Neutrophils and macrophages accumulate sequentially following Achilles tendon injury

Structural damage and inflammation occur following tendon injury. The purpose of this study was to determine the time course of inflammatory cell accumulation in two animal models of acute tendinopathy. In the first model, rat Achilles tendons were exposed by blunt dissection, injected with collagenase and sacrificed at 1, 3, 7, 14 and 28 days. In the second model, collagenase was injected percutaneously and rats were sacrificed after 1 and 3 days. Sham animals were sacrificed at 1 and 3 days in both models. Neutrophil and ED1 macrophage populations increased by 46- and 18-fold, respectively, after 1 day in surgically exposed Achilles tendons (EAT) injected with collagenase. Neutrophils dropped by 70% while the concentration of ED1 macrophages remained constant at day 3 post-injury. Neutrophils and ED1+ macrophages returned to control values after 7 and 14 days, respectively. ED2+ macrophages showed a tendency to increase at day 28 although no significant difference was observed relative to ambulatory controls. Collagenase injected percutaneously reduced the extent of inflammation compared with operated animals. Thus, injured tendons exhibited a specific sequence of inflammatory cell accumulation which varied in intensity according to the modality used for collagenase injection.