The most important
phosphates involved in
urinary stone disease are
carbonate apatite,
brushite, and
struvite. Overall,
phosphate stones account for 12-20% of all stones, with a downward trend for
struvite and an increase in
carbonate apatite being observed in the last decade. The physicochemical basis for the formation of
phosphate calculi is supersaturation. Once the solubility product has been exceeded, a metastable process of supersaturation begins, with slow crystalline growth. If a critical limit of supersaturation is exceeded, large-scale spontaneous precipitation of crystals occurs in a second stage. No
urinary tract infection is involved in
brushite stone formation. Although
infection is not a prerequisite for the formation of
carbonate apatite stones, infective conditions favor
carbonate apatite formation.
Struvite is the characteristic
infection calculus, formed as a result of
urinary tract infection with
urease-producing bacteria. During the first episode of
urinary stone disease a definitive diagnosis of the type of stone involved is very difficult without analysis of the latter by infrared spectroscopy or X-ray diffraction. In recurrent disease, appropriate treatment can be initiated on the basis of the previous stone analysis in the majority of cases. The best means of preventing recurrent disease involving any type of
phosphate stone is definitive
calculus removal by
shock-wave
lithotripsy, percutaneous stone removal, or open surgery (especially in children). Chemolysis via acidification of the urine with Suby G
solution or hemicidrin supported by oral acidification, achieved by the metabolism of
L-methionine, and
antibiotic therapy (especially for infectious stones) are important adjuvant modalities of
therapy. After
therapy of
phosphate stones, metaphylaxis involving controlled urinary acidification with
L-methionine supports the treatment of
infection and, at a pH value of less than 6.2 and urine dilution to 2.5 l/24 h, prevents the crystallization of
struvite,
brushite, and
carbonate apatite.