Introduction

Accurate measurement of knee motion is necessary for assessment of natural joint function and in the diagnosis of pathology. In particular, precise knowledge of natural knee mechanics provides useful metrics for comparison to knee function following total knee arthroplasty (TKA). Reported measurements of natural knee kinematics during activities of daily living are rare, and often do not include both tibiofemoral (TF) and patellofemoral (PF) articulations. What's more, most studies record knee motion of younger subjects that are not necessarily representative of the age range associated with degenerative changes and TKA. The purpose of this study was to measure TF and PF kinematics of healthy older adults as they performed activities of daily living, including tasks considered more demanding for the knee [1].

“How does the knee move?” is a question of fundamental importance for treatment of knee injuries and knee replacement design. Unfortunately, we lack unambiguous and comprehensive knee function data sets and/or consensus on how healthy knees move. One can just as easily find reports stating the natural knee has a center of axial rotation in the medial compartment of the knee as in the lateral. This is due to technical and practical issues: It is extremely difficult to accurately measure knee motions during ambulatory activities and, when that can be done, very few studies have examined a range of weightbearing activities in the same study cohort. The purpose of this study is to report knee kinematics in a cohort of healthy older subjects whose motions were examined during four different movements, three of them weightbearing ambulation, using a high-speed stereoradiographic system.

Six healthy consenting subjects (age = 61 ± 5 years, body mass = 75 ± 8 kg, BMI = 27 ± 4) were observed using a high-speed stereoradiographic system while completing four tasks. Subjects were instructed to perform an unloaded, seated knee extension from high flexion to full extension; to walk at a self selected pace; to step down from a 7 inch platform; and to walk and perform a 90° direction change (pivoting). Stereoradiographic images (1080 × 1080 pixels) were acquired at 100 images/second using 40cm image intensifiers and pulsed x-ray exposures. The three-dimensional knee kinematics were measured using the XROMM software suite (xromm.org, Brown University). Post-processing of the kinematics was performed in custom Matlab programs, and included fitting spheres to the posterior condylar surfaces of each knee, and then tracking the motions of the sphere centers relative to a fixed tibial reference frame (Figure 1). The motions of these flexion-facet centers, were used to determine an average center of axial rotation (CoR) over each activity as previously reported by Banks and Hodge.

Average CoRs for all four activities were in the posterior-medial quadrant of the knee, with the CoR for open-chain knee extension being the most medial and gait the most lateral (Table 1, Figure 2). One-way ANOVA showed average CoRs are different (p « 0.001). There was considerable variation in individual CoRs, for example, with two knees showing lateral CoRs for gait and the remaining knees having medial CoRs.

It should not be surprising that natural knee motions vary with dynamic activity, yet knee kinematics often are presented as being one stereotypic, monolithic pattern of motion. Our data show that the same healthy subjects performing different dynamic activities manifest different knee motions, with open-chain knee extension having the most medial CoR and gait the most lateral. This finding is consistent with previous reports comparing stair climbing and gait in knees with various implant designs. Additional experimental data and, ultimately, validated numerical simulations should facilitate an increasingly accurate process for designing improved treatments for diseased and damaged knees.