The chemical composition of stones depends on the chemical imbalance in the urine. The five most common types of stones are comprised of calcium—calcium oxalate and calcium phosphate, uric acid, struvite, and cystine.
Approximately 85 percent of stones are composed predominantly of calcium compounds. The most common cause of calcium stone production is excess calcium in the urine (hypercalciuria). Extra calcium is normally removed from the blood by the kidneys and excreted in the urine. In hypercalciuria, excess calcium builds up in the kidneys and urine, where it combines with other waste products to form kidney stones. Low levels of urinary citrate and magnesium, high levels of oxalate, sodium and uric acid, and inadequate urinary volume may also cause calcium stone formation.
Calcium stones are composed of calcium that is chemically bound to oxalate (calcium oxalate) or phosphate (calcium phosphate). Of these, calcium oxalate is much more common. Calcium phosphate stones typically occur in patients with metabolic or hormonal disorders such as hyperparathyroidism and renal tubular acidosis.
Increased intestinal absorption of calcium (absorptive hypercalciuria), excessive hormone levels (hyperparathyroidism, hypervitaminosis D), and renal calcium leak (kidney defect that causes excessive calcium to enter the urine) can all cause hypercalciuria. Prolonged inactivity also increases urinary calcium and may cause stones. (This is why astronauts in space tend to lose bone mass and form kidney stones.)
Renal tubular acidosis (inherited condition in which the kidneys are unable to excrete acid) significantly reduces urinary citrate (a natural urinary antacid) as well as total urinary acid levels and can lead to stone formation, usually calcium phosphate.
Uric Acid Stones
Digestion and metabolism of meat protein produces uric acid. If the acid level in the urine is high, the uric acid may not dissolve and uric acid stones will form.
Genetics may play a role in the development of uric acid stones. Approximately 10 percent of patients with kidney stone disease develop this type of stone. This is the only type of stone that can be dissolved just with medication. Alkalinizing medicines like potassium citrate and sodium bicarbonate can allow uric acid stones to dissolve. To do this, the urinary pH (a measurement of urinary antacid activity) must be maintained between 6.5 and 7.0. pH can be measured with special dipsticks and the dosage of the alkalinizing medications adjusted accordingly.
This type of stone, also called an infection stone or triple phosphate stone, develops when a urinary tract infection (e.g., bladder infection) affects the chemical balance of the urine. Bacteria in the urinary tract release chemicals that neutralize urinary acid, which enables bacteria to grow more quickly and promotes struvite stone development.
Struvite stones are more common in women because they have urinary tract infections more often than men. The stones usually develop as jagged structures called "staghorns" and can grow to be quite large.
Cystine is an amino acid in protein that does not dissolve well. Some people inherit a rare, congenital (i.e., present at birth) condition that results in large amounts of cystine in the urine. This condition (called cystinuria) causes cystine stones that are difficult to treat and requires life-long therapy.