How Do Kidney Stones Form?

There are several conditions that can contribute to the formation of kidney stones:

• A high concentration of stone-forming mineral salts in the urine. When high levels of stone-forming substances such as calcium oxalate, calcium phosphate, or struvite are present in the urine, one or more crystals may form and become trapped within the urinary tract. The crystal may attract other crystals and bind together with them, growing into a stone.

• An uneven balance of acid in the urine. The acidity or alkalinity of the urine affects the ability of stone-forming substances to remain dissolved. Some types of stones will form in acidic urine; others will form only in alkaline urine.

• A lack of substances in the urine that prevent the growth of crystals. Normally, substances that inhibit the formation and growth of crystals - such as pyrophosphates, citrates, and magnesium - are present in the urine. A decrease in or absence of these substances may cause a stone to develop.

Why Do Kidney Stones Form?

A variety of factors can create the conditions that allow kidney stones to form. These factors include:

• Some disturbances in the body's metabolism

• Certain inherited defects

• Abnormalities within the body

Sometimes, however, it's impossible to determine why a kidney stone has developed.

Each type of stone has its own requirements for formation.

Calcium stones form from the following:

• Most calcium stones form for unknown reasons, although a genetic basis is suspected.

• Certain foods can upset the balance of acid in the urine.

• Cancer can cause the body to produce an abnormally large amount of parathyroid hormone, which regulates calcium levels in the body. High levels of this hormone can break down bone and releases too much calcium into the blood. As a result, calcium saturates the urine.

Uric acid stones form from the following:

• Gout, the result of a genetic defect, is a disease that increases the body's production of uric acid. High levels of uric acid in the urine can cause stones to form.

• An inherited tendency can lead to the development of this type of stone, although the specific reason is unknown.

• Certain gastrointestinal conditions, such as ulcerative colitis, also can lead to the formation of uric acid stones.

Cystine stones can form from high amounts of cystine in the urine, the result of a rare in inherited abnormality.

Struvite stones may form in the kidney or bladder as a result of infection from certain bacteria.