Microscopic feature of different types of renal stones Surgery
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urinary crystals microscopy calcium oxalate struvite uric acid cystine
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"kidney calculi"[MeSH Terms] AND "composition"
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Renal stones are identified microscopically by the crystals in urine sediment and by polarized-light or crystallographic analysis of the stone. This matters in surgery because crystal type predicts stone type, which guides the choice of procedure and the work-up.
Microscopic features by stone type
| Stone type | Share of stones | Urine crystal appearance | Urine pH | Gross stone features |
|---|---|---|---|---|
| Calcium oxalate (monohydrate or dihydrate) | About 80% (with calcium phosphate) | Dihydrate (weddellite): tetrahedral "envelope" crystals. Monohydrate (whewellite): oval, dumbbell or biconcave forms (my addition, not from the retrieved texts). Both are birefringent under polarized light. | Acidic or neutral | Dihydrate: sharp, spiculated, yellowish. Monohydrate: brown, multi-lobed, concentric layers. Radiopaque. |
| Calcium phosphate (hydroxyapatite, carbapatite, brushite) | Included in the 80% above | Brushite: needle-shaped. Apatite: amorphous granules (my addition). | Alkaline | Smooth, pale tan. Radiopaque. |
| Struvite (magnesium ammonium phosphate) | About 10% | Rectangular "coffin-lid" prisms | Alkaline (pH above 7) | White, branching, forms staghorn casts. Radiopaque. |
| Uric acid | About 6-7% | Amorphous shards, irregular plates, rhomboids and rosettes (my addition) | Acidic (pH below 5.5) | Small, rounded, yellow-orange. Radiolucent. |
| Cystine | About 1-2% | Flat, colorless, hexagonal plates | Acidic | Yellow-brown, hard, waxy. Mildly radiopaque because of sulfur. |
The crystal descriptions come from Campbell-Walsh Urology, Table 92.2, and Henry's Clinical Diagnosis. Campbell-Walsh states: "Tetrahedral 'envelopes' are seen in calcium oxalate lithiasis, and rectangular, 'coffin-lid' crystals are often seen in patients with struvite calculi. Hexagonal crystals confirm cystinuria; uric acid crystals may be seen as amorphous fibers or as irregular plates."
Key points for each type
- Calcium stones
- Oxalate precipitates at acid or neutral pH. Calcium phosphate, as hydroxyapatite, forms at the normal urinary pH of 6.0 to 6.5 and in alkaline urine (Henry's Clinical Diagnosis, p. 576-577).
- About half of patients with calcium stones have hypercalciuria without hypercalcemia (Robbins Basic Pathology, p. 527-528).
- Struvite
- These form with urea-splitting organisms such as Proteus, Klebsiella and staphylococci, which make the urine alkaline.
- They are the usual cause of staghorn calculi. Robbins describes these as branching masses that cast the renal pelvis and calyces.
- Quick Compendium of Clinical Pathology notes that 75% of staghorn calculi are struvite.
- Infected stones contain far more endotoxin and often harbour organisms different from those in the preoperative bladder urine. In Campbell-Walsh, infected stones had 36 times more endotoxin.
- Uric acid
- These form in persistently acidic urine (pH below 5.5). They are linked to gout and high cell turnover, such as leukemia and its treatment.
- About half of patients have neither hyperuricemia nor hyperuricosuria (Robbins).
- They are radiolucent on plain film. This differs from the other types and is why CT is needed.
- Cystine
- These are due to a genetic defect in renal amino-acid transport. Hexagonal crystals confirm cystinuria.
- The stones are very hard, so they respond poorly to ESWL.
- Cystine crystals can be mistaken for hexagonal uric acid crystals. Henry's notes they are soluble in water at pH below 2 or above 8.
- Organic matrix: every stone contains mucoprotein matrix, about 2.5% by weight (Robbins).
Surgical relevance
- A urine pH above 7.0 suggests infection stones or renal tubular acidosis. A pH below 5.5 suggests uric acid stone risk (Campbell-Walsh).
- If cultures are positive, treat with antibiotics before any stone procedure. Operating during active infection risks bacteremia or sepsis (Campbell-Walsh).
- Gross stone morphology:
- Most stones are small (2-3 mm), smooth or jagged, and unilateral in about 80% of patients.
- Staghorn stones can fill the whole collecting system.
- Treatment options (Bailey & Love, p. 20):
- ESWL for stones under 2 cm.
- Ureteroscopy with fragmentation.
- PCNL for stones over 2 cm, ESWL-refractory stones and multiple stones.
I ran a PubMed search for recent (last 2 years) systematic reviews and meta-analyses on stone composition. It returned nothing that contradicts or updates the textbook material above.
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