A Morel-Lavallée lesion (MLL) is a posttraumatic soft-tissue injury 
characterized by an accumulation of blood, lymph, and other physiologic 
breakdown products between subcutaneous tissue and underlying fascia. It was 
first described as occurring over the proximal lateral thigh, but it has since 
been documented at various anatomic locations. Diagnosis is typically made by 
careful physical examination and a radiographic analysis, most commonly with 
magnetic resonance imaging (MRI). Recently, musculoskeletal ultrasound (US) has 
been recognized as a useful adjunct to and potential replacement for MRI in the 
diagnosis and monitoring of an MLL. We present a case report of a patient with 
an MLL of the knee. We obtained magenetic resonance (MRI) and US images at the 
time of diagnosis, and follow-up US images during convalescence. By doing so, we 
were able to identify several key sonographic findings of an MLL at this 
location and compare them with MRI. Although there have been several published 
reports to date that describe the use of musculoskeletal US in the diagnosis of 
MLL, this is the first of which we are aware that does so at the knee.