Effects of mechanical stimulation with different intensities on the early healing of Achilles ten-don-calcaneus interface in rats

Gao Shang; Shi Benhai; Shen Linhua; Yang Zhijin; Tao Xu; Tang Kanglai

doi:10.3969/j.issn.1009-4237.2025.10.004

Abstract：Objective To investigate the effects of mechanical stimulation with different intensities on the early healing of the Achilles tendon-calcaneus interface in rats,via using a rat model of Achilles tendon insertion re-pair surgery under varying treadmill training intensities.Methods After 1 week of adaptive treadmill training,60 rats were randomly divided into three groups after establishing the calcaneal suture model(1)control group:cage fed,free to move;(2)low-intensity exercise group:treadmill training at 10 m/min for 20 min/day;and(3)high-intensity exercise group:treadmill training at 17 m/min for 20 min/day.On postoperative days 7 and 10,three rats from each group were randomly euthanized,and the right Achilles tendon-calcaneus interface was harvested as the in-flammatory-phase specimen for gross observation.The day-10 rat samples were further processed for histological as-sessment of the inflammatory phase.At 4 weeks post-surgery,an open-field test was performed on the remaining rats in each group,after which the right Achilles tendon-calcaneus interface was harvested for gross inspection,histologi-cal evaluation,and biomechanical testing.Results At 7 and 10 days post-surgery,all groups exhibited the formation of abundant loose paratenon connective tissue.The low-intensity group showed significantly less paratenon connective tissue compared to the control and high-intensity groups,with the tissue being denser and more organized.Histologi-cally,at 10 days post-surgery,all groups displayed substantial inflammatory tissue filling the space between tendon and bone tissues.The low-intensity group exhibited more regular tendon-bone contact and relatively less inflammatory tissue compared to the control and high-intensity groups.At 4 weeks post-surgery,the low-intensity group demonstra-ted significantly longer movement distance(cm)in the open field test(1097.8±216.1),compared to the control(610.3±107.1)and high-intensity(286.6±56.5 cm)groups(both P<0.001).The low-intensity group also showed the least scar formation and tissue swelling at the tendon-bone interface among all groups.Histologically,the low-intensity group exhibited a denser tendon-bone interface tissue structure compared to the other two groups.The interface contained a more uniform distribution of fibrocartilage-like tissue,with a higher content than the other two groups.Biomechanically,the low-intensity group showed significantly greater maximum load(N,43.5±9.4)and e-lastic modulus(N/mm,17.1±6.7),compared to the control(N,33.4±4.2,P<0.01;N/mm,11.0±4.2,P<0.05)and high-intensity(N,30.1±2.5,P<0.001;N/mm,9.2±3.5,P<0.01)groups.Conclusion Low-in-tensity mechanical stimulation,compared to no stimulation or high-intensity stimulation,promotes early healing of the tendon-bone interface following Achilles tendon-calcaneus repair in rats.