of nine cadavers with the robotic testing system, we found that anterior cruciate ligament deficiency not only altered anterior translation and axial rotation of the tibia, but it also increased the medial translation of the tibia as well. The in vivo dual fluoroscopic imaging analysis of tibiofemoral kinematics in ten anterior cruciate ligament-deficient patients revealed analogous findings: an increased medial translation of the tibia of approximately 1 mm between 15° and 90° of flexion was found in anterior cruciate ligament-deficient knees, in addition to an increased anterior translation (approximately 3 mm) and internal rotation (approximately 2°) of the tibia at low flexion angles. In a subsequent study of tibiofemoral cartilage contact, we found that the cartilage contact points shifted posteriorly—as was expected on the basis of the increased anterior tibial translation—as well as laterally on the surface of the tibial plateau.
The data demonstrate how rupture of the anterior cruciate ligament initiates a cascade of events that eventually results in abnormal tibiofemoral cartilage contact in both the anteroposterior and mediolateral directions. If the restoration of normal knee homeostasis is the ultimate goal of ligament reconstruction, the normal function of the anterior cruciate ligament should be restored as closely as possible in all degrees of freedom.
1Bioengineering Laboratory, Massachusetts General Hospital/Harvard Medical School, 55 Fruit Street - GRJ 1215, Boston, MA 02114. E-mail address for G. Li: firstname.lastname@example.org