From the case:
Normal alignment of distal humerus and capitellum (diagram)
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Normal alignment of the distal humerus and capitellum
A line drawn along the anterior border of the distal humerus (green) should intersect the long axis of the capitellum (purple) in the mid-third (area shaded light green).
Case Discussion
In the setting of a painful elbow following a fall, if this line does not intersect the green area, but rather the red area then a supracondylar fracture should be suspected.