What is the diagnosis for Torn Anterior Cruciate Ligament
(Torn ACL or ACL Injury)?

The knee is a hinge joint where the thighbone (femur) connects to the shinbone (tibia). The quadriceps muscles in the front of the thigh and the hamstrings in back help to stabilize the knee, but stabilization occurs primarily from the knee ligaments. There are four ligaments that keep the knee stable and moving in the proper direction. The medial and lateral collateral ligaments keep the knee from shifting side to side, while the anterior and posterior cruciate ligaments keep the knee from sliding front to back. The anterior cruciate ligament (ACL) is the most commonly injured knee ligament. The ACL attaches the lateral femoral condyle to the tibia just in front of the anterior tibial spine. Some of its fibers also blend into the medial meniscus. There are actually two bundles of fibers that make up the ACL and allow it to help stabilize the knee in flexion (bending), extension (straightening), and rotation. Picture of the anatomy of the knee
Injured ligaments are called sprains, and they can be classified based upon their severity. A grade 1 sprain occurs when ligament fibers are stretched but not torn. Grade 2 sprains have some fibers torn, but the ligament remains functionally intact. A grade 3 sprain occurs when the ligament is completely torn.

Source: http://www.emedicinehealth.com