This type of metaplasia is a rare condition in which there is a transformation of the normal endometrial tissue into bone. It is an uncommon clinical finding with an incidence of 0,3/1000 and most cases occur after miscarriage or abortion. The presence of bone in the endometrium was first described by Virchow who related this condition to a spontaneous differentiation of fibroblasts into osteoblasts. Typically, this type of metaplasia occurs during reproductive years and more than 80% of reported cases ocurr after pregnancy.
There are two main theories to explain the existence of bone fragments in the endometrial tissue. The one by Thaler, who relate this entity to retention of osseous fetal parts after abortion or miscarriage after 12 weeks of pregnancy. This first theory cannot explain cases that occurr in patients without previous pregnancies. The second theory is that of a true endometrial osseous metaplasia, in which there is a osseus transformation of the endometrial stromal cells, this metaplasia is consequence of irritative, toxic or hormonal stimuli. Probably both theories are right, with cases of true metaplasia and cases in which retained bones, causes an endometrial inflammation which leads to a secondary osseous metaplasia.