W. Michael Pullen*
Christopher S. Smith*
MICHAEL J. GARDNER
Christiaan N. Mamczak

*The views expressed in this chapter are those of the author(s) and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense or the United States Government. W. Michael Pullen and Christopher S. Smith are military service members and this work was prepared as part of their official duties. Title 17 U.S.C. 105 provides that ‘Copyright protection under this title is not available for any work of the United States Government.’ Title 17 U.S.C. 101 defines a United States Government work as a work prepared by a military service member or employee of the United States Government as part of that person’s official duties.

Bony Anatomy

The ankle joint is a complex hinge joint consisting of two articulations, the talocrural joint and the distal tibiofibular joint, with bony contributions from the distal tibia, distal fibula, and the talus.
The distal tibia is subcutaneous on its anteromedial aspect contributing the medial malleolus and medial articular surface, the distal articular surface or plafond, and the tibial incisura. The articular surface is concave with distal extensions anteriorly and posteriorly.
The distal fibula is subcutaneous laterally at its distal aspect and sits within the tibial incisura contributing the lateral malleolus with an irregular convex articular surface.
The talus contributes a more uniform convex articular surface, which is wider at its anterior portion when contrasted with the posterior portion.
Ankle stability is provided by ligamentous structures, with the deep deltoid ligament being the primary medial stabilizer, the syndesmotic complex being the primary stabilizer of the tibiofibular joint, and the lateral collateral ligament complex being the primary lateral stabilizer.¹

Radiographic Anatomy

Anteroposterior (AP) View

The AP view, in the intact ankle, demonstrates congruent articular surfaces with overlap of the distal tibia and fibula within the incisural notch (Fig. 25-1).
With ankle fractures, there can be incongruent articular surfaces, decreased distal tibia and fibular overlap, and/or loss of fibular length. Note the double density of the displaced posterior malleolus, fibular shortening, and loss of tibiotalar joint space associated with this AP image of a trimalleolar fracture with posterior dislocation (Fig. 25-2).
Postreduction AP imaging should demonstrate restoration of the articular congruity, fibular length, and distal tibiofibular overlap. Shortening of the fibula in relationship to the talocrural joint or the talus as well as decreased tibiofibular overlap may be concerning for malreduction.
Tibial plafond, or pilon, fractures involve various degrees of articular involvement and may be associated with substantial comminution and shortening as depicted below (Fig. 25-3):

Gardner1e-ch025-image001 — Figure 25-1 Normal AP projection of a right ankle.

Gardner1e-ch025-image002 — Figure 25-2 AP projection with a trimalleolar ankle fracture. Note the double density due to the posterior malleolus fracture, overlap in the joint space due joint dislocation, and fibular shortening.

Gardner1e-ch025-image003 — Figure 25-3 AP projection of a comminuted pilon fracture.

Mortise View

The mortise view is performed with the ankle in dorsiflexion to neutral, to place the widest portion of the talus within the joint, and with the tibia internally rotated 10 to 15 degrees (Fig. 25-4).
This view is utilized to evaluate the medial clear space and distal tibiofibular joint relationship as surrogates of (deltoid and/or syndesmotic) ligamentous integrity to evaluate for the stability of the ankle fracture either preoperatively or intraoperatively. This mortise view demonstrates a bimalleolar ankle fracture with lateral talar subluxation and fibular shortening (Fig. 25-5).
Normal medial clear space is typically <4 mm measured at the proximal and medial aspect of the talus; however, in the absence of a scaled image to account for image magnification, this measurement should be symmetric to the superior tibiotalar distance. This asymmetry can be seen on static images or on stress radiographs.
Postreduction mortise images should demonstrate restoration of the articular congruity, reduction of the medial clear space, and restoration of fibular length.
The “dime sign” is a radiographic marker best visualized on the ankle mortise view where a dime placed laterally should uniformly touch the peroneal groove of the distal fibula and the lateral talar process. Asymmetry is usually associated with fibular fracture shortening that must be reconstructed intraoperatively. Even a few millimeters of fibular shortening will have a profound effect on ankle mortise congruity and ligamentous stability (as depicted below) (Fig. 25-6A and B).

Gardner1e-ch025-image004 — Figure 25-4 Normal mortise view of a right ankle.

Gardner1e-ch025-image005 — Figure 25-5 Mortise view of a bimalleolar ankle fracture. Note the lateral subluxation of the talus and fibular shortening. An increase in the medial clear space can be seen when comparing the distance between the superior tibiotalar distance to the distance between the medial border of the talus and the medial shoulder of the tibia.

Gardner1e-ch025-image006a — Figure 25-6 A: Ankle fracture malreduction which can be confirmed by inadequate dime sign. Note the persistent fibular shortening and mortise malreduction. B: After revision, the dime sign is appropriately in contact with the peroneal groove and lateral process of the talus. Note improved reduction of the ankle and restoration of fibular length.

Gardner1e-ch025-image006b — Figure 25-6 A: Ankle fracture malreduction which can be confirmed by inadequate dime sign. Note the persistent fibular shortening and mortise malreduction. B: After revision, the dime sign is appropriately in contact with the peroneal groove and lateral process of the talus. Note improved reduction of the ankle and restoration of fibular length.

Lateral View

The lateral view is performed with the ankle in neutral dorsiflexion and directed perpendicular to the AP projection, with the medial and lateral dome of the talus superimposed, which typically requires slight adduction and internal rotation.
On a perfect “dome” lateral, three distinct anatomic ankle projections should always be evaluated: (1) the medial malleolus with anterior and posterior colliculi (blue), (2) the posterior malleolar congruity or fracture reduction (yellow), and (3) the posterior syndesmotic tibiofibular overlap (orange), which should be matched to a saved image of the patient’s contralateral, uninjured ankle (Fig. 25-7).
In ankle fractures, the lateral view is also utilized to determine the obliquity of the fracture to determine fracture classification as well as guide proper lag screw placement and plate position. The presence or absence of a posterior malleolar fragment can also be appreciated (Fig. 25-8).
The degree of articular impaction is greatly appreciated on the lateral view of pilon injuries (Fig. 25-9).

Gardner1e-ch025-image007 — Figure 25-7 Lateral fluoroscopic projection demonstrating the medial malleolus (*blue*), posterior malleolus (*yellow*), and posterior tibiofibular overlap (*orange*).

Gardner1e-ch025-image008 — Figure 25-8 A: Lateral fluoroscopic imaging demonstrates an oblique fracture from proximal posterior to distal anterior. No posterior malleolus fracture is seen on this projection. This can help guide lag-screw fixation trajectory as seen in the post-fixation image **(B)**.

Gardner1e-ch025-image009 — Figure 25-9 Lateral projection of a comminuted pilon fracture demonstrated significant articular impaction.

Preoperative Imaging

The radiographic indication for operative treatment of an ankle fracture is articular impaction or incongruity and mortise instability secondary to fractures of two or more malleoli, a combination of osseous injury with ligamentous injury, or severe ligamentous injury. By the nature of their articular disruption, most pilon fractures are treated surgically to restore joint congruity.
Standard radiographic views including an AP, mortise, and lateral are good screening tools for ankle fractures; however, bony overlap may obscure posterior malleolus fractures. Advanced imaging such at CT scans may be necessary to elicit the extent of articular involvement (see above).
Stress radiographs, performed via a manual external rotation or gravity rotation, can demonstrate ligamentous instability.² A stress mortise view may show medial clear space widening and lateral displacement of the fibular fracture indicating deltoid ligament injury and/or syndesmotic sprain. Intraoperatively, these maneuvers, or a Cotton test, are performed following osseous fixation to evaluate for persistent ligamentous instability that may require further fixation.
A staged protocol of treating pilon fractures with an index closed reduction and ankle spanning external fixation is a widely accepted method for allowing the soft tissue swelling to subside before definitive fracture fixation. CT scans should be performed after the external fixation frame when the tibial length has been restored noting that slight overdistraction plays favorably into achieving subsequent fracture reduction when several weeks of soft tissue rest are necessary to achieve a positive wrinkle sign (Figs. 25-10 and 25-11).