A 6-year-old girl was referred to us for ultrasound evaluation of a progressive suprapatellar soft tissue swelling in the left knee. She had sustained a skiing injury 10 months earlier.
Magnetic resonance imaging (MRI) performed in an outside hospital immediately after the injury had shown significant soft tissue edema without evidence of any intra-articular knee lesion (Fig. 1, axial PD-fatsat).
The ultrasound examination performed at our department 10 months after the injury showed an oval mass in the suprapatellar soft tissues, with associated Doppler flow (Fig. 2A, sagittal view). To better characterize the soft tissue mass, an additional MRI was requested (Fig. 2B, sagittal postcontrast T1-fatsat), which confirmed the presence of a well-circumscribed hypervascular soft tissue mass. It seemed to be located between the subcutaneous tissue and the deep fascia of the suprapatellar knee.
These imaging findings, combined with the remote trauma history, were highly suggestive of a Morel-Lavallée lesion. Surgical debridement was performed one month later because of an unfavorable clinical evolution with skin necrosis. Histopathology of the resection specimen showed inflamed granulation tissue.
Morel-Lavallée lesions or closed degloving injuries represent a traumatic separation of the hypodermis from the underlying fascia by tangential shearing forces, causing rupture of small perforating vessels and filling of the newly created potential space with blood, lymph and debris. [1,2]
Patients with Morel-Lavallée lesions typically present with a progressive superficial soft tissue mass. Although this lesion is considered to have a traumatic etiology, an appropriate history may be absent, and the diagnosis may be missed or delayed, increasing the risk of possible complications, such as skin necrosis or surinfection. The principal differential diagnostic consideration of Morel-Lavallée lesions around the knee is prepatellar bursitis. Given the fact that both entities can develop after trauma, and may give a similar clinical presentation, imaging plays a crucial role in distinguishing these conditions. Another diagnosis that may have to be excluded in patients presenting with a slow-growing soft tissue swelling is a soft tissue tumor, such as vascular masses of sarcomas. [2-6]
Morel-Lavallée lesions present a variety of imaging appearances depending on the age of the lesion. MRI is generally better suited than other imaging modalities to evaluate these lesions because of its superior soft tissue contrast. In 2005, Mellado and Bencardino proposed a MRI classification system of Morel-Lavallée lesions, separating them into six types based on their signal intensity characteristics, which are believed to change with age. Our patient presented with a vascularized mass mimicking a soft tissue tumor, which is quite unusual given that Morel-Lavallée lesions generally appear as a well-circumscribed cystic structure. Because there are no vascularized solid-tissue elements within a Morel-Lavallée lesion, the typical lesion will not show internal enhancement. More chronic lesions however may display patchy internal enhancement, corresponding to inflamed granulation tissue, as was the case for our patient. [2, 3, 7]
The treatment of Morel-Lavallée lesions mainly depends on the stage at which the injury is detected, with acute lesions being more amenable to conservative measures. Indications for surgical debridement include more chronic cases with formation of a fibrous capsule, as well as severe skin necrosis or deep infection. 
In conclusion, radiologists should be aware of this relatively rare traumatic soft tissue injury, in order to warrant a timely diagnosis and treatment.
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