Knee joint stiffness following immobilization and remobilization: A study in the rat model.

Deficits in extension can limit the function and performance of the knee joint. The range of motion (ROM) deficit in knee extension is often measured and reported at a single torque value applied in the flexion-extension axis. This static measurement of ROM omits key details about the biomechanical properties of the knee, such as its mechanical stiffness. Our objectives were (1) to quantify knee extension stiffness after various periods of immobilization and remobilization, and (2) to evaluate how stiffness correlated with the length of the posterior knee capsule. Two hundred fifty-six male Sprague Dawley rats had one knee immobilized at a 45° angle in flexion using a Delrin® plate for 6 different durations ranging from 1 to 32 weeks. Remobilization was initiated by removing the plate and lasted for 0-48 weeks. The contralateral knee and unoperated age-matched rats were used as controls. An automated arthrometer extended the knee at four pre-determined torques and these data were used to calculate mechanical stiffness. The stiffness of knees immobilized for 8 or more weeks was significantly greater than controls and persisted despite remobilization (p < 0.05). Remobilization after 16 and 32 weeks of immobilization resulted in a progressive increase in mechanical stiffness (p < 0.05). The length of the posterior capsule significantly correlated with knee stiffness in extension (p < 0.05). Deficit in knee extension was characterized by increased stiffness, which was irreversible upon unassisted remobilization.