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Friday, February 15, 2002

Researchers identify knee's function axis

Knee alignment is generally expressed in terms of anatomic or mechanical alignment. Investigators have defined the functional axis of the knee, which is not coincident with either anatomic or mechanical axes. The results of their study are presented in scientific exhibit 22.

Investigators evaluated six pairs (12 limbs) of human cadaver femur-tibia-foot specimens using a motion-analysis testing apparatus with an open-chain model that moved the knee with force through the quadriceps tendon. A three-dimensional (3-D) digitizer collected the position data, which was used to determine the finite helical axes of motion for each specimen. The average flexion-extension axis of the knee was then calculated in motion-analysis space.

Computed tomography (CT) scans of each limb were used to construct a 3-D model that was related to the motion-analysis space with radiographic markers and an orthogonal matrix transformation. The calculated finite axes and the average flexion-extension axis were placed in the 3-D limb model. A plane, perpendicular to the average flexion-extension axis, was then constructed and the proximal and distal extensions of that plane identified.

Investigators found that the upper end of the functional plane intersects a region of the hip just lateral to the anterior inferior iliac spine in the region of the anatomic origin of the rectus femoris muscle. It intersects the knee through the depth of the trochlear groove of the femur and the tibial tubercle of the tibia. At the lower end, the functional plane intersects the neck of the talus at a point approximately one third of the way between the medial malleolus and the lateral malleolus.

This is significantly different from the anatomic axis, which moves along the shafts of the femur and the tibia, as well as from the mechanical axis, which involves the head of the femur, the middle of the knee and the middle of the ankle. However, all axes describe a straight line in the normal knee.

"Reconstructing the functional axis when performing a total knee replacement allows the use of stable, conforming components," said the researchers.

Investigators included William S. Simons, PhD; J. David Blaha, MD, and Vince Kish, all of Morgantown, W.V. Funding for the research project was provided by Wright Medical Technology.

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Last modified 05/February/2002