Changes in Arthrogenic Stiffness Associated with Prolonged Joint Immobilization.

Objectives: This study aimed to investigate how changes in torque curves and histological alterations in joint structures occur in relation to the duration of immobilization, using a rat knee joint contracture model. Methods: A total of 58 male Wistar rats were divided into 6 groups: 3 immobilization groups with knee joints immobilized for 4, 8, or 16 weeks and 3 control groups receiving only standard housing until the same age. After each immobilization period, the torque curves during knee extension were measured. From the torque curves, the knee extension angle at 10, 30, 60, and 120 mNm and the slope of the linear region were calculated and compared among groups. Subsequently, the articular complex structures were histologically examined. The length of the posterior joint capsule and the flatness of the tibial articular surface were calculated from tissue images and compared among groups. Results: As the immobilization duration increased, the knee extension angle significantly decreased at all torque values, and the slope of the linear region became significantly gentler. The length of the posterior joint capsule was significantly shorter in the immobilized groups than in the control groups. Additionally, the tibial articular surface was significantly flatter in the immobilized groups than in the control groups. However, no significant differences were observed between the immobilized groups. Conclusions: With joint immobilization, the shape of the torque curve changes, which is influenced by the posterior joint capsule and other factors.