The SER-IV Pivot: MCS and LTS Increases on Computed Tomography Are Correlated to External Rotation of the Talus

BackgroundSupination-external rotation (SER) variant ankle fractures can have unstable injury patterns indicating surgical treatment. Medial clear space (MCS) on 2-dimensional radiographs is 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 computed tomography (CT) scans to investigate the 3-dimensional talar displacement with this novel measurement. This axial rotation measurement of the talus may be a better measure than current standards, as the rotational component of these ankle fractures is underestimated.MethodsThis study is a retrospective review that initially identified 158 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). Ultimately, 20 patients fit our inclusion criteria. 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. Post hoc power analysis was subsequently performed.ResultsThe mean axial rotation angle was noted to be 6.20 ± 3.79 degrees in external rotation. Axial MCS and axial LTS were 3.64 mm ± 1.10 mm and 2.20 mm ± 1.41 mm, respectively. Coronal MCS and coronal LTS were 3.15 mm ± 1.28 mm and 1.08 mm ± 1.61 mm, respectively. There were significant correlations between both MCS and LTS with respect to talar external rotation on axial CT (r = 0.85 and 0.78, respectively). Regression analysis predicted that for each 1 mm of MCS increase, there are 1.5 degrees of increased talar external rotational displacement. Similarly, for every 1 mm LTS increase, there are 2 degrees of talar external rotational displacement.ConclusionLateral talar subluxation has previously been described as a useful measurement tool in identifying ankle instability and the potential need for surgical intervention. However, plain radiographs have limitations as the talus undergoes displacement in 3-dimensional positional planes. Using CT imaging, we found that both increased MCS and increased LTS positively correlated with increased talar external rotational as well as other displacement surrogates. There may be unrecognized rotational displacement that persists after SER ankle fracture injury, and careful consideration should be given by the surgeon for this phenomenon.Level of Evidence:Level III, retrospective cohort study.