MRI scan knee

MRI scan of the knee can be a good examination

MRI scan of the knee

If you have a problem with your knee, an MRI scan of the knee can often help with the diagnosis. For example, MRI scans are good at detecting meniscus injuries, cruciate ligament injuries and other ligament injuries. Unlike in the past, MRI scanning is now relatively inexpensive. If you have had knee pain for a long time, you should always undergo an MRI scan. As a general rule, you shouldn’t undergo keyhole surgery without an MRI scan first. An exception might be a locked knee due to an acute meniscus injury.

What is a knee MRI scan?

This is a magnetic scan. MRI stands for magnetic resonance imaging. By exposing the body to various high-powered multi-fields, the machine is able to convert the signals into images in an incredible way. There are both closed and open scanners. Typically, enclosed scanners have a larger magnetic field and therefore produce better images. You need to lie down and bring your knee into the machine. Rarely, claustrophobia is an issue as your head is outside when your knee is scanned.

How accurate is an MRI scan

In most cases, the MRI scan is accurate. However, there are a number of exceptions that you may need to be aware of. Firstly, the MRI scan is not a blueprint. In fact, the diagnosis can often be made quite accurately with a good examination of the knee. This means both a combination of how those genes came about and what genes you have. This should then be supplemented with a physical examination of the knee, where things can be checked. Note that a arthroscopy can be more accurate than an MRI scan, but it’s not a magic bullet. Unfortunately, every now and then I see things that are missed during a arthroscopic surgery. In fact, there are also some things that can’t be seen with an arthroscope. 

Who analyzes an MRI scan?

The outcome of the scan depends on who is looking at the images. Usually, a radiologist evaluates the images and provides a report. The radiologist is a specialist in seeing images. Unfortunately, radiologists don’t always know everything and can miss things. In particular, things can be missed if the orthopedic surgeon hasn’t specified the exact problem. Some orthopedic surgeons don’t see the images themselves, but simply look at the radiologist’s description. I often see patients who have had an MRI scan and everything is found to be normal. Despite this, the patient continues to experience pain that no one can explain. However, there is often a cause for the pain, which can be revealed by a more detailed examination of the images.

What is typically missed on MRI scans

If you have anterior knee pain and have been told everything looks normal, don’t settle. See below

Anterior knee pain

Probably the most common cause of anterior knee pain not seen on MRI is a pinched synovial fold. This is what we call a clinical diagnosis. Synovial folds can be difficult and sometimes impossible to see on an MRI scan. Other things that are typically overlooked are trochlear dysplasia and patella alta (too high a kneecap).

Anterior knee pain and MRI scans are for nerds

It requires special knowledge on how to measure this. Another oddity is that some MRI departments fail to perform axial imaging. They deem them unnecessary. Axial recordings are also not important for the most common things, but for anterior knee pain they are indispensable. The axial images show trochlear dysplasia, lateral trochlear inclination angle, trochlear asymmetry and TT-TG distance. If these things are not mentioned in your MRI report and you have anterior knee pain, there is a risk that something has been missed. If the orthopedic surgeon hasn’t seen these things either and can’t explain what it means, then you need to find another orthopedic surgeon.

Meniscus injuries

If you suspect you have a meniscus injury, you can be about 90% sure that if the MRI scan says there is no meniscus injury, there isn’t one. An MRI scan creates cross-sectional images through your knee. There is about 3 mm between each incision and if you are unlucky enough to have a meniscus injury right between these 2 incisions, the injury will be missed. 

Meniscus tear lesion

If you have an ACL injury, there is also a risk that you have a meniscus ramp lesion (rim lesion). This type of injury is often missed on MRI scans. The meniscus ramp injury can be difficult and often impossible to see on an MRI scan. Partly because radiologists and orthopedic surgeons are not familiar with this type of injury and because it can sometimes only be seen during an arthroscopy. It can also be missed during arthroscopy if the orthopedic surgeon is not familiar with this type of injury. The last commonly overlooked injury is a meniscus root injurywhich is the most serious type of meniscus injury.

Meniscus ramp lesion – or meniscus rim lesion

Here on the left you can see a very clear meniscus ramp lesion, but it is rare to see it so clearly on an MRI scan and often there can be a little scar tissue both on the MRI scan and during the arthroscopic surgery, so it hides the injury.

Meniscus root injury as seen in a arthroscopic surgery before and after repair

Meniscus root injury as seen on an MRI scan, which also shows the meniscus slipping over the tibia

Cruciate ligament injury

An cruciate ligament injury is a ligament injury and usually the MRI scan is very accurate. In most cases, the radiologist and orthopedic surgeon can see where the injury is located in the ACL. However, there are exceptions. If the ligament is not torn, but has simply become too long, the damage is not visible. This is often the case if you have already undergone ACL reconstruction, as the new ligament is often not torn, but simply becomes slightly lax.

Cartilage damage

MRI scans can often see larger cartilage injuries, while smaller injuries can be difficult to see. Here, the arthroscopic surgery is very precise. 

Important

If you come to me for a preliminary examination, it is important that we have the MRI scan entered into our pacs system, which is our local image processing system. Unfortunately, we don’t have automatic access to scans performed at other hospitals. It’s not enough for me to read the description of your MRI scan – I need to see the images for myself!

You will need to ask our secretary to request the MRI scan from the hospital where your scan was performed.

MRI scan of the knee

Normal cross-section of a knee

A normal kneecap and a normal groove for the kneecap are seen here 

Lateral trochlear inclination angle

Here you can see that the lateral trochlear inclination angle is 7 degrees, which is below 11 degrees and is the limit for when you call it trochlear dysplasia. Normally, the angle is 21 degrees. It is important that the images used for measurement are with the top image, where both femoral bones are visible. This is because the angle varies depending on where you measure.

Patella Alta  – too high kneecap

Patellotrochlear index. This measures the overlap between the groove (trochlea) and the kneecap (patella cartilage) – this is the only correct way to measure. There are some other methods (Insall-Salvati and Caton-Dechamp) but these are outdated and far too imprecise, and they don’t say anything about the “articulation” between the kneecap and its guide groove.

Patella Alta and Hoffa fat pad Impingement

Above you see a knee where the kneecap is too high and therefore the fat body is being squeezed (Hoffa impingement) and the arrow shows that it turns white due to swelling. You can also see that there is no overlap between the cartilage on the kneecap and the trochlear groove. This means that the kneecap only enters the groove when the knee bends and it’s too late. 

Classic bone bruise as seen in acute ACL injury – that’s the white in the bone that you can see in the middle of the femoral condyle and at the back of the tibial condyle, represents increased fluid in the bone, like a bruise. This is where the bones come together at the moment of injury.