A 40-year-old female patient came with complaint of swelling in the right gluteal region for 1-2 months. There was no history of acute trauma; however, she had a history of a fall and injury in the right hip region two years before. She had experienced pain for a few days but did not seek medical advice.
MR imaging of the right gluteal region was performed on a 3 T system (Achieva, Philips Medical system) using SENSE 6 channel-phased array torso coil and high-resolution matrix. It revealed a well-defined, encapsulated oval lesion measuring 4.3 x 4.4 x 3.8 cm (AP x Trans X CC) in the right gluteal region in the subcutaneous plane. It was predominantly hyperintense on T2-weighted images (Figure 2) and hypointense on T1-weighted images. Multiple foci appearing hyperintense on T1-weighted imaging were seen in the periphery of the lesion which were suppressed on STIR images, suggestive of fat (Figures 3, 4; curved arrow). Few thin internal septae were also seen (Figures 3, 5; straight arrow). Mild surrounding subcutaneous oedema was also noted (Figures 2, 3). Visualised bones, muscle and joint were normal.
Morel-Lavallée lesions are post-traumatic lesions occurring due to interfacial split between subcutaneous soft tissue and muscle  with resultant potential space that is filled by haemolymphatic fluid. These are frequently associated with degloving injuries from blunt shearing or tangential forces . This is an uncommon entity with unknown exact incidence rate. Male to female predilection of this lesion is 1:1 . Originally these lesions were described in the region of greater trochanter associated with acetabular fractures . Nowadays multifocal locations of Morel-Lavallee lesions in pelvis , lumbosacral , gluteal thigh  and calf  regions is well accepted.
Radiograph, ultrasonography and CT are initial modalities for evaluation of Morel-Lavallée lesions; however, MRI is considered best for identification, characterisation and anatomical extension . Radiographic appearances of these lesions are not very characteristic and they appear as irregular soft tissue density. On ultrasonography Morel-Lavallée lesions are located in between deep fat and overlying subcutaneous fascia with acute-subacute lesion revealing irregular margins, and heterogeneous hypoechoic to anechoic cystic appearance . Chronic lesions (>18 months) appears as well-defined, fusiform collection with smooth margins and homogenous fluid echogenicity .
On CT, acute lesions show irregular margins, no capsule and hyperdense to heterogeneous hypodense attenuation with internal fat globules . On evolution, chronic Morel-Lavallée lesions reveal a well-marginated pseudo capsule, and homogeneous fluid density with post-contrast enhancement of the capsule . However, this appearance is not specific as it is similar to contusions or simple haematomas.
MRI is the modality of choice for detailed evaluation and diagnosis. Morel-Lavallée lesion is categorised in 6 different types [1, 10] depending upon its appearance on MR imaging (Figure 1). These lesions are localised between superficial and deep fascia.
Many treatment strategies have been established for these lesions like compression banding, percutaneous drainage, incision and evacuation with or without sclerotherapy and open debridement . In early stages compression bandage, ultrasound-guided percutaneous drainage and sclerotherapy treatment with talc, alcohol and doxycycline can be attempted. However, open debridement is considered necessary in chronic lesions refractory to above treatment, chronic lesions with a pseudo capsule and in acute cases with an underlying open fracture . MR imaging plays an important role not only in the diagnosis and characterisation of Morel-Lavallée lesions but also helps with the selection of an appropriate treatment strategy.
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