Another extremely common misdiagnosis for FOP is aggressive juvenile fibromatosis. This is a benign but highly aggressive condition where connective tissue cells called fibroblasts proliferate in various tissues including muscle, tendon, ligament, and fascia. These lesions can invade adjacent soft tissues and cause much pain and disability. They are often difficult to remove. They often grow slowly and are not associated with the type of swelling one sees with FOP lesions, but often the aggressive juvenile fibromatosis lesions do not come to the doctors attention until they are fairly large. Thus, many doctors who see a patient with FOP and see a soft tissue swelling might think that it looks like an aggressive juvenile fibromatosis lesion, especially since they arise from similar tissues.

Once again, if a biopsy is done, it might be mistaken for aggressive juvenile fibromatosis.

The reason is that the very early FOP lesions (which we call fibroproliferative lesions) look identical under the microscope to the fibroproliferative lesions of aggressive juvenile fibromatosis.

In aggressive fibromatosis, the lesions do not progress beyond the connective tissue growth phase, whereas in FOP, they mature through an endochondral process to form cartilage and bone. Once cartilage or bone cells are seen in a lesion, they can no longer be mistaken for aggressive juvenile fibromatosis. Also, patients with aggressive juvenile fibromatosis do not have malformed toes, so once again that is the key.