Prepatellar Morel-Lavallée effusion

Abstract

Lateral radiograph of the left knee shows prepatellar soft tissue swelling. MR images demonstrate a T2-hyperintense unilocular prepatellar fluid collection located between the subcutaneous fat and underlying fascia. The collection extends beyond the transverse and craniocaudal boundaries of the prepatellar bursa, with the medial and lateral margins reaching the level of the femoral epicondyles at the midcoronal plane, and the superior and inferior margins extending from the mid-thigh to beyond the tibial tubercle. These findings are indicative of a prepatellar Morel-Lavallee effusion and are distinguished from prepatellar bursitis by location beyond the anatomic confines of the prepatellar bursa. Quadriceps muscle signal intensity is normal, excluding muscle contusion as the cause of the patient’s pain. Other post-traumatic subcutaneous lesions in the differential diagnosis such as fat necrosis, pseudolipoma, and coagulopathy-related hematoma are unlikely given the anatomic location, imaging characteristics, and clinical history [1]. The patient was successfully treated with activity restriction, ice, and compression wraps. Maurice Morel-Lavallee first described closed degloving injuries of the hip and pelvis in 1853 [2]. Excessive shearing force or repetitive compressive trauma causes separation of the skin and subcutaneous fat from underlying deep fascia resulting in disruption of perforating vessels and formation of fluid collections containing blood, fat, and lymph [1–3]. Morel-Lavallee lesions adjacent to the greater trochanter, proximal femur, buttock, and lower back are well known [4, 5], and MR imaging has an established role in their diagnosis. The appearance of Morel-Lavallee effusions on MRI depends on lesion acuity and the amount of blood product, fat, and lymph tissue within it [1]. Mellado et al. described six types of Morel-Lavallee effusions ranging from seroma to infected collections, with a unilocular T2 hyperintense fluid collection being the most common finding of the basic subtypes [1]. Neither blood product, fat, nor a fibrous pseudocapsule was identified in this particular case. Of these findings, the presence of an enhancing pseudocapsule has prognostic value, suggesting possible benefit of therapeutic aspiration or debridement to facilitate resolution of the lesion [1, 2]. Despite the established diagnosis of Morel-Lavallee effusion in the pelvis and hips, its occurrence in the prepatellar region has only recently been described in the orthopedic [6, 7] and radiology literature [8, 9]. A recent case series of 24 The case presentation can be found at doi:10.1007/s00256-012-1399-0.