The formation of urinary stone crystals is a complex process that involves multiple factors. These factors include lithogenic ions, inhibitors and promoters of crystallization, urine pH and urinary tract anatomy.
The tendency for urine to form stone crystals is a function of lithogenic ions such as calcium and oxalate. The relative concentration of these free ions determines whether stone formation will take place. In addition, the presence of other inorganic ions and organic molecules may serve to promote or inhibit the creation of stones.
Urine pH also seems to be a factor in stone formation. The solubility of uric acid and cystine stones is enhanced by increasing the urine pH to 6 and 7.5, respectively. Acidification of the urine pH less than 6.0 decreases the solubility of calcium phosphate stone formation. Calcium oxalate solubility is not altered by changes in urinary pH within the physiologic range.
Inhibitors that interfere with stone formation by reducing urinary saturation include magnesium, citrate, and pyrophosphate. These inhibitors block spontaneous nucleation and growth of calcium crystals. Citrate may also inhibit stone formation by complexing with calcium, thus reducing the free calcium available for interacting with oxalate.
Finally, congenital obstructive lesions of the urinary tract can predispose to stone formation by promoting urinary stasis and infection. Infection, therefore, serves as a nidus for crystal formation and allows stone formation to occur.