[Diagnostic imaging and therapy of chondroblastoma].

The chondroblastoma is a rare lytic osseous lesion, which is typically to be found in the epiphyses of the long tubular bones. We present a case report with the differential diagnostic imaging and the surgical therapy of a chondroblastoma of the proximal tibial epiphysis. A 16-year-old male presented in the traumatological accident and emergency centre with knee pain of unclear origin that had been increasing for several months. He had no memory of any acute trauma. For further clarification, a conventional radiograph in two planes, a thin-layer CT, a multiplanar MRI before and after Gd-DTPA, and a bone scan were performed. The conventional radiological diagnostics show a smoothly circumscribed, osteolytic, eccentric lesion with marginal sclerosis, which not exceeds the epiphyseal seam of the proximal tibia. The thin-layer CT reveals an eccentric osteolysis, with a typical, narrow sclerotic seam and central calcifications. In the high-resolution MRI, the T2-weighted sequences show a locally limited, epiphyseal, lobulated lesion with a heterogeneous, in part raised signal. A further signal increase can be seen in the T1-weighted sequences after administration of Gd-DTPA. Perifocally, there is an epiphyseal edema and discrete (intraarticular fluid collection) fluid in the inner knee. In the skeletal scintigraphy, there is a strong focal and diffuse increased activity from the proximal, lateral tibial epiphysis. After the diagnosis had been rendered, curettage of the defect was performed and then, after histological confirmation of the diagnosis, completing spongiosaplasty was conducted. Taking into account the localisation and the patient's age, a virtually certain diagnosis can already be rendered using conventional imaging. A supplementary MRI or CT can be helpful for final clarification, while 3-phase skeletal scintigraphy is superfluous. The therapy of choice is curettage and subsequent filling of the defect with autologous spongiosa.