Anterior cruciate ligament rupture translates the axes of motion within the knee.

OBJECTIVE: To quantify the dynamic effects of anterior cruciate ligament deficiency on human knee joint motion.
DESIGN: Three-dimensional motion was assessed by measuring the kinematics of intact and anterior cruciate ligament deficient knee joint specimens during simulated flexion-extension cycles.
BACKGROUND: It is known that the anterior cruciate ligament plays an important role in controlling three-dimensional knee joint motion. Nevertheless, dynamic effects of deficiency are not fully understood.
METHODS: Six cadaveric knees were tested in a knee joint motion and loading apparatus prior to and after anterior cruciate ligament resection. To determine if the kinematic results depended on additional loading, internal and external rotation moments of 3.4 Nm were axially applied to the tibia. The kinematics were analysed in terms of finite helical axes.
RESULTS: Sectioning the anterior cruciate ligament had little effect on the orientations of the finite helical axes. However, applying moments did affect the axes orientation. In contrast, relative translations of the axes were significantly affected by the deficiency for all rotational moments applied. Referring to the individual knee anatomy the largest translation of 12.5 mm (median) occurred in medial/lateral direction.
CONCLUSIONS: Anterior cruciate ligament rupture primarily causes a translation of the finite helical axes in medial/lateral direction. Consequently, increased anterior excursion of the tibia occurs (subluxation) and therefore dynamic instability.