Case report A 23-year-old woman had a fall 8 years previously, and she was admitted to the hospital because of a Pilon frac- ture of the right tibia. She underwent surgery for open reduction and internal fixation with a plate and screws without bone grafting. Six years later, the patient com- plained of persistent pain and sensitivity in pressure in the tibia approximately 10 cm cranially to the tibiotalar joint. A clinical diagnosis of posttraumatic osteoarthro- sis was suggested. The patient complained of increasing pain even during the night not responding to analgesics during the past 2 years. The clinical examination reveal- ed sensitivity in the distal tibial diaphysis. The complete biochemical work-up was normal. A plain X-ray reveal- ed periosteal reaction with internal osteolysis. Since os- teoid osteoma was not compatible with the age of the patient, an MRI study was scheduled for further clarifi- cation. The MRI examination revealed a posttraumatic os- teoarthrosis of the ankle and a diaphyseal intracortical area of intermediate and high signal intensity on T1- and T2-weighted images, respectively, and soft tissues and marrow edema Fig.1). In order to better delineate the bony structures, a CTwas performed. The CT exam- ination showed periosteal thickening along with a lucen- cy with a central small focus of dense bone Fig.2). The patient underwent operation. Eur. Radiol. 10, 1507±1508 2000) Ó Springer-Verlag2000 Quiz case of the month European Radiology A. H. Karantanas 1 , A. H. Zibis 2 1 Department of CT±MRI, Larissa General Hospital, 1 Tsakalof Street, GR-41221 Larissa, Greece 2 Department of Orthopaedic Surgery, Larissa University Hospital, GR-41221 Larissa, Greece Received: 25 January 2000; Accepted: 29 February 2000 Correspondence to: A.H. Karantanas 1a 1b 2 Fig 1. a Coronal T1-weighted spin-echo image reveals an old Pilon fracture of the tibia with subsequent osteoarthrosis and a smooth and thick periosteal reaction of the distal diaphysis of the tibia with a lyt- ic focus containing a tiny low signal in- tensity bony sequestrum. There is also oedema of the parosteal soft tissues. b Axial T2 turbo spin-echo image with fat suppression shows the high signal in- tensity round area in the periosteal thickening, an endosteal oedematous zone and a linear oedematous area in the parosteal soft tissues. Anterior to the lyt- ic area there is a high signal intensity lin- ear structure apparently representing an intracortical sinus tract Fig. 2. A 3-mm-thick axial CT scan with bone algorithm reconstruction shows the periosteal thickening, the central lytic abscess and the tiny sequestrum. Anteri- or to the lytic area there is a linear hypo- dense structure representing an intracor- tical sinus tract