Chronological changes in the collagen-type composition at tendon–bone interface in rabbits

OBJECTIVES The purpose of this study was to evaluate chronological changes in the collagen-type composition at tendon-bone interface during tendon-bone healing and to clarify the continuity between Sharpey-like fibres and inner fibres of the tendon. METHODS Male white rabbits were used to create an extra-articular bone-tendon graft model by grafting the extensor digitorum longus into a bone tunnel. Three rabbits were killed at two, four, eight, 12 and 26 weeks post-operatively. Elastica van Gieson staining was used to colour 5 µm coronal sections, which were examined under optical and polarised light microscopy. Immunostaining for type I, II and III collagen was also performed. RESULTS Sharpey-like fibres comprised of type III collagen in the early phase were gradually replaced by type I collagen from 12 weeks onwards, until continuity between the Sharpey-like fibres and inner fibres of the tendon was achieved by 26 weeks. CONCLUSIONS Even in rabbits, which heal faster than humans, an observation period of at least 12 to 26 weeks is required, because the collagen-type composition of the Sharpey-like fibre bone-tendon connection may have insufficient pullout strength during this period. These results suggest that caution is necessary when permitting post-operative activity in humans who have undergone intra-bone tunnel grafts.