Measuring stiffness in arthritic knees is not easy. Most often, doctors rely on the patient's report of stiffness. The most common way to measure stiffness is through the use of a self-report tool called the WOMAC.
The WOMAC is a survey including 24 questions about pain, function, and stiffness in patients with knee osteoarthritis (KOA). In this study, physical therapists use a three-camera computerized video system to measure stiffness of the knee.
Two groups of adults were tested. The first group had X-ray diagnosis of KOA and reported pain in the last month. The second (control) group were healthy adults matched by age and sex.
One test was performed on each person in both groups. The test was repeated a second time within two weeks of the first test. For each trial, the person sat on an exam table in a relaxed position. The thigh was supported by the table. The knee was bent with the lower leg dangling off the table.
With the person relaxed, the tester straightened the leg and then released it. As the knee fell into flexion, a Motion Analysis system recorded the pendulum motion for five seconds. The angle of knee motion was used to calculate stiffness and damping. Damping reflects how long it takes the knee to stop swinging.
The authors report this pendulum test is a reliable test to measure stiffness in patients with KOA. Their study did not have enough people in it to report valid findings. Further study is needed to confirm these results.
Carol A. Oatis, PT, PhD. Knee Joint Stiffness in Individuals With and Without Knee Osteoarthritis: A Preliminary Study. In Journal of Orthopaedic & Sports Physical Therapy. December 2006. Vol. 36. No. 12. Pp. 935-941.'