This paper presents a planar model of passive knee motion-FORM2D-utili
zing a novel modeling technique. For a given knee flexion, the envelop
e of passive knee joint motion is determined by applying the femoral t
ranslation necessary to minimize the anterior-posterior force in the j
oint. Unlike existing planar models, FORM2D accounts for all four of t
he major ligaments (ACL, PCL, MCL and LCL) simultaneously. Results fro
m FORM2D on ligament strain and its clinical applicability are present
ed.