Eye On Imaging - Mink Radiology

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NOVEMBER 2010 Volume 1: Issue 4
Eye On Imaging
T
MR IMAGING OF COMMON ENTRAPMENT
NEUROPATHIES OF THE FOOT AND ANKLE
arsal tunnel syndrome refers to entrapment of the tibial nerve and/or its branches
within the confines of a fibro-osseous tunnel
along the medial aspect of the ankle. This
tunnel is bounded laterally by the talus and
calcaneus and medially by the flexor retinaculum. Within the tarsal tunnel pass the tendons
of the posterior tibial, flexor digitorum longus
and flexor 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
findings.
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 specific and intrinsic muscle
motor loss can be difficult to assess. Normal
EMG studies do not exclude the diagnosis. MR
is the optimal imaging study for direct visualization of the nerves, retinaculum, and tunnel
contents.
MR studies are particularly well suited to the
identification of space occupying lesions such
as varicosities, soft tissue and perineural
ganglia, tumors, and accessory muscles ( such
as accessory flexor digitorum and soleus
muscles). (Figure 1). MR identification 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 suffer from entrapment of the
inferior calcaneal nerve. Clinically, the condition typically manifests as intractable heel
pain. It can be difficult to diagnose this entity
clinically and to differentiate 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.
(Figure 1A) Sagital image through the medial aspect of the ankle demonstrating a multi-septated
ganglion within the confines of the tarsal tunnel. (Figure 1B) Obliqe axial section demonstrates the
multilobular mass within the confines of the tarsal tunnel.
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Three sites of possible entrapment have been
described and include: (1) deep to or adjacent
to the fascial edge of a hypertrophied abductor 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 reflects a
clinically missed entrapment of the first
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.
BIOGRAPHY
Dr. Vu Bui and,
Dr. Jerrold H. Mink
Dr. Vu Bui (top) was
recruited to Mink Radiology from the University of
Colorado Health Sciences
Center where he was an
Associate Professor of
Radiology. Prior to that, he
had taken a musculoskel
fellowship at the Brigham and Womens Hosptial in
Boston. In addition to Dr. Bui’s expertise in musculoskeletal 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 first specialty texts on the knee and foot and
ankle. His general text, MRI of the Musculoskeletal
System,
was
reviewed in the New
England Journal of
Medicine as the “essential textbook to own for
anyone
interpreting
musculoskeletal
MRI”.
Dr. Mink currently directs
the Mink Radiologic
Centers in Beverly Hills
and Marina Del Rey.
MInk Radiologic
imaging
NOVEMBER 2010 Volume 1: Issue 4
MR IMAGING OF COMMON ENTRAPMENT
NEUROPATHIES OF THE FOOT AND ANKLE
Decreased bulk, fatty atrophy, and increased signal on fluid
sensitive images of the intrinsic muscles of the foot in a
diabetic patient are commonly secondary to peripheral
neuropathy.
Rest, orthotics, anti-inflammatory medication, corticosteroid injections and night splints are all part of the first
course of treatment. Surgical release of the nerve is
attempted if the pain is persistent.
The clinical diagnosis of Morton neuroma is often straightforward but on occasion diagnostic difficulty exists and
other causes of metatarsalgia (e.g. intermetatarsal bursitis,
synovitis, inflammatory arthritis, stress fracture, Freiberg’s
infraction, true neuroma) need to be differentiated. MR
imaging has been shown to be useful in narrowing the wide
differential diagnosis of forefoot pain. The accuracy of MR
has been reported with a sensitivity and specificity of 87%
and 100% respectively. The most typical MR appearance is
that of a low signal intensity (reflecting the predominant
histological composition of dense fibrous 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 reflecting 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
fibrotic 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 metatarsalgia 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.
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.
(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 fluid. (Figure 3B) (Right) Following the injection of
contrast material, the volar mass diffusely enhances consistent
with a Morton neuroma. The dorsal mass (arrow) demonstrates
peripheral enhancement consistent with a distended IM bursa.
4. Oztuna V. et.al Nerve entrapment in painful heel syndrome. Foot Ankle
Int 2002; 23 (3) : 208-211.
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.
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