Should the Tibiotalar Angle Be Measured Using an AP or Mortise X-ray?

Abstract

Category: Basic Sciences/Biologics Introduction/Purpose: The tibiotalar angle(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) is when the first line is along the anatomical axis of the tibia, and the second line is along the superior articular surface of the talus. Another method measures a line along the lateral border of the tibia, and a line along the superior articular surface of the talus (the lateral TTA, LTTA). The aims of the study were to compare the two angles as measured on mortise and AP radiographs in normal and pathological cases.We also compared the MTTA and LTTA, to see if they are comparable and if both methods are reliable and reproducible. Methods: A retrospective radiograph review was performed of sequential ankle radiographs taken between 2016 and 2017 across 4 specialist orthopaedic centres in the United Kingdom. Patients were categorised into two distinct groups. In the Normal Group (NG), patients had no evidence of injury and normal radiological appearances. In the Arthritis Group (AG), patients had radiographic changes as per the Kellgren-Lawrence scale 2 to 4. All radiographs were weightbearing and classified as either AP or mortise views based on the position of the talus and overlap of the tibia and fibula. The MTTA and the LTTA were measured on each radiograph. Results: There were 320 radiographs for review;158 normal radiographs and 162 radiographs had arthritis.There were 117 AP and 203 mortise radiographs. The overall mean MTTA was 88.7±5.1 degrees(range 77-104), and the mean LTTA was 87.5±5.2 degrees(range 73-104);p<0.01.There was no significant difference between the MTTA and LTTA in the normal group. There was a significant difference(p<0.01) when comparing the MTTA and LTTA in the arthritis group(Table 1). There was no significant difference when the MTTA was measured between the AP and Mortise radiographs. There was a significant difference in the LTTA between AP and mortise radiographs(p=0.04). There was no significant difference between the MTTA or LTTA when measuring the angles on AP radiographs(p=0.09).However, there was a significant difference when measuring these angles in the mortise radiographs(p=0.02). Conclusion: Understanding the tibiotalar angle is key to planning for deformity correction. It is important that consistency of methods for measurement are used for reporting. In this paper we have shown the MTTA to be a reliable and reproducible tool for measuring the TTA, in both normal ankle radiographs and in patients with ankle arthritis. There is no significant difference when the MTTA is measured using an AP or a mortise radiograph. In contrast, we have shown that the LTTA to be unreliable and statistically different when measured on both AP and mortise radiographs.