Collagen Type I Increases Bone Remodelling around Hydroxyapatite Implants in the Rat Tibia

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Abstract
The early interface reaction of cancellous bone to a nanocrystalline hydroxyapatite (HA) cement containing 3 wt% collagen type I (HA/Coll) with a setting under physiological temperature and pH was observed using immunohistochemical techniques. Pure HA served as a control. Cylinders with a diameter of 2 mm were implanted into the proximal tibia of 72 adult Wistar rats. Histological sections of 6 animals were prepared after 1, 2, 4, 6, 14 and 28 days. First, osteoblast-like cells as well as a marked reaction for osteonectin, osteopontin and its ligand CD44 were observed as early as 2 days after implantation at the interface around HA/Coll implants. Further, reactivity for ED1 and cathepsin D, both markers for phagocytotic cells, appeared earlier and stronger around HA/Coll. In cell counts, a significantly higher average number of ED1- and cathepsin D-positive phagocytotic cells was observed around the HA/Coll implants on days 6 (p < 0.01), 14 and 28 (p < 0.05). The number of osteopontin-positive cells was significantly higher around HA/Coll implants at days 6 and 14 (p < 0.05). Two weeks after the implantation, first islands of newly formed woven bone were observed around the HA/Coll implant, but not around the control implant. The amount of direct bone contact after 28 days averaged 28% around pure HA and 51% around HA/Coll implants (p < 0.05). While both implants displayed a good osteoconductivity, a higher bone remodelling activity was observed around collagen-containing HA implants compared to pure HA implants. It appears that the addition of collagen to HA implants can enhance both phagocytotic and osteogenic processes. This may result in an earlier acceptance and better osseointegration of the HA/Coll implants into the surrounding tissue.