What is the process of renal stone formation?
Renal stone formation occurs in six steps: (1) Initially, urinary crystallization or precipitation of sparingly soluble salts and acids occurs. (2) Nucleation follows as crystals and urinary matrix ions form a stable framework for crystal enlargement through (3) growth and (4) aggregation. After the crystals sufficiently enlarge they become trapped and (5) retained in a narrow portion of the urinary collecting system at the end of the collecting ducts and near the renal papilla. Unless washed away by increased urinary flow, these aggregated crystals formed in the medullary interstitium are extruded and (6) adhere to the renal papillae to form a calcium-phosphate–based Randall’s plaque nidus for further crystal accumulation and stone growth. Once stone growth occurs, the stone may detach from the renal papilla, move distally, and cause obstruction. Common sites for obstruction are the ureteropelvic junction (UPJ), mid-ureter (where it crosses over the iliac artery), and ureterovesical junction (UVJ).