Common Peroneal Nerve Abnormalities

A) Anatomy:

B) Trauma:

C) Peripheral nerve lesions:

This include peripheral nerve sheath tumors and ganglionic cyst.

Reports, most notably by Spinner and collaborators, have detailed MR imaging signs that allow accurate diagnosis of intraneural ganglia of the CPN.  These signs are based on a 12-hour clock face that is superimposed at various levels of the fibular head and neck. At the level of the mid-portion of the fibular head, the signet ring sign (cyst within the CPN) and tail sign (connection of the cyst to the proximal tibiofibular joint) can been seen at the 4 to 5 o’clock and 11 to 12 o’clock positions, respectively (Figure 10).

The transverse limb sign (cyst within the articular branch of the CPN) is best seen at the fibular neck level, between the 12 to 2 o’clock positions.15 (Figure 9b and c).

Determining whether a ganglion cyst at the anterior aspect of the proximal tibiofibular joint is intraneural or extraneural requires an understanding of the differences in appearance between these two types of ganglion cysts. The site from which extraneural ganglion cysts arise is more variable than the site of origin of intraneural ganglion cysts, with extraneural ganglion cysts often more superiorly located along the proximal tibiofibular joint as compared with intraneural ganglia. Additionally, the shape and direction of cyst extension are more variable in cases of extraneural ganglion cysts compared with intraneural ganglia, with the intraneural ganglia following a more predictable concave upward arc. Extraneural ganglion cysts may develop in close proximity to the CPN, abutting the nerve in such a way that their relationship results in a pseudo-signet ring sign; however, close inspection will demonstrate a fat plane between the cyst and nerve and no cyst within the articular branch of the nerve. A true signet ring sign will exhibit no fat plane between the ganglion and the nerve and the location is classically at the 4 to 5 o’clock position.

Reference