A standard lateral radiograph is the first step in the diagnostic workup in patients with posterior ankle pain. Because of overprojection by other structures at suboptimal radiographic projection angle, often an os trigonum is not discovered or erroneously be mistaken for a hypertrophic posterior talar process. The aim of this study was to identify the projection angles at which a radiograph is optimal for detecting bony impediments in patients suffering from posterior ankle impingement.
Using ankle CT scans of patients with posterior ankle impingement, digitally reconstructed radiographs (DRRs) simulating 13 different radiographic projection angles were generated. The ankle CT scans served as a reference for the detection of an os trigonum and hypertrophic posterior talar process. Members of the Ankleplatform Study Group were invited to assess the DRRs, for presence or absence of an os trigonum or hypertrophic posterior talar process. Diagnostic accuracy and interobserver reliability were estimated for each projection angle. In addition, the diagnostic accuracy of the standard lateral view in combination with the rotated views was calculated.
High sensitivity for detecting an os trigonum was found for +15° (90.3%), +20° (81.7%) and +25° (89.7%) degrees of exorotation. Specificity in this range of projection angles was between 89.6% and 97.8%. Regarding the presence of a hypertrophic posterior talar process, increased sensitivity was found for +15° (65.7%), +20° (61.0%), +25° (60.7%), +30° (56.3%) and +35° (54.5%). Specificity ranged from 78.0% to 94.7%. The combination of the standard lateral view in combination with exorotated views showed higher sensitivity. For detecting an os trigonum, a negative predictive value of 94.6% (+15°), 94.1% (+20°) and 96.1% (+25°) was found.
This study underlines the additional diagnostic value of exorotated views instead of, or in addition to the standard lateral view in detecting an osseous impediment. We recommend to use the 25° exorotated view in combination with the routine standard lateral ankle view in the workup of patients with posterior ankle pain.
Level of evidence