Assessing the Medial Distal Tibial Angle Based on a Long Ankle View Radiograph - Reliability of and Differences Between Three Approaches

Background: Lower limb alignment on plain radiographs is commonly examined in orthopedics. Measurement of the medial distal tibial angle (MDTA) requires determining the tibial axis. Traditionally, in orthopedic practice, anatomical or surgical axes have been used. Due to the increasing availability of stitching tools, assessing the mechanical axis of the tibia in a lower leg mortise view radiograph has arisen as a third option. This study evaluated the reliability of three approaches to determine the MDTA on whole lower leg radiographs and the magnitude of the disparities between the approaches.

Methods: Thirty-eight patients were assessed on weightbearing, anteroposterior radiographs. The images included an anterior-posterior image ('mortise view') of the ankle joint, including the entire tibia. The radiographs were captured twice in each patient to analyze the reliability of the technique. The MDTA was measured according to the three approaches by two independent examiners.

Results: The mean absolute difference between MDTAs across two images was 0.8° for the anatomical axis and 0.74° for the mechanical. It was distinctly larger for the surgical axis with 1°. The MDTA differed by more than 2° between the anatomical and mechanical axes in 22 % of the patients and between the mechanical and the surgical axis in 10 % of the patients.

Conclusion: Weightbearing, lower leg mortise view radiographs centered on the proximal tibia and ankle joint provide reproducible values for the MDTA based on the mechanical axis. This approach should be preferred over the anatomical axis in future. The tibial tuberosity should not be used as a reference to determine the MDTA. The choice of the axis can lead to differences in the MDTA of clinically relevant magnitude.

Level Of Evidence: Level II, Prospective Cohort Study.