A prepatellar Morel-Lavallée lesion in a pedestrian vs automobile collision.

A small truck collided with a 67-year-old female pedestrian. She sustained blunt, closed trauma to her right knee, and developed a prepatellar Morel-Lavallée lesion (MLL). A MLL is a closed soft tissue degloving injury, resulting from high-energy shearing forces, which separate the skin and subcutaneous tissue from the underlying fascia. The resultant space collects fluid and is prone to infection, tissue necrosis, and symptoms of ongoing mass effects. The diagnosis is elusive because of its rarity and often subtle initial symptoms. Prompt diagnosis is critical, given the potentially severe complications when missed, and less invasive and more successful treatment when found early. Most reported cases are proximal to the pelvis, whereas the few reported peri–knee MLLs involve young athletes or postsurgical complications. To our knowledge, this is the third reported case of a non–sport-related MLL of the knee, all of which involved high-inertia force to the knee. Therefore, MLL of the knee should be considered in patients with knee trauma, particularly in the setting of pedestrians struck by motor vehicles.