Early anchoring collagen fibers at the bone-tendon interface are conducted by woven bone formation: light microscope and scanning electron microscope observation using a canine model.

To clarify the early process of recovery at the bone-tendon interface, we used light microscopy and SEM to examine the process of anchoring of collagen fibers to bone in a canine model. At two weeks, tendon, scar tissue, woven bone and lamellar bone were present at the insertion site. SEM revealed anchoring of collagen fibril bundles of the scar to the woven bone. By 4 weeks, the number of anchoring fibers had increased and a parallel arrangement of fibers was observed. SEM demonstrated deep penetration of fibers into the woven bone layer. In addition, the fibers were observed to project into and intermingle with the scar tissue. By 6 weeks, the anchoring fibers had developed fully and were distributed densely over the interface. SEM also revealed that the collagen fibril bundles in the scar tissue had connected with the collagen fibrils of the woven bone by way of the anchoring bundles. The woven bone was identifiable throughout the early stages of recovery as the interface between soft tissue and hard tissue. Throughout all experimental periods, no staining was observed at the interface of the tendon and bone by Saffranin-O. The formation of woven bone was important during early recovery of the tendon-bone interface prior to the completion of fibrocartilage-mediated insertion.