MRI OF THE FOOT AND ANKLE
47
Neil Roach, MD
1. What are the four compartments around the ankle, and what do they contain?
The four compartments are medial, lateral, anterior, and posterior. The compartmental organization is helpful in thinking
of tendon anatomy.
" The medial compartment contains (anterior to posterior) the posterior t ibialis tendon (PTT), flexor digitorum longus
(FDL) tendon, and flexor hallucis longus (FHL) tendon:  Tom, Dan, and Harry. (The  an in Dan refers to the posterior
tibial artery and nerve.)
" The lateral compartment contains the peroneal brevis and longus.
" The anterior compartment contains (medial to lateral) the extensor t ibialis tendon, extensor hallucis longus tendon, and
extensor digitorum longus tendon:  Tom, Harry, and Dick.
" The posterior compartment contains the Achilles tendon.
2. What is the most commonly injured ankle tendon?
The Achilles tendon is most commonly injured. It usually tears 3 to 5 cm superior to its calcaneal insertion, where the
blood supply of the tendon is poorest. Tears may also occur at the musculotendinous (usually the lateral gastrocnemius)
junction, but this is much less common. Tears occur least commonly at the calcaneal insertion secondary to injury or
ill-fitting shoes. Normal tendon signal is very low on T1-weighted and T2-weighted MR images. The morphologic features
of the tendon should be evaluated on sagittal and axial images, with the anterior surface of the normal tendon flat or
concave on axial images.
3. Describe the magnetic resonance imaging (MRI) appearance of Achilles tendon tears.
Achilles abnormalities on MRI tend to cover a progressive sequence of disorders:
" Peritendinitis: The Achilles tendon does not have a sheath; only peritendinitis, not synovitis, can occur. Peritendinitis is
seen as infiltrative changes in the fat surrounding the tendon.
" Tendonopathy: Any internal tendon signal is indicative of tendonopathy.
" Chronic tendonopathy: Chronic tendonopathy is visualized on MRI as an enlarged and abnormally shaped tendon.
" Interstitial tears: Interstitial tears are depicted on MRI as longitudinally oriented internal signal changes first seen on
T1-weighted images. As these tears become more severe, T2-weighted images are abnormal also.
" Partial tears: Partial tears are horizontally oriented signal abnormalities.
" Complete tears: Complete Achilles disruption with retraction of the torn ends is similar in appearance to the end of a mop.
4. Which of the flexor or medial tendons is most commonly torn?
The PTT is the most common of the medial tendons to tear. These tears most often occur in middle-aged women. PTT
disruption results in  adult-onset flat foot. PTT tears usually occur near the tendon insertion site and are associated with
accessory navicular or a cornuate navicular bones. Normally, the PTT is twice the size as the FDL or FHL tendon. One
must suspect a PTT tear if there is tenosynovitis or if the tendon is greatly enlarged. Comparing the size of the PTT with
the FDL or FHL tendon can be useful.
5. Name the three ligaments that form the lateral collateral ligament.
The anterior talofibular, calcaneofibular, and posterior talofibular ligaments form the lateral collateral ligament.
6. Which of these three ligaments tends to tear first?
The anterior talofibular ligament is the most commonly injured ligament. The calcaneofibular ligament is the next most
commonly injured ligament. The posterior talofibular ligament is easily identified on coronal MR images, but is rarely
injured.
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7. Which bones form the hindfoot, midfoot, and forefoot?
" The hindfoot comprises the calcaneus and talus.
" The midfoot comprises the navicular, the three cuneiform bones, and the cuboid.
" The forefoot comprises the metatarsals and phalanges.
8. What are the articulations of the subtalar joint?
" The posterior facet is the largest and the  true subtalar joint.
" The middle facet is formed by the sustentaculum tali (of the calcaneus) and the anterior talus. It has a flat joint
surface.
" The anterior facet (vestigial) may be independent of or contiguous with the middle facet and has a rounded
surface.
9. Describe the tarsal canal and the sinus tarsi.
The tarsal canal and sinus tarsi form the boundary between the posterior subtalar joint and the anteriorly located
talocalcaneal navicular joint. The tarsal canal and sinus extend from posteromedial to anterolateral. The tarsal canal
extends to the medial aspect of the foot, posterior to the sustentaculum tali. The canal widens to form the sinus tarsi,
which is cone-shaped and opens laterally. The sinus tarsi contains fat, blood vessels, and the interosseous and cervical
ligaments and branches of the tibial nerve.
10. Which ligament around the ankle is least likely to tear?
The medially located deltoid ligament is very strong and uncommonly tears. Eversion injuries, rather than ligament tears,
usually cause avulsion fractures of the medial malleolus. The deltoid is composed of four ligaments two superficial
and two deep.
11. What are the three plantar compartments, and why are they important?
The plantar compartments of the foot are important for the evaluation of infection and tumor extent. The three plantar
compartments are separated by fascial planes.
" The medial compartment (first ray) contains the abductor hallucis muscle, the flexor hallucis brevis muscle, the
tendon of the flexor hallucis longus muscle, the medial plantar nerve and vessels, and the first metatarsal bone.
" The central compartment (second, third, and fourth rays) contains the flexor digitorum brevis muscle, the quadratus
plantae, the four plantar lumbricales muscles, the tendon of the flexor digitorum longus muscle, a portion of the
tendon of the flexor hallucis longus muscle, and the lateral plantar nerve and vessels. The central compartment also
contains the second, third, and fourth metatarsals. The thick, longitudinally oriented plantar aponeurosis forms the
floor of the central compartment. This compartment is important because it communicates with the calf and can
serve as a pathway for spread of infection.
" The lateral compartment (fifth ray) contains the fifth metatarsal and the abductor digiti minimi and flexor digitorum
minimi brevis muscles.
12. What is tarsal coalition?
Tarsal coalition is fusion of one or more intertarsal joints. The fusion can be osseous, fibrocartilaginous, or fibro-osseous.
Patients usually present in adolescence with a spastic flatfoot. It can be bilateral in 20% to 25% of cases. Hereditary
forms exist. Calcaneonavicular fusion is the most common type based on plain film experience. Computed tomography
(CT) and MRI show that talocalcaneal fusion may be more common. Secondary signs of tarsal coalition include talar
beaking and a ball-and-socket ankle joint. MRI diagnosis of an osseous coalition, shown by continuity of the bone
marrow, is straightforward (Fig. 47-1). Fibrocartilaginous or fibro-osseous coalitions require evaluation of articular
surfaces for irregularities and for low signal fibrous tissue bands that are more difficult to appreciate.
13. What are common sites for stress fractures of the foot?
Calcaneal and metatarsal stress fractures are the most common stress fractures in the foot. On T1-weighted images,
the edema and hemorrhage of the fracture appear as bands of low signal traversing the normally high signal bone
marrow. Fat-suppressed T2-weighted or short tau inversion recovery (STIR) images show areas of increased signal
intensity in the region of the fracture (Fig. 47-2).
14. Which part of the dome of the talus is typically injured in repetitive trauma?
What part of the talar dome is typically involved if there is a single bad traumatic
event?
Osteochondral injuries are most common in adolescents and in males. If the injury is caused by repetitive injury, the
medial talar dome is usually affected. If the injury is caused by a single bad traumatic event, the lateral talar dome is
usually involved.
MUSCULOSKELETAL RADIOLOGY 321
15. MRI and magnetic resonance arthrography
of the ankle may be useful in evaluating
the different types of osteochondral
injuries. How are osteochondral injuries
classified?
" Stable: Ill-defined rim of high T2 signal is present around
the lesion, but the cartilage is intact.
" Loose in situ: The cartilage is injured.
" Partially loose: Incomplete but well-defined ring of high
T2 signal is present around the lesion.
" Loose complete: Well-defined ring of signal surrounds
the fragment or a cyst associated with the lesion or both.
" Free: Intra-articular body has an empty crater.
16. Describe characteristics used to
differentiate between osteomyelitis and
neuropathic arthropathy?
The distinction between neuropathic joint arthropathy
and osteomyelitis is based on distribution of disease, the
association with an ulcer, and the MRI signal characteristics
of the bone marrow. Neuropathic arthropathy usually
affects the Lisfranc, metatarsophalangeal, and subtalar
joints. Osteomyelitis can affect any bone, but it is usually
associated with contiguous spread from an ulcer and
abnormal adjacent soft tissues. STIR sequences are very
sensitive to edema in bones, whether the edema is due
to osteomyelitis or to neuropathic/osteoarthritic changes.
Because bone edema is not specific for bone infection,
STIR images may lead to false-positive diagnoses of
osteomyelitis, and the finding of bone edema must be
Figure 47-1. Coronal T1-weighted image of the ankle shows
interpreted in light of other MRI findings. Postgadolinium
an osseous calcaneal-talar coalition (arrow) at the level of the
images are also helpful in evaluating edema versus
middle facet of the subtalar joint.
cellulitis.
AB
Figure 47-2. A, T1-weighted axial image of the forefoot shows a stress fracture of the second metatarsal. B, Coronal fat-suppressed
T2-weighted image outlines the soft tissue and bone marrow edema from the stress fracture in A.
322 MRI OF THE FOOT AND ANKLE
17. What MRI findings are related to
plantar fasciitis?
Plantar fasciitis is seen as increased T2 signal
adjacent to the proximal insertion of the plantar
fascia with or without a plantar enthesophyte. The
fascia may be thickened. Refractory cases can
show bone marrow edema adjacent to the plantar
fascia insertion on the calcaneus (Fig. 47-3).
18. What is plantar fibromatosis?
Patients present with pain on weight bearing and
have palpable nodular thickening along the course
of the plantar fascia. Plantar fibromatosis can be
bilateral in 10% to 40% of patients and may occur in
any age group; males are affected twice as often as
females. On T1-weighted images, the lesion can be
hypointense or isointense to muscle. On T2-weighted
images, the lesion may be hypointense or slightly
hyperintense. These lesions enhance intensely with
gadolinium and are usually bright on STIR images.
19. What are Morton neuromas?
Morton neuromas are small, focal masses of fibrosis
Figure 47-3. Sagittal STIR image of the ankle shows soft tissue and
and do not represent a true neuroma involving the bone marrow edema in a patient with plantar fasciitis (arrows).
plantar digital nerves. They occur most commonly
between the third and fourth metatarsals and can be bilateral in 25% of cases. Women 25 to 50 years old are the most
likely to be affected. T1-weighted and T2-weighted images show low signal intensity secondary to fibrosis.
20. What characterizes the tarsal tunnel?
The tarsal tunnel is located on the medial side of the ankle. It begins superiorly at the medial malleolus and extends
inferiorly to the navicular bone. The lateral side of the tunnel is formed by the calcaneus, including the sustentaculum
tali, and the talus. The flexor retinaculum forms the medial wall, and the abductor hallucis muscle forms the floor of the
tunnel. The posterior tibial nerve and its divisions the posterior tibial artery and vein and the PTT, FDL tendon, and FHL
tendon are within the tunnel.
21. What is tarsal tunnel syndrome?
Patients with tarsal tunnel syndrome usually present with pain and paresthesia along the distribution of the posterior
tibial nerve and its branches. Patients typically complain of pain and burning on the plantar aspect of the foot. Half of
cases are idiopathic, with the remainder secondary to scarring, osseous deformities, soft tissue masses, or varicose veins.
22. What is sinus tarsi syndrome, and what does it look like on MRI?
In sinus tarsi syndrome, there is loss of the normal fat within the sinus tarsi. In acute or subacute cases, there is low
T1 signal and high T2 signal secondary to inflammation. In chronic cases, there is low T1 and T2 signal secondary to
fibrosis. Patients complain of weakness and instability in the ankle with palpable tenderness over the sinus tarsi. Most
cases are idiopathic, but some patients have a history of ankle inversion injury. Other causes of sinus tarsi include
inflammatory conditions, such as rheumatoid arthritis or gout, and ganglion cysts.
23. Of the bones of the foot and ankle, which is the most likely to develop avascular
necrosis? What is the typical cause of this?
The talus is the most likely to develop avascular necrosis related to fracture of the neck of the talus and loss of the blood
supply to the dome. Avascular necrosis eventually leads to deformity and collapse of the dome.
Key Points: MRI of the Foot and Ankle
1. Tendon tears are generally related to a progressive
sequence of disorders.
2. MRI can commonly detect fractures in the foot and ankle
that are not appreciated on radiographs or CT scans.
3. The deltoid ligament practically never tears. Avulsion
fractures of the medial malleolus generally occur before
deltoid ligament tears.
4. Tarsal coalitions are usually seen in children and
adolescents.
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