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.