Dynamic Portable Ultrasound Cutoff Values for Diagnosing Medial Ankle Instability in Supination External Rotation Ankle Injury: A Cadaveric Study

CATEGORY: Ankle; Trauma INTRODUCTION/PURPOSE: In the setting of an isolated lateral malleolar fracture, stability is predicated by whether the deep deltoid ligament is intact. Portable ultrasound (P-US) has also been used in the clinical setting, allowing dynamic and non-invasive evaluation at the point of care. This study aimed to determine the cutoff values, as well as its sensitivity and specificity for diagnosing supination external rotation (SER) ankle injury stage IVb (with complete deltoid ligament rupture) using the P-US. METHODS: Ten fresh-frozen cadaveric specimens were used. Medial ankle instability was assessed by the P-US. The assessment was performed with all structures intact and later with sequential transection of the AITFL, Weber-B fibular fracture, the PITFL, the superficial and deep deltoid ligament. In all scenarios, three loading conditions were considered, including the GST, the external rotation stress test (45N), and the simulated weight-bearing (750N). The P-US measurement of the MCS was assessed at the anteromedial aspect and inferomedial aspect of the ankle joint. The P-US measurement of the MCS was assessed by placing the ultrasound probe perpendicularly to the medial gutter at the anteromedial aspect and inferomedial aspect of the ankle joint. Three different MCS distances were measured, as demonstrated in Figure 1. To determine the sensitivity and specificity of the diagnosis of SER injury type IV using the P-US, a receiver operating characteristic (ROC) curve analysis was performed. RESULTS: The MCS optimal cutoff point to distinguish SER ankle injury stage IVb from other injury stages ranged from 10.4 to 13.8 during the GST, 4.2 to 7.9mm during the weight-bearing stress, and 10.6-17.9mm during the external rotation stress test (Table 1). All P-US cutoff values provided acceptable sensitivity (60-100%), specificity (84-100%), and the area under ROC curve (0.8-1.0) for diagnosing SER ankle injury stage IVb. CONCLUSION: MCS cutoff points assessed with the P-US under the GST, weight-bearing, and the external rotation stress test demonstrate high sensitivity and specificity for detecting SER ankle injury stage IVb. The cutoff values vary depending on which loading condition is being used. Additional clinical studies are necessary in the in vivo setting to better understand the implications of these test conditions.