Fig. 1

Normal anatomy of the peroneal nerve at the level of the posterolateral corner of the right knee. Schematic drawing of a sagittal (a) and coronal (b) view with corresponding axial T1-WI of a right knee; the levels are indicated by the transparent boxes. On the schematic drawing the nerve is seen branching off the sciatic nerve, turning around the biceps femoris muscle (BF), passing through the peroneal tunnel between the insertion of the peroneus longus muscle (PL) and the fibula. As it exits the tunnel, it trifurcates in a deep (D) and superficial peroneal nerve (S) and a recurrent or articular branch (A). The articular branch is the entrance port for intraneural ganglia originating from the proximal tibiofibular joint (see the section on intraneural ganglia). Axial T1-WI at the level of the distal femur (c) shows the common peroneal nerve (white arrow) and the tibial nerve (black arrow) as they branch off the sciatic nerve. Note the intimate relationship of the common peroneal nerve with the medial side of the biceps femoris muscle (B). Axial T1-WI at the level of the femoral condyles (d) shows the common peroneal nerve (white arrow) between the short head of the biceps femoris (B) and the lateral head of the gastrocnemius muscle (G), a site of possible entrapment in case of variant course of the short head of the biceps femoris. The fascicles of the deep and superficial peroneal nerve can sometimes be discerned from this level on, the former more anteriorly, the latter more posteriorly, corresponding to the location of the anterior and lateral compartment of the lower leg. Axial T1-WI at the level of the fibular head (e). The common peroneal nerve (white arrow) is found posteriorly and can be traced by the fat around it. The peroneus longus (*) and anterior tibial muscle (**) are already seen at the most proximal parts of the lateral and anterior compartments. Axial T1-WI at the level just below the fibular neck (f) shows the superficial and deep peroneal nerve (white arrow). Because of their more horizontal course at this level, they are more difficult to discern from each other. At the 12 o’clock position the articular branch can be visualised as a small black dot surrounded by hyperintense fat (arrowhead). a adapted with permission from ref [50]. b adapted with permission from ref [10]