External knee joint design based on normal motion

There are several advantages to accurate reproduction of knee motion in an external joint assembly such as a knee brace: reduction of pistoning forces, better ligament protection, kinematic compatability. The geometry and kinematics of the normal human knee were studied and the results applied to external joint design. Geometrically, the posterior portions of the femoral condyles were found to be spherical in shape. These spherical surfaces are projected in sagittal plane radiographs as circles with center points coincident with those of the spheres. A line connecting these centers defines an axis system and enables three-dimensional orientation of the femur on the tibia to be determined using sagittal-plane radiographs. Knee kinematics was determined as a function of flexion angle for 14 fresh cadavers and 8 volunteers. Results were in the form of eulerian rotations and displacements. The data were normalized to the size of the average knee and the results from the 22 trials were averaged. The most obvious motion was internal rotation of the tibia with flexion; however, varus rotation and posterior translation of the origin were also evident. An external joint system was then designed to mimic "average" knee motion during flexion. The joints have been incorporated into a knee brace, and clinical evaluation has begun. Other applications include cast bracing and hinge distraction.