Quick answer: a knee MRI shows the bones, cartilage, meniscus, ligaments, tendons, muscles and blood vessels of the knee in detail, without radiation. Together with x-rays, it is usually the best test for knee injuries, including meniscus tears, ACL and other ligament tears, cartilage damage and fractures that x-rays miss. It usually takes 30 to 60 minutes, you go in feet first, and for most people the head stays outside the scanner.
Knee scans are among the most relaxed MRIs I do: the patient goes in feet first, and the head usually stays out of the tube. What patients worry about is the result, usually «is it torn?». Here is what a knee MRI can and cannot tell you.
What a knee MRI can detect
A knee MRI can help diagnose or evaluate:
- meniscus tears and damage to the cartilage
- ligament injuries, such as tears of the anterior cruciate ligament (ACL), and tendon injuries
- sports injuries, including sprains and torn ligaments, cartilage or tendons
- fractures that do not show on x-rays or other tests
- arthritis damage
- fluid build-up in the joint
- infections of the bone, such as osteomyelitis
- tumors affecting the bones and joint
- dead bone (loss of blood supply to the bone)
- kneecap injuries and pain
- a knee that gives way, locks or has reduced movement
- problems after surgery, including complications from implants
Doctors also use knee MRI to decide whether arthroscopy or another surgery is needed, and to check progress after surgery.
Knee MRI vs x-ray
X-rays show bones well and are often the first test. MRI adds the soft tissues that x-rays cannot show: meniscus, cartilage, ligaments and tendons, as well as small fractures and bone bruising. That is why doctors often order both.
With or without contrast? And what is an MR arthrogram?
Most knee MRIs are done without contrast. A special version called an MR arthrogram uses contrast injected directly into the knee joint, rather than into a vein, to give the radiologist a closer look at the structures inside the joint. If your doctor ordered an arthrogram, the injection is done just before the MRI. See [link: MRI With vs Without Contrast].
How to prepare
- Food, drink and medications: usually no restrictions.
- Metal and implants: tell the team about any implants, especially from previous knee surgery, and any metal fragments such as shrapnel. Knee replacements and screws are usually not a safety problem, but they can distort the images; some centers use special techniques to reduce this. See [link: Can You Have an MRI With Metal Implants, Tattoos or a Pacemaker?].
- Clothing: sports leggings and knee braces can contain metal threads; expect to change.
- Piercings and jewelry: remove them, including ankle bracelets and toe rings.
The full checklist is in [link: How to Prepare for an MRI].
What the scan feels like
- Positioning. You lie on your back and go in feet first. Your knee is placed inside a coil, a plastic frame that fits around the joint, and padded so it stays still.
- Your head usually stays outside. For a knee scan, only your lower body goes into the tube. This is why knee MRIs have some of the lowest rates of scans stopped because of claustrophobia.
- Hearing protection is still provided.
- Staying still. Each sequence lasts from under a minute to several minutes. Keep the leg relaxed and avoid tensing the thigh muscles.
From the scanner room: [TU EXPERIENCIA: detalles reales de la colocación de rodilla: cómo se inmoviliza, qué hacer si la rodilla no se puede estirar del todo, qué molestias son normales tras un rato.]
How long does a knee MRI take?
Usually 30 to 60 minutes. Plan extra time for check-in and changing, and more if an arthrogram injection is done first. See [link: How Long Does an MRI Take?].
Limitations
- Movement: shaking, coughing or pain can blur the images.
- A knee that cannot straighten is harder to image well.
- Metal implants can create distortions near the knee.
- Size: very large patients may not fit in some scanners.
Getting your results
A radiologist reviews the images and sends a signed report to your doctor, who will explain what it means for treatment. Knee reports often describe findings such as meniscus «degeneration» or small amounts of fluid. Some of these are common, especially with age, and are not always the cause of your pain. Your doctor combines the report with the physical examination.
Tips from the technologist
[TU EXPERIENCIA: 3-4 consejos propios para la RM de rodilla. Ideas: ropa cómoda sin metal, avisar si la rodilla no se estira, relajar el muslo, cómo pasar el rato si la cabeza queda fuera.]
Frequently asked questions
Can a knee MRI show a torn meniscus?
Yes. MRI is the standard imaging test to look for meniscus tears.
Can a knee MRI show an ACL tear?
Yes. MRI shows the ligaments of the knee, including the ACL, and helps doctors decide whether surgery is needed.
Will my head be inside the machine?
Usually not. For a knee MRI you go in feet first, and in most cases your head stays outside the scanner.
Can I have a knee MRI with a knee replacement?
Usually yes, but tell the team. The implant can distort images near it, and some centers use special metal-reduction techniques.
How much does a knee MRI cost?
It depends on your insurance and the facility. See [link: How Much Does an MRI Cost Without Insurance?].
This article is for general information and does not replace advice from your doctor or imaging team.