ph: 310.358.2100 NOVEMBER 2010 Volume 1: Issue 4 Eye On Imaging MR IMAGING OF COMMON ENTRAPMENT NEUROPATHIES OF THE AND

Three sites of possible entrapment have been T arsal tunnel syndrome refers to entrap- MR studies are particularly well suited to the described and include: (1) deep to or adjacent ment of the tibial and/or its branches identi cation of space occupying lesions such to the fascial edge of a hypertrophied abduc- within the con nes of a bro-osseous tunnel as varicosities, soft tissue and perineural tor hallucis muscle, (2) along the medial edge along the medial aspect of the ankle. This ganglia, tumors, and accessory muscles ( such of the quadratus plantae muscle, or (3) tunnel is bounded laterally by the talus and as accessory exor digitorum and soleus adjacent to the medial calcaneal tuberosity. and medially by the exor retinacu- muscles). (Figure 1). MR identi cation of the MR can be useful in detecting the presence lum. Within the pass the tendons cause and location of entrapment is also used and location of nerve entrapment. of the posterior tibial, exor digitorum longus in preoperative assessment to determine the and exor hallucis muscles, the tibial artery extent of required release and for determining MR detection of denervation edema and and veins, and the and its terminal causes for failed tarsal tunnel surgery. atrophy of the abductor digiti quinti muscle, branches (medial calcaneal nerve, medial often incidental in our experience, is not , ). The most BAXTER NEUROPATHY uncommon and most likely reects a common presenting complaint is intractable clinically missed entrapment of the rst chronic heel pain. Sensory loss along the Entrapment of the inferior calcaneal nerve branch of the lateral plantar nerve. (Figure 2) plantar aspect of the foot and a positive Tinel (Baxter neuropathy), may be associated with hypertrophy and sign at the tunnel are the most helpful clinical ordinary activities but nearly half of the cases plantar fasciitis with medial calcaneal spur ndings. are secondary to athletic activity particularly formation and adjacent soft tissue edema distance running. It has been estimated that are also suggestive of nerve entrapment in The three most common causes of tarsal up to 15% of athletes with chronic unresolving Baxter’s neuropathy. tunnel syndrome are trauma (related to heel pain suer from entrapment of the scarring after sprains and fractures), space inferior calcaneal nerve. Clinically, the condi- BIOGRAPHY occupying lesions, and foot deformities with tion typically manifests as intractable heel Dr. Vu Bui and, the etiology unknown in 20-40% of cases. pain. It can be dicult to diagnose this entity Dr. Jerrold H. Mink Clinical diagnosis can be challenging as the clinically and to dierentiate from other Dr. Vu Bui (top) was pain may be non speci c and intrinsic muscle causes of heel pain. Electrodiagnostic tests recruited to Mink Radiol- motor loss can be dicult to assess. Normal may not be able to distinguish lateral plantar ogy from the University of EMG studies do not exclude the diagnosis. MR nerve entrapment within the tarsal tunnel Colorado Health Sciences is the optimal imaging study for direct visual- from inferior calcaneal nerve entrapment Center where he was an ization of the , retinaculum, and tunnel further distally. Baxter neuropathy is also Associate Professor of contents. commonly seen in association with plantar Radiology. Prior to that, he fasciitis which can further confuse clinical had taken a musculoskel diagnosis. fellowship 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 and foot and ankle. His general text, MRI of the Musculoskel- 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 (Figure 1A) Sagital image through the medial aspect of the ankle demonstrating a multi-septated Centers in Beverly Hills ganglion within the con nes of the tarsal tunnel. (Figure 1B) Obliqe axial section demonstrates the and Marina Del Rey. multilobular mass within the con nes of the tarsal tunnel. www.minkrad.com MInk Radiologic NOVEMBER 2010 Volume 1: Issue 4 imaging

MR IMAGING OF COMMON ENTRAPMENT NEUROPATHIES OF THE FOOT AND ANKLE Decreased bulk, fatty atrophy, and increased signal on uid The clinical diagnosis of Morton neuroma is often straight- sensitive images of the intrinsic muscles of the foot in a forward but on occasion diagnostic diculty exists and diabetic patient are commonly secondary to peripheral other causes of metatarsalgia (e.g. intermetatarsal bursitis, neuropathy. synovitis, in ammatory arthritis, stress fracture, Freiberg’s infraction, true neuroma) need to be dierentiated. MR Rest, orthotics, anti-in ammatory medication, corticoste- imaging has been shown to be useful in narrowing the wide roid injections and night splints are all part of the rst dierential diagnosis of forefoot pain. The accuracy of MR course of treatment. Surgical release of the nerve is has been reported with a sensitivity and specicity of 87% attempted if the pain is persistent. 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.

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

MORTON NEUROMA

Intermetatarsal (Morton) neuroma is not a true tumor but rather a degenerative process of the nerve resulting in a (Figure 3A) (Left) There is a complex dumbell shaped mass in the brotic nodule caused by damage to the interdigital nerve 3rd MT interspace. The plantar aspect measures 16mm x13mm. by either entrapment of the nerve against the transverse The dorsal aspect (arrow) demonstrates homogeneous high signal metatarsal ligament or by nerve ischemia. Intermetatarsal suggesting uid. (Figure 3B) (Right) Following the injection of neuroma is one of the most common causes of metatarsal- contrast material, the volar mass diusely enhances consistent gia and is most commonly seen in middle aged women, with a Morton neuroma. The dorsal mass (arrow) demonstrates possibly related to the wearing of high heeled tight boxed peripheral enhancement consistent with a distended IM bursa. shoes. Clinically it is characterized by intermetatarsal pain, numbness, and sensory disturbances that radiate to the 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.

1. Rosenberg ZS, Cavalcanti C. Entrapment Neuropathies of the Lower 4. Oztuna V. et.al Nerve entrapment in painful heel syndrome. Foot Ankle Extremity in Stoller DW. Magnetic Resonance Imaging in Orthopedics and Int 2002; 23 (3) : 208-211. Sports Medicine. Lippincott Williams Wilkins Philadelphia 2007. pp1088 1093.

2. Jackson DL, Haglund B. 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.

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