There are several types of stones that are not made out of calcium. These are the three main types of noncalcium kidney stones.
Uric acid stones result from supersaturation of the urine with uric acid. An abnormally large amount of uric acid may or may not be present in the urine. However, the most common finding associated with uric acid stones is an acidic urine. Uric acid stone formation requires three conditions--an acidic urine, a low amount of total urine output, and uric acid excretion. Any condition that causes an abnormally acidic urine can lead to uric acid stone formation. These conditions include gout, cancer, inflammatory bowel disease, and dehydration (such as from chronic diarrhea, ileostomy, or strenuous exercise).
Cystine stones form in patients with the disease cystinuria. In cystinuria, the kidneys do not absorb cystine well, and stones form because of the poor solubility of cystine in the urine. Cystine stones can be suspected when the hexagonal shaped cystine crystals are seen in the urine.
Struvite stones are composed of magnesium ammonium phosphate mixed with carbonate. These stones form when urine is alkaline (the opposite of acidic) and ammonium is in the urine. These conditions occur when the urine is infected with certain bacteria. Since struvite stones can only form in infected urine, they are called infection stones. Another condition that contributes to struvite stones is the presence of a foreign body in the urinary tract, such as staples from previous surgery, sutures from previous surgery, or hairs. Bacteria adhere to foreign bodies, therefore struvite stones form on these materials. Since bacteria live inside the stones, antibiotics cannot penetrate the stones to kill all the bacteria, and the infection cannot be completely eliminated with just antibiotic medication. Surgery is usually required to remove the stones completely in addition to antibiotic therapy.
