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What You Should Know About Your Knee MRI

The anterior cruciate ligament, or ACL, is one of the body’s most important ligaments that help stabilize the knee.

But each year, there are an estimated 200,000 ACL ruptures in the US alone, making it the most commonly injured knee ligament.

Typically, a diagnosis is made after a clinical exam and a knee MRI. However, it’s common for physicians unfamiliar with ACL injuries to misdiagnose them. This can result in overdiagnosis, where the patient receives unnecessary treatment that can actually cause more harm, or a missed diagnosis, where the ACL is left torn, making it prone to further damage. Fortunately, ensuring your knee MRI is interpreted by a subspecialty trained musculoskeletal radiologist can help ensure an accurate diagnosis.

To dive a little deeper into why subspecialty interpretation is so important, we spoke to leading MSK radiologist, Dr. Samir Mehta. From common pitfalls in diagnosing ACL injuries to how patients can benefit from a subspecialty second opinion, here’s what you should know about your knee MRI.

Experiencing a different type of sports injury? Check out our piece on sports injury diagnosis.

[DocPanel] Why is MRI the preferred tool for diagnosing a torn ACL?

[Dr. Mehta]

Magnetic resonance imaging (MRI) is a useful noninvasive tool to evaluate the soft tissues of musculoskeletal structures, which include ligaments and tendons. This is due to superior contrast and spatial resolution of the soft tissues compared to CT or x-Ray.

As the ACL is a ligament, it is better seen on MRI. CT and x-rays are sometimes also used alongside MRI to evaluate the bone and to look for fractures or other abnormalities.

[DocPanel] What advice do you have for someone who just had a knee MRI for a suspected ACL tear?

[Dr. Mehta]

Similar to an orthopedist in charge of your knee or a cardiologist in charge of your heart, a musculoskeletal radiologist should look over your knee MRI. The main reason is that they have seen hundreds if not thousands of knee MRIs and thus, ACLs.

Tears can come in all shapes, sizes, and subtleties. A musculoskeletal radiologist has the proper experience and training to accurately diagnose an ACL tear.

[DocPanel] What are some common pitfalls in reading a knee MRI and diagnosing ACL injuries?

[Dr. Mehta]

Common pitfalls in diagnosing ACL tears can be related to suboptimal positioning of your knee during your exam due to considerable swelling and pain, or technical issues with the MRI scanner. Typically, a 1.5 or 3.0 Tesla scanner is preferred.

Additional factors that make it difficult to diagnose an ACL tear are:

  • scarring of the ligament if the MRI is performed too long after an injury
  • blood surrounding the ACL
  • pre-existing degeneration of the ligament

These circumstances make it challenging to accurately interpret a knee MRI. It’s where experience really comes into play. Because general radiologists aren’t getting exposure to different presentations of musculoskeletal injuries and conditions – they do not have the familiarity required to identify these factors, which can lead to an inaccurate reading.

[DocPanel] Are ACL tears easily misdiagnosed? If so, what other injuries might they mimic?

[Dr. Mehta]

In some cases, an ACL tear can appear similar to ACL degeneration, which is a more long-standing process. A partial tear of the ACL may also be misdiagnosed as a normal ACL since these injuries can be subtle in appearance. ACL tears are also often misdiagnosed as collateral ligament knee strains.

With that being said, when a trained musculoskeletal radiologist is reading the case, an ACL tear or other abnormality is rarely missed. This is why a radiologist with subspecialty training is recommended to look at your MRI.


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[DocPanel] How can a patient be sure whether or not they need surgery? Do have any advice for patients navigating this decision?

[Dr. Mehta]

The decision to undergo surgical reconstruction of the ACL is a conversation to have with your orthopedist. Typically, patients with low physical demand will not undergo reconstruction. Patients who are younger or more active will. Again, a conversation with your orthopedist, along with the results of your knee MRI, is the best way to determine whether or not surgery is a good option.

In addition to talking the decision through with your orthopedist, I advise patients to make sure their scans have been read by an experienced subspecialty-trained radiologist. If a partial tear in a young patient is misdiagnosed as a complete tear – you could end up getting a surgery that is not only unnecessary and expensive but also potentially harmful. Also, if it is a partial tear, that too needs to be accurately diagnosed and managed. Otherwise, you risk overuse injury which could end up resulting in a full tear.

This is why a second opinion can be of extreme value. An ACL strain, partial tear, and full tear can all feel similar in terms of pain – but each requires its own specific course of treatment ranging from at-home care to surgical repair and extensive rehabilitation.

Getting a second opinion on your knee MRI from an MSK trained radiologist can help prevent misdiagnosis and provide you with peace of mind that you’ve chosen the best form of treatment. You can get a second opinion easily online, which helps save time and money. Even if your scan was initially reviewed by an MSK specialist, it doesn’t hurt to have another set of eyes look over your scans. This is important if you are indeed planning to undergo surgery, which is a complex procedure. It generally takes a patient anywhere from six to nine months to recover from ACL surgery. For some, it can take even longer.

[DocPanel] In what other instances can a second opinion help ensure an accurate ACL tear diagnosis?

[Dr. Mehta]

Frequently, a knee MRI will be evaluated by someone without specialized musculoskeletal radiology training. The lack of experience and training in evaluating ACL tears can be profound. There can be a high rate of false positives or missed diagnoses.

Additionally, ACL tears can occur with other injuries that should not be missed. Obtaining a second opinion from a dedicated musculoskeletal radiologist can provide peace of mind that the entire knee is being adequately assessed.

[DocPanel] What happens if a torn ACL is left untreated?

[Dr. Mehta]

The ACL helps stabilize the knee. If it is left untreated, the knee can begin to feel unstable which can then lead to sensations of buckling or giving way.

For active individuals, leaving an ACL torn can lead to further knee damage as it becomes more unstable, including meniscus injuries. Studies also show that patients with an ACL deficient knee end up decreasing their overall activity level.

While ACLs can take months to heal, with proper diagnosis and careful treatment, there is a road to recovery. But keep in mind, the signs and symptoms of an ACL injury are not always the same. If you suspect you may have a tear, it’s important to see a doctor or get a second opinion on your knee MRI.


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