MOC is a viscous colloidal suspension of calcium salts. MOC is reported mainly with calyceal diverticulum or renal cysts and less frequently in the dilated calyces of hydronephrotic kidney. Urinary obstruction and stasis may lead to super-saturation of calcium salts which form calcium microliths but it is not clear in literature, why it does not form stones.
The colloidal suspension of MOC allows deposition of the calcium salts in a dependent manner due to gravitational effect sparing the more liquid part anteriorly. This explains the semilunar pattern or the anterior horizontal fluid level detected radiologically in MOC particularly prominent on CT scan. It is a rare type of stones that needs high degree of suspicion to diagnose preoperatively. When stones demonstrate a low Hounsfield unit, are arranged in dependent positions within dilated calyces and exhibit a fluid level or semilunar pattern on non contrast CT, milk of calcium stones should be considered. Association of infection, failure of ESWL to clear the MOC stones make PCNL an ideal approach to treat MOC stones after definitive treatment of the cause of obstruction.
Written by:
El-Shazly M.
Urology Department, Menoufia University, Shebin El-Kom, Egypt.
Abstract: Milk of calcium stones: Radiological signs and management outcome