The effect of foot rotation on measuring ankle alignment using simulated radiographs: a safe zone for pre-operative planning.

Aim: To assess whether variation in foot rotation, in relation to camera position, affects the reliability of measurement of hindfoot alignment on radiographs and to define a "safe zone" where measurement of the alignment axis and thus preoperative planning is not affected by foot rotation.
Materials and Methods: Healthy volunteers were recruited of whom double-sided lower-leg weight-bearing computed tomography (CT) was acquired. Weight-bearing was simulated by means of providing axial compression force equal to the weight of the healthy volunteers. The scans were uploaded into custom-made three-dimensional analysis software to create digitally reconstructed radiographs. For each CT examination, a coordinate system was determined, which defines the neutral position of the leg. Rotation about the z-axis of this coordinate system simulates endo- and exorotation of the foot. Subsequently, radiographs were reconstructed for the leg between 30° of endorotation and 45° of exorotation, and the relation between the observed alignment axis and foot rotation was determined.
Results: A total of 20 healthy volunteers were included, 10 males (mean age 37.7±11.1) and 10 females (mean age 34±10.3). Per 5° of leg rotation, the alignment axis translated with a mean of 6.86% (SD ±13.1). No significant difference in position of the alignment axis was seen between 10° of endorotation and 10° of exorotation compared to the neutral ankle position.
Conclusion: The "safe zone" for imaging the hindfoot alignment axis, is between 10° endo- and 10° exorotation of the foot.
(Copyright © 2019 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.)