The effect of nontraumatic immobilization on ankle dorsiflexion stiffness in rats.

Experimental evidence supports the general conclusion that 6 or more weeks of immobilization will cause joint contractures due to changes in the capsules of animals whose limbs have been traumatized. There is controversy whether contractures will form in 6 weeks in limbs which are free of trauma. The purpose of this study was to determine if range of motion would be lost in ankle joints of rats following nontraumatic immobilization of the hindlimb for 2 or 6 weeks. The right hindlimb of each animal was immobilized in a plaster cast for 2 or 6 weeks with the left hindlimb serving as a matched control (N = 8). Nonimmobilized rats served as additional controls (N = 4). Following 2 or 6 weeks of immobilization, each rat was sacrificed and the lower leg removed at the knee. An electrogoniometer measured the change in dorsiflexion as torque was applied in increasing increments to the ankle. Load-deformation tests were performed to determine: 1) torque to end range of dorsiflexion, 2) joint excursion with application of 3.57 mNm torque for 2.5 seconds, and 3) joint excursion with prolonged application of 3.57 mNm torque. Only the group casted for 6 weeks demonstrated a significant (p < 0.05) decrease in both degrees of dorsiflexion and joint compliance. The group casted for 6 weeks required five times more torque to achieve end range than the other groups and had a 70% decrease in ankle dorsiflexion when a fixed torque of 3.57 mNm was applied. No significant differences were noted among the remaining groups. These findings suggest that in nontraumatically immobilized joints of rats, dense connective tissue remodels in such a way that mobility is unaffected after 2 weeks, but becomes quite limited by 6 weeks.