Resorbability and solubility of zinc‐containing tricalcium phosphate

Abstract

Using zinc-containing tricalcium phosphate (ZnTCP) as the zinc carrier for zinc-releasing calcium phosphate ceramic implants promoted bone formation around the implants. Because no quantitative information was available on the equilibrium solubility and resorbability of ZnTCP, in vitro equilibrium solubility and in vivo resorbability of ZnTCP were determined and compared quantitatively in this study. The solubility of ZnTCP decreased with increasing zinc content. The negative logarithm of the solubility product (K(sp)) of ZnTCP was expressed as pK(sp) = 28.686 + 1.7414C - 0.42239C(2) + 0.063911C(3) - 0.0051037C(4) + 0.0001595C(5) in air, where C is the zinc content in ZnTCP (mol %). The solubility of ZnTCP containing a nontoxic level of zinc (<0.63 wt %) decreased to 52-92% of the solubility of pure tricalcium phosphate (TCP) in the pH range 5.0-7.4. However, the in vivo resorbed volume of ZnTCP containing the same amount of zinc was much lower than that expected from the in vitro solubility, becoming as low as 26-20% of that of TCP. Cellular resorption of TCP is substantially a process of dissolution in a fluid with an acidic pH that is maintained by the activities of cells. Therefore, the reduction of the resorbability of ZnTCP could be attributable principally to its lowered cellular activation property relative to that associated with pure TCP.

Interlaboratory studies on in vitro test methods for estimating in vivo resorption of calcium phosphate ceramics

“…The zinc incorporated in TCP decreases the equilibrium solubility of TCP [33]. The osteoclastic resorption of calcium phosphate is a process of cell-mediated dissolution in an acidic fluid in the resorption pits [19].…”

“…From the area of resorption, the apparent resorbed volume of the implant per unit lateral area of the implant (V) was calculated according to a method described elsewhere [33]. The measurement was performed on bone specimens with no femoral fracture, in-process implant fracture or deformation.…”

The most appropriate (Ca+Zn)/P molar ratio to minimize the zinc content of ZnTCP/HAP ceramic used in the promotion of bone formation

“…5 Moreover, it is well established that the solubility of TCP/HAP biphasic ceramics depends on their Ca/P ratios. [6][7][8][9] From the viewpoint of material safety, the less the zinc content is, the less the risk of an accidental burst of zinc from the material there will be.…”

Improving the bone forming ability of synthetic bone substitutes using trace element additions

“…For example, the change of grain size was noticed when introducing zinc into calcium phosphate materials, but this change was not further considered in the resulting biological effect [132,323]. Another example is silicon, which has been suggested to stimulate the biological performance of calcium phosphate ceramics mainly via (1) increasing the dissolution rate, (2) generating an electronegative surface and (3) finer microstructure [121].…”

“…The introduction of trace element in synthetic calcium phosphate materials can provoke physicochemical changes such as an increase in HA content (Chapter 2) and water affinity (Chapter 3) as well as the alteration of surface microstructures [141,323] and apatite crystal aspect ratio [258]. An essential reason behind is that the atom of trace elements has different atomic radius and electromagnetic forces as compared to the ion they replace and the insert of foreign ions may interrupt the growth of the crystal structure and water affinity [43].…”