Total Knee Arthroplasty Design Without Cruciates to Achieve Anatomic Femoral-Tibial Motion and Laxity.

Background: A frequently stated goal of an artificial knee arthroplasties is to achieve normal kinematics. However, this is not easily defined based on variations in motions previously measured for a range of activities. For activities such as crouching up and down, a fan pattern has been measured, where the lateral femoral contact displaces progressively posteriorly with flexion, and the medial contact remains almost constant. In walking and other activities, femoral-tibial contacts vary considerably in position, and even lateral pivoting has been measured at the start of the motion cycle. Fluoroscopic studies of total knee arthroplasty (TKA) patients have shown that such kinematics is not usually achieved for most TKA designs. In recent years, there has been an increasing interest in noncruciate retaining knee arthroplasties, where both cruciate ligaments are resected. A challenge with such designs is to define the design criteria, taking account of the extensive kinematic data of normal knees, as well as clinical factors.

Methods: A TKA design was formulated where the main bearing surfaces produced medial stability and lateral mobility, but where the addition of an offset cylindrical bearing surface in the center induced progressive axial rotation and lateral "rollback" with flexion. At the same time, anterior-posterior and rotational laxity were provided, as in the normal knee. The new design was compared experimentally with four types of contemporary noncruciate total knee arthroplasties. There were three-dimensional printed models fabricated. A test machine was constructed where shear and torque forces were applied at a range of flexion angles, and contact positions were determined.

Results: It was found that the design with the intercondylar cylindrical surface satisfied the design criteria more closely compared with the other designs.

Conclusions: For noncruciate designs to produce more normal motion characteristics, some mechanical configuration acting in concert with the lateral and medial condyles is likely to be necessary.