Knee swelling after a recent car accident

Abstract

A 40-year-old man presented with knee swelling following a motor vehicle accident 14 hours before MRI. Imaging demonstrated a post-traumatic fluid collection along the interface between the subcutaneous tissues and deep fascia, characteristic of a Morel-Lavallée lesion. This closed degloving injury results from shearing forces that disrupt perforating vessels and create a potential space filled with blood, lymph, and fat debris. MRI is useful for defining the extent of the collection and characterizing its contents, which vary with the age of hemorrhage. Recognition of the typical perifascial location helps distinguish this lesion from other post-traumatic soft-tissue abnormalities around the knee.