arsal tunnel syndrome refers to entrap-ment of the tibial nerve and/or its branches within the con�nes of a �bro-osseous tunnel along the medial aspect of the ankle. This tunnel is bounded laterally by the talus and calcaneus and medially by the �exor retinacu-lum. Within the tarsal tunnel pass the tendons of the posterior tibial, �exor digitorum longus and �exor hallucis muscles, the tibial artery and veins, and the tibial nerve and its terminal branches (medial calcaneal nerve, medial plantar nerve, lateral plantar nerve). The most common presenting complaint is intractable chronic heel pain. Sensory loss along the plantar aspect of the foot and a positive Tinel sign at the tunnel are the most helpful clinical �ndings.

The three most common causes of tarsal tunnel syndrome are trauma (related to scarring after sprains and fractures), space occupying lesions, and foot deformities with the etiology unknown in 20-40% of cases. Clinical diagnosis can be challenging as the pain may be non speci�c and intrinsic muscle motor loss can be di�cult to assess. Normal EMG studies do not exclude the diagnosis. MR is the optimal imaging study for direct visual-ization of the nerves, retinaculum, and tunnel contents.

MR studies are particularly well suited to the identi�cation of space occupying lesions such as varicosities, soft tissue and perineural ganglia, tumors, and accessory muscles ( such as accessory �exor digitorum and soleus muscles). (Figure 1). MR identi�cation of the cause and location of entrapment is also used in preoperative assessment to determine the extent of required release and for determining causes for failed tarsal tunnel surgery.

BAXTER NEUROPATHY

Entrapment of the inferior calcaneal nerve (Baxter neuropathy), may be associated with ordinary activities but nearly half of the cases are secondary to athletic activity particularly distance running. It has been estimated that up to 15% of athletes with chronic unresolving heel pain su�er from entrapment of the inferior calcaneal nerve. Clinically, the condi-tion typically manifests as intractable heel pain. It can be di�cult to diagnose this entity clinically and to di�erentiate from other causes of heel pain. Electrodiagnostic tests may not be able to distinguish lateral plantar nerve entrapment within the tarsal tunnel from inferior calcaneal nerve entrapment further distally. Baxter neuropathy is also commonly seen in association with plantar fasciitis which can further confuse clinical diagnosis.

Three sites of possible entrapment have been described and include: (1) deep to or adjacent to the fascial edge of a hypertrophied abduc-tor hallucis muscle, (2) along the medial edge of the quadratus plantae muscle, or (3) adjacent to the medial calcaneal tuberosity. MR can be useful in detecting the presence and location of nerve entrapment.

MR detection of denervation edema and atrophy of the abductor digiti quinti muscle, often incidental in our experience, is not uncommon and most likely re�ects a clinically missed entrapment of the �rst branch of the lateral plantar nerve. (Figure 2) Abductor hallucis muscle hypertrophy and plantar fasciitis with medial calcaneal spur formation and adjacent soft tissue edema are also suggestive of nerve entrapment in Baxter’s neuropathy.

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MR IMAGING OF COMMON ENTRAPMENTNEUROPATHIES OF THE FOOT AND ANKLE

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(Figure 1A) Sagital image through the medial aspect of the ankle demonstrating a multi-septated ganglion within the con�nes of the tarsal tunnel. (Figure 1B) Obliqe axial section demonstrates the multilobular mass within the con�nes of the tarsal tunnel.

etal System, was reviewed in the New England Journal of Medicine as the “essen-tial textbook to own for anyone interpreting musculoskeletal MRI”. Dr. Mink currently directs the Mink Radiologic Centers in Beverly Hills and Marina Del Rey.

Dr. Vu Bui and,Dr. Jerrold H. MinkDr. Vu Bui (top) was recruited to Mink Radiol-ogy from the University of Colorado Health Sciences Center where he was an Associate Professor of Radiology. Prior to that, he had taken a musculoskelfellowship at the Brigham and Womens Hosptial in Boston. In addition to Dr. Bui’s expertise in musculo-skeletal imaging, he has extensive experience with musculoskeletal interventional procedures such as spinal interventional procedures and biopsies. Jerrold Mink (bottom), MD, has written more than forty original articles that have been published in the radiology, orthopedic, sports medicine, and rheumatology literature. In addition, he has co-authored four textbooks on musculoskeletal applications of MRI including the �rst specialty texts on the knee and foot and ankle. His general text, MRI of the Musculoskel-

BIOGRAPHY

NOVEMBER 2010 Volume 1: Issue 4ph: 310.358.2100

1. Rosenberg ZS, Cavalcanti C. Entrapment Neuropathies of the Lower Extremity in Stoller DW. Magnetic Resonance Imaging in Orthopedics and Sports Medicine. Lippincott Williams Wilkins Philadelphia 2007. pp1088 1093.

2. Jackson DL, Haglund B. Tarsal tunnel syndrome in runners. Sports Med 1992; 13 92) :146-148.

3. Weishaupt D. et.al. Morton Neuroma: MR imaging in prone, supine, and upright weight bearing body positions. Radiology 2003:226(3)849-856.

4. Oztuna V. et.al Nerve entrapment in painful heel syndrome. Foot Ankle Int 2002; 23 (3) : 208-211.

Decreased bulk, fatty atrophy, and increased signal on �uid sensitive images of the intrinsic muscles of the foot in a diabetic patient are commonly secondary to peripheral neuropathy.

Rest, orthotics, anti-in�ammatory medication, corticoste-roid injections and night splints are all part of the �rst course of treatment. Surgical release of the nerve is attempted if the pain is persistent.

MORTON NEUROMA

Intermetatarsal (Morton) neuroma is not a true tumor but rather a degenerative process of the nerve resulting in a �brotic nodule caused by damage to the interdigital nerve by either entrapment of the nerve against the transverse metatarsal ligament or by nerve ischemia. Intermetatarsal neuroma is one of the most common causes of metatarsal-gia and is most commonly seen in middle aged women, possibly related to the wearing of high heeled tight boxed shoes. Clinically it is characterized by intermetatarsal pain, numbness, and sensory disturbances that radiate to the toes and are exacerbated by standing and walking. The symptoms can be relieved by rest and shoe removal. In up to 80% of patients, intermetatarsal neuromas may be associated with forefoot deformities such as hallux valgus, hammertoe, or pes planus.

The clinical diagnosis of Morton neuroma is often straight-forward but on occasion diagnostic di�culty exists and other causes of metatarsalgia (e.g. intermetatarsal bursitis, synovitis, in�ammatory arthritis, stress fracture, Freiberg’s infraction, true neuroma) need to be di�erentiated. MR imaging has been shown to be useful in narrowing the wide di�erential diagnosis of forefoot pain. The accuracy of MR has been reported with a sensitivity and speci�city of 87% and 100% respectively. The most typical MR appearance is that of a low signal intensity (re�ecting the predominant histological composition of dense �brous tissue) dumbbell shaped mass located in the intermetatarsal space and often extending into the plantar subcutaneous fat (Figure 3). Of note, the MR detection of a Morton neuroma does not necessarily imply symptomatology as the entity has been reported in up to 33% of asymptomatic patients. It appears that the larger lesions (greater then 5mm in diameter) are both more commonly to be symptomatic and more likely to be associated with a good surgical outcome.

NOVEMBER 2010 Volume 1: Issue 4

(Figure 3A) (Left) There is a complex dumbell shaped mass in the 3rd MT interspace. The plantar aspect measures 16mm x13mm. The dorsal aspect (arrow) demonstrates homogeneous high signal suggesting �uid. (Figure 3B) (Right) Following the injection of contrast material, the volar mass di�usely enhances consistent with a Morton neuroma. The dorsal mass (arrow) demonstrates peripheral enhancement consistent with a distended IM bursa.

MR IMAGING OF COMMON ENTRAPMENT NEUROPATHIES OF THE FOOT AND ANKLE

(Figure 2) Moderate edema within the abductor digiti minimi muscle re�ecting acute denervation related to entrapment of the inferior calcaneal nerve.

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