Mechanically Induced Phase Transformation of α- and β-Tricalcium Phosphate

The reactivity of calcium phosphates for self-setting bone cements is normally adjusted by altering the particle size and hence the specific surface area of the compounds. In this report we show that prolonged high energy ball milling of tricalcium phosphates (β-TCP, α-TCP), led to mechanically induced phase transformation from the crystalline to the amorphous state with first order reaction kinetics. The process increased the thermodynamic solubility compared to the unmilled materials and accelerated the normally slow reaction with water to form single component cement systems. By using a 2.5% Na₂HPQ₄ solution setting times were reduced to 5-16 minutes rather than hours. X-ray diffraction analyses indicated that the amorphous fraction within the materials was responsible for the primary setting reaction and hardening of the cements, while the crystalline fraction remained unreacted (β-TCP) or converted only slowly to hydroxyapatite (αTCP). Mechanically activated α- and β-TCP cements were produced with compressive strengths of up to 50 - 70 MPa and initial setting times between 5-16 min. In the powder:liquid (P:L) ratio range investigated, mechanical properties were found to be linearly related to activation time for β-TCP, because of a higher degree of conversion to hydroxyapatite after setting. In contrast mechanical strengths of α-TCP cements decreased with prolonged ball milling, probably because of a much faster setting reaction of the mainly amorphous materials with the formation of smaller HA-crystals and a hence less homogeneous cement microstructure.