MRI scan of the knee - this can be a very good idea
MRI scan of the knee
In case you a having a knee problem, in most situations it will be helpful for you to have an MRI scan of the knee - to make a diagnosis or to support the diagnosis or to exclude other reasons for your problem. MRI scans are really good for seeing meniscus tears, anterior cruciate ligament (ACL) tears and other ligament tears. The price has also declined in the past years and today it is close to 400 euro in Denmark. If your main problem is knee pain, I believe you should in most circumstances have an MRI scan of the knee and you should not have an arthroscopy without the MRI.
What is an MRI scan of the knee?
MRI scan is a magnetic resonance scan and by having a different magnetic field gradients it can generate images of the knee. Both open and closed MRI scanners exist and as far as I know, the quality from the closed ones is always better. You are supposed to lie supine, and claustrophobia is seldom a problem since your head is out site the machine.
How precise is an MRI scan of the knee?
In most situations it is very precise, however there is a number of exceptions. I will try to explain some of those here and what you need to be aware of. First of all an MRI of the knee not a answer book. A good clinical examination, in combination with your history about how the problem started and what type of pain or problem you might have, can been even more precise. Some things cannot be found by either a clinical examination or by an MRI scan and you may need to undergo an arthroscopy of your knee.
One of the problems with MRI scans is that the result relies on who is looking at the pictures. Normally the radiologist very good, but they may overlook things and some things might be so special that they even do not even know about it. Sometimes the radiologist has not received the appropriate information from the orthopaedic surgeon that is actually needed to look for a specific problem. The radiologist sends his report to the orthopaedic surgeon. Some orthopaedic surgeosn only look at this report and do actually look at the MRI scan pictures themselves. If they do, they may not know precisely what to look for if it is something rare. The main reason why I write this is not to scare you, but to make you aware that if you are in a situation with continuous pain and you have been told that the MRI is absolutely normal, you might need a second opinion.
What is typically overlooked by an MRI scan of the knee?
Anterior knee pain
I believe that one of the most common reasons to have anterior knee pain, and where also the findings seems to be normal, might be a synovial plica, since this structure can be difficult to evaluate on an MRI. Also trochlear dysplasia is often overlooked as well as patella alta. So the reason for this might be that the radiologist and/or the orthopaedic surgeon do not know how to precisely measure this. At least in Denmark some departments have cancelled the axial view, in order to save time and money. Nevertheless the axial view is the most important view in respect to kneecap instability and anterior knee pain, since this is where the trochlear dysplasia is noticed and the lateral trochlea inclination angle, the trochlear asymmetry and the TT-TG or TT-PCL is measured. So if those measurements are not mentioned and you are dealing with kneecap problems, ask the orthopaedic surgeon about those and if he does not know how to measure, perhaps you should ask for another orthopaedic opinion.
If a meniscus tear is suspected you can be 90% or more sure that the MRI will reveal this tear. However in some situations you can be unlucky that the tear is localised just between two MRI slices and therefore not visible. You might also have what is called a meniscus ramp lesion, that means that the meniscus is torn from the capsule in the rear part of the knee and is frequently associated with anterior cruciate ligament (ACL) injuries. The meniscus ramp lesions can be difficult to see both on MRI and during arthroscopy and are often overlooked. Also meniscus root tears might be overlooked on the MRI scan and "just" called a radial meniscus tear; however this type of tear can be very detrimental to the knee and in most cases surgery is indicated and that means the tear needs to be fixed or cut away.
Meniscus ramp lesion
In the figure on the left you see the knee from the side and there is a meniscus ramp lesion; however it is rather seldom the lesion is so clearly seen. Often such a lesion can be overlooked based on a little scar that covers the lesion, and this does also apply during the arthroscopy, and therefore careful exploration might be necessary.
Meniscus root tear before and after fixation.
Meniscus root tear and seen on MRI and you also see how the meniscus extrude (glide away over the edge of the tibia)
Anterior cruciate ligament tear - ACL tear
In most situations an ACL tear is seen very precisely on MRI; however there are exceptions. In some cases the ligament is not torn, but just overstretched and has becoming too long. This can be difficult to see on an MRI, but a clinical examination will in most situations reveal this. If you already have had an ACL reconstruction and your knee has become loose again, this can be difficult to see on an MRI, since the ACL graft will in most situations not become torn, but just slowly gets longer and longer.
MRI scan can be very clear on large cartilage lesions, however smaller lesions can be difficult to detect and in those situations is the arthroscopy much more precise.
Just a little about MRI scans of the knee and what you should be aware of
Normal axial view of a left knee
Lateral trochlear inclination angle
Here you see a lateral trochlear inclination angle at 7 degrees, which is a little below 11 degrees that is the threshold for calling it trochlear dysplasia. The normal angle is 21 degrees. It is important that the measurement of the inclination angle is done at the most proximal axial view, where both of the posterior femur condyles are seen. The lateral trochlear inclination angle varies substantially from the top of the groove (caudal) to the bottom (distal).
Patella Alta - high riding kneecap
The patellotrochlear index is a more clinically relevant measurement of patellar height than other types of measurements. It is a measurement that says how big is the overlap between the trochlear groove and the kneecap. This is the only correct way to measure the height of the kneecap, and old measurements like the Insall-Salvati and Caton-Dechamp indexes are obsolete and imprecise. Those measurements do not say anything about the articulation between the patella and trochlea, and that is what patella alta is all about.
Patella Alta & Hoffa's fat pad Impingement
Above you see a figure that demonstrates a high-riding knee cap (patella alta). You also see an arrow that points to a white area. This is what is called Fat Pad Impingement. The white area is increased water content in the fat pad and this means inflammation. The inflammation is caused by the late engagement of the patella in the trochlear groove. This is due to a high riding Patella. The patella will first engage into the trochlea area when the knee is flexed several degrees. Since the pressure is not on the patella, it causes too much load on the fat pad. The increased load causes the inflammation in the Fat Pad. The solution is to bring down the kneecap. This is done with a distalisation of the tibial tubercle.