Lateral ankle radiographs: do we really understand what we are seeing?

OBJECTIVES: To evaluate the validity of using lateral intraoperative fluoroscopic imaging to assess the reduction of the tibial plafond articular surface, two hypotheses were tested: 1) the distal tibial subchondral shadow on the lateral ankle radiograph is created equally by the medial, central, and lateral portions of the distal tibia; and (2) displacement of a 5-mm width osteochondral fragment is consistently recognizable on lateral fluoroscopic imaging.
METHODS: Six human fresh-frozen tibial plafond cadaveric specimens were sagitally sectioned in 5-mm increments after removal of the anterior soft tissue and stabilization of the position of the ankle through external fixation. To test the first hypothesis, a perfect lateral radiograph was taken after sectioning the specimens. The sagittal sections were then removed sequentially from medial to lateral. A perfect lateral radiograph was taken after each change. The sagittal sections were then removed beginning laterally and moving medially. A perfect lateral radiograph was taken after each change. The images were then compared with specific evaluation of the change in the subchondral shadow density. To test the second hypothesis, three malreductions were created by displacing a 5-mm osteochondral segment. After each malreduction, a perfect lateral radiograph was saved. These saved fluoroscopic images were placed in random order with lateral images of normal specimens. Four experienced ankle surgeons were then asked to determine whether the radiographs revealed displacement. Inter- and intraobserver reliability was then evaluated.
RESULTS: First, the subchondral shadow of the distal tibia appears to be created by an equal confluence of the subchondral bone of the medial, central, and lateral aspects of the tibial plafond. Second, fellowship-trained observers experienced in pilon fracture treatment correctly identified malreduction only 45% of the time. Intraclass correlation coefficient revealed very poor interobserver reliability with an alpha reliability statistic of 0.183. Intraobserver reliability across all four observers yielded an alpha statistic of 0.474, indicating inconsistencies in observers' evaluation of identical images at separate viewings.
CONCLUSIONS: It is difficult to discern rotational or translational displacement of a 5-mm osteochondral fragment on a perfect lateral fluoroscopic view of the ankle. Even with what appears to be a perfect lateral fluoroscopic view intraoperatively, displacement may still be present. When small osteochondral fragments are present, direct visualization of the articular surface is necessary to confidently establish that an anatomic reduction has been achieved.