Achilles tendon healing in rats is improved by intermittent mechanical loading during the inflammatory phase

Tendons adapt to changes in mechanical loading, and numerous animal studies show that immobilization of a healing tendon is detrimental to the healing process. The present study addresses whether the effects of a few episodes of mechanical loading are different during different phases of healing. Fifty female rats underwent Achilles tendon transection, and their hind limbs were unloaded by tail suspension on the day after surgery. One group of 10 rats was taken down from suspension to run on a treadmill for 30 min/day, on days 2-5 after transection. They were euthanized on day 8. Another group underwent similar treadmill running on days 8-11 and was euthanized on day 14. Continuously unloaded groups were euthanized on days 8 and 14. Tendon specimens were then evaluated mechanically. The results showed that just four loading episodes increased the strength of the healing tendon. This was evident irrespective of the time point when loading was applied (early or late). The positive effect on early healing was unexpected, considering that the mechanical stimulation was applied during the inflammatory phase, when the calluses were small and fragile. A histological study of additional groups with early loading also showed some increased bleeding in the loaded calluses. Our results indicate that a short episodes of early loading may improve the outcome of tendon healing. This could be of interest to clinical practice.