Cementum-like matrix in solitary bone cysts: a unique and characteristic but yet underrecognized feature of promising diagnostic utility.

Solitary bone cysts (SBCs) are benign, intraosseus, cystic lesions, which generally involve metaphysis of long bones during the period of skeletal immaturity. Histologic features are nonspecific, but identification of amorphous cementum-like material provides a significant diagnostic clue. This material is unique to SBC with reported frequency of 10% to 70% and has been described as an immature form of bone. We retrieved and reviewed 41 cases of SBC reported in the last 10 years. The ages of patients ranged from 4 to 64 years (mean, 16 years), with a male-to-female ratio of 3:1. Humerus and femur were the most common sites. Cementum-like matrix was observed in 26 cases (63.4%). This material was seen in different phases of deposition and progression, ultimately transforming into mature bone as seen in 4 cases. Cyst wall lining was observed in 70.7% of cases along with several other nonspecific histologic features including reactive bone formation, hemosiderin macrophages, hemorrhage, multinucleated giant cells, foamy macrophages, fibrin, cholesterol clefts, and granulation tissue. Cementum-like material in the wall of SBCs is a specific and fairly consistent finding of diagnostic significance in cases where cyst wall lining is deficient. We also histologically demonstrate, for the first time, transformation of cementum-like material into reactive and mature bone, which further validates the immature osteoid nature and finding of other authors.