Dynamic three-dimensional motion analyses of 15 fresh human knee joints subjected to combinations of flexion velocity and moment, internal and external femoral torque, and horizontal shear before and after sectioning the ACL were performed. ACL deficient specimens demonstrated marked anterior instability without rotational instability. The pivot shift phenomenon occurred with an isolated ACL deficiency and was the result of anterior instability. The pivot shift was accentuated by external femoral torque, decreased by internal femoral torque, and was present in the absence of any applied torque. The pivot shift produced a sudden directional change in the motion of both femoral condyles and may be responsible for the meniscal degeneration that accompanies chronic ACL deficiency.

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