Ultrasonography (US) is an established method for peripheral nerves imaging.
In the short axis,
the nerve has a honeycomb-like appearance,
with multiple rounded hypoechoic areas in a homogeneous hyperechoic background.
In the long axis,
it has a striated appearance composed of multiple parallel hypoechoic and hyperechoic bands.
This image resembles that of an electric cable.
This type of US appearance can be defined as fascicular structure.
The transverse image (short axis) is much more frequently used in clinical practice,
as it allows for the nerve to be examined by the so-called elevator technique which consists of finding the set nerve at a characteristic anatomic point and ‘tracking it’ either proximally or distally.
In this way,
it is possible to assess the nerve’s shape,
echogenicity and thickness and its relation to the surrounding tissues,
the surface area of the nerve and its vasculature.
If an abnormality is seen in the transverse view,
the nerve should be examined in the longitudinal view.
The common peroneal nerve is the lateral terminal branch of the sciatic nerve.
It supplies the motor innervation of the anterior and lateral compartments of the legs and the sensory innervation of the skin of the anterolateral aspect of the leg and of the dorsum of the foot.
The two terminal branches of the common peroneal nerve are deep and superficial peroneal nerve. In the popliteal fossa,
the common peroneal nerve runs obliquely passing along the medial border of biceps femoris muscle; then distally it lies between the tendon of the biceps femoris (medial) and the lateral head of the gastrocnemius muscle (lateral) until reaching the posterior surface of the head of the fibula and then passes over the peroneal head. Then the common peroneal nerve moves anteriorly perforating the lateral intermuscular septum and enters in a tunnel between the long peroneal muscle and the proximal metaphysis of the fibula,
passing in the anterior compartment of the leg.
At this location,
the common peroneal divides into two terminal branches: the superficial and deep peroneal nerves. In the popliteal fossa,
the common peroneal nerve gives off articular branches and a small communicating branch,
together with a reciprocal communicating branch from the tibial nerve,
forms the sural nerve.
The sural nerve supplies sensory innervation to the posterior and lateral surfaces of the distal third of the leg. The common peroneal nerve can be entrapped during its passage through the fibular tunnel or the biceps femoris tunnel or during its superficial course around the knee.
This clinical condition can present as foot drop,
due to the denervation of the anterior compartment of the leg.
The deep peroneal nerve runs inferiorly close to the fibular neck between
the long peroneal muscle and the bony surface of the fibula,
on the anterior aspect of the interosseous membrane of the leg. At the level of the ankle joint it passes longitudinally through the anterior tarsal tunnel beneath the extensor retinaculum.
Approximately 1.5 cm above the ankle joint,
in the dorsum of the foot,
the deep peroneal nerve splits into a medial and a lateral branch.(Fig10)
The deep peroneal nerve supplies the muscles of the anterior compartment of the leg that allow dorsiflexion and inversion of the foot.
It supplies sensory innervation to the first web space and the adjacent sides of the first two toes and the tarsal and metatarsophalangeal joints of the middle three toes.
It provides sensation to the ankle joint and the sinus tarsi.
Deep peroneal nerve palsy causes an inability to dorsiflex the ankle and loss of sensation in the first dorsal web space.
The symptom is clinically described as “foot drop.” Causes of compression or lesion of the deep peroneal nerve could be ganglion cyst,
osteophytes from the talonavicular joint,
subluxations or fractures,
an external pressure on the dorsum of the foot due to a long history of wearing shoes with tight laces,repeated extreme plantar flexion of the foot (e.g.,
in ballet dancers),
and pes cavus.
The superficial peroneal nerve is the smaller of the two terminal branches of the common peroneal nerve.
It supplies motor innervation to the lateral compartment of the leg,
innervating the peroneus longus and brevis muscles which originate from the fibula.
It also provides sensory innervation to the lateral compartment of the leg and dorsum of the foot.
The superficial peroneal nerve emerges at the bifurcation of the common peroneal nerve and runs between the peroneal muscles,
supplying them with motor innervation. Initially it runs along the peroneus brevis muscle,
under the peroneus longus.
In the distal part of the leg,
it runs underneath the fascia of the calf and descends between the peroneus muscles and the extensor digitorum longus. In the lower third of the leg,
it pierces the deep fascia through a short fibrous tunnel,
to enter the subcutaneous tissue.
This exit point is located at the level of a defect in the crural fascia,
typically located about 10 cm above the ankle joint.(Fig11)
More distally it divides into two cutaneous terminal branches: the medial dorsal cutaneous nerve and the intermediate dorsal cutaneous nerve,
which supply the dorsal foot.
The Superficial peroneal nerve is the only nerve in the human body that can be visible in the dorsum of the ankle.
Depending on the location of the entrapment or lesion of superficial peroneal nerve,
symptoms may vary:proximal location in the lateral compartment of the leg it causes weak ankle eversion and dorsiflexion due to affected innervation of the peroneus longus and brevis muscles;at level of the crural fibrous tunnel,
it causes pain in the anterolateral leg,
or paresthesia over the dorsal foot,
sparing the first web space.