Basic MRI Evaluation of an ACL Tear
Updated: Sep 27, 2020
The Classic ACL Injury
A typical ACL injury occurs as a non-contact injury with a planted foot on the ground. Many times the athlete is shifting directions, where the body is moving in one direction and the knee is moving in the opposite direction. Usually, the athlete will report a "pop" or the sensation of the knee giving away. Athletes will typically have swelling and fluid over the entire knee, not just the inner aspect or outer aspect. The swelling can be described as "like a water balloon" or "like a grapefruit" and should be apparent within the first 24 hours of the injury. Many times an ACL tear is associated with other injuries, such as a meniscus tear or lateral collateral ligament tear. Although the diagnosis can be confidently made by physical exam, an MRI is generally accepted as the appropriate test to confirm the presence of an ACL tear. This post takes a more in-depth view on a practical approach to diagnosing an ACL tear on MRI.
Inside of Knee Joint
First, you need to get yourself oriented to the anatomy of the knee. We have the femur as the upper thigh bone and the tibia as the lower shin bone that meet together to create the knee joint. We then have the ACL that runs directly through the middle of the knee acting as the major stabilizer of the knee joint. We can then compare these structures to the MRI of the exact same area.
MRI Cross Section of Knee (Normal)
Now we can see the anatomy within the knee joint and compare to the model above (note the knee is pointing in the opposite direction). Of note you can see the black line cutting through the central portion of the knee joint that is representative of the ACL.
MRI Cross Section of Knee (Torn ACL)
In contrast, in the picture below there is no clear black line cutting through the central portion of the knee. The lack of this line indicates that the ACL is torn. Keep in mind this is a simplistic explanation of the analysis on an MRI and significant expertise is needed to confirm the diagnosis and any other associated injuries.
Return to Play
The data on return to play is ever-changing in regards to ACL reconstruction. In the past, the general goal for return back to full sports participation was reported between 6 to 9 months. However, recent data suggests that we should potentially be waiting 1 to 2 years for return back to full contact sports participation. The focus has shifted away from a fixed time toward the need to meet functional criteria before returning back to contact sports.