The cause of non-ossifying fibroma is unknown

Non-ossifying fibroma is a metaphyseal hamartoma composed of tissue fibroblasts, also known as metaphyseal fibrous defect, non-osteoblastic fibroma, histiocytic fibroma, metaphyseal hamartoma, and fibrous cortical defect.

The cause of nonossifying fibroma is unknown.

Non-ossifying fibroma has clear tumor boundaries, is offset, is solid or tough, and has a yellow or dark brown cross-section. It is composed of yellow and brownish-yellow fibrous connective tissue containing lipids. There are brown-yellow areas mixed in the tumor tissue, and the local bone cortex is thinned and swollen, with a clear boundary from normal bone, surrounded by sclerotic bone or fibrous bone. The adjacent bone cortex is intact unless a pathological fracture occurs. If the tumor is composed of multiple lesions, it can be leaf-shaped. Generally, there is no periosteal reaction.

Microscopically, the histological features are spindle-shaped fibroblastic proliferation, arranged in a whorl or storiform pattern, with a small amount of collagen fibers and fibroblasts. Reticular fibers are generally abundant. Sometimes there is bleeding and hemosiderin deposition in the interstitium, the latter of which can be seen in spindle cells and multinucleated giant cells. Focal distribution of clustered foam cells is very prominent, and the tissue cells that engulf lipids and hemosiderin are foam cells transformed from fibroblasts. Foam cells can be seen in 1/3 of cases. However, in some cases, fibrous tissue and cells are more common, while foam cells are rare. Non-ossifying fibromas that are about to degenerate have thickened collagen bundles, similar to fibroblasts.

The third component is multinuclear giant cells, which generally have 3 to 10 nuclei. A few cells are large and have more nuclei. They are scattered in the tissues, sparsely distributed, and small in size. They are different from the giant cells of giant cell tumors. Lymphocytes and plasma cells can also be seen between tissues. There is no osteogenic activity in the tumor, which can be distinguished from fibrous dysplasia. The edge can produce reactive bone sclerosis due to the expansion of the tumor. The histological image is consistent with benign fibrous histiocytoma. These characteristics indicate that this tumor is a tumor of fibrous histiocytic origin.