Purpose: Self-reported knee instability is highly prevalent in knee osteoarthritis (OA) and affects daily physical function. An association between knee instability and muscle strength has been demonstrated in cross-sectional studies. However, no information is available on change in self-reported knee instability over time and whether change in muscle strength is associated with change in self-reported knee instability. Muscle strength itself is associated with knee joint proprioception and knee pain. Whether these factors are influencing the association between change in muscle strength and change in self-reported instability is unknown. Therefore, the aims of the study were (i) to determine the change in self-reported knee instability over two-year time, and (ii) to study the association between change in muscle strength and change in self-reported knee instability, controlled for the change in proprioception and pain over two-year time in patients with established knee OA. Methods: A total of 201 patients from the Amsterdam Osteoarthritis (AMS-OA) cohort were followed-up during two years. Self-reported knee instability, muscle strength, proprioception, pain and patient characteristics were assessed at baseline and 2 years later. Self-reported knee instability was defined as the sensation of an episode of buckling, shifting or giving way of the knee in the previous three months. Patients reporting knee instability were additionally asked for the number of episodes of instability, whether these episodes concerned the left, right or both knees, if any episodes had resulted in a fall, and the particular activity that induced an episode of instability. Muscle strength of the left and right leg was measured isokinetically (EnKnee, Enraf-Nonius, Rotterdam, Netherlands) at 60 /second. The mean muscle strength (i.e., extension and flexion) per leg was calculated to obtain a measure of overall leg muscle strength (Nm). Joint proprioception was measured as the threshold of detection of passive joint movement. Knee pain over the past week was assessed by an 11 point numeric rating scale (0e10), with higher scores representing more pain. A series of demographic variables were obtained including age, gender, height, weight, and duration of complaints. Difference between baseline and two-year values of self-reported knee instability, muscle strength, proprioception and pain was calculated with Student's t-test or when appropriate with Chi-square test. Linear regression analysis was used to determine the association between the change in knee instability and change in muscle strength. Results: Self-reported knee instability, muscle strength, proprioception and knee pain changed significantly over two-year time (p < .001, respectively). Univariate regression analysis showed that the change in muscle strength was significantly associated with change in knee joint stability, indicating that an increase in muscle strength over time resulted in an improvement in self-reported knee stability. Changes in proprioception and knee pai...

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