Measuring Deltoid Insufficiency after Supination-External Rotation Ankle Fracture with Lateral Talar Subluxation on Gravity Stress View

CATEGORY: Trauma; Ankle INTRODUCTION/PURPOSE: Supination-external rotation (SER) variant ankle fracture is common, and identifying the unstable SER variants requiring surgery has been controversial. Stress view radiographs are frequently obtained to assist in decision-making. Measurement of the medial clear space (MCS) is used to determine medial structure integrity. SER variant ankle injuries are characterized by talus external rotation, and posterolateral translation causing a posterior displaced distal fibula fracture, with possible medial and posterior injury at further subluxation events. We propose that the MCS measurement is flawed by measuring from the point of rotational pivoting. Measurement of lateral displacement of the lateral talus versus the tibial plafond will be a more sensitive measurement tool for an injury mechanism characterized by external rotation and posterolateral subluxation. METHODS: IRB-approved, retrospective review of ankle injury radiographs for acute SER-variant isolated lateral malleolar fractures during a 3-year consecutive period. Patients were included if they had a gravity-stress view (GSV) evaluation. Patients were excluded for different fracture morphology, pediatric fractures, no GSV films, or prior history of ankle or foot arthritis or trauma. A priori power analysis was performed demonstrating 32 patients would be required to detect a change in 2mm. 102 patients underwent analysis. GSV were analyzed with measurements taken of the medial clear space (MCS), superior clear space (SCS), and we propose two novel measurements of lateral talus subluxation (LTS) by comparison to the anterior ankle syndesmosis marked by a vertical line at the tibial plafond. Standardized measurements were acquired by foot-and-ankle fellowship-trained orthopaedic surgeons and musculoskeletal radiologists. ICC data was obtained. Surgeons separately provided blinded decisions- for-surgery which patients appeared to have ankle instability requiring surgical stabilization. RESULTS: Measurements were performed for 102 patients, there were no significant differences in baseline demographics. There was a statistically significant increase in mean MCS, SCS, and LTS within the operative group (MCS 7.3 vs 5, p=0.005, SCS 4.6 vs 3.9, p=0.02, LTS 6.1 vs 2.9, p <0.001) with the LTS having the highest statistical significance. Pearson correlation analysis demonstrated positive and significant correlations between MCS and SCS (0.43, 0.37, P<0.001), MCS and LTS (0.51, p<0.001). MCS >=5mm had a sensitivity of 95% and specificity of 60% in our cohort, LTS >4mm demonstrating the same sensitivity (95%) with improved specificity to 75%. Receiver-operator-characteristic analysis for MCS vs LTS demonstrates an area under the curve (AUC) 0.786 for MCS >4 versus LTS (AUC) 0.918. CONCLUSION: The LTS measurement is superior to MCS measurement as a surrogate for medial ankle structure stability. We propose that ankle joint subluxation can be more accurately identified by measuring nearer to the point of maximal talar displacement in an external rotation and posterolaterally displacing injury pattern. The LTS measurement elevation >4mm was found to be superior to the MCS measurement >4mm in all categories, with high sensitivity and specificity, and superior performance in ROC analysis. We propose this will be a useful tool for clinicians when evaluating SER variant ankle fractures.