| Abstract
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Implants of tricalcium phosphate ceramic (Ca3(PO4)2, TCP) and hydroxyapatite (Ca(PO4)6(OH)2, HAP) labelled with Ca were implanted into long bones of turkeys for up to six months. These synthesized compounds are analogues of biologically relevant minerals that are deposited, remodelled, and removed by the body as required during growth, tissue repair, and maintenance: an amorphous calcium phosphate of similar composition to TCP has been suggested to play an early role in mineralization; a modified HAP is the major inorganic constituent of bone. Retrieved bone sections containing these implants were thinned to approximately 50 microns, dipped into photographic emulsion, and subsequently developed. Microscopic examination of the location and pattern of the reduced silver grains suggest that different mechanisms distribute the calcium ions. After two months calcium ions from both implants were present within newly deposited bone that surrounded and penetrated the porous implants. At six months, both ceramics were being removed. Calcium ions spread uniformly into adjacent original bone in TCP-treated, but did not diffuse within HAP-treated, birds. However, silver grains were transported selectively within cells and channels of nutrient distribution within bone in the six-month HAP specimen suggesting that cellular processes, rather than physical dissolution, dominate HAP resorption at later time periods. Mechanical properties of porous, sintered implants were determined in compression to verify broad literature values. For test samples of 50 percent theoretical density, TCP has a 2.8-, 1.3-, and 6.2-fold advantage over HAP in compressive strength, Young's modulus of elasticity, and toughness. These advantages increase with density, suggesting that pore sizes and pore size distributions can be manipulated more effectively in TCP implants. Expected logarithmic relationships were found between these properties and porosity and were independent from pore size distribution. These data confirm the dominance of thermal decomposition products in effecting changes in the compressive strength of calcium phosphate ceramics.
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