Objective To develop and validate an in vitro method suitable for the quantitative investigation of the growth of calcium oxalate stones through to a clinically significant size. Materials and methods Small fragments of calcium oxalate calculi were suspended in a mixed suspension/ mixed product removal crystalliser supplied with artificial urine supersaturated with calcium oxalate. The fragments were weighed at regular intervals until they reached ª 500 mg. The results were plotted as weight against time and fitted to equations corresponding to constant increase in diameter, surface area-controlled and constant-deposition growth patterns. The choice of the most appropriate model was based on the squared regression coefficient ( r 2 ).Results Eight fragments (2-6 mm in diameter) were grown to ª 10 mm in diameter over periods from 137 to 369 h. Seven of the growth curves were best-fitted ( r 2 ≥ 0.988) by the equation w = kt (3/2) + c , where w is the weight, k is a growth constant, t is the time and c is a constant approximating to the initial weight. This corresponds to a surface area-dependent mechanism. Conclusions The growth of these small fragments to a clinically significant size accelerated throughout the experimental period in a way which was consistent with a surface area-dependent mechanism. We have developed a resilient model suitable for studying the kinetics of calcium oxalate stone growth in vitro .
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