Updated Threshold Values Needed for Arthroscopic Diagnosis of Syndesmotic Instability
Key findings
- This systematic review identified eight cadaveric and three clinical studies that explored what amount of fibular displacement correlates with syndesmotic instability after a high ankle sprain
- Many studies used 2 mm as a cutoff value for distal tibiofibular diastasis in the coronal plane, but others suggested this may overdiagnose syndesmotic instability and that 3 mm would be a better threshold
- In four cadaveric studies that had a similar design, the weighted mean associated with instability in the coronal plane was 2.9 mm at the anterior portion of the distal tibiofibular joint and 3.4 mm at the posterior portion
- Regarding the sagittal plane, data from a cadaveric study suggested cutoffs of 2.2 mm of posterior fibular translation when performing an anterior-to-posterior hook test and 2.6 mm of anterior translation when performing a posterior-to-anterior hook test
Ankle arthroscopy has been proposed as the gold standard for diagnosing subtle syndesmotic instability, but the amount of fibular motion that correlates with instability remains unclear. Most studies use a cutoff value between 2 mm and 3 mm.
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Noortje Hagemeijer, MD, lead researcher in the Foot and Ankle Research and Innovation Laboratory in the Department of Orthopaedic Surgery at Massachusetts General Hospital, Gregory Waryasz, MD, foot and ankle surgeon and sports medicine physician, Daniel Guss, MD, MBA, orthopedic foot and ankle surgeon, Christopher W. DiGiovanni, MD, chief of the Foot and Ankle Center at Mass General, and colleagues have conducted the first systematic review of published literature to explore what amount of fibular displacement correlates with syndesmotic instability after a high ankle sprain. Their results suggest a cutoff of 2.0 mm may lead to overtreatment, and in Knee Surgery, Sports Traumatology, Arthroscopy, they suggest more evidence-based thresholds.
Study Methods
The researchers searched EMBASE, PubMed, CINAHL, Web of Science and Google Scholar from the earliest citations through June 18, 2019. Studies had to be full-text, English-language reports on the arthroscopic evaluation of fibular displacement in humans in at least one of the three planes after a syndesmotic ligament injury. Eight cadaveric and three clinical studies were identified.
Coronal Plane
Most of the studies identified evaluated syndesmosis primarily in the coronal plane while applying a lateral fibular "hook test." Proposed cutoff values ranged from 1 mm to >4 mm, and there was inconsistency about whether stress should be applied.
Many studies used 2 mm as a cutoff, but others suggested this may overdiagnose syndesmotic instability and 3 mm would be a better threshold in the coronal plane. In four cadaveric studies that had a similar design, the weighted mean associated with instability in the coronal plane was 2.9 mm at the anterior portion of the distal tibiofibular joint and 3.4 mm at the posterior portion.
Sagittal Plane
Fibular translation in the sagittal plane was less well studied. However, data from a cadaveric study suggested thresholds of 2.2 mm of posterior fibular translation when performing an anterior-to-posterior hook test and 2.6 mm of anterior fibular translation when performing a posterior-to-anterior hook test.
Rotational Plane
Fibular external rotational stability was rarely assessed arthroscopically. One clinical study evaluated it by measuring the difference between the distance from the anterior border to the incisura and the distance from the posterior border to the incisura. This value can be confounded by concomitant coronal and sagittal plane translation, though, so arthroscopy may not be a suitable method for determining fibular rotation.
Recommendations for Surgeons
2.9 mm measured at the anterior portion of the incisura and 3.4 mm at the posterior portion may be the best cutoffs for distal tibiofibular diastasis in the coronal plane. At the very least, since 3-mm arthroscopic probes are readily available, surgeons should use them in lieu of 2-mm probes.
Other considerations suggested by this review:
- Measurements made in the posterior third of the incisura may result in higher values than those taken anteriorly
- The use of an ankle distractor can mask syndesmotic instability, so distraction should be released at the time of measurement
- Stress forces >100 N do not result in additional diastasis, and numerous studies have standardized a 100-N force applied to the fibula 5 cm proximal to the tibiotalar joint in either the coronal or sagittal plane
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