Lipoma of the Infrapatellar Fat Pad: A Report of Three Cases

Although lipoma is the most common soft tissue tumor, lipoma in the infrapatellar fat pad (IFP) is rare. Herein, we report three cases of lipoma in the IFP that penetrated the joint capsule and extended subcutaneously. All patients presented with unusual MRI findings. Patients 1, 2, and 3 were 63, 74, and 64 years old, respectively, and all were female. Their chief complaint was a knee mass; however, they did not experience pain or limitations in range of motion. The interval from initial awareness of the mass to first consultation was long (3, 8, and 13 years, respectively). MRI revealed that the mass had extended subcutaneously from the IFP through the lateral, medial, and bilateral joint capsules of the patellar tendon, respectively. On MRI, the masses appeared as lipomatous tumors with scattered low signal areas on T1-weighted imaging. The tumors excised after biopsy had long axes of approximately 6, 7, and 7 cm, respectively. Histological examination revealed lipomas with partial fibrosis in all three tumors and cartilage metaplasia in the tumor from patient 3. Lipomas in IFPs often exhibit secondary changes, such as fibrosis and cartilage metaplasia, resulting in MRI findings that differ from those of typical lipomas. In the present cases, biopsy was necessary to differentiate them from atypical lipomatous tumors or Hoffa disease.