When Stones Escape: A Rare Radiologic Finding of Retroperitoneal Stone Extrusion Following American Association for the Surgery of Trauma (AAST) Grade IV Blunt Renal Trauma in a Chronically Hydronephrotic Kidney

Traumatic injury of a hydronephrotic kidney is uncommon, and extrusion of calculi beyond the renal capsule is exceptionally rare. We describe a middle-aged man with severe left hydronephrosis who sustained a low-impact fall, resulting in an American Association for the Surgery of Trauma (AAST) grade IV renal laceration. Contrast-enhanced CT revealed urinary extravasation, inflammatory urothelial enhancement consistent with uretero-pyelitis, and multiple displaced calculi within the retroperitoneum. A double-J stent was inserted after the initial scan. Follow-up CT demonstrated a well-circumscribed perirenal collection with peripheral enhancement during the venous phase, consistent with a superinfected urinoma. Conservative management led to full recovery. This case highlights how chronic obstruction weakens the renal parenchyma, predisposing to rupture and stone migration even after minor trauma.