Imaging Features of Morel-Lavallée Lesions

Objectives: To review the imaging characteristics of Morel-Lavallée lesions with both ultrasound and magnetic resonance imaging (MRI). Materials and Methods: We retrospectively analyzed 31 patients (mean age = 46 years), diagnosed with a Morel-Lavallée lesion, on ultrasound (n = 15) or MRI (n = 16). On ultrasound the echogenicity, internal septations, hyperechoic fat globules, compressibility and Doppler signal were evaluated. On MRI, T1- and T2-signal intensity, capsule presence, internal septations, enhancement, mass-effect and fluid-fluid levels were assessed. The MR images were classified according to the classification of Mellado and Bencardino. Results: Most of the lesions were situated peritrochanteric, around the knee or the lower leg. The majority of the lesions had a heterogeneous hypoechoic appearance with septations and intralesional fat globules. On MRI, most of the collections were hypointense on T1-weighted images and hyperintense on T2-weighted images. Half of the collections were encapsulated, and most collections demonstrated septations. The collections were classified as seroma (n = 10), subacute hematoma (n = 2) and chronic organizing hematoma (n = 5). Conclusion: Ultrasound is the imaging method of choice to diagnose Morel-Lavallée lesions. MRI can be of use in selected cases (extension in different compartments, large collections, superinfection). Characteristic imaging features include a fusiform fluid collection between the subcutaneous fat and the underlying fascia with internal septations and fat globules. On MRI, six types of ML lesion can be differentiated, with the seroma, the subacute hematoma, and the chronic organizing hematoma being the most frequently observed lesions.

Several forms of subcutaneous fluid collections may result, depending on the severity of the injury, ranging from seroma to hematoma. After some time, the blood is reabsorbed and replaced by serosanguineous fluid [11]. This hemolymphatic collection may continue to enlarge, and patients may present with a large mass, usually located over the external aspect of the thigh. Because of this enlarging nature, these collections may be mistaken as a soft tissue tumour [2,3].
In the literature, six types of ML lesions have been described based on the chronicity, tissue composition and lesion appearance on magnetic resonance imaging (MRI) [7]. Mellado and Bencardino proposed this classification based on lesion shape, presence or absence of a capsule, signal intensity (SI) on T1-and T2-weighted images (WI) and enhancement pattern (Figure 2, Table 1) [2,7].
The goal of our study was to retrospectively analyse patients with ML lesions on both ultrasound and MRI. We classified the lesions according to the above mentioned classification, and we subsequently briefly reviewed the imaging features of ML lesions.

Material and Methods
From 2005 to 2016, 31 patients [mean age = 46 years (range ] were included in this retrospective cohort study. The search terms Morel-Lavallée and closed degloving injury were inserted in the search engine of the PACS system; the imaging reports and associated MR-images were analysed and 31 patients were included in the study. Fifteen patients had ultrasound examinations available and 16 patients were evaluated with MRI. The location and morphology were analysed for all patients on both imaging modalities: 1. location: greater trochanter/hip, proximal thigh, suprapatellar region of the knee, abdominal wall, periscapular, scalp, arm, hand, lumbosacral region. 2. morphology: fusiform, oval, laminar. The ultrasound examinations (both report and images) were specifically evaluated for the following characteristics: echogenicity, internal septations, hyperechoic fat globules, compressibility and Doppler signal.
The following characteristics were evaluated on the MR-images: T1-and T2-SI, presence of a (pseudo) capsule, septations or nodules within the collection, internal enhancement (if intravenous contrast was administered), mass-effect and fluid-fluid levels. Tangential shearing force cause the relatively mobile dermis and subcutaneous fat to move relative to the fixed underlying fascia, causing disruption of perforating arteries (red), veins (blue), and lymphatics (green). (C) Formation of a hemolymphatic collection in the potential space between subcutaneous fat and fascia. Case courtesy of Dr Matt Skalski.
The patients with MR-images were classified according to the classification of Mellado and Bencardino, who proposed an MRI classification of ML lesions corresponding to their histopathological stadium [7]. Six categories have been defined, wherein the first three types are the most frequently observed. An essential factor for determining the MRI appearance of the ML lesion is the age of the blood within the collection [11].

MRI classification
A type I ML lesion is a seroma which exhibits fluid-like signal characteristics on MRI (Figure 2A). This type appears homogeneously hypointense on T1-WI and hyperintense on T2-WI, respective to muscle [13]. This lesion can be found in both acute or chronic settings, and is most often not encapsulated [7].
A type II lesion is a subacute hematoma and exhibits a predominantly homogeneous hyperintense SI on both T1-and T2-WI ( Figure 2B) [13,14]. The high SI on T1-WI is caused by the presence of methemoglobin; the methemoglobin is first observed in the periphery of early subacute hematomas and progressively acquires a more homogeneous distribution [7,15]. Subacute hematomas often have a hemosiderin-rich hypointense capsule on both T1-and T2-WI. In some subacute hematomas, internal inhomogeneity is observed caused by entrapped fat globules, internal septations or fluid-fluid levels [15]. The presence of capillaries within the lesion can lead to patchy internal enhancement, falsely positive leading to the diagnosis of a soft tissue tumour. These subacute hematomas can further be subdivided in early and late subacute hematomas. The early ones tend to be more homogeneous whereas the late ones are more often heterogeneous with internal septations and debris and develop a fibrous capsule [2].
Type III ML lesions are chronic organizing hematomas that demonstrate hypo-or intermediate SI on T1-WI and   Figure 2C). The heterogeneous signal is due to the presence of hemosiderin, granulation tissue, necrotic debris, fibrin, and blood clots [7]. Patchy internal and peripheral enhancement can be seen on post-contrast MR [13], due to neovascularization and granulation tissue. These lesions can be surrounded by a hemosiderin-rich fibrous capsule [2]. The other three categories are longstanding ML lesions and they may demonstrate more atypical MRI features. A type IV ML lesion represents a closed fatty tissue laceration in combination with a perifascial dissection, and can be associated with or without a serous/hemorrhagic collection ( Figure 2D) [7]. The collection has low SI on T1-WI and high SI on T2-WI, is not encapsulated and demonstrates variable enhancement [2].
A type V ML lesion has a small, rounded pseudonodular appearance and is perifascially located (Figure 2E). They occasionally can demonstrate irregular peripheral enhancement and skin retraction [7].
A type VI lesion is an infected ML lesion, often with a thick enhancing capsule, internal septations, inflammation of the adjacent fatty tissue and fascia, peripheral fluid leakage, and sometimes an associated sinus tract ( Figure 2F) [7].
ML lesions most often appear as well-defined oval or fusiform cystic structures with differing degrees of internal complexity on all imaging modalities [6]. In our study, the predominant morphology was fusiform (26/31 = 84%) and oval collections were only seen to a minor extent (5/31 = 16%). ML lesions may contain blood, lymph, and debris. Depending on the age of the internal blood products, they can have a variable imaging appearance. Ultrasound is the imaging modality of choice to investigate patients with clinical suspicion of a ML lesion. They appear as fluid collections, often with heterogeneous echogenicity. The echogenicity depends on the degradation stage of the blood products [19][20][21][22]; acute and subacute (<1 month) lesions will appear heterogeneous with irregular margins and lobular shape.    The more acute the hematoma, the more hyperechoic the collection will appear. Over time the blood products will liquefy and become more hypoechoic [6]. Chronic lesions (> 18 months) are more often homogeneous with smooth margins and flat or fusiform shape [21]. Generally, ML lesions are compressible with the ultrasound transducer and demonstrate no flow on colour Doppler. Underlying muscle contusion or laceration can sometimes be demonstrated with ultrasound [2,23]. Neal, et al. investigated the ultrasound characteristics of 21 ML lesions located around the hip and knee [21]. The majority demonstrated a heterogeneous hypoechoic appearance, with less than 1/3 being purely anechoic. This is similar to our observations (87% heterogeneous hypoechoic; 13% anechoic). All collections were compressible and demonstrated no vascularity, similar to the observations in our series. Neal, et al. believes that the presence of internal areas of nodular echogenicity correspond with areas of disrupted, sheared subdermal fat globules and secondary necrosis [21]. These intralesional fat globules may be seen as small hyperechoic foci [12], as we observed in 67% of the patients. Their presence is not pathognomonic, but it is believed that this imaging feature can aid to characterize the collection as an ML lesion [4,19,21]. Some lesions may contain internal septations or a fluid-fluid level. MRI demonstrates exquisite soft-tissue contrast and has the ability to gauge the chronicity of the hematoma [6]. It is the modality of choice when a more global overview of the lesion is required. It provides the clinician with a more precise assessment of the lesion's extent (e.g. underlying muscle contusion) and morphology, particularly in large lesions. The SI of the lesion varies according to the stage of degradation of the internal blood products. Each stage has its corresponding SI characteristics [12].
The differential diagnosis of ML includes soft tissue masses and other posttraumatic collections. In the group of the soft tissue masses, sarcomas are the most important differential and may cause a diagnostic dilemma. A soft tissue sarcoma may mimic a type I or III ML lesion. A distinguishing feature between sarcomas and ML lesions are intralesional areas of marked homogeneous enhancement, exclusively seen in sarcomas [2,24].
Another differential diagnosis is bursitis; the location is usually the clue to the correct diagnosis, e.g. location of a fluid collection in the prepatellar region is mostly due to bursitis. However, chronic hemorrhagic bursitis may mimic a type III ML lesion [2,24]. A prepatellar bursitis however does not extend beyond the mid-coronal plane medially or laterally or to the mid-thigh proximally, whereas prepatellar ML lesions often extend medially and laterally at the mid-to distal thigh, a location distinct from the prepatellar bursa [25,26].
Other differential diagnostic dilemmas are posttraumatic collections such as fat necrosis and coagulopathyrelated hematomas [2]. The MR appearance of fat necrosis depends on the timing after trauma. It can appear spiculated, globular or more laminar [27], and sometimes may mimic a type IV (pseudonodular) ML lesion [7].
A pseudolipoma can develop posttraumatic after blunt trauma or iatrogenic after surgery and is often preceded by a hematoma [7,19]. On MRI it presents as a nonencapsulated subcutaneous lipomatous mass without contrast enhancement [28].
Coagulopathy-related hematomas are often out of proportion to the degree of trauma, or may occur even in an atraumatic setting. Patient history and laboratory test are most of the time sufficient to lead to a correct diagnosis [7].
Treatment depends on the stadium of the ML lesion. Management options include compression banding, aspiration, or incision and evacuation, with or without injection of sclerosing agents [2,29].
In acute posttraumatic ML lesions, a conservative approach with aspiration and compression bandages first may be attempted. If the lesion does not resolve it may require percutaneous drainage. If the drainage is not sufficient, an open debridement with resection of the necrotic tissue is performed to avoid risk of infection [2,7].
In chronic lesions, percutaneous drainage with sclerotherapy can be attempted. In lesions in which a peripheral capsule has formed, capsule resection may be required in order to prevent re-accumulation of hemolymphatic fluid [12].

Conclusion
Ultrasound is the imaging method of choice to diagnose Morel-Lavallée lesions. MRI can be useful in selected cases (extension in different compartments, large collections, suspicion of superinfection, differential diagnosis with soft tissue sarcoma). Characteristic imaging features include a fusiform fluid collection with septations and fat globules between the subcutaneous tissues and the underlying fascia.
On MRI, six types of ML lesion can be differentiated, with the seroma, the subacute hematoma, and the chronic organizing hematoma being the most frequently observed lesions. Knowledge of the common locations, the specific location between the subcutaneous tissue and the fascia, the imaging signs, and knowledge of the classification system may all be helpful in the differential diagnosis with other fluid-filled collections in the soft tissues.