HOW ARE STRENGTH AND RANGE OF MOTION AFFECTED BY KNEE IMPLANT DESIGN?

Abstract

Total knee arthroplasty (TKA) seeks to reduce pain and restore function in patients suffering from degenerative joint disease. TKA has become quite common and does provide predictable pain relief and return to some level of function. However, knees with TKA are not normal knees and do not perform like normal knees, and these facts motivate efforts to improve the functional performance of TKA. In this lecture, we will briefly review observations supporting the perspective that TKA function is strongly influenced by joint mechanics and implant design. In particular, we will discuss data relating joint mechanics and implant design to maximum knee flexion and to functional strength.

A number of studies on various designs of TKA with a range of patient cohorts have identified fairly specific relationships between several aspects of knee mechanics and flexion range. First, posterior translation of the femur with respect to the tibia increases maximum flexion (about 1.4° flexion for each mm of posterior femoral translation). Second, maintaining the natural posterior condylar offset is important to retain or enhance knee flexion (6° flexion are lost for each mm less posterior condylar offset from the natural state). Finally, posterior slope of the tibia can, with some designs, increase the flexion angle before posterior impingement (1.7° more flexion for each addition 1° tibial posterior slope). Various designs can combine these ‘ingredients’ to create unique ‘recipes’ for knee function and flexion. For example, a posterior cruciate ligament (PLC) retaining design can emphasize the second two ingredients, since it is relatively less predictable how the PCL will control anterior-posterior femoral translation. Designs of this type have shown excellent flexion, and significant improvements from past designs. A posterior-cruciate substituting design can emphasize the first ingredient to achieve satisfying flexion. Many other examples can be discussed.

Functional knee strength is another critical element of TKA patient function and also has been studied for decades. In this arena, there seems to be less of a consensus about specific mechanical factors that affect functional knee strength. Posterior femoral translation with respect to the tibia is thought to increase the quadriceps moment arm, and thereby provide greater functional knee strength in demanding tasks. It also has been proposed that the shape of the femoral component can be modified to provide a posterior axis for flexion, which also could increase the quadriceps moment arm. Finally, some argue that joint stability is critical to the normal physiologic co-activation of the quadriceps and hamstrings – where unstable joints recruit greater hamstrings activity and render the quadriceps less functionally effective. Gait laboratory studies are cited to support several of these design paths. Surveys of patient preference give strongest support to the stability hypothesis.

It is clear there is opportunity to improve the function of patients with TKA. By carefully assessing the relationships between patient function and knee arthroplasty mechanics, we can continue evolving TKA designs to better meet the needs of our patients.