Epiphyseal hyperplasia caused by an ectopic ossification center of the distal tibia: presentation of a pediatric case with 13 years of radiological surveillance.

Herein, we report the case of a 4-year-old boy, who presented with a cortical lytic lesion of his distal tibia. It was located eccentrically, measured 2 cm in its craniocaudal diameter, and was associated with a lamella-like soft-tissue mineralization that resembled a periosteal reaction. Thus, the lesion's radiographic features were initially suggestive of an aggressive disease. However, further imaging revealed the lesion to be of cartilaginous origin and to extend into the metaphysis, thus creating the aspect of hyperplastic epiphyseal cartilage. This cartilaginous hyperplasia was likely caused by an ectopic ossification center, although no similar cases have yet been reported. As radiology and clinics no longer indicated the imminent threat of a high-grade malignancy, we decided against a biopsy to avoid interfering with epiphyseal growth. However, because of the unprecedented nature of the given constellation, we closely monitored the lesion radiologically to rule out an untypical manifestation of a benign but potentially expanding lesion, in addition to a growth disturbance resulting from the lesion itself. However, further imaging indicated that mineralization and ossifications, which were already present at the lesion's initial presentation, increased over time, until the lesion fully disappeared after 13 years of clinical and radiological surveillance. This case is outstanding because of its singular morphology and the long-term follow-up that illustrates its self-limiting natural course. This report provides support in differential diagnosis to help discriminate potentially self-limiting conditions from other diseases that may require invasive diagnosis and/or therapy.