A864. THE ANATOMIC RATIONALE FOR GUIDED MOTION TOTAL KNEES PETER

Abstract

The treatment of osteoarthritis using artificial knee joints is expected to expand further over the next decade. Increasingly, patients expect quicker rehabilitation, improved performance, and high durability. However, economic limitations require a reduced cost for each procedure, as well as early intervention and even preventative measures. The major goal of implant design needs to be a restoration of normal knee mechanics, whether by maximum preservation of tissues, or by guiding surfaces which replicate their function. In this paper it is proposed that total knees should exhibit anatomic knee mechanics, namely medial stability – lateral mobility.

Many studies in the past have shown that the neutral path of motion of the anatomic knee, is that the medial side remains relatively immobile in the AP direction, which will impart a feeling of stability, while the lateral side shows posterior femoral displacement with flexion, to obtain a high range of flexion. There is considerable rotational laxity about this neutral path to accommodate a range of positions and activities. Recent studies carried out in our laboratory using an up-and-down crouching machine, and other test machines, have conformed this mechanical behaviour. To further elaborate, we tested eight young male subjects in a 7T MRI machine, where compressive and shear loads were applied. AP displacements occurred laterally but not medially. We attributed this behaviour to the medial meniscus and the tibial bearing geometry under weight-bearing conditions.

On the basis of these various studies, we developed a method for the design of Guided Motion knees, which would be implanted without the cruciates, and which would restore anatomic knee mechanics. The method started with the femoral component, where the medial side had features to provide a continuous radius anteriorly, and distally to 75 degrees flexion when a post-cam would contact. This feature would prevent paradoxical anterior femoral sliding in early flexion. Multiple femoral positions were then defined for accommodating anatomic motion, in particular limited AP motion on the medial side, but posterior displacement laterally. Tibial bearing surfaces were generated accordingly.

Tests were carried out on the crouching machine and on a Desktop TKR Test machine to compare the TKR motion with anatomic. Although not accurate in all respects, the Guided Motion designs were closer than models of standard TKR’s today. Such Guided Motion designs hold the promise for restoring anatomic knee mechanics and a normal feeling knee.