Abstract
Background:
Medical imaging of the distal tibiofibular joint requires reliable and simple tools to identify disruption of the syndesmosis. We present an anatomical feature, the “tibiofibular line,” which appears on axial computed tomography (CT) images of normal ankles. This feature is a straight line that connects the anterolateral surface of the fibula with the anterolateral tubercle of the tibia at the level of the ankle syndesmosis. The purpose of this study was, first, to demonstrate that this line is a reliable anatomical feature in normal ankles and, second, to demonstrate that this line is displaced with diastasis or malrotation of the syndesmosis.
Methods:
A series of 150 normal ankle CTs were collected, negative for history of ankle injury with a normal tibiofibular overlap and clear space. Thirty ankles with a displaced syndesmosis were identified by history, CT, and abnormal tibiofibular overlap and clear space parameters. The tibiofibular line was applied to both groups and measured for its distance displaced from the anterior tibial tubercle.
Results:
All CT images in the normal ankle group had a tibiofibular line within 2 mm of the anterior tibial tubercle (77% of the tibiofibular lines were within 0 mm, 19% were within 1 mm, and 4% were within 2 mm). The tibiofibular line in the injured group was displaced anteriorly by 4 to 19 mm (minimum to maximum) from the tibial tubercle (P < .0001).
Conclusion:
The tibiofibular line was a normal anatomical feature that could be used to identify displacement of the distal tibiofibular syndesmosis.
Level of Evidence:
Level III, comparative series.
Ankle injuries are the most frequent musculoskeletal injury seen by primary care providers. 1 There are approximately 5600 ankle injuries each day in the United Kingdom and 23,000 in the United States. 9 An estimated 1% to 11% of all ankle sprains involve injury to the distal tibiofibular syndesmosis. 8 Cadaveric work by Ramsay and Hamilton 16 demonstrated that widening of the ankle mortise by 1 mm decreases the contact area of the tibiotalar joint by 42%. Accessible and reliable diagnostic imaging is needed to identify ankles with tibiofibular diastasis.
Medical imaging of the injured ankle begins with standard AP, mortise, and lateral radiographs. Harper and Keller determined the distal tibiofibular parameters used on the AP and mortise views to detect syndesmosis displacement on 12 cadavers. Tibiofibular clear space (TFCS) is the distance between the medial border of the fibula and the lateral border of the tibia as it extends into the incisura fibularis. The TFCS is measured 1 cm proximal to the plafond and should be less than 6 mm in both the AP and mortise views. Tibiofibular overlap (TFO) is the overlap of the lateral malleolus and the anterior tibial tubercle measured 1 cm proximal to the plafond. The overlap should be more than 6 mm in the AP view and more than 1 mm on the mortise view. 6
The TFCS and TFO were also measured on axial CT scans by Dikos et al 3 on 60 ankles confirmed to have no previous injury. The TFCS is the parallel interval between the medial fibula and the tip of the posterior tibial tubercle. The TFO is the parallel interval between the medial fibula and the anterior tibial tubercle. Dikos et al demonstrated a TFCS range of −0.8 to 5.8 mm and a TFO range of 1 to 6 mm in their normal ankle series. Negative values for the TFCS were found with a deep incisura.
Axial computed tomography (CT) is reported to be superior to conventional radiography in evaluating the syndesmosis.2,4,6,7,11,17 Many authors also use CT to postoperatively assess reduction. 5 Several CT techniques have been described in the literature to detect distal tibiofibular displacement or malreduction, with variable reproducibility.3,10,22 Six of these techniques found with a literature search are represented and summarized in Figure 2. CT parameters used to diagnose syndesmosis displacement and evaluate reduction are not universally accepted. 3
Axial computed tomography image used to measure tibiofibular clear space (A) and tibiofibular overlap (B).
Methods of detecting distal tibiofibular displacement or malreduction. Method 1: The angle between (B) the long axis of the fibula and (A) a line parallel to the posterior facet of the tibial incisura is determined, useful as comparison only (Marmor et al 13 ). Method 2: The distance between the fibula and the facets of the incisura anteriorly (A) and posteriorly (B) along a line perpendicular to the joint space is measured, with a difference of more than 2 mm considered incongruous. There is no reference in the paper as to how this technique was developed or verified (Gardner et al 5 ). Method 3: This method determines the angle (degrees) between the tangent to the anterior tibial surface at its most anterior point and the bisection of the vertical midline of the fibula. The tibial tangential line is approximated on the curved anterior tibia. The angle is compared to the contralateral side (Zwipp). 13 Method 4: The opening angle of the tangents of the medial fibular border (B) and the incisura at the level of the tibiofibular incisura is determined. These tangents normally form a dorsally open angle (Vasarhelyi et al 21 ). Method 5: This is the calculated ratio of the distances from the central reference point of the tibia to (A) the anterior cortex and (B) the posterior cortex of the fibula. The center reference point on the tibia is determined by using multiple bisecting lines and using the summation of the intersections as the center of the tibia. The ratio is used to compare to the contralateral side (Tang et al 20 ). Method 6: The angle between 2 tangential lines drawn at the anterior and posterior aspects of the lateral malleolus and tibia is calculated. The area between the fibula and the tibial incisura contained by the tangential lines is also measured. The angle and area are used to compare to the contralateral side (Malhotra et al 12 ).
Five of the 6 methods produce a devised angle or ratio of the ankle joint that is used to compare to the contralateral side. This assumes that the other ankle is normal and that there is minimal anatomical variation from side to side. The remaining method by Gardner et al is not validated in any way in the original article.
The purpose of this study was to demonstrate an anatomical feature seen on axial CT images of the ankle that can be used to identify a subluxated or malreduced distal tibiofibular joint. This line is referred to as the tibiofibular line (TFL) in this article. The TFL is drawn along the flat anterolateral surface of the fibula, extending it anteromedially to cross the tibia. The investigators hypothesized that this line would contact the anterior tibial tubercle in normal ankles and would be displaced in subluxated syndesmosis joints.
Methods
This study was conducted at a tertiary referral trauma center. All radiographic imaging was recorded in an Agfa WEB1000 picture archiving and communication system. With this system, a consecutive series of ankle CT scans were reviewed and collected between January 2006 and December 2012. The age, sex, and reason for the investigation were recorded. Inclusion in the study required the ankle CT to have axial reconstructions in the plane of the tibia. An axial image of the distal tibiofibular syndesmosis was selected from each patient’s series. The image for analysis was 10 mm superior to the tibial plafond, in the axial plane, which is consistent with other CT techniques. 10
An axial computed tomography image of ankle at the level of syndesmosis demonstrates the tibiofibular line.
Inclusion into the normal ankle group required a negative history for ankle injury, no malleolar fracture, and a TFCS and TFO that were within the normal range for uninjured ankles on plain radiographs. 3 The ankle CTs included in the normal group were performed for reasons of angiography, foot fractures, proximal tibial fractures, pathological lesions, and infection. The injured syndesmosis group was compiled from CT investigations of malleolar fractures and ankle sprains. Ankles with a confirmed displaced syndesmosis joint with abnormal TFCS and TFO were included.
The anterior TFL was drawn using an on-screen line tool with the imaging software. The line was placed along the straight anterolateral fibula cortex, extending it anteromedially across the anterior tibia. The distance the line was from the anterior tubercle of the tibia was recorded.
Results
The normal ankle population analyzed had an average age of 39 with 51 females and 99 males. The normal ankle group had a TFL that was within 2 mm of the anterior tibial tubercle. One hundred and fifteen (77%) had a TFL that was in contact or 0 mm from the anterior tibial tubercle, 29 (19%) had a line 1 mm from the tibial tubercle, and 6 (4%) were 2 mm from the tubercle.
In the 30 injured ankle CT scans, the average age was again 39; there were 11 females and 19 males. All the ankles had a TFCS and/or TFO outside the range of normal. The TFL was consistently displaced anteriorly, with a range of 4 to 19 mm (minimum to maximum).
One of the ankles examined with an undisplaced medial malleolus fracture demonstrated a normal syndesmosis on CT scan, an intact TFL, and normal TFO and TFCS measurements. Magnetic resonance imaging (MRI) was subsequently performed, which demonstrated injury to the anterior tibiofibular ligament of the syndesmosis. This highlights the limitations of static CT measurements of the syndesmosis, which will identify displacement or malreduction but not subtle injuries that have left the joint in a normal position.
Discussion
The mechanism of syndesmotic injury involves an external rotation force applied to the foot relative to the tibia. The injury may be purely ligamentous, or there may be an associated fracture. Associated fractures include pronation-external rotation ankle fractures (Weber type C fracture), supination-external rotation ankle fractures (Weber type B fracture), and fractures of the proximal fibula (Maisonneuve fracture). 14 The distal fragment has been shown to externally rotate with both supination–external rotation and pronation–external rotation types of injuries. 20 As a straight line, the TFL is sensitive to an externally rotated fibula, making the test consistent with the mechanism of injury. As a single oblique line, it will detect displacement in both the anterior-posterior axis and the medial-lateral axis.
Coronal and sagittal computed tomography images demonstrating the plane and height of the axial image used to measure the syndesmosis.
Results for the Normal and Injured Ankle Groups Including Number in Each Group and the Average and Standard Deviation of Tibiofibular Line (TFL) Displacement (P < .0001).
Axial images of 2 ankles with abnormal tibiofibular overlap and an anteriorly displaced TFL.
The TFL as a tool is applied to the anterolateral fibula, which is commonly a flat cortical surface. The anatomical feature is often not strictly a flat geometric line, however, which is reflected in the 0- to 2-mm variation in results when measuring the TFL. The key anatomical feature noted in the 150 normal ankle CT images is a smooth transition of cortical bone from the anterolateral fibula to the tibia at the 10-mm axial level. Placing a line on this surface helps define this normally linear relationship at the anterior tibiofibular joint.
There are several published CT techniques to diagnose syndesmosis abnormality, as presented in Figure 2. Knops et al 10 reviewed the reliability and accuracy of 4 leading CT methods examining malrotation of the syndesmosis. These methods derive angles or ratios using computer software on axial CT images to determine whether there is malrotation, by comparison to an assumed normal contralateral side. Vasarhelyi et al 21 described the techniques as susceptible to anatomic variation or difficult to perform. They require the use of calculations and are vulnerable to interobserver variability. 10 In this study, anatomic variation appears to be within the limit of 2 mm in the sample population, 96% being within 1 mm. Visual inspection with a straight line is adequate to confirm that the TFL is intact. However, interobserver reliability using the TFL was not assessed.
An axial magnetic resonance image of the ankle at the level of the syndesmosis demonstrating an intact tibiofibular line but increased signal at the anterior tibiofibular ligament. There is also a medial malleolus fracture.
Vectors of the tibiofibular line reveal displacement in the medial-lateral and anterior-posterior planes.
Image of ankle with Bristow instrument laid flat against the anterolateral fibular surface, demonstrating the linear transition of the fibula to the tibia. The Bristow in this position is the tibiofibular line in gross terms, 10 mm level above joint marked with pen.
The TFL is also useful following syndesmosis surgery to confirm adequate and correct reduction. Many surgeons are using limited CT scans of bilateral ankles to assess reduction postoperatively. Bilateral image comparison reduces anatomic variation. 21 We commonly perform postoperative limited ankle CT scans, using the TFL to confirm adequate reduction of the distal tibiofibular joint following internal fixation. Attention to the TFL is a useful landmark in single joint analysis or when evaluating bilateral CT scans.
There are limitations to all CT-based techniques in measuring the syndesmosis in ankle injuries. First, the ankle is not stressed or weight loaded and so there may be dynamic instability that is undetected on supine CT images. As demonstrated in this series, an injured syndesmosis joint may have an intact anterior TFL. Nielson et al 15 demonstrated that normal TFO and TFCS measurements did not preclude syndesmotic injury. This finding underscores the importance of clinical history, physical examination, and intraoperative stress testing to determine stability of the syndesmosis.
The TFL was not compared with a gold standard but rather with ankles confirmed to show diastasis using the Dikos et al 3 TFCS and TFO parameters. These measurements were chosen because the TFCS and TFO are well-known concepts when measuring the syndesmosis.
MRI has been shown to offer excellent specificity and sensitivity of injury when compared with observed arthroscopic changes, but there remains the question of what parameters indicate internal fixation of the syndesmosis is required. 19 Partial tears detected by MRI may be suitable for nonoperative management; it is not clear what degree of changes seen on MRI should be managed operatively. MRI is less accessible than CT for such common injuries as ankle sprains and fractures. Diastasis detected on CT is an agreed-upon indication for reduction and internal fixation of the distal tibiofibular joint. 23
The TFL offers an accessible method of accurately detecting static syndesmosis diastasis preoperatively and can be used to detect malreduction and malrotation postoperatively. The advantage of the TFL over other CT-based techniques is that it uses anatomical landmarks of the fibula’s relationship to the tibia, partially avoiding population and contralateral variation. The key advantage of the TFL is the simplicity of the single linear line measurement, which we use daily without computer software.
In 1902, Shenton 18 described a line of the pelvis, “broken continuity of which infallibly indicates displacement or trouble in the hip region.” The TFL is similarly simple to use and can identify “displacement or trouble” in the distal tibiofibular joint.
Conclusion
The TFL uses an observed anatomical feature of the distal tibiofibular joint to diagnose malposition of the fibula within the incisura tibialis. In this study, the TFL was consistently observed to be within 2 mm of the anterior tibial tubercle in normal ankles. Clinically, this line offers a simple way of examining the syndesmosis after ankle injury or postoperatively looking for diastasis or malreduction.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.