URIC ACID CALCULI

The study of the composition and structure of 41 stones predominantly composed of uric acid was complemented by in vitro investigation of the crystallization behaviour of uric acid. The solubility of anhydrous (UAA) and dihydrated (UAD) forms of uric acid in urine-like liquor at 37°C was calculated as a function of pH using reported dissociation constants and solubility products. UAD precipitates from urine under physiological conditions when the medium is supersaturated with respect to this compound, though UAA represents the thermodynamically stable, and hence, less soluble, form. Solid UAD in contact with liquid, transforms into UAA within 2 days. This transition is accompanied by development of hexagonal bulky crystals of UAA and the appearance of cracks in the original prismatic UAD crystals. Uric acid calculi can be classified into three distinctly different groups, differing in outer appearance and inner structure. Formation mechanism of individual types of uric acid calculi was inferred from results of performed investigation. Thus, type I includes stones with a little central core and a compact columnar UAA shell, and stones with interior structured in alternating densely noncolumnar layers developed around a central core; both of these stones are formed mainly by crystalline growth at low uric acid supersaturation. Type II includes porous stones without inner structure and stones formed by a well developed outermost layer with an inner central cavity; type II stones are formed mainly by sedimentation of uric acid crystals generated at higher uric acid supersaturation. Type III includes stones whose structure and mechanism of formation is a mixture of types I and II.