Enlargement of calcium oxalate stones to clinically significant size in an in-vitro stone generator.

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 ≈ 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 (r2). Results  Eight fragments (2–6 mm in diameter) were grown to ≈ 10 mm in diameter over periods from 137 to 369 h. Seven of the growth curves were best-fitted (r2 ≥ 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.