A 3-year-old boy was found to have a painless mass over his right anterior lateral knee for the previous 6 months. The mass was hardly identified during knee extension and became visible upon squatting. There was no specific finding on ultrasound imaging over his right knee at the supine extended position. During squatting, ultrasound imaging showed an ill-demarcated hyperechoic mass protruding next to the cartilage overlying the distal femur towards the subcutaneous layer through a slit over the lateral patellar retinaculum. Herniation of the fat pad through a defect in the lateral patellar retinaculum was diagnosed. Our case highlights the usefulness of ultrasound examination as regards the lateral patellar retinaculum defect in pediatric knees, as well as its capability for dynamic scanning to capture the exact "pathological moment".

Center for Regional Anesthesia and Pain Medicine Wang Fang Hospital Taipei Medical University Taipei 11600 Taiwan

Department of Orthopaedic Surgery National Taiwan University Hospital Taipei 10048 Taiwan

Department of Physical and Rehabilitation Medicine Hacettepe University Medical School Ankara 06100 Turkey

Department of Physical Medicine and Rehabilitation College of Medicine National Taiwan University Taipei 10048 Taiwan

Department of Physical Medicine and Rehabilitation National Taiwan University Hospital Bei Hu Branch Taipei 10845 Taiwan

Department of Rehabilitation Medicine Charles University 1st Faculty of Medicine and General University Hospital Prague 12800 Prague Czech Republic

Physical and Rehabilitation Medicine Unit Luigi Sacco University Hospital ASST Fatebenefratelli Sacco 20157 Milan Italy

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