Improving the detection of radiographically occult ankle fractures: positive predictive value of an ankle joint effusion.

Objective: To assess the value of an ankle effusion on plain radiographs as a predictor of radiographically occult fracture after acute ankle trauma.

Methods: Consecutive patients with acute ankle trauma and radiographic evidence of an ankle joint effusion. Patients were excluded if ankle trauma was sustained more than 48 h previously or if a fracture was visible on initial photographs. Methods: All subjects (n = 26) underwent computed tomography (CT) of the ankle in sagittal and coronal planes. Ankle effusion size was measured from initial lateral ankle radiographs.

Results: Twelve patients (46%) had radiographically occult fractures identified with CT. Fracture sites included: posterior or lateral malleoli (n = 4), calcaneus (n = 1), or talus (n = 7). Ankle effusion size was 11.2 mm in the group without fracture and 17.1 mm in the group with fracture (P < 0.0001). The positive predictive value of an effusion 15 mm or greater was 83%. CT detected significant soft-tissue injuries in four (15%) patients including peroneal retinaculum tear (n = 1), anterior talofibular ligament avulsion (n = 1), and tears of the peroneus longus (n = 1) and tibialis posterior (n = 1) tendons.

Conclusions: The presence of a large ankle effusion of radiographs after acute ankle trauma suggests an underlying fracture. An ankle effusion of > or = 15 mm is a reasonable threshold to prompt additional imaging. Computed tomography provides good visualization of subtle bone injuries and may detect clinically imported soft-tissue injuries.