Acoustic and mechanical properties of artificial stones in comparison to natural kidney stones

Purpose: Standardized and reproducible artificial kidney stone models are i mportant for performing comparative studies of different lithotripsy modali ties. The acoustic and mechanical properties of renal calculi dictate the m anner by which stones interact with the mechanical stresses produced by sho ck wave lithotripsy (SWL) or intracorporeal lithotripsy modalities. We have developed a novel artificial kidney stone model that is made of natural su bstances found in real kidney stones. These stone models appear to be much closer in physical properties to natural kidney stones than previously used stone models. Materials and Methods: The acoustic and mechanical properties of six groups of artificial stone models were compared to corresponding natural stones o f similar compositions. Moreover, three groups of artificial stone models m ade of plaster-of-Paris were compared to their natural counterparts. In ter ms of acoustic properties, stone density was measured using a pycnometer ba sed on Archimedes' principle, whereas longitudinal and transverse (or shear ) wave propagation speeds were measured using an ultrasound pulse transmiss ion technique. These values were used to calculate wave impedance and dynam ic mechanical properties (bulk modulus, Young's modulus, and shear modulus) of the stones. The microhardness of the stones was measured and the effect of composition on stone fragility was evaluated. Results: Artificial stones, when compared to natural stones of similar comp osition, showed similar trends in longitudinal and transverse wave speeds, wave impedance, and dynamic elastic moduli. However, values for the artific ial stones were uniformly low compared to those of natural stones, suggesti ng that these artificial stones may be more amenable to shock wave fragment ation. The results of SWL on stone fragmentation of artificial and natural stones also revealed similar trends with the exception of artificial cystin e stones which were found to be the most resistant to shock wave fragmentat ion. Conclusions: The results indicate that the physical properties of artificia l stones made of natural stone materials are comparable to renal calculi of the same chemical composition. The data suggests that these stone phantoms are suitable for performing standardized and reproducible in vitro investi gations, especially with regards to fragility of kidney stones of different chemical compositions during SWL.