ACL is a most common knee injury occurring due to low velocity, deceleration injuries and direct contact or collision. Normally, ACL injury happens when knee is hyperextended, straightened beyond its normal limits twisted or bent side to side.
Anatomy: Femur or the end of thigh-bone is located where it meets the top of thigh-bone, or tibia. Medial collateral ligament (MCL) runs along the inside of the knee and prevents the knee from bending in. Similarly, Lateral Collateral Ligament (LCL) runs along the outside of the knee and prevents the knee from bending out. ACL is in the middle of the knee. It prevents the shin-bone from sliding out in front of the thigh bone. Posterior cruciate ligament (PCL) works with the ACL and it prevents the shin-bone from sliding backwards under the femur. The cruciate ligaments are located inside the knee joint in the form of a cross ‘X’.
There are three clinical tests normally done to diagnose an ACL Injury. Pivot Shift Test, Lachman Test and Anterior Drawer Test.
In Pivot Shift Test, the ACL-deficient knee is extended resulting in anterior tibial translation during flexion. Shifting or pivoting of the tibia is observed with this examination.
Lachman Test is considered as the most sensitive of tests for acute rupture of the ACL, the Lachman test requires a 30 deg. flexion of the knee while, an Anterior Drawer Test, requires a knee flexion of 90 deg. which attempts to displace the tibia from the femur. A tibial displacement of more than 6 mm indicates the presence of an ACL tear.
MRI Scan: MRI helps in identifying extent of bone bruising and aids in the confirmation of ACL tear. Soft tissue images are obtained with greater clarity with the MRI.
X-ray If there is an injury, the x-ray will confirm presence of an injury to the bone.