Introduction

An equivocal fracture appearance should be considered in the context of other information such as

• Patient history
• Clinical presentation
• Radiographic soft tissue signs

This page will examine the radiographic soft tissue signs of bony injury of the ankle- what they are, their plain film appearances, their limitations, and their utility.

Ankle Effusion- The Teardrop Sign

An ankle effusion suggests a significant injury to the ankle joint. The anterior and posterior extra-capsular region of a normal ankle joint should appear as a fat-like density. In the presence of an ankle effusion, the capsule can become distended and may appear to have a more fluid-like density.

Normal ankle demonstrating fat-like density (arrow).

Abnormal ankle demonstrating fluid-like density (arrow).

This patient presented to the ED following fall. There is extensive soft tissue swelling. There is an ankle effusion with the anterior and posterior recesses visibly distended with fluid.

Same image with the anterior and posterior ankle joint recesses marked. Note that it is difficult to be sure of the exact contour of the posterior ankle joint recess. In the absence of an ankle joint effusion, the area bounded by the dotted line would show a fat density rather than a fluid density.

This is an ankle arthrogram which simulates an ankle effusion. The distension of the anterior recess (AR) and posterior recess (PR) of the ankle joint are simulated.

“(ankle) Fat pads are consistently visualized on the plain radiograph. Anteriorly the pretalar fat pad usually appears crescentic, and is cradled in the neck of the talus. There are two posterior fat pads, a thin juxtaarticular and a larger pre-Achilles fat pad. The juxtaarticular fat pad is closely apposed to the posterior recess. It appears as a thin, linear structure about 1 -2 mm wide, that courses in a nearly straight line from the posterior aspect of the tibia to the posterior superior part of the calcaneus. The larger posterior fat pad is the triangular, pre-Achilles fat pad” (1).

Ankle Effusion- Correlation with Occult Ankle Fracture

Clark et al. (1) investigated the correlation between an ankle effusion and occult ankle fracture. They concluded that the presence of an ankle effusion suggested underlying fracture. Furthermore, they quantified the correlation as follows.

Clarke et al (2) reported that “An ankle effusion of I 3 mm (d1 + d2) or greater in total capsular distension has a positive predictive value of 82% for occult fracture and is a reasonable threshold to prompt additional imaging.”

References
1. Richard Towbin1, J. Scott Dunbar1, Jeffrey Towbin2, Robert Clark3. Teardrop Sign: Plain Film Recognition of Ankle Effusion. AJR:134, May 1980.

2. Timothy W. I. Clark, Dennis L. Janzen, Kendall Ho, Anton Gnunfeld, Douglas G. Conneli. Detection of Radiographically Occult Ankle Fractures Following Acute Trauma : Positive Predictive Value of an Ankle Effusion AJR 1995;164:1185-1189.