Ectopic renal ossification as nucleus of urinary stones.

Areas of ectopic ossification were found in thin sections from 19 of 1,624 calculi submitted to a petrographic procedure, an incidence of 1.16 per cent. Only 8 calculi of renal, pelvic or ureteral origin with osseous tissue in its structure have been described since the first report by Phemister in 1923.