From the case:
Anterior cruciate ligament injury (illustrations)
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This diagram demonstrates a complete ACL tear (figure 1) and ACL avulsion (figure 2) with the arrows indicating the "pivot-shift" twisting mechanism of the knee. Under stress, (classically with athletic activity) this motion may result in an ACL tear. In children, avulsion fracture of the tibial insertion of the ACL is not uncommon.
Case Discussion
See article: Anterior cruciate ligament tear