Should the Tibiotalar Angle Be Measured Using an AP or Mortise Radiograph? Does It Matter?

Understanding the tibiotalar angle (TTA) is key to planning for deformity correction. The TTA is an important radiographic tool to determine alignment or malalignment of the ankle and hindfoot. Two methods of measuring the TTA have been described: the midline TTA (MTTA) and the lateral TTA (LTTA). The aim of this study was to compare the 2 angles as measured on mortise and anteroposterior (AP) radiographs in a series of normal and pathological cases. A radiographic review was performed of sequential ankle AP and mortise radiographs taken between January 2016 and September 2017 across 4 specialist orthopedic centers. Patients were categorized into a normal group, where patients had normal radiological appearances, and an arthritis group, where patients had radiographic arthritis. The MTTA and the LTTA were measured. The overall mean ± standard deviation MTTA was 88.7° ± 5.1°, and mean LTTA was 87.5° ± 5.2° (p < .01). There was no statistically significant difference between the MTTA and LTTA in the normal group or on AP radiographs alone (p = .09). There was a statistically significant difference between the MTTA and LTTA in the arthritis group (p < .01) and when measured on mortise radiographs (p = .02). The MTTA had no difference when measured on the AP and mortise radiographs. There was a statistically significant difference in the LTTA between AP and mortise radiographs (p = .04). We have shown the MTTA to be a reliable and reproducible tool in all patients, on AP and mortise radiographs. The type of radiograph does not alter the measurement of deformity. In contrast, we have shown the LTTA to be unreliable and statistically different when measured on AP and mortise radiographs.