The Process of Tendon Regeneration in an Achilles Tendon Resection Rat Model as a Model for Hamstring Regeneration After Harvesting for Anterior Cruciate Ligament Reconstruction

PurposeThe purpose of this study was to clarify the mechanism of tendon regeneration by investigating macroscopically, histologically, and biomechanically.MethodsFifty, adult, female Sprague-Dawley rats were used. The Achilles tendon in the left hind limb was removed totally by use of the tendon-stripping device. Rats were killed at 2, 7, 30, 90, and 180 days after surgery, and the regenerate tendons were dissected. Contralateral Achilles tendons were used as normal controls. Gross anatomic changes, microscopic remodeling, and recovery of biomechanical properties of regenerate tendons were investigated. The expressions of type I collagen, type III collagen, and transforming growth factor β1 were also investigated by immunohistochemistry.ResultsThe regenerate tendons formed in all specimens. In the early phase, hematoma and soft granulation tissue were observed at the harvest defect. These gradually matured with time, and the microscopic structure became quite similar to normal at 180 days after surgery. These findings occurred uniformly along the entire length of the regenerate tendon. However, the biomechanical properties were significantly inferior to the normal tendons (P < .05). Transforming growth factor β1 was well co-localized with inflammatory cells and fibroblasts in the regenerate tendons. The type I–type III collagen ratio in the regenerate tendon was significantly decreased in the early phase (P < .05) but gradually increased with time.ConclusionsTendon regeneration and maturation occurred uniformly along the length of regenerate tendons. The hematoma that initially occupies the harvest defect acted as a scaffold for fibroblast precursor cells from the surrounding peritendinous tissue and tendon sheath. The mechanical properties of regenerate tendon were significantly inferior to contralateral control even at 180 days after surgery, and the alteration of the collagen composition would have an influence on mechanical properties of regenerate tendon.Clinical RelevanceClinicians should be cautious about using reharvested hamstring tendons for ligament reconstruction surgery.