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European Journal of Radiology 82 (2013) 27– 37

Contents lists available at ScienceDirect

European Journal of Radiology

jo ur n al hom epage: www.elsev ier .com/ locate /e j rad

maging of the nerves of the knee region

. Damareya,b,∗, X. Demondiona,b, G. Wavreilleb,c, V. Pansinia, V. Balbia, A. Cottena

Service de Radiologie et d’Imagerie musculosquelettique, CCIAL, Hôpital Roger Salengro, CHRU de Lille, Rue Emile Laine, 59037 Lille Cedex, FranceLaboratoire d’anatomie, Faculté de médecine, CHRU de Lille, 1 Place de Verdun, 59045 Lille Cedex, FranceService d’orthopédie, Hôpital Roger Salengro, CHRU de Lille, Rue Emile Laine, 59037 Lille Cedex, France

r t i c l e i n f o

rticle history:eceived 15 March 2011ccepted 29 March 2011

a b s t r a c t

Peripheral neuropathies are a frequent, but often underdiagnosed, cause of pain and functional impair-ment. The clinical symptoms can be subtle, and other neurologic or non neurologic clinical entities areoften evoked. MRI and ultrasonography are the imaging modalities of choice for depicting nerves and

eywords:ltrasoundRIerventrapment

assessing neuropathies. Common neuropathies in the knee area involve the saphenous, the tibial, thecommon peroneal and the sural nerves. The most frequent mechanisms of nerve injury in this area arenerve entrapment and nerve stretching. A perfect knowledge of the normal imaging anatomy is essentialfor accurate assessment of neuropathies. In this article, we will review the anatomy of the nerves aroundthe knee, and their normal and pathological appearance.

nee

. Introduction

Peripheral neuropathies are a frequent, but often underdiag-osed cause of pain and functional impairment, which are oftennderdiagnosed. The clinical symptoms can be subtle and non spe-ific, and other neurologic or non neurologic clinical entities areften evoked. MRI and ultrasonography are the best imaging tech-iques to depict nervous structures. Ultrasonography has a veryigh-resolution and can image long nerve segments. It is mostuitable for the evaluation of small superficial nerves. MRI is lessapable of depicting small structures but it can better image deeptructures.

The most frequent mechanisms of nerve injury are nerve entrap-ent and nerve stretching. Nerve entrapment is usually well

ecognized at imaging because the nerve may demonstrate dis-lacement, focal swelling and changes in its signal intensity orchogenicity. On the contrary, imaging signs of stretching neu-opathies are often subtle or unremarkable despite clear clinicalvidence of nerve dysfunction [1]. Whatever the nature of the nerveysfunction, denervation edema or fat infiltration of muscles has toe searched for in the territory of a motor nerve.

Four nerves are present in the knee area: the saphenous, theibial, the common peroneal and the sural nerves. The aims of this

aper are to review the anatomy of the nerves around the knee ando present the main nerve disorders of this region.

∗ Corresponding author at: Service de Radiologie et d’Imagerie musculosquelet-ique, CCIAL, Hôpital Roger Salengro, CHRU de Lille, Rue Emile Laine, 59037 Lilleedex, France. Tel.: +33 320446440; fax: +33 320446135.

E-mail address: benjdam@hotmail.com (B. Damarey).

720-048X/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved.oi:10.1016/j.ejrad.2011.04.035

© 2011 Elsevier Ireland Ltd. All rights reserved.

2. Saphenous nerve

2.1. Normal anatomy

The saphenous nerve is the longest and the largest branch of thefemoral nerve [2]. It is a purely sensory nerve that supplies innerva-tion to the anteromedial aspect of the lower leg from the knee to thefoot. During its course in the thigh, the nerve runs in the adductorcanal (Hunter’s canal) and migrates along with the sartorius mus-cle, from a relatively anterior position in the proximal thigh to amedial location in the knee area [3–6]. Within the adductor canal,the saphenous nerve lies anteromedially to the femoral artery andvein. In addition to the saphenous nerve and the femoral vessels,the canal also includes the nerve to the vastus medialis and othermotor branches from the femoral nerve. The saphenous nerve thenexits the adductor canal through a fascial veil approximately 10 cmproximal and 7 cm medial to the superior pole of the patella. Thenthe nerve continues along the medial side of the knee, covered bythe sartorius muscle [6] and divides into its terminal branches: theinfrapatellar and the sartorial branches (Fig. 1). The infrapatellarbranch becomes superficial to the sartorius tendon, crossing mostcommonly posterior (75% of cases), rarely through (22%) or ante-rior (3%) to the sartorius tendon [7]. Then it curves antero-inferiorlybelow the patella and divides in several small branches to supplysensory innervation to the anteromedial aspect of the knee and theproximal part of the leg. The sartorial branch takes a vertical courseas it travels down the medial knee before becoming subcutaneous

by piercing the fascia between the sartorius and gracilis tendons.The site where the nerve can cross the fascia is up to 37 mm proxi-mal to 30 mm distal to the joint line [5]. Then the sartorial branch islocated along the great saphenous vein in the subcutaneous fat of

28 B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37

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prolonged kneeling position [12] and in surfers who can com-press their nerves by gripping the surfboard between their knees[13]. Idiopathic nerve entrapment can occur at the exit of the

ig. 1. (A and B) Cadaveric dissections of the medial side of a right knee. The saphhen becomes superficial between the sartorius (S) and gracilis (G) tendons. The inf the anteromedial aspect of the knee. The sartorial branch (SB) lies close to the gr

he medial part of the leg and descends to the ankle. Its several col-ateral branches supply innervation to the skin of the medial aspectf the leg and a small portion of the medial arch of the foot and theallux [4].

.2. Normal imaging

Examination of the saphenous nerve is challenging because ofts small size. MR or US axial images obtained at the middle thirdf the thigh can show the nerve in the adductor canal using theartorius muscle and femoral vessels as landmarks. The saphenouserve is seen as a small, round structure within a fat plane run-ing on the deep surface of the sartorius muscle (Fig. 2). In a moreistal location, the sartorial branch can be seen in the medial kneeiercing the fascia between the sartorius and gracilis tendons toeach the subcutaneous tissues (Figs. 3–8). In the medial leg, theranches of the saphenous nerve can be visualized along the greataphenous nerve. Detection of the thin infrapatellar branch is diffi-ult. Its course runs in the medial and inferior subcutaneous tissuesf the knee.

.3. Pathologic conditions and imaging

Patients who suffer from saphenous neuropathy complain ofensory loss, paresthesias or pain in the medial aspect of the kneend the leg that can extend to the medial part of the foot and theallux [4].

Injury to the saphenous nerve accounts for the majority of

eurovascular complications after surgical medial knee incisions8]. It has been reported in 7–22% of meniscal repair and maynvolve either the infrapatellar or sartorial branches [5]. The inci-ence of infrapatellar branch injury in anterior cruciate ligament

nerve (SaN) lies deep to the sartorius muscle (S) which is displaced in figure A. Itellar branch (IPB) of the saphenous nerve supplies sensory innervation to the skinphenous vein (GSV).

reconstruction has been reported to be as much as 50% with theBone-Patellar-Tendon-Bone autograft technique and 30% to 59%with the harmstrings technique [9]. Direct blunt trauma, like dash-board injury, or accidental lacerations can also cause nerve injuryto the infrapatellar branch [10]. Saphenous neuropathy has alsobeen reported as a complication of pes anserinus bursitis [11],

Fig. 2. Axial ultrasound image at the distal third of the right thigh. The saphenousnerve can be easily found against the deep side of the sartorius muscle, along thefemoral artery (FA) (FV: femoral vein).

B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37 29

Figs. 3–8. Axial T1-weighted images (A) and axial cadaveric sections (B) of a right knee. The saphenous nerve lies proximally against the deep side of the sartorius muscle(Figs. 3 and 4). The sartorial branch continues the course of the saphenous nerve and pierces the fascia between the sartorius and gracilis tendons (Figs. 5 and 6) beforebecoming subcutaneous, along the great saphenous vein (Figs. 7 and 8). The infrapatellar branch is not visualized. The tibial, peroneal, saphenous and sural nerves can also

be seen (nerves, yellow: ( ) tibial nerve, ( ) common peroneal nerve, ( ) lateral sural nerve, ( ) medial sural nerve, ( ) saphenous nerve, ( ) superficial peronealnerve, ( ) deep peroneal nerve; veins, blue: ( ) great saphenous vein, ( ) small saphenous vein. S: sartorius, G: gracilis, ST: semi tendinous, SM: semi membranosus,B l bicepa

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F: femoral biceps, SHBF: short head of femoral biceps, LHBF: long head of femorand vein).

dductor canal (saphenous nerve), through the fascia between theartorius and gracilis tendons (sartorial branch) or through the sar-orius (infrapatellar branch) [6,14,15] (Table 1).

Neuromas of the saphenous nerve or its branches have beenescribed after trauma or surgery [8,10,16]. They can be visualizedith MR or US imaging (Fig. 9).

The saphenous nerve, infrapatellar nerve and sartorial nerve cane successfully blocked with the use of ultrasonographic guidance,

n Hunter’s canal, at the medial femoral condyle and along the greataphenous nerve respectively [17–19]. These techniques are nowommonly used to control post operative pain.

able 1tiologies of saphenous neuropathy at knee level.

Knee surgery+++TraumaCompression (Surfer’s neuropathy)Kneeling positionPes anserinus bursitisIdiopathic

s, GL and GM: lateral and medial heads of gastrocnemius, A and V: femoral artery

3. Tibial nerve

3.1. Normal anatomy

Formed by the L4–S3 nerve roots, the sciatic nerve is thelargest nerve in the body [2,3]. The nerve exits the greater sci-atic foramen as distinct tibial and peroneal divisions, enclosedin a common nerve sheath [20]. The peroneal and tibial divi-sions of the sciatic nerve physically split at or above the poplitealfossa to form the common peroneal nerve and the tibial nerve[21].

The tibial nerve is the medial and the largest terminal branchof the sciatic nerve. It provides motor innervation to the flexormuscles of the toes and ankle. Most of the sensory and motor inner-vation of the foot is supplied by the tibial nerve.

The tibial nerve branches off from the sciatic nerve at the apexof the popliteal fossa, which appears like a diamond-shaped fatty

space delimited by the biceps femoris and the semi membranosusmuscles superiorly and the two heads of the gastronecmius mus-cles inferiorly (Figs. 3–8 and 10). During its vertical course inthe popliteal fossa, the nerve is virtually median, typically just

30 B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37

Fig. 9. Axial T1-weighted (A) and PD-weighted fat-suppressed (B) images, sagittal PD-weighted fat-suppressed (C) and ultrasound (D) images of a right knee. Post surgeryn es suv

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euroma of the sarorial branch of the saphenous nerve at the point where it becomein, GSV: great saphenous vein).

osterior to the popliteal vein whereas the popliteal artery is ante-ior to the vein. At this location, the nerve is covered by the poplitealascia. At the end of the popliteal fossa, the tibial nerve runs pos-erior to the popliteal muscle and anterior to the fibrous arcadef the soleus muscle to enter the posterior compartment of theeg.

Collaterals of the tibial nerve in the knee region includehe posterior articular branch for the femoro-tibial joint, theerve supplying the popliteus muscle with a branch for theroximal tibiofibular joint, the nerves supplying the soleususcle, the medial and lateral gastrocnemius muscles, the cru-

al interosseous nerve and the medial sural cutaneous nerve1].

.2. Normal imaging

On MR axial images, the tibial nerve is relatively large, oval andf intermediate signal intensity on T1-weighted sequences and ofildly brighter signal intensity on fluid-sensitive sequences. Fas-

icles of the nerve can be appreciated on high-resolution axialmages [3]. Whereas MR imaging is usually limited to the kneeegion, the standard ultrasound examination of the tibial nerveenerally begins in the middle third of the thigh, where the dis-

perficial between the sartorius (S) and gracilis (G) tendons (SSV: small saphenous

tal portion of the sciatic nerve is identified and assessed [1].On US axial images, the sciatic nerve appears as an oval struc-ture with a fasciculated appearance [22]. Its mean cross-sectionalarea at this level is 53 mm2 (range 25–80 mm2) in healthy indi-viduals [23]. The nerve is medial to the biceps femoris musclewhich is the main anatomic landmark. The sciatic nerve mustbe assessed from the middle thigh since it can bifurcate prox-imally into its main branches. Rarely a small persistent sciaticartery can be seen following the sciatic nerve from the sciaticforamen to the level of the knee. Most commonly, the sciaticnerve divides into the tibial and peroneal nerves at the apex ofthe popliteal fossa [1]. The tibial nerve has a vertical and some-what median course, following the course of the sciatic nerve(Figs. 3–8). The nerve runs in a posterior position to the poplitealvein which serves as a useful landmark. The nerve is surroundedby fat between the biceps femoris and semimembranosus mus-cles. More distally the nerve continues its downward descent,posterior to the popliteal muscle, first between the lateral andmedial heads of the gastrocnemius muscle (Fig. 11) and finally

anterior to the fibrous arcade of the soleus muscle, accompaniedby popliteal vessels. The mean cross-sectional area of the tibialnerve is 35.3 mm2 (range 15–56 mm2), estimated with ultrasoundexamination [23].

B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37 31

Fig. 10. Cadaveric dissection of the popliteal fossa of a right knee. The sciatic nerve(SN) splits into the tibial nerve (TN), medial, and the common peroneal nerve (CPN),l(

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Table 2Etiologies of tibial neuropathy at knee level.

TraumaExtrinsic compression

Baker’s cystExtraneural ganglion cystPopliteal venous or artery aneurysmTumorPopliteus muscle strain or hypertrophyGastrocnemius hypertrophyAnomalous band between gastrocnemius heads

Nerve tumorIntraneural ganglion cystIatrogenic

Surgery

ateral. The lateral sural nerve (LSuN) originates from the common peroneal nerve�: popliteal artery).

.3. Pathologic conditions and imaging

Proximal tibial neuropathy is less common than peroneal neu-

opathy, owing to the deep location of the nerve and abundanturrounding fat and muscle tissue. It manifests as a weakness ofhe plantar fexors and invertors of the foot. Sensory loss is noted in

ig. 11. Axial ultrasound image of the posterior aspect of a right knee. The tibialerve (arrow), lies between the two heads of the gastrocnemius muscle with theopliteal vessels (A: artery, V: vein). From anterior to posterior: popliteal artery,opliteal vein and tibial nerve.

InjectionsOssificans neuritis

the heel and occasionally along the distribution of the sural nerve[21].

Several causes of tibial neuropathy in the knee area have beenreported. Most of them are related to space-occupying lesions inthe popliteal fossa, such as tumor, popliteal cyst, ganglion cyst,popliteal venous or artery aneurysm [21,24–28]. Surgery, localinjections or trauma have also been reported [29–31]. Some condi-tions are rare, such as ossificans neuritis, popliteal or gastrocnemiusmuscle hypertrophy, anomalous bands between gastrocnemiusheads or popliteal strain [32–36] (Table 2).

Imaging is useful for the depiction of a mass in the poplitealfossa and can play a crucial role in the assessment of tibial neu-ropathy. Direct signs are nerve swelling, US hypoechoic change orMR signal abnormality. Indirect signs of tibial neuropathy are repre-sented by edema or fatty infiltration of the posterior compartmentcalf muscles. These abnormalities are often evident with MR imag-ing, whereas they are more subtle with ultrasonography. Sparing ofhamstring muscles or the anterior and lateral compartment mus-cles can be helpful to distinguish tibial neuropathy from sciatic orplexus palsy [3].

Tibial nerve tumors consist habitually of nerve sheath tumorsand ganglion cysts. Schwannomas or neurofibromas of the tibialnerve are not infrequent (Fig. 12). Ganglion cysts of the tibial nerveare rare when compared with peroneal intraneural ganglion cysts.Tibial intraneural ganglion cysts can arise from an articular branchto the tibiofibular joint which is derived from the oblique descend-ing branch to the popliteus muscle, or from the extension of aperoneal intraneural cyst to the tibial nerve [24,37,38]. In the firstcase, the articular branch arises from the posterior aspect of thesuperior tibiofibular joint (Fig. 13). Similar to peroneal intraneu-ral ganglia, a ganglion cyst arising from the superior tibiofibularjoint can dissect the articular branch, which is the conduit fromthe joint to the tibial nerve [37,38]. Because the articular branch isderived from the descending branch to the popliteus muscle, dener-vation of popliteus muscle is an early marker of neuropathy. As thecompression neuropathy continues, denervation changes developin muscles innervated by the parent nerve [24]. Extraneural gangliaarising to the posterior portion of the superior tibiofibular joint canrarely compress the tibial nerve extrinsically [26].

4. Common peroneal nerve

4.1. Normal anatomy [21]

The common peroneal nerve (CPN) is the lateral terminal branch

of the sciatic nerve [2]. The CPN and its branches provide motorinnervation to the extensor muscles of the foot and toes and per-oneus muscles, and sensory innervation to the anterolateral aspectof the leg and the dorsal of the foot [1].

32 B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37

Fig. 12. Sagittal T1-weighted (A), PD-weighted fat-suppressed (B) and ultrasound (C) images. A tibial nerve tumor is seen at the lower part of the popliteal fossa, eccentricto the nerve trunk and surrounded by a fat plane. The tumor has a target-like appearance. These features are typical of a schwannoma of the tibial nerve.

Fig. 13. Sagittal PD-weighted fat-suppressed (A–C) and axial PD-weighted fat-suppressed (D) images of a right knee. The tibial ganglion cyst originates from the proximaltibio-fibular joint.

B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37 33

Fig. 14. Axial ultrasound image of the posterolateral aspect of a right knee. Thetibial nerve ( ) is medial to the lateral head of the gastrocnemius muscle (GL). Thecommon peroneal nerve ( ) and the lateral sural nerve ( ) are posterior to thel(

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Table 3Etiologies of peroneal neuropathy at knee level [3].

Direct traumaFibular head fractureCrush injuryKnee dislocationSurgery

Stretch injuryAnkle sprainRepetitive inversion and pronation (runners, cyclists, machine operators)

Extrinsic compressionProlonged immobilization (Saturday night palsy, anesthesia and surgery)Habitual leg crossing, especially following significant weight loss (slimmer’s

paralysis)Lithotomy position during childbirtha

Prolonged squatting (strawberry picker’s palsy)a

Tight castTumorsOsteochondromasSynovial or meniscal cystsArterial or venous aneurysmsIntraneural and extraneural gangliaFibrous bands of the deep or superficial heads of the peroneus longus muscleGastrocnemius muscle herniaBiceps femoris or lateral gastrocnemius muscle hypertrophyAberrant muscle

ateral head of the gastrocnemius muscle and medial to the biceps femoris muscleLFC: lateral femoral condyle).

During its course in the popliteal fossa, the CPN descendsbliquely downward and laterally along the medial border of theiceps femoris muscle and tendon. As it exits the popliteal fossa,he nerve runs superficial to the lateral head of the gastrocnemius

uscle (Figs. 3–8 and 10). At this location, the common peronealerve typically lies within abundant fat. Nevertheless, in approxi-ately 23% of individuals [39], the nerve travels in a narrow passage

etween the lateral head of the gastrocnemius muscle and theiceps femoris muscle. Nerve entrapment can occur in this tun-el in case of hypertrophy of the muscles or variant distal extent ofhe long or short head of the biceps femoris. The common peronealerve then winds around the neck of the fibula. Here the nerve isuperficial, covered only by subcutaneous nerve fat and skin, andhus is predisposed to direct compression. The CPN then pierces theateral intermuscular septum to enter the anterior compartment

f the leg, between the tendinous origins of the peroneus longususcle. At this level, tethering of the nerve may occur, making it

usceptible to stretch injury.

ig. 15. Axial ultrasound image of the lateral aspect of a right knee, at the level ofhe fibular head. The superficial and deep peroneal nerves (SPN and DPN) are closeo the fibular head, as they wind around it. They are covered by the peroneus longus

uscle (PL). The deep peroneal nerve is anterior and closer to the fibula than theuperficial peroneal nerve.

IdiopathicNerve tumor

a Bilateral symptoms.

The CPN trifurcates into the recurrent articular branch, super-ficial peroneal nerve (SPN) and deep peroneal nerve (DPN). Thistrifurcation most commonly occurs at or distal to the fibular neckbut can also occur above (10% of cases) or up to 3 cm below theknee joint (less than 9% of cases) [1,21]. The deep and superfi-cial branches of the CPN can usually be distinguished from eachother as they descend together in the posterior knee before theyphysically separate from one another. The DPN is situated moreanterior and hugs the fibula more closely relative to the SPN[3,21].

The SPN runs along the lateral aspect of the fibula between theperoneal longus muscle and the anterior intermuscular septum. Itinnervates both peroneal muscles. At the midpoint of the leg, it per-forates the crural fascia and enters the subcutaneous tissues. Thedeep peroneal nerve runs close to the fibular neck before perfo-rating the anterior intermuscular septum. It runs with the anteriortibial artery, initially passing between the tibialis anterior and theextensor digitorum longus muscles and more distally betweenthe tibialis anterior and the extensor hallucis longus muscles. Atthis point, its course continues anteriorly to the intermuscularmembrane, and it innervates the tibialis anterior, extensor longusdigitorum, and extensor hallucis longus muscles [1].

In the upper popliteal fossa, the peroneal division of the sci-atic nerve sends out the communicating peroneal nerve and lateralsural cutaneous nerve, which together with the medial sural cuta-neous nerve (branch of the tibial nerve) provide sensation to thelateral leg.

4.2. Normal imaging

Similarly to tibial nerve examination, ultrasound assessment ofthe common peroneal nerve begins proximally to the sciatic splitat the middle third of the thigh. Above the knee joint, the commonperoneal nerve can be traced as it migrates from the apex of thepopliteal fossa to a more lateral position posterior to the lateralhead of the gastrocnemius muscle (Figs. 3–8). During its course,

the nerve runs just medially to the short head and then the tendonof the biceps femoris muscle (Fig. 14). At this level the commonperoneal nerve is separated from the tibial nerve by a large amountof fat filling the popliteal space.

34 B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37

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ommon peroneal nerve (arrows).

Below the knee joint, the nerve runs posterior and then lateralo the fibular neck to enter the anterior compartment of the leg in

tunnel formed by the origins of the peroneal longus muscle. Theeep and superficial branches of the nerve usually diverge fromach other at the level or just proximal to this tunnel. Typically, theeep peroneal nerve is anterior to the superficial peroneal nervend hugs the fibula more closely (Fig. 15). A high-resolution probend a large amount of gel are useful because of the superficialocation of the nerve, especially around the fibular neck. With MRmaging, it is sometimes difficult to appreciate the CPN and itsranches as they travel around the fibular neck. The two branchesre better seen as they dive underneath the peroneus longususcle, especially in older or heavy patients, in whom a fat plane

an be identified between the muscle and adjacent fibula aroundhe fibular neck, deep to the peroneus longus muscle [3].

The CPN is habitually much smaller than the tibial nerve. Itsean cross-sectional area is 11.7 mm2 (range 2.5–20.9) in the

opliteal fossa and 11.2 mm2 (range 4.6–17.8) at the fibular neckith ultrasound examination [3].

.3. Pathologic conditions and imaging

Common peroneal neuropathy is the most frequent mononeu-opathy in the lower extremity. Patients often suffer frequentripping related to a footdrop. Pain may be present at the site ofompression. Sensory disturbances include paresthesia and anes-hesia along the lateral lower leg and dorsal foot. Upon physicalxamination, patients demonstrate weak foot extension (anterioribial muscle), weak foot eversion (peroneus longus and brevis

uscles), and loss of sensation in the lower lateral two-thirds ofhe leg and the dorsum of the foot.

Common peroneal neuropathy usually occurs as the nerverosses the fibular neck or as it pierces the peroneus longus muscle.he superficial location of the nerve and the fact that it is fixed at thebular neck make it susceptible to injury. There are many causes of

ommon peroneal neuropathy including trauma, postural habits,apid weight loss, intrinsic or extrinsic nerve tumors, compressiony a synovial cyst, extra- or intraneural ganglion, soft tissue tumor,sseous mass or large fabella [25,29,39–48] (Table 3).

An intraneural ganglion, derived from the tibiofibular proximal joint, dissects the

Intraneural ganglia is a classic cause of neuropathy and has beenwell described [48,49]. The peroneal intraneural ganglion cyst is themost common, derived from the anterior portion of the superiortibiofibular joint (Fig. 16). It dissects proximally along the recur-rent articular branch and typically extends into the deep peronealcomponent of the common peroneal nerve, producing the charac-teristic symptom of footdrop (from the preferential compressionof the deep peroneal fascicles). With increased intra-articular pres-sure, the intraneural cyst may extend into the peroneal division ofthe sciatic nerve, in some cases reaching the buttock [50]. Occasion-ally, extraneural cysts arising from the superior tibiofibular jointmay cause extrinsic compression of the peroneal nerve. Intraneu-ral ganglia and the occasional concomitant adventitial cysts of thearticular branches of the anterior tibial vessels should not be misin-terpreted as varicosities. When communication between the jointand the ganglion cyst cannot be established using ultrasound or MRimaging, one can perform CT or MR arthrography. Nerve entrap-ment is possible in the fibrous tunnel underneath the peroneuslongus muscle. Diabetes mellitus increases the risk of entrapmentin that location [3]. Nerve entrapment can occasionally occur ina narrow fatty tunnel between the lateral head of the gastroc-nemius muscle and the short head of the biceps femoris muscle,especially in the case of anatomic variation or muscle hypertrophy[39]. Traumatic injury may occur secondary to a fibular head frac-ture such as Maisonneuve fracture, knee dislocation, ankle sprainswith stretching of the nerve, surgical procedures and application ofskeletal traction or a tight cast. Repetitive inversion and pronation,such as occurs in runners, cyclists and machine operators, may alsostretch the nerve against the fibular head and the fibrous arch ofthe peroneal tunnel [3].

In cases of compressive neuropathy, imaging can show the nervedeviated by various masses, such as a tumor, ganglion, or anoma-lous muscle. Swelling of the nerve is a frequent finding. On MRimaging, increased signal intensity within the common peronealnerve can be easily appreciated on axial fluid-sensitive images.

On the contrary, in many instances the cause of the neuropathyis not evident, especially in cases of strain neuropathy [51]. In suchcases, indirect signs of common peroneal neuropathy, includingedema or fatty infiltration within muscles from anterior and lateral

B. Damarey et al. / European Journal of Radiology 82 (2013) 27– 37 35

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ig. 17. Axial ultrasound images of the anterior compartment of the leg (A) and teft foot drop a few months after an ankle sprain. Although the anterior compartmeeroneal nerve.

ompartments of the leg, are the only remarkable abnormalitiesFig. 17).

Imaging can also play an important role in evaluating chronic

eg pain in the athlete and in differentiating CPN entrapment fromther common causes of chronic leg pain, including stress fracture,hronic exertional compartment syndrome, and popliteal arteryntrapment syndrome.

ig. 18. Axial PD-weighted fat-suppressed (A), Sagittal PD-weighted fat-suppressed (B), Toma of the lateral sural nerve. The lateral ( ) and medial ( ) sural nerves can be seeerve).

mon peroneal nerve at the level of the fibular neck (B). The patient suffers fromthe leg shows fatty infiltration (�), there is no abnormality of the common or deep

5. Sural nerve

5.1. Normal anatomy

The sural nerve is a purely sensory nerve that innervates thelateral aspect of the ankle, heel and the lateral border of the foot.Typically, it begins as a branch of the tibial nerve (medial sural

1-weighted (C) and ultrasound (DC) images image of a right knee. Typical schwan-n superficially, covered by the deep fascia ( : tibial nerve, : common peroneal

36 B. Damarey et al. / European Journal

Table 4Etiologies of sural neuropathy [52].

Baker’s cystExternal compression or blowTumorsGangliaFracture of the fifth metatarsalFibrosis after ankle sprain or other traumaIntraneural gangliaOsteochondroma

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Myositis ossificansIatrogenic (small saphenous vein ablation or Achilles tendon repair procedure)

erve) in the popliteal fossa and descends between the heads ofhe gastrocnemius beneath the deep fascia [52]. It then exits theeep fascia at the level of the midcalf and is joined by the peronealommunicating nerve which is a branch of the lateral sural nerver the lateral sural nerve itself. The sural nerve then lies in closeroximity to the small saphenous vein [53]. This description, inhich the sural nerve has contributions from both the medial and

he lateral sural cutaneous nerves, accounts for 40–84% of casesFig. 17). Less commonly, the sural nerve may originate directlyrom the tibial nerve (13–54% of cases) or from the common per-neal nerve (0–16% of cases) (Figs. 3–8 and 10) [52,54]. The pointhere the medial and lateral sural cutaneous nerves meet is also

ariable: it occurs most often in the lower third of the leg but canlso occur in the popliteal fossa, the middle third of the leg or at thenkle [53,55]. The nerve continues distally and laterally alongsidehe Achilles tendon, and then curves inferior to the lateral malle-lus and passes along the lateral border of the foot, splitting intoedial and lateral branches near the base of the fifth metatarsal

52].

.2. Normal imaging

Ultrasonography is more suitable for sural nerve assessmenthan MRI imaging, except in cases of nervous tumor, owing to themall size and the superficial location of the nerve. At the level ofhe midcalf, the nerve usually lies near the small saphenous vein,

ost frequently laterally to the vein. Its diameter is about 1–2 mm.nce the nerve has been identified, the nerve course can be fol-

owed proximally until it splits into its two origins. The lateral suralerve joins the common peroneal nerve in the fat plane betweenhe biceps femoris tendon and the lateral head of the gastrocne-

ius muscle (Fig. 4A). The medial sural nerve joins the tibial nerven the middle of the popliteal fossa. Variant anatomical patterns arerequent and only the tibial or the common peroneal componentan be visible proximally (Figs. 3–8) [53].

.3. Pathologic conditions and imaging

Patients with sural neuropathy suffer pain and paresthesiaslong the lateral foot, which are often worse at night and are exac-rbated by exercise. Upon examination, symptoms can be provokedy plantar flexion and inversion of the foot.

Entrapment of the sural nerve occurs almost exclusively inhe leg or in the foot secondary to trauma, such as direct con-usion or fracture of the distal fibula, talus, calcaneus or base ofhe fifth metatarsal, surgery such as small saphenous vein abla-ion or Achilles tendon repair procedure (Table 4). Other causesnclude severe ankle sprains, gastrocnemius tear, tendinosis of theeroneal or Achilles tendons or osteochondroma [52,56]. An intra-eural ganglion derived from the calcaneocuboid joint has been

escribed [52].

Sural nerve entrapment in the popliteal fossa is rare butan occur secondary to space-occupying lesions like Baker’s cystBRYAN). Nerve tumors like schwannoma can also be seen (Fig. 18).

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of Radiology 82 (2013) 27– 37

6. Conclusion

Nerves around the knee can be assessed with MR and ultrasoundimaging. Entrapment neuropathies of the knee most commonlyaffect the common peroneal nerve. MR may show direct nerve dis-ease and indirect signs of muscle denervation. Ultrasound is moresuitable to image superficial small nerves like the saphenous or thesural nerve. Nerve assessment should be included in standard kneeimaging, especially when clinical symptoms are equivocal.

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