Weight-bearing CT Detects Subtle Syndesmotic Instability in Presence of Fibular Fracture
Key findings
- This study evaluated the ability of weight-bearing computed tomography (WBCT) to diagnose syndesmotic instability in patients with unilateral Weber B lateral malleolar ankle fractures without medial clear space widening
- Bilateral 1D, 2D and 3D measurements were obtained for adult patients undergoing surgical fixation of ankle fracture: 23 who had syndesmotic instability identified intraoperatively, and 18 who were deemed intraoperatively to have a stable syndesmosis
- WBCT was able to identify an unstable ankle syndesmosis even in the presence of fracture, with useful measurements including middle tibiofibular distance, fibular rotation and 3D volumetric measurements on WBCT
- Other commonly used WBCT 1D and 2D measurements for diagnosing syndesmotic instability (e.g., anterior tibiofibular distance, posterior tibiofibular distance and syndesmotic area) were not significantly different from side to side
- Measuring the 3D volume of the distal tibiofibular articulation from the tibial plafond to a height of 5 cm proximally was the best test for diagnosing subtle syndesmotic instability, and can do so even in the presence of a fractured lateral malleolus
Tenderness to palpation is an unreliable test of the ligamentous injury and instability that can accompany Weber B ankle fractures (distal fibular fractures). Therefore, clinical examination is almost always supplemented with imaging, but traditional modalities vary in their ability to depict syndesmosis under stress.
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Previous research at Massachusetts General Hospital established bilateral weight-bearing computed tomography (WBCT) as an effective tool to distinguish stable from unstable syndesmotic injuries because it allows direct comparison of the injured and uninjured sides in three dimensions (distance, area and volume) while under physiologic load.
A new study expands on those findings, showing that WBCT can diagnose syndesmotic instability among patients who have lateral malleolus fractures but symmetric medial clear space (MCS) on initial radiographs. These patients are normally treated conservatively, so there is no chance for an intraoperative stress evaluation of syndesmosis.
Rohan Bhimani, MD, MBA, research fellow in the Foot and Ankle Research and Innovation Laboratory in the Department of Orthopaedic Surgery at Mass General, Daniel Guss, MD, MBA, a foot and ankle surgeon in the department, Christopher DiGiovanni, MD, chief of the Foot and Ankle Service, and colleagues report in the Journal of the American Academy of Orthopaedic Surgeons.
Methods
The researchers reviewed bilateral foot and ankle WBCT scans that were attained at Mass General between 2016 and 2020. They identified 41 patients with unilateral Weber B lateral malleolar fracture and symmetric MCS who had another indication for surgery (intra-articular loose bodies, syndesmosis tenderness with or without posterior malleolar fracture or fibular displacement and shortening):
- Patient group—23 adults who had syndesmotic instability identified intraoperatively that required surgical fixation
- Control group—18 subjects who were deemed intraoperatively to have a stable syndesmosis
Injured vs. Uninjured Side
Measurements were made on one-dimensional, 2D and 3D (volumetric) images:
- Patient group—The following were significantly increased on the injured side compared with the uninjured side: the volume of the distal tibiofibular articulation measured from the plafond to a height of either 3 or 5 cm proximally, MCS volume, lateral clear space volume, middle tibiofibular distance, fibular rotation and distance from fibular tip to plafond
- Control group—Only MCS volume and navicular to floor distance were significantly increased on the injured side, and the distance from the fibular tip to the plafond was decreased on the injured side
WBCT for Diagnosis
Findings of the value of WBCT for detecting syndesmotic instability were:
- 3D syndesmotic measurements had a larger area under the receiver operating curve (AUC) than measurements obtained from 1D or 2D scans
- Measuring the 3D volume of the distal tibiofibular articulation from the tibial plafond to a height of 5 cm proximally was an outstanding diagnostic test (AUC, 0.96)
- With that test, the optimum cutoff for predicting syndesmotic instability was 10.2 cm3 (sensitivity, 90%; specificity, 95%; accuracy, 93%)
- When the volume was measured from the plafond to 3 cm, the AUC was 0.91 (at a cutoff of 4.8 cm3 the sensitivity was 90%; specificity, 90%; accuracy, 90%)
Guidance for the Surgeons
To detect subtle syndesmotic instability, measuring distal tibiofibular volume is a sensitive test for diagnosing syndesmotic instability, and measuring to 5 cm may be a better strategy than shorter distances such as 3 cm.
WBCT need not be used for patients with tibiotalar subluxation and MCS widening because they will definitely undergo surgical fixation and therefore can have intraoperative stress evaluation of the syndesmosis intraoperatively. Instead, WBCT is expedient when the tibiotalar relationship appears to be intact but there is concern that syndesmotic instability exists and may require stabilization.
Most current imaging systems will need software upgrades in order to obtain WBCT measurements such as area and volume in a manner accessible to most clinicians.
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