A Novel Surgical Approach to Induce an Ankle Joint Contracture in Rats.

Our primary aim was to establish a novel surgical immobilization approach to induce a persistent ankle plantar-flexion contracture in the Wistar rat. A stainless steel suture was placed in experimental animals (n = 14) and pulled taut between the calcaneal bone and the knee joint, restricting dorsal-flexion while still allowing for limited ankle plantar-flexion, muscle activity, and loading. The immobilization method was well tolerated; adverse events were limited and manageable. After 4 weeks, immobilization was released by cutting the suture. After 4 weeks of immobilization, ankle angle at 5 Nmm passive torque shifted 48° towards plantar-flexion, plantar-flexion torque and stiffness at 55° ankle angle increased 4.5 and eightfold, respectively (n = 8). Exploratory tissue analyses ex-vivo revealed significant atrophy of the medial gastrocnemius muscle-belly (34% smaller mass than controls, n = 4). Ankle angle at 5 Nmm, torque, and stiffness at 55° angle showed no signs of recovery 4 weeks post-release of the immobilization (n = 3). Morphological and mechanical assessments at this timepoint revealed persistent medial-gastrocnemius atrophy (19% smaller mass and 30% lower distal-tendon length in experimental animals (n = 3) compared to controls, n = 4). Also, passive and active length-force relationships of medial gastrocnemius were shifted towards lower lengths (i.e., plantar-flexion) compared to controls. Taken together, our novel surgical approach elicits an ankle plantar-flexion contracture that appears to persist for at least 4 weeks, providing a window for an intervention study. This model may be used to improve our understanding of the biomechanical, morphological and neurological changes following joint contractures, and to evaluate therapeutic interventions.