Synthesis of Tricalcium Phosphate as Bone Replacement Material

Tricalcium phosphate (TCP) is one of the main constituents of bone replacement materials which find wide use in medical and dental applications for bone grafting and for implants. This article shows how TGA/DSC and TMA can be used to investigate the synthesis of tricalcium phosphate and to determine the transition temperatures of different TCP polymorphs.

Introduction

From the chemical point of view, bones consist of 60% calcium phosphate. It therefore seems obvious to use synthetic calcium phosphate compounds as bone replacement material. Bone replacement materials are needed for the production of ceramic bone implants and to repair bone defects.

A particularly important aspect of bone replacement materials is their absorbability by newly produced bone material in the body. A prerequisite for this is biocompatibility of the bone replacement material. The calcium phosphates used for bioceramics are mostly hydroxylapatite or hydroxyapatite (HA), alpha-tricalcium phosphate (α-TCP), beta-tricalcium phosphate (β-TCP) or biphasic calcium phosphate (HA+ β-TCP).

Hydroxylapatite is the slowest to be absorbed and exhibits the greatest mechanical stability. In contrast, α- and β-TCP are more soluble in the body’s bone material. This increases their absorption rate and shortens the healing process. For this reason, α-TCP and β-TCP are often the main constituents in bone replacement material used to fill bone defects that for example can occur in the insertion of dental implants [1] or in ceramic bone implants [2, 3].

This article shows how the synthesis and the phase behavior of TCP can be investigated by TGA/DSC and TMA.

The starting material for the production of TCP is a stoichiometric mixture of calcium hydrogen phosphate (CaHPO₄) and calcium carbonate (CaCO₃). The overall reaction is shown in eq 1.

2·CaHPO₄ + CaCO₃ → Ca₃(PO₄)₂ + H₂O + CO₂ (1)

Conclusions

The formation of TCP from a stoichiometric mixture of CaCO3 and calcium hydrogen phosphate was investigated by TGA/DSC and TMA. TCP exists in different polymorphic forms. The temperatures for the corresponding solid-solid transitions were determined using TMA and TGA/DSC.

TGA measurements showed that TCP is formed as the result of a solid-solid reaction between CaO and calcium pyrophosphate. On the basis of the TMA measurements, it can be concluded that the reaction takes place between about 900 and 1000 °C. This reaction yields β-TCP. Above about 1250 °C, β-TCP changes to α-TCP. The transition from α-TCP to α'-TCP occurs above 1460 °C.

These temperatures therefore represent the maximum or minimum temperatures that must be reached in order to synthesize a particular polymorph of TCP.

_{Synthesis of Tricalcium Phosphate as Bone Replacement Material | Thermal Analysis Application No. UC 446 | Application published in METTLER TOLEDO Thermal Analysis UserCom 44}