Inflammation, cellularity, and fibrillogenesis in regenerating tendon: implications for tendon rehabilitation

The initial three weeks of tendon healing were followed via electron microscopy to elucidate the process of inflammation, fibrillogenesis, and the cellular and subcellular events in tenotomized Achilles tendons, a model that is commonly used to determine the biomechanical effects of electrical stimulation, physical activity, ultrasound, and other forms of physical therapy. The right Achilles tendons of 18 rabbits were tenotomized, sutured, and immobilized. On each of postoperative days 5, 7, 12, 15, 18, and 21, the right Achilles tendons of three experimental rabbits were excised and processed for electron microscopy. To compare these tendons to normal tendons, the Achilles tendons of three control rabbits were excised bilaterally without prior tenotomy and processed for electron microscopy. Electron micrographs thus obtained revealed 1) an initial period of inflammation lasting at least five days, 2) a subsequent period of fibroplasia and fibrillogenesis, and 3) a third period of progressive alignment and organization of the collagen fibrils into bundles that were oriented in the longitudinal axis of the tendon. Although healing in rabbits may not translate directly to healing in humans, the findings of this study indicate that healing begins soon after tenotomy and that the regenerating Achilles tendon undergoes different stages of healing. Because each stage entails a different set of ultrastructural events, therapeutic interventions should be modified to address the specific events of each stage.