The term “stone” in medicine refers to a hardened, mineralized mass that develops inside a body organ or duct. Medically, these formations are known as a calculus, with the plural form being calculi. These concretions are typically mineral salts that the body fails to keep dissolved in its fluids. The presence or formation of these masses, a condition known as lithiasis, is often associated with disease and can cause significant pain or obstruction.
The Process of Concretion
The formation of a calculus begins at a microscopic level, driven by fluid chemistry. The initial step requires the bodily fluid, such as urine or bile, to become supersaturated with stone-forming material. Supersaturation means the fluid contains a higher concentration of a substance than can normally remain dissolved at that temperature and pH.
This unstable state provides the necessary drive for crystal formation. The next step is nucleation, where dissolved ions spontaneously associate to form a tiny, stable solid structure, or crystal nucleus. This initial crystal may form freely in the solution or attach to a pre-existing structure within the organ or duct.
Once a nucleus is formed, crystal growth and aggregation begin, allowing the mass to become a clinically detectable stone. More material precipitates out of the supersaturated fluid and deposits onto the surface of the existing crystal. These microscopic crystals clump together to form larger, macroscopic masses that can eventually obstruct the normal flow of bodily fluids.
Anatomical Locations and Specific Types
The most common calculi form within the excretory and digestive systems, and their naming reflects the anatomical site of origin. Stones that form within the kidney are known as renal calculi, or nephrolithiasis. These masses originate in the urine-collecting areas of the kidney, but often cause symptoms when they migrate.
If a stone leaves the kidney and lodges in the ureter, the tube connecting the kidney to the bladder, the condition is called ureterolithiasis. The pain associated with this movement is known as renal colic, which occurs as the stone attempts to pass through the narrow tube. This type of stone commonly impacts at three naturally narrowed points along the urinary tract.
In the digestive system, the gallbladder is a frequent site of stone formation, leading to cholelithiasis. The stones formed here are commonly referred to as gallstones. Gallstones can remain in the gallbladder or pass into the bile ducts, causing inflammation or obstruction.
A less frequent type of stone occurs in the salivary glands, a condition known as sialolithiasis. These salivary calculi primarily form in the submandibular glands. Their presence can cause swelling and pain, particularly during mealtimes when saliva production is stimulated.
Chemical Classification of Calculi
The chemical composition of a calculus is the primary factor for determining its underlying cause and guiding preventative strategies. The majority of stones, particularly those in the urinary tract, are composed of calcium compounds.
Calcium Stones
Calcium oxalate stones are the most prevalent type, accounting for about 70 to 80 percent of all kidney stones. They are often linked to high levels of calcium or oxalate in the urine, stemming from dietary habits or metabolic disorders like hyperparathyroidism. Calcium phosphate stones are less common (5 to 10 percent) and are sometimes associated with conditions that cause the urine to become abnormally alkaline.
Uric Acid Stones
Uric acid stones develop when the concentration of uric acid is high and the urine’s pH is consistently low (acidic), often below 5.5. These stones are frequently observed in individuals with gout, those who consume large amounts of protein, or people with certain metabolic syndromes. Unlike calcium stones, uric acid stones do not contain calcium but are a product of purine metabolism.
Struvite Stones
Struvite stones, also called infection stones, are composed of magnesium ammonium phosphate. Their formation is directly tied to chronic urinary tract infections caused by specific bacteria that produce the enzyme urease. Urease breaks down urea into ammonia, raising the urine’s pH and promoting the rapid precipitation of struvite crystals.
Cholesterol Stones
Cholesterol stones are the most common type of gallstone. They typically form when the bile contains too much cholesterol and not enough bile salts to keep it dissolved. They are composed primarily of hardened cholesterol, and their formation is distinct from the mineral-based stones of the urinary tract.