Hyperextension forces. When a person falls on the outstretched upper extremity, the wrist and fingers are forced into hyperextension (solid arrow), which places tension on the forearm flexor muscles. The sudden tension along with the normal valgus carrying angle tends to place a strong avulsion force on the medial epicondyle (open arrow).
Reproduced with permission from: Beaty JH, Kasser JR. The elbow: Physeal fractures, apophyseal injuries of the distal humerus, avascular necrosis of the trochlea, and T-condylar fractures. In: Rockwood and Wilkins' Fractures in Children, 6th ed, Beaty JH, Kasser JR (Eds), Lippincott Williams & Wilkins 2006. Copyright © 2006 Lippincott Williams & Wilkins.
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