What are staghorn calculi (struvite stones)?

Staghorn calculi (struvite stones) are a type of kidney stone that may occur with frequent kidney infections. They can be more serious than other types of kidney stones, because they are large stones and often occur with an infection. They are called "staghorn" because on X-rays they look like deer (stag) horns. Medical treatment, including antibiotics and surgical removal of the stone, is usually needed. Women are affected more than men by staghorn calculi because of their higher risk for urinary tract infections.

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. 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.