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Morel‐Lavallee Lesions of the Knee: Update and Imaging Review

The Morel-Lavallée lesion (MLL) is a closed, traumatic, soft-tissue degloving injury, classically over the external aspect of the thigh, but it is also used to describe similar lesions elsewhere. It was first described by the French physician Victor-Auguste-François Morel-Lavallée in 1863. These lesions are characterized by a collection of hemolymphatic fluid between the hypodermis and the underlying fascia, which results from a shear force that disrupts the perforating vascular and lymphatic structures between these tissue layers. The body responds by releasing inflammatory and metabolic products that increase cellular permeability and potentiate further fluid collection. Studies of macroscopic contents within MLLs have shown combinations of blood clot, fibrin, and fat globules. In 1997, Hak et al found 46% (11 of 24) of patients with MLLs of the hip had bacterial colonization independent of time from injury to debridement. Whether the shearing trauma is a single acute injury or repetitive microtrauma, the general evolution of MLLs can be divided into four stages. In the first stage, the dermis is separated from the underlying fascia. In the second stage, damage to the subdermal plexus causes exsanguination from the vasculature and lymphatics,which produces afluid collection of blood, lymph, and fatty debris. Over time the third stage begins as the initial fluid collection is replaced by serosanguinous fluid, and the collection enlarges. The fourth stage occurs if the lesion is left untreated, with local inflammation causing a pseudocapsule to form as the body attempts to isolate the fluid-filled space. MLLs that become chronic can have undesirable consequences, such as infection, pseudocyst formation, and cosmetic deformity. In addition, the development of an expanding hematoma in some severe cases can compromise skin vascularity if not drained promptly. Epidemiology