Abstract
In different contemporary posterior-stabilized (PS) total knees, there are considerable variations in condylar surface radii and cam-post geometry. This is expected to result in differences in kinematics and functional outcomes in patients. The hypotheses of our study were: 1. Current PS design will show symmetric motion which is different from anatomic motion, and 2. An asymmetric PS design will produce motion closer to normal anatomic motion than symmetric designs.
A special machine was constructed which could implement the ASTM standard test on constraint, by measuring the laxities. The rational for the test is to predict functional laxity ranges which will affect the kinematics in vivo. The machine set the knee at the required flexion angles and applied combinations of compressive, shear, and torque forces, to represent a range of everyday activities. The femorotibial contact points, the neutral path of motion, and the AP and internal-external laxities were used as the motion indicators. The benchmark was the motion data from anatomic knee specimens tested under the same conditions.
Four contemporary PS designs with a range of geometries was selected for the tests, together with a design where the medial side was more constrained, the lateral side was less constrained, and the post was rounded. The output motions were compared between themselves, while all designs were compared with the anatomic data. The PS designs showed major differences in motion characteristics among themselves including the neutral path of motion and the AP and rotational laxities. These differences were related to the constraints of the condyles, and the cam-post designs. The four PS designs showed motion different from anatomic, including symmetric mediolateral motion, susceptibility to excessive AP medial laxity, and reduced laxity in high flexion. The asymmetric Guided Motion design alleviated some but not all of the abnormalities; in particular, while the lateral rollback with flexion and the near-constant position of the medial femoral condyle resembled anatomic behaviour, the rotational laxity was still limited in high flexion. The latter ws observed to be due to the ‘entrapment’ of the femoral condyles between the upwards posterior lip of the tibial plastic, and the posterior of the cam-post, a phenomenon seen on all designs.
The conclusion of the study is that an asymmetric PS design may provide a path to achieving a closer match to anatomic kinematics. This may improve functional outcomes, and even provide a better ‘feel’ to the patient. However, there are still inherent challenges in PS design to closely achieve this goal. Other design configurations have also been formulated which could even more closely reproduce anatomic motion. However a pre-clinical testing method such as presented here, is one method for evaluation and can be used hand-in-hand with computational methods to produce an optimal design. The importance of the benchmark of the anatomic knee and the identification of the important parameters of the ASTM standard, notably the neutral path of motion and the laxity about the neutral path, are important aspects of the design methodology.
