Abstract
Placing total knee components in the proper position in all three planes (frontal, sagittal and transverse) is important for correct functioning of the arthroplasty. Appropriate position for the femoral component is parallel to and equidistant from the “average flexion extension axis” of the knee. An axis drawn from one epicondyle to the other is an appropriate approximation for the average flexion-extension axis of the knee. Surgeons have experienced problems, however, finding the epicondyles with certainty making this set of landmarks difficult to use. A line down the trochlear groove (AP axis – most often attributed to Whiteside, sulcus axis, Eckhoff)) usually intersects the TEA at right angles. Using both the palpated epicondyles and the AP axis should make finding the proper position easier, but still there is a need for instrumentation that can readily and reproducibly find the correct position of the flexionextension axis.
Kinematic work (Blaha, Simons et al.; Eckhoff et al.) has confirmed that the average flexion-extension axis of the human knee, for the majority of the flexionextension arc, is very nearly parallel to the TEA though slightly displaced from it. The knee joint then moves in a plane perpendicular to the flexion-extension axis. This functional plane is neither coincident with the anatomic axis (i.e., shafts of the bones) nor the mechanical axis (i.e., femoral head – center knee – center ankle). Rather the plane intersects four critical functional points: the lateral border of the acetabulum (origin of the rectus femoris muscle), the trochlear groove, the tibial tubercle, and the neck of the talus. These points now define a different axis for the knee joint: the functional axis. Finding the functional axis will appropriately position the components of a total knee replacement so that the axis of the replaced knee matches that of the native knee.
The AP clamp attaches to the femur at the most posterior- superior part of the intercondylar notch* with an acutely curved portion and to the trochlear groove with a more gently curved portion. Tests in our laboratory have determined that the clamp finds the functional plane (and thus the functional axis) with less than 1° of variability. A line drawn perpendicular to the clamp parallels the flexion-extension axis of the knee and is thus the proper rotational axis (transverse plane) of the femoral component as well as varus-valgus axis (coronal plane). The clamp has been used in both cadaver and live operating situations and has been found to correctly determine not just the rotational position of the components but also the position in the frontal plane. Clinical experience with the clamp has confirmed that when a total knee prosthesis is placed in the proper position relative to the flexion-extension axis the limb will have correct alignment, stability and kinematics.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
