Lateral talar subluxation (LTS) in SER ankle fractures is more responsive than medial clear space widening: A cadaveric biomechanical study

CATEGORY: Ankle; Trauma INTRODUCTION/ PURPOSE: Supination-external rotation (SER) ankle fractures are characterized by posterolateral translation and external rotation. In isolated fibula fracture SER variants, medial soft tissue structure incompetence can be identified with medial clear space (MCS) widening or with lateral talar subluxation (LTS), frequently obtained on stress view imaging. This assessment is a key determination of which patients may require surgical management to restore ankle joint stability. Current radiographic studies demonstrated increased sensitivity and specificity of LTS versus MCS which may be due to increased rotational movement occurring in the direction of the talar displacement. Biomechanical data has not yet been established. The purpose of this study was to investigate MCS and LTS displacements in a cadaveric biomechanical study to confirm this rotational movement principle in a controlled setting. METHODS: An open approach was utilized to surgically create an SER isolated fibula fracture in five below-knee cadaveric specimens. Open anteromedial and lateral ankle approaches were created for visualization of the medial and lateral structures, also allowing direct measurement of the MCS and LTS with a ruler. There was no ankle instability prior to injury, MCS and LTS were zero in each specimen prior to experimental model creation. An SER-pattern fibular osteotomy was made with an oscillating saw. The superficial and deep deltoid were then released from the medial malleolus, and the foot was displaced maximally creating an SER-4b variant ankle fracture. The MCS and LTS were then measured. Differences were analyzed using a paired Student’s t-test. Effect size (Cohen’s d) was measured to characterize statistical significance and determine relative importance in the degree of change. RESULTS: There was no underlying ankle stability, deformity, or arthritis in any of the 5 cadaveric ankle specimens. In our simulated SER-4b ankle fractures, the mean MCS widening using medial landmarks was 7.2 mm (+/- 1.3mm). The mean lateral displacement utilizing the LTS landmarks was 11.2mm (+/- 1.3mm). The difference between medial and lateral displacement was found to be statistically significant (p = 0.0067, 95% CI [1.85, 6.15]). Cohen’s d was equal to 3.1 (95% CI [1.24, 4.90]), demonstrating a large effect size, showing the mean to be different by 3 standard deviations. The increase in mean lateral displacement was 43% greater when compared to the mean medial displacement. CONCLUSION: In this cadaveric study, LTS displacement was significantly greater than MCS widening following simulation of SER type IV ankle fracture injuries. The effect size of 3.1 confirms statistical significance. This data supports that talus external rotation is a significant factor in SER pattern displacement. The rotational moment arm is largest laterally with external rotation, and posterolateral displacement. Our study supports the idea of measuring the LTS radiographically as a marker to assist in surgical decision-making. Further characterization of the displacement pattern of SER injuries will be helpful to assist surgical decision- making in these common ankle injuries.