The influence of aqueous and PEG 400 solvent vehicles on hydroxyl ion release from calcium hydroxide medicaments

Introduction Hydroxyl ion release from calcium hydroxide is responsible for the key attributes of broad spectrum antimicrobial activity, penetration into biofilms, inhibition of endotoxins, and dissolution of organic tissues.1,2 The maximum solubility of calcium hydroxide in water at 25C is only 0.159 g/100mL (0.16%), and this reduces to 0.140 g/100 mL (0.14%) at 40C, with an accompanying decrease of 0.033 pH unit/C with increasing temperature.3 Calcium hydroxide is much less soluble in water than sodium hydroxide or potassium hydroxide, however both the latter too caustic to soft tissues for clinical use.4 To gain the greatest antimicrobial actions, release of hydroxyl ions from endodontic pastes must be optimized. Traditional water-based calcium hydroxide pastes have a long history of clinical use, with PulpdentTM paste (Pulpdent Corporation, Watertown, Massachusetts, USA) having been on the market since 1947. In products with a waterbased vehicle (e.g. distilled water or saline), the common ion effect operates, since water already contains some free hydroxyl ions. Most calcium hydroxide endodontic pastes which are water-based calcium hydroxide contain a large excess of calcium hydroxide, well beyond that which can be dissolved.2 Placing calcium hydroxide into water in amounts above the solubility limit will not elevate the pH which can be achieved, since no further material can dissolve. The undissolved material will act as a filler or thickener in the paste, and as hydroxyl ions diffuse into the surrounding environment, it can also act as a reservoir. C L I N I C A L