Causes of phosphate stone formation and the importance of metaphylaxis by urinary acidification: a review

The most important phosphates involved in urinary stone disease are carbona te apatite, brushite, and struvite. Overall, phosphate stones account for 1 2-20% of all stones, with a downward trend for struvite and an increase in carbonate apatite being observed in the last decade. The physicochemical ba sis for the formation of phosphate calculi is supersaturation. Once the sol ubility product has been exceeded, a metastable process of supersaturation begins, with slow crystalline growth. If a critical limit of supersaturatio n is exceeded, large-scale spontaneous precipitation of crystals occurs in a second stage. No urinary tract infection is involved in brushite stone fo rmation. Although infection is not a prerequisite for the formation of carb onate apatite stones, infective conditions favor carbonate apatite formatio n. Struvite is the characteristic infection calculus, formed as a result of urinary tract infection with urease-producing bacteria. During the first e pisode of urinary stone disease a definitive diagnosis of the type of stone involved is very difficult without analysis of the latter by infrared spec troscopy or X-ray diffraction. In recurrent disease, appropriate treatment can be initiated on the basis of the previous stone analysis in the majorit y of cases. The best means of preventing recurrent disease involving any ty pe of phosphate stone is definitive calculus removal by shock-wave lithotri psy, percutaneous stone removal, or open surgery (especially in children). Chemolysis via acidification of the urine with Suby G solution or hemicidri n supported by oral acidification, achieved by the metabolism of L-methioni ne, and antibiotic therapy (especially for infectious stones) are important adjuvant modalities of therapy. After therapy of phosphate stones, metaphy laxis involving controlled urinary acidification with. L-methionine support s the treatment of infection and, at a pH value of less than 6.2 and urine dilution to 2.5 1/24 h, prevents the crystallization of struvite, brushite, and carbonate apatite.