- Foot and ankle injuries occur frequently and may be subtle to identify on radiographs. Radiologists must have detailed knowledge of normal anatomy and an understanding of mechanisms of injury in order to make the correct diagnosis. Because several injuries can occur in the same traumatic event, it is important to adopt a detailed, organized search pattern that is applied consistently to every case so that subtle and multiple injuries are not overlooked. The radiologist should not only be familiar with current imaging and orthopedic literature, they should also know what information the orthopedic surgeon needs in order to facilitate proper management and surgery.
- For example, the search pattern on an ankle MR might start with the ankle ligaments, examining the lateral collateral ligament, the spring ligament, deltoid deep and superficial components, then the distal tibiofibular syndesmosis. Attention is then turned to articular cartilage and bones, to the tendons and their retinacula, and then to the muscles and plantar fascia. The tarsal tunnel and tarsal sinus are examined next. Finally, the "corners of the image" are reviewed. Radiologists are divided on when to view the clinical history. Some prefer to know the history before case review so that the search pattern can be tailored to the mechanism of injury. Others believe that knowing the history in advance may bias the reader and cause additional findings to be missed.
- Early ankle MR literature often focused on injuries to a single structure. However, the interrelated nature of the structures in this region has become ever more apparent as knowledge has advanced. A posterior tibial tendon abnormality, for example, is often associated with tears of the flexor retinaculum and spring ligament as well as plantar fasciopathy and preexisting pes planus. Information about associated abnormalities is critical to surgical planning. The "associated abnormalities" section in each chapter is designed to focus attention on this common phenomenon.
- MR imaging or US should not be interpreted without pertinent radiographs. Small bone fragments can be difficult to distinguish from soft tissue injuries on MR or US but are easily seen on radiographs or CT. In addition, weight-bearing radiographs can yield important information about ligament injuries that can determine whether an injury requires surgical management and also alert the radiologist to areas of concern on MR and US.
- In the ankle and foot, the axial and sagittal planes are considered to be the continuation of the axial and sagittal planes of the leg, and the coronal plane is perpendicular to them. Since the foot has a normal plantar declination, planes are usually angled slightly from those used in the leg. The axial plane is along the long axis of the talus, and the coronal plane is the short axis, perpendicular to the talus.
- An alternative descriptive approach is to employ terminology that does not depend on the standard axial/coronal orientations. The short axis of the foot displays the metatarsals in cross section, while the horizontal long-axis plane is similar in orientation to a PA radiographic view of the foot.
- When performing or interpreting US of tendons and ligaments, imaging planes are described with respect to the transducer's orientation to the tendon or ligament. For example, the term "long axis" refers to a tendon imaged along its longitudinal plane. The term "short axis" refers to a tendon imaged in cross section.
- Ankle fractures can be divided into 3 groups. Pilon fractures are axial load injuries, disrupting the articular surface of the distal tibia, known as the tibial plafond. Malleolus fractures are twisting injuries, involving the medial, lateral, &/or posterior malleoli. They commonly occur in conjunction with collateral ligament injuries. Finally, twisting injuries of the tibia may extend into the articular surface.
- The ankle joint is a pure hinge joint formed by the tibia, fibula, and talus. Its stability is maintained by 3 major ligament complexes: the syndesmotic ligaments, the lateral collateral ligaments, and the deltoid ligament. Injury to these ligaments puts the patient at risk for instability and premature osteoarthritis.
- The syndesmotic and lateral collateral ligament complexes are in close proximity, but they have opposing functions and should be analyzed on MR with that difference in mind. More superiorly, the syndesmotic ligament complex consists of the anterior inferior and posterior inferior tibiofibular ligaments and the interosseous ligament. The anterior inferior tibiofibular ligament usually has an accessory band, the Basset ligament, slightly inferior to the primary ligament. The posterior inferior tibiofibular ligament has a separate component inferiorly, the intermalleolar (transverse) ligament. The syndesmotic ligaments maintain the normal relationship of the fibula in the fibular notch of the tibia. They are usually injured with an eversion (pronation) stress or a rotational injury. More inferiorly, the ankle joint is stabilized from inversion, anterior translation, and rotational stresses by the lateral collateral ligament complex, consisting of the anterior and posterior talofibular ligaments and the calcaneofibular ligament.
- Medially, the primary ligamentous stabilizer of the ankle joint is the deltoid ligament. It can be divided into deep and superficial components. The deep component extends from the deep surface of the medial malleolus (primarily, the posterior margin of the anterior colliculus) to a fovea on the medial surface of the talar body. It stabilizes the ankle from eversion stress. The superficial component has multiple, somewhat variable bands, which extend from the superficial surface of the malleolus to the talus, navicular, spring ligament, and sustentaculum tali of the calcaneus.
- The foot is divided into the hindfoot (talus and calcaneus), the midfoot (navicular, cuboid, and cuneiforms), and the forefoot (metatarsals and phalanges). It has both transverse and longitudinal arches.
- The subtalar joint consists of 3 facets: posterior, middle, and anterior. The posterior subtalar facet forms a separate joint cavity (the posterior subtalar joint), which is continuous with the ankle joint in ~ 15% of the population. Fifty percent of weight bearing is through the posterior subtalar joint. Inversion, eversion, and a gliding motion occur at the posterior subtalar joint. The middle and anterior subtalar facets have a common joint cavity (the anterior subtalar joint) and communicate with the talonavicular joint; this complex is often called the talocalcaneonavicular joint. The subtalar joints are stabilized by interosseous ligaments as well as the deltoid and the calcaneofibular ligaments.
- The talonavicular and calcaneocuboid joints have separate joint cavities but act together in supination and pronation, and they are collectively known as the Chopart joint.
- The tarsometatarsal joints are collectively known as the Lisfranc joint. The Lisfranc ligament is crucial to the stability of the Lisfranc joint and has 3 bands that extend from the 1st cuneiform to the medial base of the 2nd metatarsal. It can be injured with relatively minor trauma due to forcible plantarflexion of the midfoot.
- The Ottawa rules were designed to avoid unnecessary imaging of the ankle and foot. Radiographs of the ankle are recommended by the Ottawa rules only if the patient is unable to bear weight for 4 steps both immediately after the injury and in the emergency department, and physical exam shows tenderness to palpation at the distal 6 cm of the posterior tibia, medial malleolus, or lateral malleolus. Radiographs of the foot are recommended if the patient is unable to bear weight for 4 steps both immediately after the injury and in the emergency department and has tenderness to palpation along the 5th metatarsal or the navicular. Unfortunately, the foot rules do not include criteria to aid in recognition of the important and often missed isolated Lisfranc ligament rupture, which presents with tenderness and soft tissue swelling at the medial margin of the 2nd metatarsal base.
- Ankle sprains and suspected ankle fractures are the most common indication for obtaining radiographs of the ankle. Hindfoot injuries can be divided into axial load and twisting injuries. Axial load creates the pilon fracture with disruption of the articular surface of the tibial plafond; talus fractures; ankle and subtalar dislocations; and compressive fractures of the calcaneus. Twisting injuries create malleolus fractures, ligament tears, and multiple small fractures, most commonly osteochondral, lateral process talus, anterior process calcaneus, and base of 5th metatarsal fractures.
- Stress fractures are common in the foot, and, initially, they may be radiographically occult. In the metatarsals, they cause a sharp, focal pain, and diagnosis can be made clinically even when radiographs are normal. However, in the hindfoot and midfoot, they often present with vague, ill-defined pain. Malleolus stress fractures are less common. In the setting of negative radiographs, MR is very useful in identifying an occult stress fracture and excluding soft tissue and articular causes of pain.
- Ligament and tendon injuries may occur with acute trauma or chronic overuse injuries. Tendon injuries range from tendinopathy, or tendon thickening or thinning, to partial tears to ruptures. Tendon ruptures usually occur in the setting of chronic injury.
- Whenever possible, weight-bearing radiographs should be obtained so that alignment can be assessed properly. Careful positioning of the patient is critically important in the evaluation of foot and ankle injuries. Incorrect positioning can lead to missed or false diagnoses. A technologist's manual, such as Merrill's Atlas of Radiographic Positioning and Procedures, should be available at every center where musculoskeletal imaging is performed.
- The standard ankle series consists of AP, mortise (15° internal oblique), and lateral views. The standard foot series consists of AP, oblique, and lateral views. Additional views are sometimes useful, such as the axial (Harris) view for calcaneus fractures, the hindfoot alignment view, and the gravity stress view to look for medial clear space widening in the setting of malleolar fractures.
- Ligament injuries often can be inferred on radiographs based on the location of soft tissue swelling, osseous or avulsive injury, &/or malalignment. For instance, an isolated fracture of the medial or posterior malleolus implies injury to the distal tibiofibular syndesmosis, which is often accompanied by a Maisonneuve fracture of the proximal fibula.
- Multidetector CT scanning is performed acquiring thin (0.6- to 1.0-mm) sections of the hindfoot or forefoot with 2-mm sagittal and coronal reformations. A soft tissue algorithm should be generated in addition to bone algorithm to permit diagnosis of tendon, retinaculum, and ligament injuries, which are often associated with ankle fractures. 3D CT can be helpful in understanding complex fracture patterns.
Bone Scan Considerations
- Tc-99m bone scanning has a very limited role in evaluating trauma of the foot and ankle. It will show areas of hyperemia and areas of increased osteoblast activity, but these are nonspecific parameters. In cases of cryptogenic foot pain, MR is a more effective tool for identifying the site and etiology of the patient's pain.
- MR with FOV covering the entire foot is generally not needed. Higher resolution will be achieved with smaller FOV (10-14 cm) centered on the area of concern. It is useful to have standard protocols for the hindfoot, midfoot, and forefoot. Each protocol requires ~ 25 minutes and includes sagittal T1 and STIR images, axial T1 and PD FS, and coronal PD and T2 FS. Slice thickness varies from 2.0-3.0 mm. A multichannel knee coil or dedicated chimney coil can be used.
- In our experience, patients are most comfortable with supine positioning, but some authors prefer to image them in the prone position.
- The ankle/hindfoot protocol utilizes an FOV that extends from above the ankle joint to the metatarsal bases in order to include the entire distal course of the posterior tibial and peroneal tendons.
- The midfoot protocol covers the talar neck to the proximal metatarsal shafts. This protocol is used primarily for Lisfranc ligament injury.
- The forefoot protocol includes the region from the Lisfranc joint to the tips of the toes and is excellent for evaluation of stress fractures, Morton neuroma, and osteomyelitis.
- The toe protocol utilizes a wrist coil or a flexible surface coil with a small FOV centered on the area of concern. Thinner slices are used, often covering only the toe of concern and the 2 adjacent toes. This protocol is useful for suspected ligament injuries of the MTP joints. However, the FOV is smaller than desirable for evaluation of osteomyelitis.
- MR is usually performed for the evaluation of chronic ankle and foot pain and disability or to assess for occult fracture. Radiographs and CT are the primary means of diagnosing acute injuries. However, MR has a role to play in some acute soft tissue injuries, especially in athletes. Acute ankle sprains do not usually warrant MR unless there is concern for a syndesmosis injury in a high-level athlete.
- Cartilage in the foot is thin and often tightly curved, and osteochondral injuries are easily missed on MR. MR arthrography improves sensitivity in the diagnosis of hyaline cartilage injury and may also improve visualization of ligament injuries.
- In the hands of experienced operators, US is useful for the assessment of tendon, ligament, and plantar fascia injuries and to locate foreign bodies. High-resolution, linear array transducers of at least 12 MHz are preferred. Given the small FOV provided by US transducers, it is imperative that the operator have a detailed knowledge of anatomy and osseous landmarks so as to avoid confusion while scanning. It is crucial that the operator knows how to optimize technical parameters in order to obtain excellent images. It is vitally important to be familiar with commonly encountered US artifacts, such as anisotropy, shadowing, posterior acoustic enhancement, and reverberation so as to not mistake normal anatomy for pathology.
- The beauty of US is that it can be focused to the area of pain or palpable abnormality. When a finding is of questionable significance, it is easy to examine the contralateral limb for comparison. US provides the advantage of dynamic assessment, which may be particularly helpful in the planning the surgical treatment of Achilles tendon rupture or peroneal tendon subluxation/dislocation. Findings are made in "real-time" and can be communicated directly to the patient. US is not optimized to evaluate fractures since the sound beam is attenuated by bone.
- Therapeutic joint, tendon sheath, or bursa injections performed with fluoroscopic or US guidance are useful in the diagnosis and treatment of ankle pain. Injection can assist the surgeon in verifying pain generators; if performed for this purpose, care should be taken to evaluate for communications between joints. For instance, if the ankle and subtalar joints communicate, pain relief after injection is a less specific finding.
- Mosher TJ et al: ACR appropriateness criteria acute trauma to the ankle. J Am Coll Radiol. 12(3):221-7, 2015
- Goost H et al: Fractures of the ankle joint: investigation and treatment options. Dtsch Arztebl Int. 111(21):377-88, 2014
- Thompson JC: Netter's Concise Orthopaedic Anatomy. 2nd ed. Elsevier, 2010
- Cerezal L et al: MR arthrography of the ankle: indications and technique. Radiol Clin North Am. 46(6):973-94, v, 2008
- Collins MS: Imaging evaluation of chronic ankle and hindfoot pain in athletes. Magn Reson Imaging Clin N Am. 16(1):39-58, v-vi, 2008
- Frank ED et al: Merrill's Atlas of Radiographic Positioning and Procedures. 11th ed. Elsevier, 2007
- Choplin RH et al: CT with 3D rendering of the tendons of the foot and ankle: technique, normal anatomy, and disease. Radiographics. 24(2):343-56, 2004
- Newman JS: Diagnostic and therapeutic injections of the foot and ankle. Semin Roentgenol. 39(1):85-94, 2004
- Stiell IG et al: Decision rules for the use of radiography in acute ankle injuries. Refinement and prospective validation. JAMA. 269(9):1127-32, 1993
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