High-frequency impedance measurement as a relevant tool for monitoring the apatitic cement setting reaction

This work reports the development of a relevant and general method, based on high-frequency impedance measurements, for the in situ monitoring of the alpha-tricalcium phosphate to calcium-deficient hydroxyapatite transformation that is the driving force of the hardening processes of some calcium phosphate cements (CPCs) used as bone substitutes. The three main steps of the setting reaction are identified in a non-invasive way through variation of the dielectric permittivity and dielectric losses. The method is also likely to characterize the effect of the incorporation of additives (i.e. antiosteoporotic bisphosphonate drugs such as Alendronate) in the CPC formulation on the hydration process. It allows us not only to confirm the retarding effect of bisphosphonate by an accurate determination of the setting times, but also to assess the phenomena that take place when Alendronate is added in the liquid phase or combined to the solid phase of the cement composition. Compared to the conventional Gillmore needle test, the present method offers the advantage of accurate, user-independent, in situ and real-time determination of the initial and final times of the chemical hardening process, which are important parameters when considering surgical applications.