Comparison between cotton test and tap test for the assessment of coronal syndesmotic instability: A cadaveric study

Abstract

Introduction: In surgically treated rotational malleolar fractures, residual syndesmotic instability is typically assessed following fixation with the widely used intraoperative Cotton test. However, due to its dynamic nature, there are inconsistencies of the magnitude and direction of the distraction force when attempting to pull the fibula away from the tibia using a bone hook. The novel Tap test advances a cortical tap through a drilled hole in the fibula with a stable, unidirectional distraction force applied to the tibia. The objective of this cadaveric study was to compare the Cotton and Tap tests as diagnostic tools for coronal plane syndesmotic instability.

Methods: Tibiofibular Clear Space (TFCS) of 10 cadaveric specimens was measured for: intact, non-stressed; intact, stressed; injured, non-stressed; and injured, stressed (Tap and Cotton tests). In injured conditions, the syndesmotic ligamentous complex was sectioned using an anterolateral longitudinal approach. Perfect fluoroscopic Mortise images were acquired for all conditions. Two independent and blinded Orthopaedic Foot and Ankle Surgeons measured TFCS 1 cm proximal to the ankle joint line. Intra and interobserver reliabilities were assessed by Intraclass Correlation Coefficient. Syndesmotic TFCS values for all conditions were compared by paired Wilcoxon. Diagnostic performance of the Cotton and Tap tests was assessed using a relative increase of TFCS > 2 mm when comparing intact stressed and injured stressed conditions. P-values <0.05 were considered significant.

Results: The intraclass correlation coefficient for intraobserver and interobserver reliability was respectively 0.96 and 0.79. TFCS measurements were similar in intact non-stressed, intact stressed (both Cotton and Tap tests) and injured non-stressed conditions, with mean values and 95% Confidence Intervals of: intact non-stressed, 3.5 mm; intact stressed, 3.6 mm (Cotton test) and 4.0 mm (Tap test); injured non-stressed, 3.8 mm. The Cotton test and Tap test had, respectively, 73.3% and 70% sensitivity, 100% and 90% specificity, 86.7% and 80% diagnostic accuracy.

Conclusions: Our cadaveric study compared the Cotton and Tap tests for detection of coronal plane syndesmotic instability. Both tests demonstrated similar increases in the TFCS measurements in stressed injured conditions when compared to intact non-stressed and stressed conditions, as well as injured non-stressed conditions.

Keywords: Ankle fracture; Cotton test; Malleolar fracture; Syndesmotic injury; Syndesmotic instability.