The SER IV Pivot- CT Analysis Demonstrates External Rotation Drives MCS and LTS Increases in Unstable Ankle Fractures

Submission Type: Ankle Fractures Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose: Supination-external rotation (SER) variant ankle fractures can have unstable injury patterns indicating surgical treatment. Medial clear space (MCS) on 2-dimensional radiographs are often variable and validity of this measurement is argued. Recently, lateral talar subluxation (LTS) has been posed to be a more specific measurement than MCS likely because the pattern of injury is an external rotation pivoting displacement mechanism. However, there have been no studies using CT scans to investigate the 3-dimensional talar displacement with this novel measurement. Methods: This study is a retrospective review that identified 115 patients with unstable (SER IV) bi- or trimalleolar ankle fracture variants with injury radiographs and CT scans over a consecutive 10-year period from 2013 to 2023. Standardized measurements of radiographic MCS and LTS were acquired as well as CT scan measurements of the Axial MCS, Axial LTS, and a novel measurement of Axial rotation angle which is the amount of talar rotatory fracture deformity relative to the tibia. Coronal MCS, and Coronal LTS were also measured (Figure 1). The primary outcome measure investigated was the degree of external rotation of the talus in SER IV patterns. Secondary outcomes were other measurement surrogates and their correlations. Regression analysis was performed to predict the amount of rotational displacement by MCS or LTS abnormalities. Power analysis was performed. Results: The mean axial rotation angle was noted to be 8.62° +/- 6.29° external rotation. Axial MCS and Axial LTS were 4.49 mm +/- 1.81mm and 2.19 mm +/- 2.34 mm, respectively. Coronal MCS and Coronal LTS were 3.84 mm +/- 1.66 mm and 1.56 mm +/- 2.23 mm, respectively. There were significant correlations between MCS and LTS with R=0.58 and 0.35 (p < 0.00001) as seen in Figure 2. Regression analysis predicted that for each 1 mm of MCS increase, there is 2 degrees of increased talar external rotational displacement. Similarly, for every 1 mm LTS increase, there is 1 degree of talar external rotational displacement. Conclusion: CT imaging revealed a pivoting external rotation mechanism of the talus in SER IV patterns as a significant contributor of the deformity. As horizontal displacement increased through MCS and LTS measures, so did the degree of rotational displacement confirming the pivoting displacement mechanism of SER IV variant ankle injuries. This highlights the importance of the rotational aspect of the injury which is inadequately assessed with conventional radiographs. Regression analysis predicts more external rotational deformity with worse radiographic MCS and LTS measurements. Limitations of this study include being retrospective and variability in CT indications.