Kinetic factors influencing the dissolution behavior of calcium oxalate renal stones: a constant composition study

A constant composition method has been used to examine the dissolution kinetics of calcium oxalate renal stones over a wide range of undersaturation in vitro. Demineralization experiments have been carried out with the concentrations of calcium and oxalate ions and ionic strength (hence the solution undersaturation) held constant by the potentiometrically controlled addition of medium electrolyte solution as diluent, triggered by a calcium ion electrode. Kinetic data for renal stones have been compared with results obtained for synthetic calcium oxalate. In addition, constant composition results have been directly compared with results obtained using conventional dissolution methods for both calculi and synthetic calcium oxalate. Overall, calcium oxalate renal stones exhibited markedly different kinetic dissolution behavior as compared with synthetic controls. The renal stone samples dissolved more slowly at all undersaturations, exhibited increased kinetic orders of reaction, and showed reduced sensitivity to solution hydrodynamics. Stones composed of mixed hydrates of calcium oxalate (mono- and di-) came to dihydrate equilibrium in conventional experiments and underwent net dissolution in solutions supersaturated to monohydrate under constant composition conditions. No conversion of di- to monohydrate was observed under these experimental conditions. These results indicate that stone dissolution is strongly influenced by adsorbed inhibitors, presumably including matrix components, which may complicate efforts to develop systemic and/or irrigation measures effective for in situ solubilization.