What is Renal Calculus?
A renal calculus (kidney stone) is a mineral deposit that forms in the renal calyces or pelvis, either free-floating or attached to the renal papillae, composed of crystalline and organic components that develop when urine becomes supersaturated with minerals. 1
Composition and Formation
Calcium oxalate is the primary constituent of most kidney stones, with many forming on a foundation of calcium phosphate deposits called Randall's plaques located on the renal papillary surface. 1 Other stone types include:
- Struvite (magnesium ammonium phosphate) - infection stones associated with urease-producing bacteria 2
- Calcium phosphate - often mixed with calcium oxalate 2
- Uric acid - can form pure or mixed stones 2
- Cystine - associated with genetic disorders 2
Clinical Significance and Natural History
The prevalence of kidney stones has increased dramatically, with current rates up to 14.8%, and the recurrence rate reaches 50% within 5 years of the initial stone episode. 1 This represents a 70% increase from historical prevalence rates of 5.2%. 2
Critical Complications
Untreated stones, particularly staghorn calculi, can lead to kidney destruction and life-threatening sepsis. 2 The disease is increasingly linked to systemic conditions including:
- Obesity and metabolic syndrome 2
- Hypertension 2
- Chronic kidney disease progression 1
- End-stage renal disease 1
Types of Renal Calculi
Staghorn Calculi
Staghorn calculi are branched stones that occupy a large portion of the collecting system, typically filling the renal pelvis and branching into several or all calyces. 2 These are most frequently composed of struvite/calcium carbonate apatite mixtures and are strongly associated with urinary tract infections from urease-producing organisms. 2
Punctate Stones
Punctate stones represent the earliest stage of stone formation, appearing as small, dot-like calcifications within the renal parenchyma or collecting system. 3 Most are asymptomatic and discovered incidentally, but they indicate underlying metabolic abnormalities requiring evaluation. 3
Pathophysiology
Stone formation occurs through crystallization and crystal aggregation from supersaturated urine, rather than excessive organic matrix production. 4 Key mechanisms include:
- Supersaturation of urine with stone-forming minerals 1
- Absence of normal urine inhibitors of calcium salt precipitation 4
- Crystal nucleation and aggregation on existing structures 5
- Bacterial biofilm formation in infection stones, where bacteria reside within the stone itself 2
Infection Stones Mechanism
Urease-producing bacteria generate ammonia and hydroxide from urea, creating an alkaline urinary environment with high ammonia concentration that promotes struvite crystallization. 2 This process incorporates exopolysaccharide biofilm and mucoproteins into the stone matrix. 2
Risk Factors
Intestinal hyperabsorption of calcium causes the hypercalciuria seen in 30-40% of patients with calcium oxalate stones. 4 Additional risk factors include: