Kidney stones (also called nephrolithiasis or urolithiasis) affect approximately 12 percent of men and 5 percent of women by age 70. Fortunately, treatment is available to effectively manage most stones and steps can be taken to prevent their recurrence. Recurrence can occur at a rate of up to 5 percent per year in untreated individuals

A brief overview of the anatomy of the urinary tract will help in the understanding of kidney stones. The urinary tract is composed of two kidneys and ureters, a bladder, and a urethra. Urine is produced by the kidneys, which are located towards the middle of the back, below the ribs. The kidneys remove waste products and excess fluid from the blood and convert this to urine. The urine passes out of the kidney through small tubules into the hollow portion of the kidney (renal pelvis) and then into the ureter, a narrow tube connecting the kidney to the bladder (show figure 2). The urine collects in the bladder until it passes out of the body through the urethra. A kidney stone can form when substances such as calcium, oxalate or uric acid, are present in the urine in relatively high concentrations, but stones can also form if these substances are at normal levels. The substances form crystals, which become anchored in the kidney and gradually increase in size, forming a kidney stone.

Most kidney stones are formed of calcium-containing material, primarily calcium oxalate and less commonly as calcium phosphate. Stones can also be made of other substances, such as uric acid, struvite (magnesium ammonium phosphate), or cystine. Knowing the stone composition is important as this information may influence the recommendations for treatment.

RISK FACTORS Certain diseases and habits can affect a person's risk for developing kidney stones. These include:

History of kidney stones — Patients who have had a kidney stone in the past have the highest risk of future stone formation. It has been estimated that for patients who have previously had a stone, the likelihood of forming a second stone is about 5 to 10 percent at one year and 50 percent or higher at 10 years.
Family history of stones — Persons with a family history of kidney stones are at increased risk for developing stones.
Low fluid intake — A low fluid intake (less than 1.5 quarts or 48 ounces per day) leads to a low urine output, which can promote stone formation by increasing the concentration of stone-forming substances in the urine.
Calcium — There is little evidence that a high intake of calcium (from dietary sources) increases the risk, or that low intake of calcium decreases the risk of developing kidney stones. In fact, avoiding dairy products is likely to increase the risk of kidney stones. An individual should consume the amount of calcium that is recommended for bone health based on their age and gender.
Other medical conditions — Some medical conditions are associated with an increased risk for stone formation, including:

Conditions that increase the absorption of oxalate from the gastrointestinal tract (like short bowel syndrome, chronic diarrhea, or previous bowel surgery)
Conditions that increase the chance of urinary tract infection, such as anatomic abnormalities of the urinary tract or difficulties with bladder emptying
Hyperparathyroidism and sarcoidosis, which can be associated with high blood levels of calcium
Gout, which may result in high concentrations of uric acid in the urine
Diabetes, which may be associated with increased urine acidity
Cystinuria, an inherited condition that is associated with an increased level of cystine in the urine