New Measurement Proposed for Detecting Syndesmotic Instability in 3D
Key findings
- This retrospective case–control study investigated the clinical value of weight-bearing CT (WBCT) volume measurements of syndesmotic instability compared with area measurements—in essence, it compared the value of three-dimensional versus two-dimensional methods
- The cases were 24 adults who had surgically confirmed syndesmotic instability and who had undergone preoperative WBCT; the controls were 24 adults who underwent WBCT for midfoot or forefoot conditions
- The team determined cutoff values for each method (3D and 2D) in terms of measurement in metric scale and also as a percentage of change (increase) in the value when comparing each patient's injured side to their uninjured side
- Using volumetric measurement extending up to 5 cm above the tibial plafond showed the highest sensitivity, specificity and accuracy for recognizing subtle unstable syndesmosis
Syndesmotic instability occurs as a three-dimensional (3D) process, since the fibula may translate in the coronal, sagittal and/or rotational planes depending on the severity of the injury—and weight-bearing computed tomography (WBCT) of the ankle affords a 3D view of this alteration in anatomy. Currently, most routinely available and utilized methods for identifying instability at the tibiofibular articulation use simple two-dimensional (2D) x-rays, and those that do employ WBCT imaging have typically not capitalized on this new methodology, as they have historically been performed as 1D (they measure distal tibiofibular distance) or 2D (they measure distal tibiofibular area) analyses.
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Researchers at Massachusetts General Hospital have developed a new 3D method of WBCT image analysis—the first to be applied in a clinical setting—that relies on "percentage of change," rather than using an absolute, metric measurement. From Mass General's Foot and Ankle Research and Innovation Laboratory (FARIL), Soheil Ashkani-Esfahani, MD, the lead investigator, Foot and Ankle surgeons Daniel Guss, MD, MBA, and Gregory Waryasz, MD; Christopher DiGiovanni, MD, chief of the Foot and Ankle Center, Bart Lubberts, MD, PhD, former R&D director; and Rohan Bhimani, MD, MBA, Foot and ankle research fellow at the Department of Orthopaedic Surgery; explains the details in the Journal of Orthopaedic Research.
Study Methods
The cases in this study were 24 adults who underwent WBCT at Mass General between 2015 and 2020 and had no syndesmosis appreciated on imaging. All experienced continued symptoms, and later had unilateral syndesmotic instability diagnosed via arthroscopy or intraoperatively. The study controls were 24 adults who underwent WBCT for midfoot or forefoot conditions unrelated to ankle pathology.
On scans of the 72 uninjured ankles (24 in the cases and both ankles of the 24 controls), the researchers measured the area of the joint (2D) at 0, 1, 3, 5 and 10 cm cross-sections proximal to the tibial plafond. They also created volumetric shapes (3D) of the distal tibiofibular interspace starting at the tibial plafond and extending to heights of 1, 3, 5 and 10 cm proximally.
The team determined cutoff values for each method—both in terms of measurement in metric scale, and also as a percentage of change (increase) in the value in contrast with the uninjured, contralateral ankle.
Results
Volumetric measurements up to 5 cm above the tibial plafond showed the best level of statistical significance (P<0.001 vs. the other methods) for detection of syndesmotic instability. At 5 cm, corresponding to a cutoff value of 11.6 cm3 and 25% change in the volume of the syndesmotic space:
- Sensitivity—96%
- Specificity—83%
- Positive predictive value—86%
- Negative predictive value—95%
- Accuracy—90%
Implications for the Clinic
The optimal method for assessing syndesmotic instability appears to be employment of WBCT with volumetric (3D) figuring that spans the interspace between the distal fibula and tibia and extends from the plafond to a height of 5 cm proximally. The percentage of change values provided in this paper can easily be used to compare the injured and uninjured sides on bilateral WBCT images and determine whether the syndesmosis is unstable.
Percentage of change is a more unified and easier-to-use diagnostic value than metric measurement because measurement units differ around the world (inch vs. centimeter), a percentage is easier to use to create threshold values and there is no need for calibration of the measurement tools based on image magnification.
As physicians become better able to identify subtle asymmetries between the injured and uninjured ankles, treatment of syndesmotic instability might evolve to be more tailored to each individual patient's anatomy.
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