Education And Debate

ABC of Emergency Radiology: THE ANKLE

BMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6924.331 (Published 29 January 1994) Cite this as: BMJ 1994;308:331
  1. D O'Keeffe,
  2. D A Nicholson,
  3. P A Driscoll,
  4. D Marsh

    A large proportion of the radiographs performed in accident and emergency departments are for injuries to the ankle. This article describes an effective system by which non-specialists can request appropriate radiographs and interpret them. It is essential to be familiar with the anatomy of the ankle and understand how it can be damaged.

    FIG 1
    FIG 1

    Normal anteroposterior ankle radiograph and line diagram.

    FIG 2
    FIG 2

    Normal lateral ankle radiograph and line diagram.

    Two views of the ankle are required for proper assessment

    Important anatomical considerations

    Adult

    The talus is the key to understanding the ankle. This bone is surrounded by a circle made of bones and ligaments. Superiorly the distal tibia is joined to the distal fibula by three ligaments (fig 3). These are the posterior and anterior tibiofibular ligaments and the interosseous membrane.

    FIG 3
    FIG 3

    Line diagram of important ligaments in ankle.

    Ankle movements

    • Dorsiflexion and plantar flexion occur at the ankle joint

    • Inversion and eversion occur at the subtalar joint

    There are two important collateral ligament complexes. The collateral ligamental complexes and malleoli combine with the distal tibial articular surface to lock the talus in a mortice. Plantar flexion and dorsiflexion of the ankle occur at this joint. The talus is divided into a body (including the dome), neck, and head. The dome has a wide anterior aspect giving it a trapezoid shape. In extreme dorsiflexion the talus is wedged between the malleoli and all the associated ligaments are taut. Therefore in this position there is little movement of the ankle mortice. This is an important factor in various types of injuries.

    The talus has a vulnerable blood supply similar to the scaphoid in the wrist. As the talus has no muscular or tendinous attachments it relies on the integrity of the capsule for nutrition. The proximal part (body) is supplied into the distal aspect (head), and a fracture of the waist can compromise the supply to the body, causing necrosis. Congruity of the articular surfaces is critical to the function of the joint. A 1 mm discrepancy in alignment due to a fracture leads to a 47% reduction in load bearing surfaces with consequent predisposition to degenerative joint disease.

    Children

    The developing ankle can have numerous accessory centres of ossification and named ossicles - for example, the os trigonum, which is present in about a quarter of the population. There is also a wide variety of normal variants that at first sight resemble fractures. Separate ossification centres have a characteristic appearance but comparison views of the other ankle are sometimes required. Injury to a developing physis may result in premature closure of all or part of the physis with consequent deformity. Children often break the physis whereas adults strain ligaments before they break bones or dislocate joints. In children injury to a growth plate (Salter-Harris classification) requires orthopaedic attention sooner rather than later (fig 4).

    FIG 4
    FIG 4

    Anteroposterior and lateral views of child's ankle showing the importance of the two views. The anteroposterior view looks normal (note normal appearances to the epiphyses) but the lateral view shows a fracture of the posterior malleolus with posterior displacement of the epiphysis and a spiral fracture through the fibula.

    Stability of the ankle

    Stability is lost if the circle of bones and ligaments that makes up the ankle mortice is disrupted in two or more places. Often one of the breaks will be in the supporting soft tissue and therefore not radiologically visible.

    The stability of the ankle should always be investigated as this determines future management. Stability should be assessed from the mechanism of injury and the radiological findings

    Mechanism of injury Twisting ankle injuries.

    The ankle is almost always damaged because of abnormal movement of the talus. Occasionally only one type of movement occurs. Usually, however, a combination of these movements coexists at the time of injury. This gives rise to complex patterns of skeletal and ligamentous damage. Three patterns are distinguished according to the level and type of fibular fracture.

    Movements of the talus leading to ankle injury

    • Movement in the coronal plane - abduction or adduction

    • Rotation about the long axis of the tibia - internal or external rotation

    • Vertical compression

    Horizontal fracture at the ankle joint or distal to it - This occurs when the main movement of the talus is adduction in the coronal plane (inversion). It leads to either tearing of the lateral collateral ligament or a horizontal avulsion fracture of the lateral malleolus (fig 5). In extreme cases the medical malleolus can be obliquely fractured (fig 6). The junction between the malleolus and the horizontal articular surface of the tibia may also be compressed. The tibiofibular ligaments remain intact and there is no diastasis of the tibiofibular joint. Avulsion most commonly occurs at the site of insertion of the anterior ankle joint capsule.

    FIG 5
    FIG 5

    Radiograph showing avulsion fracture of tip of fibula.

    FIG 6
    FIG 6

    Oblique fracture of medial malleolus with an inversion injury.

    Spiral fracture at the distal tibiofibular joint or distal to it - This occurs when the main movement of the talus is abduction in the coronal plane (eversion) and rotation. It leads to a spiral fracture of the lateral malleolus, usually beginning at the ankle joint. With greater degrees of force this fracture is comminuted. The stresses in the medial collateral ligament cause it to rupture or produce a horizontal avulsion fracture of the medial malleolus (fig 7). If the talar movement continues both tibiofibular ligaments are stretched. With sufficient force the insertion of the posterior ligament into the posterior malleolus (posterior aspect of tibia) can be avulsed. The tibiofibular ligaments remain intact if the fibular fracture begins below the ankle joint.

    FIG 7
    FIG 7

    Anteroposterior radiograph and line diagram showing avulsion fracture of the medial malleolus and oblique fracture of the fibula. Note the lateral shift of the talus and incongruity at the ankle joint.

    Fracture between the inferior tibiofibular joint and head of the fibula - This results from a combination of talar movements, the main one being external rotation. The fibula can fracture anywhere along its length - even at its neck (fig 8). Rarely the bone is preserved at the expense of dislocation of the superior tibiofibular joint. The stresses in the medial collateral ligament cause it to tear or produce a horizontal avulsion fracture of the medial malleolus. Stretching of the tibiofibular ligaments usually results in an avulsion of the posterior malleolus. The tibiofibular ligaments are torn or avulsed from their bony attchments.

    FIG 8
    FIG 8

    Fractured neck of fibula. The ankle was also fractured.

    Deceleration ankle injuries

    Rapid deceleration from a fall or road traffic accident can produce a force which dorsiflexes the ankle. The wide anterior aspect of the talus is driven between the malleoli, fracturing the medial one. With further dorsiflexion the anterior surface of the tibia is broken along with the lateral malleolus. If the foot is plantar flexed at the time of impact the talus strikes the posterior articular surface of the tibia, which can fracture. The fragment, however, is usually small and minimally displaced.

    Aviator's astragalus (an old word for the talus) is a fracture of the neck of the talus due to a plantar force which drives the talar neck into the anterior lip of the tibia. Any fractures or injuries causing complete dislocation of the talus are associated with avascular necrosis of the proximal fragment. This should be suspected in follow up films when the density of the proximal fragment remains constant while the remainder of the bones of the ankle become osteoporotic because of hyperaemia.

    • The higher the fibular fracture the more extensive the damage to the tibiofibular ligaments

    Reasons for taking an ankle radiograph

    Criteria for ankle radiography

    The Royal College of Radiologists 1989 guidelines recommend selective radiographs in patients presenting with an ankle injury with one or more of the following signs:

    • Deformity, crepitus, or instability

    • Bruising or severe swelling

    • Moderate or severe pain on weight bearing

    • Point tenderness on palpation

    • Injury of tendon, vessel, or nerve

    • Suspected foreign body

    • Age

    A radiograph is unlikely to show a fracture if the patient is able to continue with his or her activity, is able to bear weight with moderate ease, has no swelling or joint tenderness over bone, and has no swelling over the anterior tibiofibular ligament

    The decision when to radiograph an ankle depends on the clinical findings and the mechanism of injury. A higher index of suspicion and a lower threshold for radiological assessment is needed in elderly patients, who can sustain fractures with minimal trauma.

    Types of view

    Radiographic projections for ankle

    • Standard: Lateral

    • Anteroposterior

    • Additional: Mortice

    • Oblique

    The mortice view is a modified anteroposterior view with the foot internally rotated through 20°. When oblique views are indicated the ankle joint is rotated 45° internally and externally. Stress views and special techniques such as tenography, tomography, computed tomography, ultrasonography, and magnetic resonance imaging all have a role in investigating ligamentous tears but should not be undertaken without consultation with an experienced radiologist. Weight bearing radiographs are of little help.

    Stress views taken under medical supervision help determine the extent of ligamentous injuries. However, these should not be performed until five to seven days after injury, when the pain and swelling have subsided. Such radiographs are contraindicated in the acute situation and in patients with a fracture.

    System of radiological assessment

    Criteria for fracture

    Primary signs
    • Primary signs

    • Breaks in the cortex of the bone

    • Abnormal lucency or sclerosis in the medulla

    • Abnormal alignment of the bone

    • Abnormal shortening or lengthening

    Secondary signs
    • Fat-blood levels in the adjacent joint (if the film is taken across the table with a horizontal beam)

    • Fragments from parent bone

    • Adjacent soft tissue swelling

    A streak of high density in the trabecular bone is always abnormal and suggests an impacted fracture

    Lateral view

    We recommend using the ABCs system of radiographic interpretation.

    Check the adequacy and quality of the radiograph

    Check name, age, and sex of the patient, that the correct side has been radiographed, and that the correct region has been included. The malleoli should be superimposed on each other and the entire calcareus and bones of the mid-foot, including the base of the fifth metatarsal, visible. If the radiograph has been properly exposed, you should not need to use a bright light to see soft tissues.

    Check alignment of bones - The malleoli are superimposed with the tibia articulating with the dome of the talus. The contiguous surfaces of the tibia and talus are smooth and symmetrical. The subtalar joint is visible.

    Check bone margins and density - Follow the cortex of the bones of the ankle looking for any sudden changes in direction. Start by tracing down the posterior aspect of the tibia to the posterior aspect of the ankle joint. Follow the joint surface of the tibia anteriorly then, passing upwards, trace the anterior margin of the tibia. Look through the tibia to trace the surface of the fibula. Next trace the margins of the talus in a similar clockwise fashion. The margins are normally partly obscured in the anterior and posterior portions of the subtalar joint. Examine the internal density and structure of all the visible bones. The trabeculae in the medullary canal should be of uniform or gradually changing density.

    Check the cartilage and joints - The contiguous surfaces of the talus and distal tibia should be smooth and symmetrical with the distal tibia lying on the posterior aspect of the dome of the talus.

    Check soft tissues - Extracapsular soft tissue swelling about the ankle joint is often seen in trauma and is a non-specific sign that is usually not associated with bony injury. Anterior to the ankle joint a vertical soft tissue fat density is visible. This relates to the anterior aspect of the joint capsule. When a severe joint effusion is present the joint capsule bulges. Rupture of the Achilles' tendon may be seen in plain films, although it is usually detectable clinically if complete. If incomplete, ultrasonography or magnetic resonance imaging may be required for diagnosis.

    Anteroposterior radiograph

    Check the adequacy and quality of the radiograph - The anteroposterior radiograph of the ankle should show both malleoli and the talus (fig 1). The relation of the talus to the mortice cannot be optimally assessed. (This requires the mortice view, in which there is 20° internal rotation.)

    Check alignment of bones - The talus should be in the mortice.

    Check bone margins and density - Follow the scheme described for the lateral projection - that is, examine the cortical margins in a clockwise fashion then inspect the internal bony structure.

    Check the cartilage and joints - In this projection the distal tibiofibular joint is obscured by the superimposed portions of the tibia and fibula. The distance between the contiguous surfaces of the talus and distal tibia/fibula should be equal in its medial, superior, and lateral aspects (fig 7). Check the distance of the distal tibiofibular joint looking for diastasis. Check there is no compression fracture of the tibial plate or osteochondritis tali.

    Check soft tissues - Extracapsular soft tissue swelling about the ankle joint is often seen in trauma. It is a non-specific sign and is not usually associated with bony injury.

    FIG 9
    FIG 9

    Ankle joint effusion. Note the displaced fat plane.

    FIG 10
    FIG 10

    Osteochondritis of the talus. It usually occurs at the medial aspect of the superior surface and is identified as a separated piece of bone and associated cartilage.

    The os trigonum is a sommon normal variant of the talus is due to a separate ossification centre arising from the posterior tubercle. The appearance may resemble an old ununited fracture fragment. However, it is triangular, well corticated, in a classic location, and usually bilateral, which enables it to be distinguished from a fracture.

    Transverse, sclerotic, linear lines located at the metaphysis of growing long bones are due to short periods of growth arrest and have no clinical importance (fig 5). They may be confused with compression fractures, but again these lines are usually bilateral.

    Fibrious cortical defects are the most commonly seen benign lesions of long bones and are usually identified incidentally in radiographs taken for another reason. The defect is limited to the cortex, commonly found at the metaphysis, but may be located in the diaphysis as the bone grows. The lesion is well corticated (sclerotic margins) and usually does not produce signs or symptoms.

    Accute osteomyeltitis

    Acute bone infection produces swelling and irregular demineralisation of the affected bone but demineralisation is often difficult to detect. Periosteal new bone formation also occurs, but these bony changes are not evident for 10-14 days. The diagnosis is therefore initially clinical. Specialist advice should be sought early if this condition is suspected.

    Summary

    • Check the adequacy and quality

    • Check alignment of bones

    • Check bone margins and density

    • Check the cartilage and joints

    • Check soft tissues

    D O'Keeffe is consultant radiologist, Royal Albert and Edward Infirmary, Wigan; D A Nicholson is consultant radiologist, Hope Hospital, Salford; P A Driscoll is senior lecturer in emergency medicine, Hope Hospital, Salford; and D Marsh is consultant orthopaedic surgeon, Hope Hospital, Salford.

    The line drawings were prepared by Mary Harrison, medical illustrator.

    The ABC of Emergency Radiology has been edited by David Nicholson and Peter Driscoll.

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