Bone needs three conditions to grow: cells, signal, and a scaffold. In a bone defect site there are usually enough cells and signal (growth factor proteins) present, but a scaffold is often lacking. The most effective bone scaffolds are osteoconductive, meaning that bone grows directly on the material surface without an intervening fibrous tissue layer.
Beta Tricalcium Phosphate (ß-TCP) is well established as an osteoconductive material. A contributing factor is that ß-TCP acquires a biological carbonate apatite surface soon after implantation. Only hydroxyapatite (HA) is more stable in the presence of water which leads all other calcium phosphates to eventually transform into HA. It has been shown that the transformation of a ß-TCP surface to apatite starts almost immediately after exposure to simulated body fluid (SBF) which mimics the ion composition and concentration found in in vivo conditions. A positive correlation between the acquisition of an apatite surface in SBF with bone growth has been shown. This transformation results in a natural, bone apatite surface; unlike the highly crystalline surface of sintered HA. And a bone-like carbonated apatite surface has been shown to favor bone growth and remodeling.
In addition to having osteoconductive properties, Cytophil beta -TCP particles have macro-pores in the size range of about 5 to 300 microns. Properly sized pores that allow cell infiltration stimulate bone forming cells (osteoblasts) to grow bone, even if the scaffold surface is not inherently osteoconductive. For example, bone will grow in the pores of porous Teflon and polyethylene implants even though these materials are not even wetted by water. The ideal pore size for such bone growth is about 100 to 400 microns. In Cytophil beta-TCP the majority of the macro-pores are in the optimum size range for bone ingrowth.
In addition to the specific chemical/physical effects discussed above, calcium phosphates have been shown to directly induce osteogenic differentiation of stem cells via an adenosine signaling pathway. This is a further mechanism by which TCP promotes bone healing.
Cytophil beta- TCP also has extensive micro-porosity. Micro-pores are not accessible to cells, but they readily take up liquids. Liquid uptake helps handling by making the particle mass more cohesive. Thin liquids like saline help hold particles together by capillary action, while blood containing liquids additionally help cohesion by gelling and clotting. The table shows typical water uptake values for Cytophil beta TCP:
ß-TCP Particle size
Water absorbed per 1 gram of material
Water absorbed per 1 cc of material
1 – 2 mm
1.30 grams +/- 0.02 grams
2.55 grams +/- 0.05 grams
As well as helping handling properties, remodeling is also enhanced since the micro-pores help to facilitate the breakup of the particles over time.