Comparison Between Cotton Test and Tap Test for the Assessment of Coronal Syndesmotic Instability: A Cadaveric Study

CATEGORY: Ankle; Basic Sciences/Biologics; Sports; Trauma INTRODUCTION/PURPOSE: Diagnosis of subtle instability of the distal tibiofibular syndesmosis is challenging. In surgically treated rotational malleolar fractures, instability is typically assessed with the intraoperative Cotton test. However, this test can be unreliable due to its dynamic nature and uncontrolled distraction force. The Tap test is an alternative test where a cortical tap is advanced through the fibula with a progressive, stable, and unidirectional distraction force. The objective of this cadaveric study was to compare the DTFS widening when using the Cotton and Tap tests as diagnostic tools for coronal plane syndesmotic instability. METHODS: Ten below-knee cadaveric specimens were tested in intact non-stressed, intact stressed, injured non-stressed, and injured stressed conditions, with stressed conditions utilizing both Cotton and Tap tests. In injured conditions, the syndesmotic ligamentous complex was sectioned (anterolateral longitudinal approach). Perfect fluoroscopic Mortise images were acquired for all conditions. For the Tap test, a 2.5 drill bit was used to drill a hole through both distal fibular cortices. A blunt-edged 3.5mm cortical tap was advanced toward the tibia. For the Cotton test, a lateral distraction force was applied to the distal fibula with a towel clamp. Two observers measured Tibiofibular Clear Space (TFCS) 1cm proximal to the ankle joint line. Intra and interobserver reliabilities were assessed by Intraclass Correlation Coefficient (ICC). Syndesmotic TFCS values for all conditions were compared by paired Wilcoxon. Diagnostic performance of the Cotton and Tap tests was assessed (a relative increase of TFCS>2mm). P-values <0.05 were considered significant. RESULTS: The intraclass correlation coefficient (ICC) for intraobserver and interobserver reliability was respectively, 0.96 and 0.78.TFCS measurements were similar in intact non-stressed, intact stressed (both Cotton and Tap tests) and injured non- stressed conditions: intact non-stressed, 3.5mm (CI, 3.0 to 3.9mm); intact stressed, 3.6mm (CI, 3.1 to 4.1mm) (Cotton test) and 4.0mm (CI, 3.5 to 4.5mm) (Tap test); injured non-stressed, 3.8mm (CI, 3.3 to 4.3mm). TFCS was significantly increased (p<0.0001) in injured and stressed ankles for both Cotton and Tap tests, with values of respectively, 6.2mm (CI, 5.8 to 6.7mm) and 6.1mm (CI, 5.7 to 6.6mm). The Cotton test had 73.3% sensitivity, 100% specificity, and 86.7% diagnostic accuracy. The Tap test had 70% sensitivity, 90% specificity, and 80% diagnostic accuracy. CONCLUSION: Our cadaveric study compared the Cotton and Tap tests for detection of coronal plane syndesmotic instability. Both tests demonstrated similar increases in TFCS measurements in stressed injured conditions when compared to intact and injured non-stressed conditions. Additionally, both tests demonstrated similar diagnostic accuracy for coronal plane syndesmotic instability, with slight favor for the Cotton test. In our experience, the Cotton test can be unreliable due to the difficulty in applying a steady distraction force while maintaining a perfect Mortise view. We recommend the Tap test as a more stable, controlled, and reproducible intraoperative diagnostic test for coronal plane syndesmotic instability.