Imaging modalitiesRadiography

Radiography is often the first imaging modality used to evaluate ankle pain and/or injury. A typical ankle radiographic series includes anteroposterior (AP), mortise, and lateral views. On the AP radiograph, the X-ray beam should be centered over the talar dome or tibiotalar joint centered between the medial and lateral malleoli, and the fibula should slightly overlap the lateral talar dome. The mortise radiograph is also centered either at the talar dome or the tibiotalar joint. However, the ankle is internally rotated by 15°–20° to bring the talar dome and ankle mortise in profile. In this view, there is often slight overlap between the distal tibia and fibula at the syndesmosis, and the lower limb may need to be further internally rotated to assess the syndesmosis in profile if needed. Finally, the lateral radiograph is centered at the talar dome or tibiotalar joint, bringing the tibiotalar joint in sharp profile. Because proximal fifth metatarsal base fractures are common, many institutions include this in the imaged field-of-view on lateral ankle radiographs. Well-positioned radiographs are vital in orthopaedics since they allow standardized measurements, such as the assessment of the talar tilt angle or anterior translation, which can be compared prior to and after any intervention and followed with serial imaging. Weightbearing is the most common stress and allows assessment of alignment with physiologic load-bearing. Stress radiographs of the ankle can also be helpful to assess ligamentous and/or syndesmotic integrity. These include AP varus stress (inversion), AP valgus stress (external rotation), and lateral anterior drawer radiographs. Stress can be applied manually (either via hand or with various devices available for the purpose of stress radiograph positioning) or with gravity [1]. Gravity-stress radiography is as reliable as manual stress imaging in detecting deltoid ligament insufficiency or syndesmotic widening; avoids exposing the clinician to ionizing radiation laterally; and is more comfortable for patients since it requires less force [2, 3]. Abnormalities in measurements obtained on these various stress views may suggest underlying injury to the soft tissue stabilizers, as discussed subsequently.

Cross-sectional imaging

Computed tomography (CT) and magnetic resonance imaging (MRI) are frequently utilized to evaluate the ankle and foot. CT provides precise osseous detail and is often used for fracture evaluation, pre-operative planning, and postoperative/hardware assessment. It allows for multiplanar and 3-dimensional reformats, which are particularly helpful for pre-operative planning. Compared with radiographs, CT provides better 3-dimensional evaluation and is not limited by projection or overlapping structures.

Until recently, CT imaging could not be performed with weightbearing, and thus, was unable to provide an accurate representation of the physiologic alignment of a patient’s foot. However, new advances in cone-beam CT (CBCT) have led to the development of weightbearing CT scanners, in which a large detector array and a pyramid-shaped X-ray beam (“cone” beam) allow CT data to be obtained with a single rotation around the patient [4]. With this configuration. patients can stand for the CT acquisition, placing their feet in more physiologic positions, resulting in more accurate measurements and precise surgical planning [5]. Compared with whole-body multidetector CT (MDCT) systems, CBCT has a similar geometric accuracy, and may even be slightly better than MDCT for small fields-of-view [6]. In addition to weightbearing acquisition, CBCT is particularly helpful in imaging the foot and ankle since it allows exquisite bony trabecular detail with greatly reduced radiation doses, and has metal artifact reduction capability; these factors make CBCT attractive for serial imaging. On the other hand, CBCT images often have lower signal-to-noise, which can greatly limit soft tissue assessments, and this technology is not widely available.

Several published articles have compared some of the common measurements obtained from weightbearing radiography with those from weightbearing CBCT with promising results. However, further work must be performed to verify the accuracy of additional measurements obtained from weightbearing CBCT that have not already been compared with radiography.

Compared with other imaging modalities, MRI is the test of choice for evaluating tendinous and ligamentous soft tissue structures of the ankle, though this modality is limited since it cannot commonly be performed with weightbearing and is therefore not generally used to make definitive ankle measurements. In clinical practice, some radiographic measurements can be applied to MRI to help suggest certain pathologies; however, the abnormalities must be confirmed with weightbearing radiography or CT to establish the diagnosis.

Measurements (Table 1)Table 1 Commonly used orthopaedic measurements in assessment of the ankleAnkle instability Talar tilt angle

The talar tilt angle is a measure of the lateral opening of the tibiotalar joint and generally reflects the integrity of the calcaneofibular and anterior talofibular ligaments. Measured on the AP or mortise view ankle radiographs, it is defined as the angle formed between lines drawn along the tibial plafond and talar dome (Fig. 1). There is wide variability in reported normal values in the literature with stress measurements ranging from 0° to 27° [7]. In practice, accepted normal values in neutral position are < 2° of varus angulation [8], and with supination/inversion stress the angle should measure < 5° [9].

Fig. 1

Measurement of the talar tilt angle on an AP radiograph of the ankle. The angle is formed from a line tangent to the tibial plafond (A, blue line) and a line along the talar dome (B, red line). In practice, accepted normal values in neutral position are < 2° of varus angulation; with supination/inversion stress the angle should measure < 5°. This measurement reflects the integrity of the calcaneofibular and anterior talofibular ligaments

Medial clear space (tibiotalar space)

The medial clear space is best assessed on the mortise radiograph and represents the horizontal linear distance between the lateral margin of the medial malleolar articular surface and the medial margin of the talar articular surface (Fig. 2). Multiple studies have validated this measurement, with reported normal values of < 4–5 mm [1, 10]. While malalignment of the ankle mortise and medial clear space widening is often evident on neutral radiographs, external rotation stress radiographs may be helpful to elicit medial clear space widening in more subtle cases [1]; an increase in the medial clear space > 2 mm on stress radiographs relative to baseline neutral radiographs has been correlated with deep deltoid ligamentous injury and syndesmotic injury [2, 10].

Fig. 2

Measurement of medial clear space on mortise view of the ankle. This represents the horizontal distance between the lateral margin of the medial malleolus and the medial margin of the talus. It should normally measure < 4–5 mm and an increase in the medial clear space > 2 mm on stress radiographs relative to baseline neutral radiographs is associated with deep deltoid ligamentous injury and syndesmotic injury

Less commonly used measurements to assess ankle stability Talocrural angle

The talocrural angle is determined on the mortise radiograph and assesses the relative length of the fibula, which can become shortened in the setting of fracture (Fig. 3). Restoring fibular length is paramount in successfully treating ankle fractures and preventing post-traumatic arthropathy. This angle can be assessed in two ways. Either it can be measured by taking a line perpendicular to the tibial plafond articular surface and a second line connecting the tips of the medial and lateral malleoli, or it can be taken from a line along the tibial plafond articular surface and a second line connecting the tips of the medial and lateral malleoli. If the first method of reporting is performed, the talocrural angle should measure 83° ± 4°, and increased angles indicate fibular shortening. On the other hand, if the second method is used, the angle should measure 8°–15°, and decreased angles suggest fibular shortening. There should be < 2°–4° difference with the contralateral side. This measurement can be affected by limb rotation or X-ray beam divergence [11, 12].

Fig. 3

Measurement of the talocrural angle on ankle mortise radiograph. The angle (yellow angle notation) is created from a line connecting the tips of the medial and lateral malleoli (yellow line) and the tibial axis (dashed line), which is perpendicular to the tibial plafond articular surface (solid red line). Comparison with the contralateral side if unaffected may help detect subtle fibular shortening. Normal values are 83° ± 4° with < 2°–4° difference with the contralateral side. When using the tibial axis for the frame of reference, increased angle measurements indicate tibial shortening. Alternatively, the angle between a line tangent to the tibial plafond (solid red line) and a line connecting the tips of the medial and lateral malleoli (solid yellow line) may be taken, with normal values of 8°–15°. When using the tibial plafond as the reference line, decreased angles suggest lateral malleolar shortening

Anterior talar translation

The anterior talar translation measurement is measured on lateral ankle radiographs and is helpful in evaluating anterior talofibular ligament integrity (Fig. 4). It measures the shortest distance between the posterior lip of the tibial plafond and the posterior talar dome. This measurement has been described under manual stress and with various mechanical devices, with variable agreement between methods and observers and a wide range of reported normal and abnormal values. Using this method of measurement, normal values in the neutral position are < 4 mm, and anterior talar displacement ≥ 4 mm suggests anterior talofibular ligament insufficiency. With anterior drawer stress, it should measure < 6 mm [13,14,15,16].

Fig. 4

Measurement of the anterior talar translation on a lateral weightbearing radiograph of the ankle. The most reliable method for evaluation of anterior talar translation measures the shortest distance between the posterior lip of the tibial plafond (A) and the posterior talar dome articular surface (B). Normal values in neutral position are < 4 mm, and < 6 mm with anterior drawer stress. An increased anterior talar translation distance indicates anterior talofibular ligament insufficiency

Syndesmosis

Injuries to the distal tibiofibular syndesmosis can have important implications for ankle stability. As the talus is typically more tightly aligned with the lateral malleolus due to the strong lateral stabilizers, even slight widening of the tibiofibular interval can lead to lateral talar shift, resulting in decreased tibiotalar contact and ankle joint instability [17]. Syndesmotic injury may occur as an isolated soft tissue injury but often occurs in combination with lateral and/or medial malleolar fractures and deltoid ligament insufficiency [18]. The radiographic relationship between the distal tibia and fibula, including tibiofibular clear space widening and decreased tibiofibular overlap, can suggest syndesmotic injury. Although syndesmotic injuries can be assessed on AP views of the ankle, the mortise view is considered more reliable clinically since it more directly visualizes the joint.

It should be noted that some studies have called into question the accuracy of the tibiofibular clear space and tibiofibular overlap in predicting syndesmotic injury, including a study by Shah et al. which indicated that approximately 5–10% of normal patients may demonstrate variant tibiofibular clear space widening and diminished tibiofibular overlap in the absence of syndesmotic injury [19]. Comparison with the normal contralateral side and interpreting radiographs within the context of the Lauge-Hansen classification of ankle injury mechanisms [19, 20] have been shown to increase diagnostic accuracy.

Tibiofibular clear space

The tibiofibular clear space is the radiographic depiction of the syndesmosis, and widening of this space indicates tibiofibular ligament insufficiency. It can be measured on AP or mortise radiographs of the ankle. The measurement is made 1 cm superior to the tibial plafond, extending a horizontal line from the distal tibial incisura fibularis to the medial margin of the distal fibula (Fig. 5). The normal tibiofibular clear space measures < 6 mm and commonly measures 4–5 mm on both AP and mortise views; a measurement ≥ 6 mm indicates syndesmotic disruption [21].

Fig. 5

Measurement of the tibiofibular clear space on mortise radiograph of the ankle. a The measurement is made between the distal tibial incisura fibularis (A) to the medial margin of the distal fibula (B) at a level 1 cm superior to the tibial plafond. The normal tibiofibular clear space measures < 6 mm, and a measurement ≥ 6 mm indicates syndesmotic disruption. b Widening of the tibiofibular clear space on mortise radiograph of the ankle in a 20-year-old male patient after a fall. The tibiofibular distance measures 7 mm, and there is widening of the medial tibiotalar clear space, indicating deltoid ligament insufficiency as well

Tibiofibular overlap

Tibiofibular overlap can also be measured on AP or mortise views of the ankle. The tibiofibular overlap is measured 1 cm superior to the tibial plafond by taking the horizontal distance between the medial contour of the fibula and the lateral contour of the anterior tibial tubercle (Fig. 6). This measurement is highly dependent on patient rotation and should be interpreted along with the tibiofibular clear space distance. Harper et al. reported that the tibiofibular overlap should be > 6 mm or approximately 42% of the fibular width on the AP view and > 1 mm on the mortise view [21]. In general, there is typically > 0 mm of tibiofibular overlap regardless of rotation [22], though Shah et al. did demonstrate 4.9% of normal patients without overlap, which may reflect variant anatomy [19]. However, absent tibiofibular overlap associated with tibiofibular and medial tibiotalar diastasis highly suggests syndesmotic instability.

Fig. 6

Measurement of tibiofibular overlap on AP radiograph of the ankle. a The horizontal distance between the medial margin of the distal fibula (A) and the lateral margin of the tibial tubercle (B) is measured 1 cm proximal to the tibial plafond. The tibiofibular overlap should be > 6 mm on the AP view and > 1 mm on the mortise view. The absence of tibiofibular overlap in conjunction with tibiofibular and medial tibiotalar diastasis suggests syndesmotic instability. b Absent tibiofibular overlap on AP radiograph of the ankle on the same patient seen on Fig. 5b, indicating syndesmotic insufficiency. Even on the AP radiograph, the tibiofibular clear space measures 6 mm, which also indicates syndesmotic disruption

Calcaneal fractures

Most calcaneal fractures are the result of high energy trauma with significant axial load resulting in talar impact onto the calcaneus, typically resulting in collapse of the calcaneus and subtalar joint depression. There are several tools and classification systems for diagnostic and prognostic evaluation of calcaneal fractures. The two most commonly reported measurements are Böhler’s angle and the critical angle of Gissane, which are important metrics for determining the degree of injury and presurgical planning. Of these two, restoration of Böhler’s angle correlates better with functional outcomes.

Böhler’s angle

Initially described in 1931 by Böhler [23], this is the most commonly used angle to evaluate the severity of calcaneal fractures. Proper measurement technique is performed on lateral foot radiographs (Fig. 7), though lateral ankle radiographs can be used for screening purposes. The angle is formed by the intersection of two lines: a line connecting the tip of the anterior calcaneal process and the superior margin of the posterior calcaneal facet; and a line connecting the superior margin of the posterior calcaneal facet and the superior margin of the calcaneal tuberosity [1].

Fig. 7

Measurement of Bohler’s angle on weightbearing lateral radiograph of the foot. The angle is formed between a line extending from the tip of the anterior calcaneal process to the superior margin of the posterior calcaneal facet (A, blue line) and a line extending from the posterior calcaneal facet to the superior margin of the calcaneal tuberosity (B, red line). Generally accepted normal values range from 25° to 40°, and values < 25° suggest depression of the calcaneal articular surface with more severe deformities

As originally described by Böhler, normal values range from 30° to 35° with a cutoff of < 28° considered pathologic [23]. However, many subsequent studies have reported different normal values for the angle, particularly with differences between ethnicities. In practice, generally accepted normal values range from 25° to 40° [24]. Lower values indicate more severe fractures with greater posterior calcaneal facet depression, and are associated with poorer outcomes. However, a normal Böhler’s angle does not exclude a non-displaced calcaneal fracture. Additionally, while restoration of Böhler’s angle is generally felt to improve postoperative function, studies have shown mixed results in terms of the prognostic value of the measurement on postoperative outcomes [25, 26].

Critical angle of Gissane

Initially described in 1946 by Gissane [27], this angle measures depression of the posterior calcaneal facet. Proper measurement technique is performed on lateral foot radiographs, though lateral ankle radiographs can be used for screening purposes. The angle is formed by lines drawn along the superior surface of the anterior calcaneal process and along the posterior calcaneal facet (Fig. 8) [1]. In severely comminuted depressed fractures, this angle is not commonly utilized, as it can be difficult or impossible to visualize the bony landmarks, and measurements are considered unreliable [28].

Fig. 8

Measurement of the critical angle of Gissane on lateral weightbearing radiograph of the foot. The angle is formed by lines drawn along the superior surface of the anterior calcaneal process (A, blue line) and along the posterior calcaneal facet (B, red line). Generally accepted normal angles range from 120° to 145°. The critical angle of Gissane enlarges with greater posterior calcaneal facet collapse

The originally reported normal values are difficult to trace, and subsequent studies have reported a wide range of normal values from 95° to 150° [28]. In practice, generally accepted normal angles range from 120° to 145° [1]. The critical angle of Gissane increases with greater posterior calcaneal facet collapse.

Posterior tibialis tendon dysfunction

The posterior tibialis tendon serves as a major stabilizer in maintaining the medial longitudinal arch of the foot and resisting valgus forces on the hindfoot. Dysfunction of the posterior tibialis tendon, may start with tenosynovitis and tendinopathy without or with tendon tearing. Loss of stabilization by the posterior tibialis tendon places increased stress on the remaining medial longitudinal arch stabilizers, and failure of this tendon can result in progressive flatfoot deformity, forefoot abduction, acquired hindfoot valgus and varus talar tilt. Posterior tibialis tendon dysfunction is the most common cause of adult acquired flatfoot deformity [29]. Comprehensive discussion of the progression of adult acquired flatfoot deformity is beyond the scope of this article. However, it has been divided into 4 clinical stages. In stage I, there may be tenosynovitis and pain without deformity. In stage II, the posterior tibialis tendon begins to elongate, and there is mild hindfoot valgus. Stage II is often divided into stage IIA, in which there is mild flatfoot deformity, and the talar head uncoverage measures between 25 and 40%, and stage IIB, which represents a more severe flatfoot deformity, including hindfoot valgus measuring > 15° and > 40% talar head uncoverage. Stage III flatfoot deformity is rigid and there is often osteoarthritis (OA) in the hindfoot joints. Finally, stage IV disease represents stage III disease with valgus deformity at the ankle [30, 31]. Initial weightbearing radiographic and CT imaging focuses on detecting associated secondary alignment changes. MRI may be helpful to directly assess the status of the tendon and suggest associated malalignment. However, any malalignment detected on MRI should be confirmed on weightbearing studies.

Hindfoot valgus and pes planus

Hindfoot valgus (tibiocalcaneal) angle

Hindfoot alignment can be assessed on radiography or cross-sectional imaging by evaluating the angle between the tibial axis and the calcaneal axis. Weightbearing is important for obtaining reproducible, physiologically relevant measurements, because the calcaneus may either be in neutral or even varus angulation when in the relaxed, non-weightbearing state. Hindfoot valgus represents a lateral deviation of the calcaneal axis from the tibial midline, leading to forefoot abduction and talocalcaneal angle enlargement. Dedicated hindfoot alignment radiographs, initially described by Cobey et al. [32], can be utilized to minimize overlap of hindfoot structures by the forefoot. The hindfoot valgus (tibiocalcaneal) angle is measured between a line along the distal tibial longitudinal axis and a line tangential to the medial calcaneal cortex (Fig. 9). The hindfoot valgus angle normally measures ≤ 6° utilizing weightbearing radiography or CBCT [33]. An MRI grading system for the hindfoot valgus angle has been proposed, which classifies mild hindfoot valgus as 7°–16°, moderate as 17°–26°, and severe as > 26°, though it is important to remember that non-weightbearing MRI may underestimate the true degree of hindfoot angulation [34].

Fig. 9

Tibiocalcaneal (hindfoot valgus) angle on coronal reformatted cone beam CT (CBCT) of the ankle: CBCT images are shown at the level of the distal tibia (a), which depicts the tibial axis (A, blue line) and at the level of the calcaneus (b), which shows the appropriate reference for the medial calcaneal cortex (B, red line). The angle is formed between lines drawn along the axis of the distal tibia (A) and tangent to the medial border of the calcaneus (B). Please note that the reference lines are drawn on two separate coronal reformatted images when using CBCT images and need to be superimposed to determine the angle. The hindfoot valgus angle normally measures ≤ 6° utilizing weightbearing radiography or CT, and larger angles indicate greater degrees of hindfoot valgus, which is associated with loss of the medial midfoot arch and posterior tibialis tendon dysfunction

Lateral talocalcaneal angle

The lateral talocalcaneal angle is used to assess hindfoot alignment and deformity. Measured on lateral weightbearing foot radiographs, it is defined as the angle between a line along the long axis of the talar neck and a line along the inferior surface of the calcaneus (Fig. 10). Normal values are between 25° and 40° with values < 25° seen in pes cavus, hindfoot varus, and congenital talipes equinovarus. Values > 40° are seen in pes planus and hindfoot valgus [1].

Fig. 10

Measurement of the lateral talocalcaneal angle on lateral foot radiograph. a The angle is formed between a line (A, red line) along the axis of the talar neck and a line (B, blue line) along the inferior surface of the calcaneus. Normal values are between 25° and 40°. Values < 25° are seen in pes cavus, hindfoot varus, and congenital talipes equinovarus, while values > 40° are seen in pes planus and hindfoot valgus. b In a 34-year-old male patient with posterior tibialis tendon dysfunction and flatfoot deformity, the lateral talocalcaneal angle measures 57°, indicating pes planus and hindfoot valgus

Lateral talus-first metatarsal (Meary) angle

The lateral talus-first metatarsal angle, initially described by Meary et al., is measured on a lateral weightbearing foot radiograph since non-weightbearing radiographs may underestimate the degree of deformity. The angle is formed by a line along the long axis of the talar head and neck and a line along the long axis of the first metatarsal (Fig. 11). Normally, the long axis of the talus is colinear with the long axis of the first metatarsal, resulting in a normal angle range of 0° ± 4° [35]. Angles greater than 4° convex downward are consistent with pes planus, and angles greater than 4° convex upward are consistent with pes cavus. Pes planus can be graded as follows: 5°–15° is mild; 16°–30° is moderate; and > 30° is severe [36].

Fig. 11

Measurement of the lateral talus-first metatarsal (Meary) angle on lateral weightbearing radiograph of the foot. a The angle is formed between lines drawn along the axis of the first metatarsal (A, blue line) and along the axis of the talus (B, red line). The long axis of the talus is either colinear or nearly colinear with the long axis of the first metatarsal, resulting in a normal angle range of 0° ± 4°. Angles greater than 4° convex downward are seen with pes planus, and angles greater than 4° convex upward are consistent with pes cavus. b In a 34-year-old male patient with posterior tibialis tendon dysfunction and flatfoot deformity (same patient as in Fig. 10b), the lateral talus-first metatarsal angle measures more than 30°, indicating borderline severe pes planus

Calcaneal inclination (pitch) angle

The calcaneal inclination (pitch) angle is used to assess arch height. Measured on lateral weightbearing foot radiographs, it is defined as the angle formed from a line along the plantar surface of the calcaneus and a line from the plantar margin of the calcaneal tuberosity to the plantar surface of the 5th metatarsal head, representing the plantar weightbearing axis of the foot (Fig. 12). Non-weightbearing lateral radiographs may underestimate the degree of the deformity. There has been significant variability in the normal values reported, with values of 20°–30° often considered normal [37, 38]. A decreased calcaneal inclination angle (< 20°) is consistent with pes planus, while an increased angle (> 30°) suggests of pes cavus.

Fig. 12

Measurement of the calcaneal inclination (pitch) angle on lateral weightbearing radiograph of the foot. The angle is formed between a horizontal line (A, blue line) from the calcaneal tuberosity to the plantar surface of the 5th metatarsal head, and a line (B, red line) along the inferior surface of the calcaneus. Reported normal values are variable although 20°–30° is often considered nor