The posterior impingement view: an alternative conventional projection to detect bony posterior ankle impingement.

PURPOSE: The purpose of the current study was to clinically evaluate the diagnostic value of the new posterior impingement (PIM) view in the detection of an os trigonum, compared with the standard lateral view, using computed tomography (CT) as a reference standard.

METHODS: Three observers, 2 experienced (orthopaedic surgeon and radiologist) and one inexperienced (resident), independently scored 142 radiographic images for the presence of an os trigonum. The diagnostic performance was assessed using the computed tomographic scan as the reference standard. Accuracy, sensitivity, specificity, positive predicted value (PPV), and negative predicted value (NPV) were calculated.

RESULTS: The PIM view had significantly superior accuracy compared with the lateral view for each observer: orthopaedic surgeon, PIM view = 90 versus lateral view = 75 (P = .013); radiologist, PIM view = 80 versus lateral view = 64 (P = .019); resident, PIM view = 90 versus lateral view = 79 (P = .039). The mean sensitivity and specificity of the lateral view for all observers was 50% and 81%, respectively. For the PIM view, this was 78% and 89%, respectively. The PPV was 50% for the lateral view and 70% for the PIM view. The NPV was 84% for the lateral view and 93% for the PIM view.

CONCLUSIONS: The PIM view has significantly superior diagnostic accuracy compared with the conventional lateral view in the detection of an os trigonum. In cases of symptomatic posterior ankle impingement, we advise that a PIM view be used instead of or in addition to the standard lateral view for detection of posterior talar pathologic conditions. LEVEL OF 

EVIDENCE: Level II, development of diagnostic criteria on basis of consecutive patients with universally applied reference gold standard.