How do staghorn kidney stones form?

Staghorn stones are stones that involve the renal pelvis and extend into at least 2 calyces of the kidney. Approximately 75% of staghorn calculi are struvite (composed of magnesium, ammonium, phosphate crystals mixed with carbonate-apatite). These stones are associated with urinary tract infections caused by bacteria that produce urease. Urease hydrolyzes (or splits) urea into ammonium and hydroxyl ions resulting in an increase in ammonium and phosphate concentrations leading to an alkaline urine. The alkaline urine leads to precipitation of magnesium ammonium phosphate crystals mixed with varying proportions of carbonate apatite. The infection stones generally grow rapidly and serve as the nidus (or center) for further stone formation. This stone matrix is believed to protect the bacteria from antibiotics. In essence, antibiotics cannot clear the associated infection unless the stone material is completely removed.