Directional growth of renal calculi.

The forces involved in shaping urinary calculi reside in crystalline characteristics and especially in local influences. Box pelves may restrict passage and, by allowing mobility, help shape a round calculus. Fixation, occurring with a rapidly growing infection stone, facilitates initial growth on a matrix mold in a funnel pelvis, further aided by pelviocaliceal paralysis from bacterial endotoxins and later by stasis. Branched calculi are further shaped by contact with the pelvic and infundibular walls. Through pressure and mucous coating they grow only at the ends. Late caliceal obstruction frees the ends for clubbing. Consideration of these forces aids in prognosis and surgical removal, and may offer opportunities for prevention.