Imaging findings in Morel-Lavallée lesion

Clinical History

30-year-old male patient with history of trauma to the right knee and lower thigh 2 weeks earlier during a soccer match. The patient complained of a swelling over the medial aspect of the right lower thigh which was progressively increasing in size.

Imaging Findings

Ultrasonography showed a well defined oval hypoechoic lesion with internal septations in the medial aspect of lower thigh with no evidence of internal vascularity entirely limited to the subcutaneous plane.

CT showed a hypodense fluid attenuation lesion in the subcutaneous plane of the lower thigh in the medial aspect causing minimal compression on the vastus medialis muscle.

MRI showed a well defined lesion in the medial aspect of the lower thigh limited to the subcutaneous perifascial plane (between subcutaneous fat and deep fascia). The lesion appeared heterogeneously hyperintense on T2W and STIR sequences and predominantly hyperintense on T1W images (suggestive of subacute hemorrhage). Internal septations and fluid-fluid levels were noted. Focal internal areas of T1 hyperintensity were noted showing suppression on STIR images- suggestive of areas of fat necrosis.

Incision and drainage confirmed the presence of altered blood within the lesion.

Imaging and operative findings were diagnostic of Morel-Lavallée lesion.

Discussion

Morel-Lavallée lesions are soft tissue injuries characterised by post traumatic fluid collections dissecting subcutaneous fatty tissue. The pathogenesis of the lesion consists of shearing or tangential forces causing separation of skin and subcutaneous tissue from the underlying fascia [1, 2, 3].The injury causes disruption of perforating vessels and creation of a potential space which gets filled with blood , lymph, debris and fat (secondary to fat necrosis) [3]. Various names have been attributed to Morel-Lavallée lesions in literature such as posttraumatic soft-tissue cysts, pseudocysts, Morel-Lavallée extravasations or Morel-Lavallée effusions [1, 2, 4]. Although initially described in the trochanteric and proximal thigh region, these lesions may also involve the abdominal wall, buttock, lower thigh and scapular region [1, 2, 5].

Imaging findings confirm the location of the lesion in the subcutaneous perifascial plane. On ultrasonography, Morel-Lavallée lesions appear hypoechoic with internal septations. CT images may demonstrate fluid/blood attenuation. On MRI, the lesions show variable signal intensity. Important diagnostic clues include fluid-fluid levels, internal septations and focal areas of fat within the lesion. The finding of internal fat is an important diagnostic tool which radiologists should actively seek [5]. The lesions may have no demonstrable capsule, a partial capsule or complete capsule which if present appears hypointense on all pulse sequences due to fibrosis or haemorrhagic byproducts [6]. Mallado et al proposed a useful MRI classification system of Morel-Lavallée lesions including six subtypes [7].

Type I: Laminar shaped, seroma-like with decreased T1 and increased T2 signal.
Type II: Oval shaped with increased T1 and T2 signal (subacute haematoma).
Type III: Oval shaped with intermediate T1 and heterogeneous T2 signal (chronic organising haematoma), thick capsule and internal/peripheral enhancement.
Type IV: Linear shaped like a closed laceration with hypointense T1 and hyperintense T2 signal.
Type V: Pseudonodular (round shape) with variable T1/T2 signal and internal/peripheral enhancement.
Type VI: Infected with variable T1 and T2 signal, sinus tract formation, thick capsule and internal/peripheral enhancement.

Prepatellar bursitis can mimic a Morel-Lavallée lesion when it occurs near the knee joint. The size of the bursal collection is usually smaller when compared to Morel-Lavallée lesions. Prepatellar bursal collections also do not cross the midcoronal plane to extend into the posterior aspect of the knee joint [5].

Chronic lesions may lead to a misdiagnosis of soft tissue tumour owing to irregular enhancement [4].

In acute settings, surgery is recommended. Other modes of treatment include compression and doxycycline sclerodesis [7, 8].

Differential Diagnosis List

Final Diagnosis

Morel-Lavallée lesion

URL:	https://www.eurorad.org/case/9991
DOI:	10.1594/EURORAD/CASE.9991
ISSN:	1563-4086