Rat Model of Knee Flexion Contracture: Progression of Joint Motion Restriction without Subsequent Changes in Joint Capsule Stiffness

Background. Joint contractures are classified as myogenic or arthrogenic. Capsular ligaments are partially thickened areas of the joint capsule, which are impossible to separate from the joint capsule. The joint capsule is an important factor limiting the range of knee extension; however, its stiffness has not been investigated. This study aimed to investigate changes in joint capsule stiffness over time after the onset of knee flexion contracture. Methods. Twenty-four 8-week-old male Wistar rats were used. The groups were clas- sified as follows: 0-week (without joint fixation), 2-week fixation, 4-week fixation, and 8-week fixation groups. First, the knee extension range of motion was measured before and after skeletal muscle removal. Next, femur-tibia joint capsule complex samples underwent tensile testing to obtain information on the waveform and stiffness of the length-tension curve of the knee joint capsule. The stiffness was the slope of the wave- form calculated from the elongation distance and resistance to elongation. Results. Despite a significant decrease in the knee extension range of motion after skeletal muscle removal due to the extension of the joint immobilization period, the stiffness of the joint capsule did not change. The length-tension curve for the 0-week group had a sawtooth waveform that repeatedly rose and dropped steeply, which was not observed in the 2-, 4-, and 8-week groups. Conclusions. The stiffness of the joint capsule did not change during knee flexion contractures.