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Friday, March 17, 2000

Mobile-bearing knee designs offer advantage in restoration of knee function

Restoration of normal knee joint function where arthroplasty is employed is dependent upon load sharing between the implant and surrounding soft tissue structures, say co-authors of a study in scientific exhibit 54.

In particular, emergent mobile-bearing knee designs offer the advantage of maximum conformal geometry while diminishing constraint forces to fixation interfaces through plateau mobility. The degree of mobility afforded by these designs in the anterior-posterior, medial-lateral and rotational directions defines the required interaction between soft tissue and design geometry to maintain a stable articulation in vivo.

The authors developed a dynamic testing system to evaluate the intrinsic directional constraints for four mobile bearing knee designs under loads simulating normal gait. The force-displacement curves were analyzed for the extent of tibial insert sliding, the shear forces generated through component interaction as well as any defining characteristics of the design.

They found that all of the mobile bearing knee designs evaluated permit normal, physiologic rotation without the engagement of mechanical stops. In contrast, none of the designs accommodated medial-lateral translation without being coupled with internal and external rotation. There were substantial differences in the degree of mobility offered by these designs in the anterior-posterior direction due to the utilization of mechanical stops on the tibial tray to avoid component subluxation.

They concluded that the mobile bearing knee systems studied evidenced mobility differences that will influence their in vivo performance. This study points to the importance of attaining the correct balance between the mobility characteristics of a device and the patient's presenting pathology. Further, the results pro-vide specific device performance criteria which should assist their regulatory evaluation in the United States.

The co-authors of the study, all of the Orthopaedic Research Laboratories, PHS Mt. Sinai Medical Center, Ohio are Christine S. Heim, BSc.; Paul D. Postak, BSc. and A. Seth Greenwald, D. Phil. (Oxon).

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Last modified 17/March/2000 by IS