Trevor disease, also known as hemimelic epiphyseal dysplasia, is a rare skeletal disorder in which abnormal proliferation of epiphyseal cartilage occurs, most commonly in the ankle or the knee. A slow-growing mass that is usually painless is the principal complaint. The diagnosis is made through imaging studies and biopsy.
Trevor disease is more commonly known as hemimelic epiphyseal dysplasia (HED) and is characterized by unilateral asymmetric overgrowth of the epiphyseal cartilage . Although the lesion is benign and does not have the potential for malignant transformation, signs and symptoms can be quite debilitating, especially if recognition is delayed for a significant period of time. The diagnosis is most frequently made during the first and second decade of life, with a marked predisposition toward male gender   . A slow-growing painless mass, most commonly located in the medial aspects of the ankle (approx 50% of all cases) or the knee, is the typical feature of trevor disease  . Distal or proximal tibia, fibula, distal femur, talus, navicular, cuneiform, cuboid, and calcaneus are other reported sites  . Profound reduction in mobility and functional impairment is noted, and symptoms such as swelling and wasting may accompany the lesion . In severe cases, the mass can become painful, leading to marked stiffness of the joint and anisomelia . However, three clinical forms have been recognized: Involvement of only one epiphysis (localized form), more than one epiphysis in the same extremity (classic form that is observed in the majority of cases); and generalized, in which the entire limb is affected by abnormal epiphyseal growth  .
Trevor disease is rarely encountered in clinical practice and its diagnosis may be difficult to obtain without a thorough workup. Patient history is important for determining the course of symptoms and their onset, whereas physical examination can note joint-related complaints and rule out other more common causes, such as trauma. Imaging studies, however, are the gold standard, and plain radiography of the affected joint or bone is a useful first-line method. In the initial stages of the disease, an irregular lesion arising from the epiphysis is observed with signs of calcification . Over time, complete ossification and fusion with the underlying bone occurs, in which case distinction from other similar bone lesions (osteosarcoma, osteochondroma, and other tumors) may be difficult  . For this reason, more advanced techniques, such as computed tomography (CT) or magnetic resonance imaging (MRI) can be employed, in order to obtain a better view of the lesion   . Biopsy and subsequent histopathological examination, frequently performed after surgical excision of the lesion, is considered to be the definite diagnostic measure that confirms trevor disease, although the numerous cartilaginous foci of endochondral ossification, which are typically detected on microscopy, are strikingly similar to osteochondroma .