DOI: 10.1148/rg.233025053
(Radiographics. 2003;23:613-623.)
© RSNA, 2003
Imaging of Foot and Ankle Nerve Entrapment Syndromes: From Well-demonstrated to Unfamiliar Sites1
Emmanuelle M. Delfaut, MD,
Xavier Demondion, MD, PhD,
Anne Bieganski, MD,
Marie-Camille Thiron, MD,
Henry Mestdagh, MD and
Anne Cotten, MD, PhD
1 From the Departments of Skeletal Radiology (E.M.D., X.D., A.B., M.C.T., A.C.) and Orthopaedic Surgery (H.M.), Roger Salengro Hospital, CHRU of Lille, Bd du Professeur Jules Leclercq, 59037 Lille Cedex, France; and the Department of Anatomy, Faculty of Medicine, Place de Verdun, Lille, France (X.D., H.M.). Recipient of a Certificate of Merit award for an education exhibit at the 2001 RSNA scientific assembly. Received March 11, 2002; revision requested May 3; final revision received July 30; accepted August 1. Address correspondence to E.M.D. (e-mail: philmanu@wanadoo.fr).
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Abstract
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Nerve entrapment at the foot and ankle involves thin and complex anatomic structures and is underdiagnosed because clinical symptoms and electrophysiologic findings may not contribute to the diagnosis. Nerve entrapment can be secondary to acute trauma or repetitive microtrauma. The latter often results from intensive sports-related activity, inappropriate footwear, or internal foot derangement. Various lesions that occur in fibro-osseous tunnels can cause nerve compression (eg, ganglion cysts, varicosities, bone and joint abnormalities, tumors, tenosynovitis, supernumerary or hypertrophic muscles). Accurate nerve examination must be performed, particularly in patients with atypical ankle pain, to detect focal tenderness or paresthesia. Ultrasonography is useful in this setting because it yields both clinical and morphologic findings. High-resolution magnetic resonance imaging provides accurate delineation of the nervous system anatomy. Furthermore, technologic developments in the field of radiology are making it possible to obtain clearer, more accurate images. Radiologists must be aware of the main nerve entrapment syndromes at the foot and ankle and be able to perform accurate nerve examinations with different imaging modalities in patients with foot and ankle pain.
© RSNA, 2003
Index Terms: Ankle, anatomy, 46.92 • Ankle, injuries, 46.40 • Ankle, MR, 46.12141 • Foot, anatomy, 46.92 • Foot, injuries, 46.40 • Foot, MR, 46.12141 • Nerves, injuries
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Describe the most common nerve entrapment syndromes at the foot and ankle.
- Correlate clinical findings with MR imaging and US findings in these nerve entrapment syndromes.
- Recognize the most frequent clinical causes of nerve dysfunction at the foot and ankle.
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Introduction
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Diagnosis of nerve entrapment relies mainly on clinical examination and electrodiagnosis. However, because the anatomy of foot and ankle nerves is variable, electrodiagnosis is not always reliable. Clinical examination can be very contributive in patients who complain of typical neuritis. Reproduction of pain at palpation and percussion of the nerve is a useful clinical and ultrasonographic (US) sign. However, sometimes only subjective symptoms (paresthesia, pain) in the nerve territory (Fig 1) or symptoms accompanied by very few objective motor or sensory signs are present. Magnetic resonance (MR) imaging is often requested for patients who complain of posttraumatic ankle pain. In these cases, the possibility of nerve injury must also be considered. Therefore, careful assessment of the various nerve entrapment sites must be made.
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Figure 1a. (a) Drawing of the dorsal aspect of the foot illustrates the territories of the deep peroneal nerve (DPN), lateral plantar nerve (LPN), medial plantar nerve (MPN), sural nerve (SN), and superficial peroneal nerve (SPN). (b) Drawing of the plantar aspect of the foot illustrates the territories of the lateral calcaneal nerve (LCN), lateral plantar nerve (LPN), medial calcaneal nerve (MCN), medial plantar nerve (MPN), and sural nerve (SN).
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Figure 1b. (a) Drawing of the dorsal aspect of the foot illustrates the territories of the deep peroneal nerve (DPN), lateral plantar nerve (LPN), medial plantar nerve (MPN), sural nerve (SN), and superficial peroneal nerve (SPN). (b) Drawing of the plantar aspect of the foot illustrates the territories of the lateral calcaneal nerve (LCN), lateral plantar nerve (LPN), medial calcaneal nerve (MCN), medial plantar nerve (MPN), and sural nerve (SN).
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Local disturbance of nerve function can be secondary to numerous clinical situations. Laceration or complete destruction of all the neural elements and the perineural connective tissue (epineurium, perineurium, endoneurium) may cause irreversible neural dysfunction (1). Neural lesions without perineural connective tissue involvement may occur secondary to acute trauma or overuse injury. With appropriate treatment, such lesions can resolve within 3 weeks to 6 months. However, in higher-grade lesions, either perineural fibrosis requiring surgical decompression or irreversible nerve disruption may occur. The most frequent injury mechanisms are nerve compression and nerve tension (2–4). Compression neuropathy can occur secondary to trauma. The nerve may be compressed by a bone fragment, hematoma, or soft-tissue edema. It can also be accidentally transected during surgery, entrapped in scar tissue, or compressed by a callus or by osteosynthetic material. A variety of lesions that occur in fibro-osseous tunnels can also cause nerve compression, the most frequently encountered of which are ganglion cysts, varicosities, bone and joint abnormalities, tumors, tenosynovitis, and supernumerary or hypertrophic muscles. Inappropriate footwear (narrow shoes, tight laces) may also cause nerve compression. Ankle sprain (acute or repetitive), internal foot derangement (flatfoot, hyperpronation), or high-heeled shoes can cause tension neuropathy. Regardless of the entrapment sites, a trial of nonsurgical treatment should be instituted (2–4), which can include rest, anti-inflammatory medication, adapted footwear, and orthoses or braces to stabilize the ankle. Physical therapy may be helpful in some cases. Local injection of lidocaine and cortisone at the point of maximum tenderness in the nerve area, but not into the nerve itself, may provide some relief. This injection, which may be performedunder US guidance, also is a diagnostic test. When a patient does not respond to conservative treatment, surgical nerve release may be indicated.
In this article, we review the anatomy of the nerves involved in foot and ankle entrapment syndromes, including the posterior tibial nerve (PTN) and its branches, the sural nerve, the deep peroneal nerve (DPN), the superficial peroneal nerve (SPN), and the nerves of the forefoot. We also discuss and illustrate various nerve entrapment sites and the factors that predispose to entrapment, particularly in athletes.
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PTN and Its Branches
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The PTN and its branches (Fig 2) are often affected at the point where they course through the tarsal tunnel. However, other entrapment sites should also be considered, particularly those that involve the inferior calcaneal nerve. The tarsal tunnel is a fibro-osseous space that extends from the posteromedial aspect of the ankle to the plantar aspect of the foot (5,6). It is divided into an upper (tibiotalar) compartment and a lower (talocalcaneal) compartment. The upper tarsal tunnel (Fig 3) is covered by the deep aponeurosis and has an osseous floor that is formed by the posterior aspect of the tibia and the talus. It comprises the posterior tibial tendon, flexor digitorum longus tendon, posterior tibial neurovascular bundle (which is located more superficially), and flexor hallucis longus tendon and muscle. The lower tarsal tunnel (Fig 4) is covered by the flexor retinaculum (created by the fusion of the superficial and deep aponeuroses of the leg) and the abductor hallucis muscle with its fascia. Its osseous floor is formed by the posteromedial aspect of the talus, the inferomedial aspect of the navicular bone, and the medial aspects of the sustentaculum tali and calcaneus. These osseous structures are partly covered by the quadratus plantae muscle. In the tarsal tunnel, the PTN trifurcates into its terminal branches 1.3–1.5 cm proximal to the tip of the medial malleolus (Fig 2) (7). These branches are the medial and lateral plantar nerves (Fig 4) and the medial calcaneal nerve. The medial and lateral plantar neurovascular bundles are separated by the interfascicular septum. The medial calcaneal nerve can also arise from the lateral plantar nerve (8). In a few cases, trifurcation of the PTN occurs superior to the tarsal tunnel.
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Figure 3a. Normal anatomy of the upper tarsal tunnel. (a) Axial spin-echo T1-weighted MR image (repetition time msec/echo time msec = 500/12) shows the lateral plantar nerve (black arrowhead), medial plantar nerve (black arrow), posterior tibial artery and veins (white arrow), flexor hallucis longus tendon and muscle (FHL), deep aponeurosis (white arrowheads), and tibia (T). (b) Corresponding transverse 13.5-MHz US scan shows the PTN branches (circled), posterior tibial artery and veins (thick white arrow), flexor digitorum longus tendon (black arrow), and posterior tibial tendon (thin white arrow).
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Figure 3b. Normal anatomy of the upper tarsal tunnel. (a) Axial spin-echo T1-weighted MR image (repetition time msec/echo time msec = 500/12) shows the lateral plantar nerve (black arrowhead), medial plantar nerve (black arrow), posterior tibial artery and veins (white arrow), flexor hallucis longus tendon and muscle (FHL), deep aponeurosis (white arrowheads), and tibia (T). (b) Corresponding transverse 13.5-MHz US scan shows the PTN branches (circled), posterior tibial artery and veins (thick white arrow), flexor digitorum longus tendon (black arrow), and posterior tibial tendon (thin white arrow).
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Figure 4. Normal anatomy of the lower tarsal tunnel. Axial spin-echo T1-weighted MR image (500/12) shows the posterior tibial artery and veins (thin white arrow), flexor digitorum longus tendon (thick white arrow), flexor hallucis longus tendon (black arrowhead), medial plantar nerve (black arrow), lateral plantar nerve (circled), and flexor retinaculum (white arrowheads).
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Patients with tarsal tunnel syndrome may complain of paresthesia or burning pain at the plantar aspect of the foot and toes. However, depending on the compression site and the nerve branch involved, clinical symptoms can be more focal, localized to the medial plantar aspect of the heel (Fig 1) or radiating proximally along the medial calf. The main clinical finding in tarsal tunnel syndrome is the Tinel sign: Gentle percussion over the course of the nerve causes pain or paresthesia that radiates in the sensory distribution of the nerve. Muscle weakness is a late and infrequent outcome (9–13). Compression neuropathy secondary to trauma or various lesions (Figs 5, 6) and tension neuropathy are the major mechanisms responsible for tarsal tunnel syndrome (14). Foot deformities that are more often involved in tension neuropathies of the PTN and its branches are varus deformity of the heel with pronated forefoot and valgus heel with abducted forefoot in flatfoot deformities (14). Tension on the PTN is significantly increased in ankles in dorsiflexion and hindfoot eversion. Medial and lateral plantar nerve branches can be entrapped distally by various abnormalities such as synovial cysts (Fig 7), ganglion cysts, bone and joint abnormalities, tumors, and tenosynovitis.
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Figure 5a. Tarsal tunnel syndrome in a 33-year-old man with pain radiating to the plantar aspect of the foot and toes. (a) Axial proton-density-weighted turbo spin-echo MR image (3,500/17) shows a ganglion cyst (*) in the tarsal tunnel. The cyst is responsible for compression of the medial plantar nerve branch (arrows). The lateral plantar nerve is circled. (b) Axial contrast material-enhanced fat-saturated spin-echo T1-weighted MR image (700/12) shows the ganglion cyst in the tarsal tunnel (*) surrounded by soft-tissue contrast enhancement (arrow).
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Figure 5b. Tarsal tunnel syndrome in a 33-year-old man with pain radiating to the plantar aspect of the foot and toes. (a) Axial proton-density-weighted turbo spin-echo MR image (3,500/17) shows a ganglion cyst (*) in the tarsal tunnel. The cyst is responsible for compression of the medial plantar nerve branch (arrows). The lateral plantar nerve is circled. (b) Axial contrast material-enhanced fat-saturated spin-echo T1-weighted MR image (700/12) shows the ganglion cyst in the tarsal tunnel (*) surrounded by soft-tissue contrast enhancement (arrow).
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Figure 6. Calcaneal fracture in a 40-year-old patient with pain radiating to the plantar aspect of the foot. Axial CT scan shows a bone fragment (arrowhead) and soft-tissue edema adjacent to the lateral plantar nerve branches (arrow). C = calcaneus.
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Figure 7a. (a) Normal anatomy of the lateral and medial plantar nerves. Coronal oblique T1-weighted MR image (500/12) obtained at the level of the metatarsal bases shows the medial plantar nerve branches (circled), oblique head of the adductor hallucis muscle (thick arrow), flexor longus hallucis and flexor digitorum longus tendons (arrowheads), flexor digitorum brevis muscle (FDB), lateral plantar nerve branches (thin arrow), abductor digiti quinti muscle (ADQM), and abductor hallucis muscle (AH). 1rst M = first metatarsal. (b, c) Distal entrapment of the medial and lateral plantar nerve branches in a 60-year-old patient with plantar midtarsal pain radiating to the medial plantar aspect of the toes. Contiguous coronal T2-weighted MR images (3,500/119) show an arthrosynovial cyst (* in b) that is responsible for compression of the medial plantar nerve branches (circled in c).
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Figure 7b. (a) Normal anatomy of the lateral and medial plantar nerves. Coronal oblique T1-weighted MR image (500/12) obtained at the level of the metatarsal bases shows the medial plantar nerve branches (circled), oblique head of the adductor hallucis muscle (thick arrow), flexor longus hallucis and flexor digitorum longus tendons (arrowheads), flexor digitorum brevis muscle (FDB), lateral plantar nerve branches (thin arrow), abductor digiti quinti muscle (ADQM), and abductor hallucis muscle (AH). 1rst M = first metatarsal. (b, c) Distal entrapment of the medial and lateral plantar nerve branches in a 60-year-old patient with plantar midtarsal pain radiating to the medial plantar aspect of the toes. Contiguous coronal T2-weighted MR images (3,500/119) show an arthrosynovial cyst (* in b) that is responsible for compression of the medial plantar nerve branches (circled in c).
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Figure 7c. (a) Normal anatomy of the lateral and medial plantar nerves. Coronal oblique T1-weighted MR image (500/12) obtained at the level of the metatarsal bases shows the medial plantar nerve branches (circled), oblique head of the adductor hallucis muscle (thick arrow), flexor longus hallucis and flexor digitorum longus tendons (arrowheads), flexor digitorum brevis muscle (FDB), lateral plantar nerve branches (thin arrow), abductor digiti quinti muscle (ADQM), and abductor hallucis muscle (AH). 1rst M = first metatarsal. (b, c) Distal entrapment of the medial and lateral plantar nerve branches in a 60-year-old patient with plantar midtarsal pain radiating to the medial plantar aspect of the toes. Contiguous coronal T2-weighted MR images (3,500/119) show an arthrosynovial cyst (* in b) that is responsible for compression of the medial plantar nerve branches (circled in c).
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The medial calcaneal nerve (Fig 8) arises from the posterior tibial or lateral plantar nerve (8,15). It pierces the flexor retinaculum and divides into multiple superficial sensory branches that innervate the skin covering the medial aspect of the Achilles tendon, the posteromedial aspect of the heel, and the plantar fat pad (Fig 1).
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Figure 8a. (a) Normal anatomy of the inferior calcaneal nerve. Axial T1-weighted MR image (400/12) shows the lateral plantar nerve (white arrowhead), flexor retinaculum (black arrowheads), inferior calcaneal nerve (black arrow), and medial calcaneal nerve and vessels (white arrow). (b, c) Suspected neuropathy of the inferior calcaneal nerve. Coronal T1-weighted MR images (500/12) of the ankle were obtained in a healthy volunteer (b) and a 55-year-old patient with posterior tibial tendinopathy, flatfoot, and painful heel (c). Note the selective fatty atrophy (arrow in c) of the abductor digiti quinti muscle (ADQM) on the pathologic MR image compared with the normal image. AH = abductor hallucis muscle, FDB = flexor digitorum brevis muscle.
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Figure 8b. (a) Normal anatomy of the inferior calcaneal nerve. Axial T1-weighted MR image (400/12) shows the lateral plantar nerve (white arrowhead), flexor retinaculum (black arrowheads), inferior calcaneal nerve (black arrow), and medial calcaneal nerve and vessels (white arrow). (b, c) Suspected neuropathy of the inferior calcaneal nerve. Coronal T1-weighted MR images (500/12) of the ankle were obtained in a healthy volunteer (b) and a 55-year-old patient with posterior tibial tendinopathy, flatfoot, and painful heel (c). Note the selective fatty atrophy (arrow in c) of the abductor digiti quinti muscle (ADQM) on the pathologic MR image compared with the normal image. AH = abductor hallucis muscle, FDB = flexor digitorum brevis muscle.
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Figure 8c. (a) Normal anatomy of the inferior calcaneal nerve. Axial T1-weighted MR image (400/12) shows the lateral plantar nerve (white arrowhead), flexor retinaculum (black arrowheads), inferior calcaneal nerve (black arrow), and medial calcaneal nerve and vessels (white arrow). (b, c) Suspected neuropathy of the inferior calcaneal nerve. Coronal T1-weighted MR images (500/12) of the ankle were obtained in a healthy volunteer (b) and a 55-year-old patient with posterior tibial tendinopathy, flatfoot, and painful heel (c). Note the selective fatty atrophy (arrow in c) of the abductor digiti quinti muscle (ADQM) on the pathologic MR image compared with the normal image. AH = abductor hallucis muscle, FDB = flexor digitorum brevis muscle.
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Repetitive microtraumas can be responsible for medial calcaneal neuropathy in joggers. Fat pad atrophy, particularly in association with sudden weight loss or diabetes, can also be involved in medial calcaneal neuropathy (16).
The inferior calcaneal nerve (Fig 8) is a mixed nerve (8,15). It sends out small motor branches to the flexor digitorum brevis, quadratus plantae, and abductor digiti quinti muscles as well as sensory fibers to the long plantar ligament and calcaneal periosteum. The inferior calcaneal nerve originates from the lateral plantar nerve at the level of the medial malleolus, courses between the abductor hallucis muscle and the quadratus plantae and along the medial border of the long plantar ligament, then turns laterally to send out motor branches to the abductor digiti quinti muscle. It also gives rise to a sensory branch for the anterior calcaneal tubercle.
Compression of the inferior calcaneal nerve (Baxter neuropathy) may occur secondary to microtrauma (calcaneal spur, internal foot derangement) or to plantar fasciitis, particularly if associated with a flexor digitorum brevis muscle and soft-tissue edema. The patient experiences medial heel pain that may be indistinguishable from pain secondary to plantar fasciitis (Fig 9). Surgical nerve release can be proposed if conservative treatment is ineffective (17). An interesting MR imaging sign secondary to this neuropathy is isolated fatty atrophy of the abductor digiti quinti muscle (Fig 8c) (18).
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Figure 9. Plantar fasciitis in a patient with heel pain who was not responding to local injection of corticosteroids and anesthetics. Coronal contrast-enhanced fat-saturated spin-echo T1-weighted MR image (700/12) shows plantar fasciitis (arrow) with soft-tissue contrast material uptake in the area of the inferior calcaneal nerve course (circled). ADQM = abductor digiti quinti muscle, FDB = flexor digitorum brevis muscle.
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Jogger’s foot is related to entrapment of the medial plantar nerve branches in a narrow space located between the abductor hallucis muscle and the anatomic crossover between the flexor hallucis longus and flexor digitorum longus tendons (master Knot of Henry) (Fig 10).
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Figure 10. Jogger’s foot. Coronal spin-echo T1-weighted MR image (500/12) shows entrapment of the medial plantar nerve branches (circled) in a narrow space between the abductor hallucis muscle (AH) and the flexor digitorum longus and flexor hallucis longus tendons (arrow). ADQM = abductor digiti quinti muscle, FDB = flexor digitorum brevis muscle, QP = quadratus plantae muscle.
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Running with excessive pronation or a high medial arch can predispose to these injuries. Patients experience pain or dysesthesia at the plantar aspect of the first and second toes. The point of maximum tenderness and the Tinel sign are located at the medial arch (19,20).
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Sural Nerve
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At the ankle, the sural nerve first runs lateral to the Achilles tendon, then inferior to the peroneal tendon sheath (Fig 11). Above the lateral malleolus, it supplies the lateral calcaneal branches, which in turn supply sensory innervation to the lateral aspect of the heel (Fig 1). At the level of the fifth metatarsal base, the sural nerve bifurcates into lateral and medial terminal branches. It provides sensation to the lateral aspects of the ankle, heel, and foot (Figs 1, 2) (5,21).
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Figure 11. Normal anatomy of the sural nerve. Proximal coronal oblique spin-echo T1-weighted MR image (400/12) shows the sural nerve and vessels (thin arrow) near the peroneus brevis (arrowhead) and peroneus longus (thick arrow) tendons.
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Patients with sural neuropathy suffer from pain and paresthesia of the lateral border of the ankle and foot. Nerve entrapment can occur secondary to acute trauma, particularly a fifth metatarsal base fracture, but also a peroneal, calcaneal, or cuboid fracture (2–4,16). In the acute phase, a bone fragment, hematoma, or soft-tissue edema may be responsible for nerve compression. In the chronic phase, compression can be secondary to callus, scarring, or the presence of osteosynthetic material. Peroneal tendon sheath disease or Achilles tendinopathy may also cause nerve compression. Recurrent ankle inversion sprain may be responsible for tension neuropathy. Secondary to ankle surgery performed with a posterolateral approach (os trigonum syndrome, peroneal tendon subluxation surgery), a branch of the sural nerve may be transected or entrapped in the scar.
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Deep Peroneal Nerve
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The DPN courses under the extensor retinaculum (Fig 12) between the extensor digitorum longus and extensor hallucis longus tendons and lateral to the anterior tibial artery. It divides about 1.3 cm above the ankle joint into a medial sensory branch for the first interspace (Fig 1) and a lateral motor branch for the extensor digitorum brevis muscle (5,7).
There are two main compression sites of the DPN. The nerve may be compressed at the anterior tarsal tunnel, resulting in anterior tarsal tunnel syndrome (Fig 13). This syndrome is secondary to DPN compression at the inferior extensor retinaculum where the extensor hallucis longus tendon crosses over it (22). Both sensory and motor DPN branches are involved. The pain is located at the dorsomedial aspect of the foot and is worst at rest. Weakness of the extensor digitorum brevis muscle may be evident. The DPN may also be compressed at the dorsum of the foot (Fig 14). The sensory component of the DPN is located in a tight tunnel beneath the extensor hallucis brevis tendon and the deep fascia at the level of the first and second tarsometatarsal joints. Contusion, soft-tissue swelling, tight footwear or a high longitudinal arch, and naviculocuneiform or cuneometatarsal osteophytes can cause acute or chronic nerve compression at this site. Sports-related injuries to the DPN have been described in skiers with tight boots (23) and dancers in pointe position, as well as in soccer players secondary to multiple blows to the dorsum of the foot (24,25).
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Figure 13a. (a) Normal anatomy of the anterior tarsal tunnel. Proton-density-weighted MR image (3,500/19) shows the anterior tibial tendon (large arrowhead), extensor hallucis longus muscle and tendon (thick arrow), inferior extensor retinaculum (small arrowheads), dorsal pedis artery, and lateral and medial terminal branches of the DPN (thin arrows). (b) Inflammatory arthritis and multiple tenosynovitis in a patient with vague foot pain and swelling. Contrast-enhanced fat-saturated T1-weighted MR image (700/12) shows tenosynovitis of the extensor digitorum longus tendon (arrowhead) and contrast material uptake surrounding the deep peroneal neurovascular bundle (arrow).
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Figure 13b. (a) Normal anatomy of the anterior tarsal tunnel. Proton-density-weighted MR image (3,500/19) shows the anterior tibial tendon (large arrowhead), extensor hallucis longus muscle and tendon (thick arrow), inferior extensor retinaculum (small arrowheads), dorsal pedis artery, and lateral and medial terminal branches of the DPN (thin arrows). (b) Inflammatory arthritis and multiple tenosynovitis in a patient with vague foot pain and swelling. Contrast-enhanced fat-saturated T1-weighted MR image (700/12) shows tenosynovitis of the extensor digitorum longus tendon (arrowhead) and contrast material uptake surrounding the deep peroneal neurovascular bundle (arrow).
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Figure 14. Normal anatomy of the distal DPN branches. Coronal oblique spin-echo T1-weighted MR image (400/12) shows the medial branch of the DPN (for the first dorsal space) (thick arrow) under the extensor hallucis longus tendon (arrowhead) and the lateral branch of the DPN (thin arrow), which supplies motor innervation to the extensor digitorum brevis muscle (EDB). N = navicular bone.
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Superficial Peroneal Nerve
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The SPN (Figs 12, 15) is a branch of the common peroneal nerve. The SPN pierces the deep fascia of the leg about 12.5 cm above the tip of the lateral malleolus (5,7). Before it divides into its terminal branches roughly 6 cm above the tip of the lateral malleolus, the SPN sends out collateral motor branches to the peroneal brevis and peroneal longus muscles. It provides sensory innervation to the dorsolateral aspect of the foot and ankle (Fig 1).
The SPN is tethered as it pierces the deep fascia of the leg. It may be overstretched during inversion or plantar flexion injuries. This overstretching can be responsible for tension neuropathy, perineural fibrosis, and chronic ankle pain (26). If the SPN pierces the deep fascia more distally, it has limited ability to stretch or to move laterally or medially. This may be a predisposing factor to nerve injury in some individuals (27).
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Forefoot Nerves
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At the dorsum of the foot, the first interspace is innervated by sensory branches of the DPN. The other spaces are innervated by branches of the SPN (Fig 16). The medial aspect of the hallux is innervated by the medial plantar hallucis and medial dorsal hallucis nerves, which are sensory branches from the medial plantar nerve and the SPN, respectively (Figs 1, 16). The medial hallucal nerves may become irritated by bunions, osteophytes, or tophi (Fig 17), thereby causing first toe pain that sometimes radiates proximally. Focal medial plantar hallucal neuritis is part of the differential diagnosis for sesamoiditis (2).
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Figure 16a. Normal forefoot nerve anatomy. (a) Distal coronal spin-echo T1-weighted MR image (400/12) demonstrates the branches of the DPN for the first dorsal space (circled) and the branches of the SPN for the second through the fourth dorsal spaces (arrows). (b) Distal coronal spin-echo T1-weighted MR image (400/12) shows the medial dorsal (thin arrow) and medial plantar (thick arrow) hallucal nerves.
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Figure 16b. Normal forefoot nerve anatomy. (a) Distal coronal spin-echo T1-weighted MR image (400/12) demonstrates the branches of the DPN for the first dorsal space (circled) and the branches of the SPN for the second through the fourth dorsal spaces (arrows). (b) Distal coronal spin-echo T1-weighted MR image (400/12) shows the medial dorsal (thin arrow) and medial plantar (thick arrow) hallucal nerves.
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Figure 17. Forefoot neuropathy in a 60-year-old patient with gout. Oblique radiograph of the forefoot shows a large tophus (arrowheads). The medial dorsal and plantar hallucal nerves may become entrapped by a tophus, as in this case.
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At the plantar aspect of the foot, the first through the third common digital nerves are branches of the medial plantar nerve and the fourth common digital nerve is a branch of the lateral plantar nerve.
Morton neuroma (Fig 18) is a well-known disorder of these digital nerves. It results from chronic nerve entrapment under the intermetatarsal ligament, particularly at the second and third intermetatarsal spaces (28–30). If the diagnosis of Morton neuroma is clinically suspected, imaging can help define the location of the neuroma, particularly when pain is atypical. US is a useful imaging method for evaluating Morton neuroma. However, small lesions (<5 mm) are better demonstrated with MR imaging (31). Interdigital nerve entrapment and neuritis can also result from conditions other than Morton neuroma: acute sprain or tearing of the metatarsophalangeal lateral collateral ligaments, metatarsophalangeal joint synovitis (especially in second ray syndrome) (Fig 19), or, in runners, isolated tarsometatarsal bursitis (31).
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Figure 18a. Morton neuroma. (a) Coronal spin-echo T1-weighted MR image (400/12) shows the plantar interdigital nerve branches (circled). (b) Coronal spin-echo T1-weighted MR image (400/12) obtained in a different patient shows the plantar interdigital nerve branches embedded in perineural fibrosis (arrow).
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Figure 18b. Morton neuroma. (a) Coronal spin-echo T1-weighted MR image (400/12) shows the plantar interdigital nerve branches (circled). (b) Coronal spin-echo T1-weighted MR image (400/12) obtained in a different patient shows the plantar interdigital nerve branches embedded in perineural fibrosis (arrow).
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Figure 19. Second ray syndrome in a 43-year-old patient. Coronal fat-saturated turbo spin-echo T2-weighted MR image (3,500/119) demonstrates synovitis of the second metatarsophalangeal joint (arrowhead) and soft-tissue edema in the second plantar metatarsal space (arrows). These pathologic conditions can cause compression of the corresponding common plantar digital nerve branches.
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Conclusions
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Foot and ankle neuropathy are frequently underdiagnosed conditions that involve thin and complex anatomic structures. Clinical symptoms are often subjective, and physicians may have difficulty differentiating neuropathy from other clinical entities. Imaging techniques are becoming more and more accurate. Therefore, radiologists must be aware of the main nerve entrapment syndromes at the foot and ankle and be able to perform, with different imaging modalities, accurate screening of the nerves that may be involved in patients with foot and ankle pain.
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Footnotes
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Abbreviations: DPN = deep peroneal nerve,
PTN = posterior tibial nerve,
SPN = superficial peroneal nerve
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M. Nagaoka and H. Matsuzaki
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1035 - 1040.
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[Full Text]
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Abstract
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