Case: We describe the case of a 3-year-old boy who presented with limited flexion in the left knee. High-resolution magnetic resonance imaging allowed visualization of a fibrotic muscle band in the anatomic position of the fifth component of the quadriceps muscle, which was confirmed by surgical findings. After surgical resection, the patient achieved full recovery of range of motion of the knee.
Conclusion: To our knowledge, this is the first reported case of a fibrotic muscle band in the anatomic position of a previously described fifth accessory component of the quadriceps muscle that caused limited knee flexion in a child.