Focal Periphyseal Edema (FOPE) zone on MRI of the adolescent knee: a potentially painful manifestation of physiologic physeal fusion?

Abstract

Objective: We have termed an MRI finding in the knees of adolescents characterized by a focal bone marrow edema pattern centered about the closing physis a "FOPE"-that is, focal periphyseal edema-zone. The cause of this appearance is unknown. Therefore, the purpose of this article is to review the MRI appearance of the FOPE zone and to postulate a causative mechanism.

Materials and methods: FOPE zones were identified on knee MRI examinations performed for pain in 12 patients (seven girls, five boys; age range, 11 years 9 months-15 years 8 months); the examinations were collected over 5 years. Clinical history, skeletal maturity, size and location of FOPE zone, and concomitant ipsilateral knee abnormalities were recorded at presentation and on follow-up MRI examinations when available. Bone ages were estimated from knee radiographs using published standards.

Results: Fifteen FOPE zones measuring 2-27 mm were identified: eight were femoral; six, tibial; and one, fibular. All were centrally located. All physes were patent, albeit narrowed. The radiographic appearance of the physes was similar for both sexes. All patients with estimated bone ages of 11 and 12 years were girls. Two other girls had bone ages closer to 14 years, whereas all boys had estimated bone ages of 13 or 14 years.

Conclusion: On knee MRI, a FOPE zone can be seen in adolescents and likely relates to the early stages of physiologic physeal closure. It may be associated with pain particularly when no other MRI abnormalities are present. When the characteristic appearance of a FOPE zone is observed on MRI, we suggest that it not be mistaken for an abnormality, requires no invasive diagnostic procedure, and does not need imaging follow-up.