HEALTHY KNEE MOTIONS AND IMPLICATIONS FOR ARTHROPLASTY DESIGN

The history of knee mechanics studies and the evolution of knee arthroplasty design have been well reported through the last decade (e.g. [1],[2]). Through the early 20009s, there was near consensus on the dominant motions occurring in the healthy knee among much of the biomechanics and orthopaedic communities. However, the past decade has seen the application of improved measurement techniques to permit accurate measurement of natural knee motion during activities like walking and running. The results of these studies suggest healthy knee motion is more complex than previously thought, and therefore, design of suitable arthroplasty devices more difficult. The purpose of this paper is to briefly review the knee biomechanics literature before 2008, to present newer studies for walking and running, and to discuss the implications of these findings for the design of knee replacement implants that seek to replicate physiologic knee motions. Many surgeons point to Brantigan and Voshell [3], an anatomic study of over one hundred specimens focusing on the ligamentous and passive stabilizers of the knee, as being an important influence in their thinking about normal knee function. M.A.R. Freeman and colleagues in London claim particular influence from this work, which motivated their extensive series of MR-based knee studies reported in 2000 [4,5,6]. These papers, perhaps more than any others, are responsible for the common impression that knee kinematics are well and simply described as having a ‘medial pivot’ pattern, where the medial condyle remains stationary on the tibial plateau while the lateral condyle translates posteriorly with knee flexion. Indeed, subsequent studies in healthy and arthritic knees during squatting and kneeling [7,8,9] and healthy and ACL-deficient knees during deep knee bends [10,11] show patterns of motion quite similar to those reported by Freeman and coworkers. These studies make a convincing case for how the healthy knee moves during squatting, kneeling and lunging activities. However, these studies are essentially silent on knee motions during ambulatory activities like walking, running and stair-climbing; activities which most agree are critically important to a high-function lifestyle. In 2008 Koo and Andriacchi reported a motion laboratory study of walking in 46 young healthy individuals and found that the stance phase knee center of rotation was LATERAL in 100% of study participants [12]. One year later, Kozanek et al. published a bi-plane fluoroscopy study of healthy knees walking on a treadmill and corroborated the findings of Koo and Andriacchi, i.e. the center of rotation in healthy knees walking was lateral [13]. Isberg et al. published in 2011 a dynamic radiostereometric study of knee motions in healthy, ACL-deficient and ACL-reconstructed knees during a weight-bearing flexion-to-extension activity, and showed consistent anterior-to-posterior medial condylar translations with knee extension, accompanied by relatively little lateral condylar translation [14]. Hoshino and Tashman reported in 2012 another dynamic radiostereometric analysis of healthy knees during downhill running and concluded “ While the location of the knee rotational axis may be dependent on the specific loading condition, during … walking and running … it is positioned primarily on the lateral side of the joint. ”[15] Finally, Claes et al. reported in late 2013 the detailed anatomy of the anterolateral ligament (ALL), another structure serving to stabilize the lateral knee compartment near extension, roughly in parallel with the anterior cruciate ligament (ACL) [16]. Studies since 2008 [9,12–16] show knee motions during walking, running and pivoting activities do not fit the “medial pivot” pattern of motion, but rather point to a “lateral pivot” pattern of knee motion consistent with the stabilizing roles of the ACL and ALL. Having a medial center of rotation in flexion and a lateral center of rotation in extension greatly complicates knee arthroplasty design if the goal is to reproduce kinematics approximating those observed in the natural knee. Consistent kinematics having a fixed center of rotation implies joint stabilizing structures or surfaces, not simply articular laxity allowing the knee to move as forces dictate. Thus, a total knee arthroplasty design seeking to reproduce physiologic motions may need to provide distinct means for controlling tibiofemoral motion in both extension and flexion. Recent studies of natural knee motions have made the implant designer9s job more difficult!