An 11-year-old boy came to our outpatient department with symptoms of pain and swelling in the right knee. He had noticed the swelling 6 months previously and since then there was a gradual increase in size. There was no history of fever or trauma. On examination, local temperature was normal. There was no tenderness. Swelling was localized to the anteromedial aspect of the right knee in the suprapatellar region. It seemed to be fixed to the patella because both were moving as a unit. It was firm on palpation. The patient had full knee extension, but flexion was restricted by 30° on the affected side in comparison with the contralateral normal knee. On radiographic evaluation, there was a bony outgrowth with associated mass originating from the superior pole of the patella. This mass had focal areas of calcification (Fig. 1). On computed tomographic (CT) scans, similar findings were noted (Fig. 2). On magnetic resonance imaging (MRI), the mass had low to intermediate signal intensity on T1-weighted images and isointense to hyperintense signal on T2-weighted images, suggestive of a cartilage component (Figs. 3 and 4). Routine blood investigations were within the normal limits. After diagnostic evaluation, an excision biopsy of the mass was planned. The patient underwent an operation that included an anterior midline incision and a medial subvastus approach. A multilobulated mass was found to be arising from the superior pole of the patella. The surface of the mass was in continuity with the articular cartilage of the superomedial aspect of the patella. The mass was excised along with its bony stalk. It measured 6.2 cm longitudinally, 3.7 cm in transverse extent, and 3 cm in depth (Fig. 5). The histology of the excised tissue is shown in Figure 6.
What is the diagnosis?
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