Vol. 11, Issue 3 (2025)

Lipomas are the most frequent benign soft-tissue tumours; however, their intra-articular occurrence is exceptionally rare compared to lipoma arborescens, which is more often a reactive synovial process linked to degenerative or inflammatory joint disease. The knee is the most common site, though occasional cases have been reported in the hip, elbow, wrist, shoulder, and spine. We present the case of a 48-year-old male who reported a progressively enlarging, painless swelling in his right knee, which later caused discomfort during deep flexion. Clinical examination revealed a mobile, non-tender suprapatellar mass without inflammatory changes. Radiographs demonstrated soft-tissue opacity, and MRI revealed a well-defined, high-intensity lesion with fibrous septa consistent with fatty tissue. The patient underwent medial parapatellar arthrotomy, during which a large encapsulated lipoma measuring 13 × 8.5 cm was excised in toto. Histopathological examination confirmed mature adipose tissue with fibrous septa, establishing the diagnosis of a true intra-articular lipoma. Postoperative recovery was uneventful, with complete restoration of knee motion and no recurrence at follow-up. Intra-articular lipomas are diagnostically challenging due to their rarity and non-specific radiographic findings. MRI is the imaging modality of choice, facilitating differentiation from mimics such as lipoma arborescens, liposarcoma, pigmented villonodular synovitis, and Hoffa’s disease. While arthroscopic excision is suitable for small lesions, large tumours may require open arthrotomy for complete removal. This case underscores the importance of considering intra-articular lipoma in the differential diagnosis of intra-articular knee masses and demonstrates that surgical excision provides excellent functional and oncological outcomes with a negligible recurrence rate.