And now for the more detailed answer ...
In almost all instances of a suspected ACL tear, United States-based orthopedic surgeons will request an MRI of the knee.
A team physician or athletic trainer who is evaluating an injured player in the first couple of minutes after an injury has some key advantages compared to the physician seeing the patient in the clinic a week after the injury.
When we evaluate the player on the field or sideline we’ll be listening for key red-flag phrases from the player. Often it will be a non-contact injury, either a forceful change of direction or an awkward landing from a jump. Players will frequently tell us they felt one or more “pops” and the knee gave out, causing the player to fall to the ground. This is suspicious for an ACL tear. Then, we are able to do some ligament tests in those first few minutes before the injured knee becomes swollen. We compare the injured knee to the non-injured knee.
A skilled doctor or trainer has about a 90% accuracy of diagnosing an ACL tear in this setting.
But in most injuries to the knee there will not be a skilled clinician able to evaluate the knee in the first 5 minutes. The more likely scenario is that you may be evaluated in an emergency room a few hours later, or see a doctor a few days later. In these common situations our diagnostic accuracy isn’t quite as high.
Swelling often sets in within the first 24-48 hours, making it difficult to examine the knee. The exam is challenging in large patients, and in patients with strong secondary muscular contractions. Partial ACL tears are also difficult to diagnose on physical examination. MRI will provide important diagnostic information about the ACL in all of these settings.
There are other very good reasons to obtain an MRI, especially to evaluate other structures such as the meniscus, and other ligaments such as the LCL and PCL. Knowing the extent of injury to other structures can affect the treatment plan and possible outcomes.
If you’re the patient considering your options you definitely want to have the most accurate information available, and for that reason as well as the points I’ve outlined above, you’ll almost certainly end up with an MRI to confirm the diagnosis suspected by your orthopedic surgeon as well as to evaluate the other structures in the knee.
• A skilled sports knee surgeon or athletic trainer has about a 90% chance of accurately diagnosing an ACL tear on the sidelines immediately after an injury without any imaging.
• From a practical standpoint, most injured athletes will see a doctor several hours or days after an injury when the knee is not as easy to evaluate, and our diagnostic accuracy is generally not as good.
• For these reasons, most USA-based orthopedic surgeons will order an MRI on a knee with a suspected ACL tear to confirm our diagnosis as well as to evaluate possible other injured areas in the knee.
(Dr. Dev K. Mishra, a Clinical Assistant Professor of orthopedic surgery at Stanford University, is the creator of the SidelineSportsDoc.com online injury-management course, now a requirement for US Club Soccer coaches and staff members. Mishra writes about injury recognition and management at SidelineSportsDoc.com Blog.)