I had to ask Google: https://en.wikipedia.org/wiki/Anterior_cruciate_ligament

Aftet knowing the maning, I could Ctrl+F the page and get

> Like the professional athletes Megan Rapinoe, Ronald Acuña Jr. and Lindsey Vonn, Ella had torn her anterior cruciate ligament. Ella, though, was only 12.

> A.C.L. injuries are among the most devastating in sports.

Technically the explanation is before the first use in the text, but it's difficult to notice and I guess it breaks whatever style manual they are using.