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Kidney Stone Types & Formation

A number of materials can form kidney stones, including calcium and uric acid. Kidney stones develop as minerals and other wastes accumulate, adding layers upon layers and eventually forming masses large enough to cause obstructions and other problems.

Kidney Stone Formation

Hard masses develop when waste materials are not dissolved adequately in urine. Microscopic crystals of waste material remain in the kidney. Over time, the crystals grow in size, until they are large enough to cause blockages or other kidney stone symptoms. These deposits are also called calculi (plural for calculus).

The most common elements of calculi are calcium, oxalate, phosphate and uric acid. Kidney stone formation occurs when excessive amounts of waste material accumulate and cannot be dissolved in the urine. Conversely, calcium and other materials may be present in normal amounts, but the body possesses insufficient water to process enough urine to dissolve them.

Certain substances that occur naturally in urine help break down and dissolve waste material, the most notable of these substances are citrate, magnesium and pyrophosphate. A deficiency of these substances can also promote the growth of stones.

Calcium Oxylate & Calcium Phosphate Stones

Calcium crystal formations are by far the most common types of kidney stone, making up approximately eighty percent of all cases. The usual cause is an excess of calcium in the urine, a condition referred to as hypercalciuria.

Calcium usually combines with other substances to form kidney stones, most often with oxalate and phosphate. Calcium oxalate is the most common form. Oxalate is a byproduct of metabolism. It is also found in a number of foods. Calcium phosphate calculi often occur as a result of metabolic or hormonal disorders.

Uric Acid Stones

Uric acid is produced when nucleic acids (the building blocks of DNA) are recycled. High levels of uric acid may result from a diet high in nucleic acids such as meat, fish, and poultry, or in cancer patients treated with chemotherapy who have a high turnover of cells. The formation of uric acid stones is more common in men than in women, and uric acid stones account for roughly ten percent of cases.

Struvite Stones

Struvite stones are also known as magnesium ammonium phosphate, triple phosphate, or "infection stones." Unlike other stones, struvite crystals are caused by a urinary tract infection. The bacteria that cause the urinary tract infection affect urine chemistry and neutralize urine acids. This allows the bacteria to grow even more, resulting in struvite crystals. They are the only types of stone that are treated medically as if they were infected foreign particles.

Struvite formations are more common in women than men, mainly because the female urinary tract system is more susceptible to infections. The stones are jagged, and can reach large sizes. "Staghorn" calculi are a form of struvite deposit that grows very large, and can fill the kidney cavity.

Cystine Stones

Only one to two percent of calculi are formed from cystine, an amino acid that is found in protein. Most cases stem from cystinuria. Cystinuria is a rare congenital disorder that produces unusually high levels of cystine. Stones formed due to cystinuria occur throughout life and are difficult to treat.

Relative Frequencies
The relative breakdown of the various types of crystal stones is:

calcium: approximately eighty percent of cases
uric acid: approximately ten percent
struvite (infection stones): ten to fifteen percent
cystine: one to two percent.

Resources

Beers, M. H., & Berkow, R. (ed). Urinary calculi. The Merck Manual of Diagnosis and Therapy, 17th Edition. Merck Research Laboratories, NJ, 1999.

Browne, G. & Plant, L. (updated 2004). Kidney stones. Renal Unit of the Royal Infirmary of Edinburgh.

Fauci, A., Braunwald, E., Isselbacher, K., Wilson, J., Martin, J., Kasper, D., Hauser, S., & Longo, D. (ed.). Harrison's Principles of Internal Medicine, 14th Edition. McGraw-Hill, NY, 1998.

Carson-Dewitt, R.S. (1999). Kidney stones. Gale Encyclopedia of Medicine.

Kidney Stone Treatment and Prevention Centers. (nd). About kidney stones. LithoLink.com.

Kidney Stone Treatment and Prevention Centers. (nd). Types of kidney stones. LithoLink.com.

National Kidney and Urologic Diseases Information Clearinghouse. (2003, April). Kidney stones in adults [NIH Publication No. 03-2495].