Mechanism: Normally a metal suture used for reconstruction is buried and not in contact with urine. If the staple migrates or erodes through the mucosa then it may become exposed to urine. It can then serve as a starting point for crystal formation. The stone usually is composed of struvite, apatite, or magnesium ammonium phosphate.
Features:
(1) There is usually no previous history of stone disease.
(2) There is a history of reconstructive surgery to the urinary tract, typically with placement of a portion of small bowel.
(3) A 24 hour urine specimen is usually negative for a metabolic abnormality.
(4) The stone can seen on radiographs because the stone is calcified.
(5) The radiograph will show the metal staple located eccentrically within the stone.
(6) The stone and staple may remain adherent to the wall, or the stone component may break away from the staple, or both may become detached.
Analysis of a detached stone will demonstrate the staple.
Differential diagnosis:
(1) stone formation about a free foreign body (foreign body is centrally located within the stone)
(2) nephrolithiasis due to a metabolic abnormality
