An anterior cruciate ligament (ACL) tear, is a knee joint injury that happens mostly while playing sports. ACL results in an unstable knee and leg pain. It is one of the most common injuries amongst athletes all over the world. The diagnosis for ACL is done through a physical exam. An X-ray can also be conducted to determine if there are other injuries present such as a bone fracture. The treatment for ACL depends on the severity of the symptoms. Treatment may include a supportive brace, surgery or physical therapy. In some cases, after a surgical treatment, rehabilitation is done. Certain tests should also be done, to check the stability of the knee, some of them include the Lachman test , drawer test  and the pivot shift maneuver. 
ACL tears are also graded.  This grading is done depending on the extent of damage done to the ligament. The grades run through from grade 1 to 3, in which damage runs from the ligament experiencing a minor stretch, which the knee is still quite stable, to a partial tear of the ACL with the damage causing the ligament to loosen and grade 3, which is the last one and the most severe, is characterized by a total tear of the ACL with a complete instability of the knee joint to the extent that surgery is almost inevitable.
Immediately after an ACL injury, you may hear a very sudden or sharp pop in the joint. This pop can be louder than expected and sometimes, even those standing by the sidelines in a game may hear the pop. After an ACL tear occurs, you may very quickly develop hemarthrosis which is a situation where you bleed into your joint.