Fig. 10
From: Peroneal nerve: Normal anatomy and pathologic findings on routine MRI of the knee
Neuroma. Schematic drawing (a) showing the two types of nerve damage. A normal nerve (1 and 4) is surrounded by a nerve sheath on both sides. A complete disruption of the nerve (2) results in disorganised sprouting of nerve fascicles and fibrous tissue (3). This creates a ‘ballon-on-a-string’ or ‘green-onion’ image, typical of a terminal (posttraumatic) neuroma. Wallerian degeneration occurs distally in the nerve. A partial laceration of the nerve (5) causes focal regeneration with hypertrophy of the nerve fascicles and fibrosis. This creates a focal asymmetrical thickening on imaging (6). Sagittal T2-WI image (b) shows a hypointense terminal neuroma (black arrow) after accidental surgical transection of the peroneal nerve. Note also associated hypointense strands of scar tissue (white arrowhead) in the adjacent subcutaneous fat