The Use of a Mechanical Model to Describe the Stiffness and Damping Characteristics of the Knee Joint in Healthy Adults

BACKGROUND AND PURPOSE: Stiffness is a common clinical complaint but is rarely quantified by clinicians. The purpose of this study was to determine the reliability of a relaxed oscillation test that yields stiffness and damping coefficients of the knee. These coefficients describe the knee joint's resistance to bending and the time-dependent nature of that resistance. SUBJECTS AND METHODS: Effects of age, gender, and knee position on these coefficients were assessed in 96 healthy volunteers aged 20 to 79 years. Measures were based on the premise that the knee joint can be modeled as a damped spring. Oscillations of the knee were recorded using an electrogoniometer with the knee oscillating through about 45 and 75 degrees of knee flexion. RESULTS: Intraclass correlation coefficients revealed moderate to high reliability in the measurements taken three times in the same test session and on three separate days. Analysis of variance showed significant increases in stiffness and damping coefficients in the male subjects as compared with the female subjects. Analysis of variance also suggested an age effect on stiffness coefficients at the 75-degree test position, with decreasing stiffness with age. Both stiffness and damping coefficients were significantly smaller when measured at the 75-degree test position compared with the 45-degree position (Student's t test). CONCLUSION AND DISCUSSION: These results demonstrate a reliable method of measuring knee joint stiffness, and they correlate well with known morphological differences related to age and gender. This measure may prove to be more useful in evaluating the function of the knee than more commonly used assessments. It may also lead to a better understanding of how the knee functions in such activities as locomotion.