Effects of the geometrical structure of a honeycomb TCP on relationship between bone/ cartilage formation and angiogenesis

A number of biomaterials have been developed, some of which already enjoy widespread clinic use. We have devised a new honeycomb tricalcium phosphate (TCP) containing through-and-through holes of various diameters to control cartilage and bone formation. However, the way in which the geometric structure of the honeycomb TCP controls cartilage and bone tissue formation separately remains unknown. In addition, an association has been reported between bone formation and angiogenesis. Therefore, in the present study, we investigated the relationship between angiogenesis and various hole diameters in our honeycomb TCP over time in a rat ectopic hard tissue formation model. Honeycomb TCPs with hole diameters of 75, 300, and 500 µm were implanted into rat femoral muscle. Next, ectopic hard tissue formation in the holes of the honeycomb TCP was assessed histologically at postoperative weeks 1, 2, and 3, and CD34 immunostaining was performed to evaluate angiogenesis. The results showed that cartilage formation accompanied by thin and poor blood vessel formation, bone marrow-like tissue with a branching network of vessels, and vigorous bone formation with thick linear blood vessels occurred in the TCPs with 75-µm, 300-µm, and 500-µm hole diameters, respectively. These results indicated that the geometrical structure of the honeycomb TCP affected cartilage and bone tissue formation separately owing to the induced angiogenesis and altered oxygen partial pressure within the holes.