Selective Water Uptake in Calcium Oxalate Monohydrate Kidney Stones

Here we report the amount and location of fluid water absorbed by calcium oxalate monohydrate (COM) kidney stones evaluated using controlled humidity and magnetic resonance imaging (MRI). COM stones subjected to rehydration in a controlled humidity chamber for 16 h at 37 °C absorb approximately 3 wt % water, most of which resides only in thin outer shells of the stones and not within their cores. Although scanning electron microscopy (SEM) shows that there are differences in texture between the inner surface of the water-absorbing shell and the outer surface of the nonabsorbing core, energy-dispersive X-ray (EDX) spectroscopy does not reveal any differences in elemental composition between the two surfaces, and X-ray diffraction (XRD) of powered samples shows them to be similar in crystalline nature. The fact that COM stones do not absorb a significant amount of fluid water beyond their thin outer coatings, which have a different (albeit unidentified) interfacial material separating them from the bulk of the stones, may be the reason for the differences in sensitivity to common medical treatment options such as shock wave lithotripsy.