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Fig. 1 Anterior view of weight-bearing computed tomography syndesmotic joint measurements in a patient with a Weber B fracture with right-sided syndesmotic instability: syndesmotic joint volume up to 5 cm above the joint line (A), and syndesmotic joint volume up to 5 cm above the joint line (B).
Courtesy of Rohan Bhimani, MD, MBA

AAOS Now

Published 9/2/2021
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Terry Stanton

Study Gauges 1D, 2D, and 3D Weight-bearing CT to Diagnose Syndesmotic Instability

A study presented Wednesday aimed to evaluate the ability of weight-bearing computed tomography (WBCT) to diagnose syndesmotic instability using one-dimensional (1D), 2D, and 3D measurements among patients with unilateral Weber B lateral malleolar ankle fractures. The patients in the study did not demonstrate widened medial clear space (MCS) on initial radiographs yet demonstrated operatively confirmed syndesmotic instability.

Presenting author Rohan Bhimani, MD, MBA, of Massachusetts General Hospital, told AAOS Now Daily Edition that the study yielded three primary findings:

  • Using the uninjured side as an internal control, syndesmotic joint volumetric measurements from the tibial plafond (extending to 3 cm and 5 cm proximally), fibular rotation, and middle tibiofibular distance were significantly increased on the injured side on WBCT in patients with unilateral syndesmotic instability with Weber B ankle fractures (Fig. 1).
  • Previously used 2D measurement such as syndesmotic area and 1D measurements such as MCS distance, as well as anterior and posterior tibiofibular distance, did not show a side-to-side difference.
  • Among patients with syndesmotic instability, the ratio of 3D volume of the distal tibiofibular articulation (measured from the plafond to a height of both 3 cm and 5 cm proximally) between the injured and uninjured side was larger than the ratios between injured and uninjured sides on non-3D measurements.

The study involved patients with unilateral, surgically confirmed syndesmotic instability accompanying a Weber B type lateral malleolar ankle fracture (n = 24) who underwent preoperative bilateral foot and ankle WBCT. A separate group of patients with unilateral Weber B ankle fractures without syndesmotic instability, and who underwent bilateral WBCT, served as a control group (n = 18). With the uninjured side serving as an internal control, measurements on bilateral WBCT images included: (1) syndesmotic area; (2) tibiofibular distance measured at the anterior, middle, and posterior aspects of the distal tibiofibular articulation; (3) fibular rotation; (4) distance from fibular tip to plafond; (5) fibular fracture displacement; and (6) MCS distance.

In addition, 3D volumetric measurements were also calculated, including syndesmotic joint volume from the tibial plafond extending to 3 cm proximal, syndesmotic joint volume from the tibial plafond extending to 5 cm proximal, MCS volume, and lateral clear space volume. Their sensitivities were compared to the aforementioned measurements.

Among patients with unilateral syndesmotic instability with Weber B ankle fractures, all WBCT measurements, except MCS distance, syndesmotic area, and anterior/posterior tibiofibular distance, were significantly greater on the injured compared to the uninjured side (P values ranging from <0.001 to 0.004). Of those measurements, 3D syndesmosis volumetric measurements spanning from the tibial plafond to a level 3 cm and 5 cm proximally had the highest relative volumetric ratio between the injured and uninjured side, suggesting high sensitivity to distinguish between stable and unstable syndesmotic injuries (P values ranging from 0.001–0.036). In the control group without syndesmotic instability, no evaluated WBCT parameters showed a significant side-to-side difference, except for MCS volume and distance from fibular tip to tibial plafond.

Dr. Bhimani said the most unexpected finding of the study was “an increase in MCS volume on the injured side compared to the contralateral healthy side, despite normal MCS distance in both groups.” Theoretically, he said, “The tibiotalar contact area does not decrease until the deltoid ligament is injured. The increases in WBCT MCS volume measurements on the injured side highlights that we do not completely understand what happens to the medial clear space when the fibular buttress is lost.”

He commented that his own experience has shown him that “3D volumetric measurement is a very sensitive tool to detect syndesmotic instability, especially when subtle.” Dr. Bhimani added that, because volume can be defined by the formula volume = area × height, 3D volumetric measurement offers “a better picture of the instability than traditional 2D [area] methods.” Because syndesmotic instability is a 3D problem, with destabilization of the distal tibiofibular articulation resulting in both translation and rotation of the fibula in varying planes relative to the tibia, he said, “It is logical that a 3D volumetric measurement rather than a 2D area measurement would more accurately reflect the anatomic processes. As a result, it perhaps more accurately captures increasingly subtle instability potentially missed by 2D measurements.”

According to Dr. Bhimani, the clinical takeaways from this report are that WBCT scan effectively diagnoses syndesmotic instability in the presence of a Weber B ankle fracture among patients with symmetric MCS who would otherwise not necessarily undergo operative intervention, and that 3D volume measurements have a higher sensitivity than traditional distance measurements in diagnosing instability. These measurements should be taken to a height of 5 cm above the tibial plafond.

Dr. Bhimani’s coauthors of “Utility of Weight-bearing Computed Tomography to Diagnose Syndesmotic Instability in Weber B Lateral Malleolar Fractures,” are Soheil Ashkani-Esfahani, MD; Bart Lubberts, MD, PhD; Gino M.M.J. Kerkhoffs, MD; Gregory Waryasz, MD; Christopher W. DiGiovanni, MD; and Daniel Guss, MD, MBA.

Terry Stanton is the senior medical writer for AAOS Now. He can be reached at tstanton@aaos.org.