Text

Objectives and questions: The Lateral Distal Tibial Angle (LDTA) is best measured on long-leg radiographs due to tibial deformities, while standard anterior-posterior (AP) ankle radiographs are preferred for assessing the Talar Tilt Angle (TTA). However, conventional long-leg radiographs do not consistently center the beam between the lateral and medial malleoli, potentially affecting TTA accuracy. Conversely, LDTA is sometimes measured on standard AP ankle radiographs, which do not include the tibial plateau, requiring approximation. This study aims to assess the accuracy of TTA measurements on long-leg radiographs and evaluate the reliability of LDTA measurements on standard AP ankle radiographs.

Material and methods: This retrospective study included patients who underwent total knee arthroplasty (TKA) for knee osteoarthritis (Kellgren-Lawrence Grade IV) between September 2020 and September 2021. Patients with preoperative weight-bearing long-leg radiographs (tri-focal stitching technique) and standard AP ankle radiographs were included. TTA (<2° considered normal) and LDTA (86°–92° considered normal) were measured on both imaging modalities. Since the mechanical axis of the tibia cannot be drawn on AP ankle radiographs due to the absence of the tibial plateau, an alternative method was used by drawing two touching circles on the distal tibia to approximate the longitudinal tibial axis. A Student’s t-test was used for statistical comparisons.

Results: A total of 69 patients (31 men) with a mean age of 70.3 ± 9.9 years and a mean height of 169 ± 10 cm were included. The TTA was 0.32 ± 1.6° on standard AP ankle radiographs and 0.57 ± 2.4° on long-leg radiographs (p = 0.42). Fifty-one patients had normal TTA values on both radiographs, with 61 classified as normal on standard AP ankle radiographs and 55 on long-leg radiographs. The LDTA was 84.6 ± 5.0° on long-leg radiographs and 86.2 ± 2.6° on standard AP ankle radiographs (p = 0.02). Twenty patients had normal LDTA values on both imaging modalities, while 24 were within the normal range on long-leg radiographs and 43 on standard AP ankle radiographs.

Discussion and conclusions: TTA measurements showed minimal differences between standard AP ankle radiographs and trifocal long-leg radiographs, indicating that TTA can be reliably assessed using either method. However, LDTA measurements exhibited significant discrepancies between the two imaging techniques, suggesting that standard AP ankle radiographs do not provide an accurate assessment of LDTA. Therefore, while TTA can be measured on trifocal long-leg radiographs, LDTA should not be assessed using standard AP ankle radiographs.