- Using a cadaveric model, researchers at the Massachusetts General Hospital Foot and Ankle Research and Innovation Lab (FARIL) conducted arthroscopic assessments of syndesmotic instability in the sagittal plane
- Syndesmotic instability appears in the sagittal plane after injury to all three syndesmotic ligaments or after partial syndesmotic injury with concomitant deltoid ligament injury
- The optimal cutoff point to arthroscopically distinguish stable from unstable injuries in the sagittal plane was 2 mm of fibular translation for the total sum of anterior-to-posterior and posterior-to-anterior translation
- Arthroscopic assessment of sagittal plane translation is a more sensitive assessment of syndesmotic instability than coronal plane translation
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It's now understood that untreated syndesmotic instability can result in prolonged time to return to daily activities and degenerative changes of the ankle joint over time. Radiography and MRI have limited ability to explicitly diagnose syndesmotic instability, especially when subtle, so arthroscopy is being used increasingly often for assessment.
In a previous arthroscopic cadaveric study in Foot and Ankle International, researchers at the Massachusetts General Hospital Foot and Ankle Research and Innovation Lab (FARIL) detected unstable syndesmosis in the coronal plane when all three syndesmotic ligaments were injured—the anterior inferior tibiofibular ligament (AITFL), the tibiofibular interosseous ligament (IOL) and the posterior inferior tibiofibular ligament (PITFL). Similar findings were also found after partial injury (AITFL and IOL) if there was a concomitant injury of the deltoid ligament (DL).
In a new study, published in Foot & Ankle International, Bart Lubberts, MD, PhD, director of research and development in FARIL, Daniel Guss, MD, MBA, orthopaedic foot and ankle surgeon, Christopher W. DiGiovanni, MD, chief of the Foot and Ankle Center at Mass General, and colleagues found that similar patterns of injury also render the syndesmosis unstable in the sagittal plane.
In fact, they report that translation measurements of unstable injuries in the sagittal plane were greater than their previous findings in the coronal plane. Thus, arthroscopic assessment of sagittal plane translation is more sensitive than the assessment of coronal plane translation for diagnosing syndesmotic instability.
The researchers studied 21 nonpaired human cadaveric specimens, randomly divided into three groups of seven. Each specimen underwent arthroscopic evaluation of the syndesmosis, first with all syndesmotic and ankle ligaments intact and then with sequential sectioning of ligaments, based on common patterns of injury:
- In group 1, the AITFL was transected first, followed sequentially by the IOL, the PITFL and DL
- In group 2, the sequence was DL, AITFL, IOL and PITFL
- In group 3, the sequence was PITFL, IOL, AITFL and DL
In all scenarios, anterior-to-posterior (AP) and posterior-to-anterior (PA) fibular translation tests were performed under a 100-N applied force. The researchers measured the AP and PA sagittal plane translation of the distal fibula relative to the fixed tibial incisura.
The average fibular translation in the sagittal plane with an intact syndesmosis was 1.1 mm in group 1, 0.9 mm in group 2 and 0.7 mm in group 3. That value did not increase significantly in any group after the transection of one or two ligaments.
However, fibular translation in the sagittal plane increased significantly after the transection of all syndesmotic ligaments. The same was true in group 2 after transection of DL followed by transection of AITFL and IOL.
The researchers performed a receiver operating curve (ROC) analysis to determine how well the different values of sagittal translation distinguished stable from unstable syndesmotic injuries. They identified a cutoff value of 2 mm (total sum of AP and PA fibular translation) as having the highest sensitivity (78%) and specificity (89%).
The area under the ROC curve reflects the accuracy of a diagnostic test, from 0.5 (a worthless test) to 1.0 (a perfect test). In this study, the area under the curve for predicting instability in the sagittal plane had a score of 0.91 (95% CI, 0.85–0.96).
Applying the Findings to Practice
These findings demonstrate that during arthroscopy, a stress maneuver performed in the sagittal plane readily identifies an unstable syndesmosis. They also have implications for the clinical setting. Maneuvers such as the fibular shuck, performed in the AP plane, can be readily performed in the office and will be especially useful if the fibular translation is quantified with an imaging modality such as ultrasound.
The sensitivity (true-positive rate) of the 2-mm cutoff value was high, 78%, but still means that a surgeon has a 22% chance of misdiagnosing an unstable injury as being stable. By using a lower cutoff value, a surgeon would err toward fixing stable injuries rather than missing unstable ones.
The fact that transection of the DL contributed to sagittal plane translation probably reflects the ability of the DL to tie the distal fibula through the talus via intact lateral ankle ligaments. Whether a DL injury should be repaired in patients with acute unstable syndesmotic injury is still being debated.
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