Chapter 17

Urinary calculi are the third most common affliction of the urinary tract, exceeded only by urinary tract infections and pathologic conditions of the prostate. They are common in both animals and humans. The nomenclature associated with urinary stone disease arises from a variety of disciplines. Struvite stones, for example, composed of magnesium ammonium phosphate hexahydrate, are named in honor of H.C.G. von Struve (1772–1851), a Russian naturalist. Before the time of von Struve, the stones were referred to as guanite, because magnesium ammonium phosphate is prominent in bat droppings. Calcium oxalate dihydrate is frequently referred to as weddellite, because it was commonly found in floor samples collected from the Weddell Sea in Antarctica. The history of the nomenclature associated with urinary stone disease is as intriguing as that of the development of the interventional techniques used in their treatment.

Urinary stones have plagued humans since the earliest records of civilization. The etiology of stones remains speculative. If urinary constituents are similar from each kidney and if there is no evidence of obstruction, why do most stones present in a unilateral fashion? Why do not small stones pass uneventfully down the ureter early in their development? Why do some people form one large stone and others form multiple small calculi? There is much speculation concerning these and other questions.

Advances in the surgical treatment of urinary stones have outpaced our understanding of their etiology. As clinicians, we are concerned with an expedient diagnosis and efficient treatment. Equally important is a thorough metabolic evaluation directing appropriate medical therapy and lifestyle changes to help reduce recurrent stone disease. Without such follow-up and medical intervention, stone recurrence rates can be as high as 50% within 5 years. Uric acid calculi can recur even more frequently. Physicians look forward to gaining a better understanding of this multifactorial disease process in hopes of developing more effective prophylaxis.

### Etiology

Mineralization in all biologic systems has a common theme in that the crystals and matrix are intertwined. Urinary stones are no exception; they are polycrystalline aggregates composed of varying amounts of crystalloid and organic matrix. Theories to explain urinary stone disease are incomplete.

Stone formation requires supersaturated urine. Supersaturation depends on urinary pH, ionic strength, solute concentration, and complexation. Urinary constituents may change dramatically during different physiologic states from a relatively acid urine in a first morning void to an alkaline tide noted after meals. Ionic strength is determined primarily by the relative concentration of monovalent ions. As ionic strength increases, the activity coefficient decreases. The activity coefficient reflects the availability of a particular ion.

The role of solute concentrations is clear: The greater the concentration of two ions, the more likely they are to precipitate. Low ion concentrations result in undersaturation and increased solubility. As ion concentrations increase, their activity product reaches a specific point termed the solubility product (Ksp). Concentrations above ...

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