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Fractures lines can be difficult to visualize after acute elbow injury, particularly in children. Below are eight sequential steps to aid in the radiographic recognition of occult signs of injury.
Search for an adequate "hourglass sign", or "figure of eight" at the distal humerus. If absent the study is not a true lateral and interpretation of steps 2 through 4 is less reliable.
Here's an example of a true lateral; note the symmetric figure of eight/hourglass sign at the distal humerus; also notice the posterior fat pad? (see below.)
Here is an imperfect lateral radiograph accompanied by a normal AP radiograph; notice how the figure of eight/hourglass is asymmetric:
A visible anterior fat pad can be normal; it is a small radiolucent shadow adherent to the anterior aspect of the distal humerus:
An abnormal anterior fat pad is described as a "sail sign" because it is unusually prominent and bows outward to form a triangular shape. After trauma, blood can accumulate in the intraarticular space and push the fat pad anteriorly; a positive sail sign in the setting of trauma is a reliable indication of an intraarticular fracture – even if no fracture line can be identified. An atraumatic sail sign implies intraarticular fluid of an inflammatory nature.
Radiographic visualization of a posterior fat pad is never normal and always signifies fluid in the intraarticular space. Again, in the setting of trauma, this strongly implies fracture of an articular surface.
Here is a radiograph with both a sail and posterior fat pad sign:
This line should intersect the middle third of the capitellum on the lateral view. Fractures usually result in displacement of the capitellum posteriorly (versus anteriorly). If the film is not a true lateral, interpretation of the anterior humeral line becomes fallible.
This radiograph depicts a normal anterior humeral line:
This radiograph demonstrates abnormal alignment of the anterior humeral line strongly suspicious for fracture. (The anterior humeral line of a toddler/child must also intersect the middle third of an ossified capitellum; also note the posterior fat pad and sail sign.)
This line is drawn through the middle of the radius posteriorly/rostrally and should bisect the capitellum on both the lateral and the AP elbow radiograph. Failure to align properly indicates a radial head dislocation that requires prompt reduction if neuro-vascular compromise is to be avoided.
Normal radio-capitellar lines:
Notice that this is not an ideal lateral making interpretation of the anterior humeral line difficult; however the radius should bisect the capitellum on all views regardless of adequacy; also note the posterior fat pad.
An abnormal radio-capitellar line is depicted below:
On both views the radius fails to bisect the capitellum indicating an obvious radial head dislocation. Also note the anterior and posterior fat pads, as well as the obvious olecranon deformity. A radial head dislocation with an olecranon fracture is called a Monteggia injury.
Careful inspection is paramount since fracture lines are often not visible; look for subtle disruptions in the cortical contour. Examine the radiograph below:
Notice how the radius bisects the capitellum on this view; however there is a subtle cortical disruption/acute angulation at the superior aspect of the distal radius indicating fracture.
Signs of humeral fracture are also commonly subtle; breakage may only be evidenced by an abnormal anterior humeral line. Exam the radiograph below:
The anterior humeral line is perhaps slightly off as it seems to intersect the anterior third of the capitellum, while the radio-capitellar line is intact. There are prominent sail and posterior fat pad signs; and on careful inspection one sees the subtle cortical disruption along the posterior aspect of the distal humerus.