Knee MRI: Meniscus Tear vs. ACL Injury

MRI 10 November, 2026

Anatomy of the knee and its structures

The knee joint connects 3 bones in your leg. It joins your thigh bone (femur) to your shin bone (tibia) and your kneecap (patella) to your tibia. A fourth lower-leg bone (fibula) sits nearby on the outside but is not part of the main knee joint.

Your knee joint has a wide range of movement, bending forwards and backwards, and rotating. This is possible because of movement between the interacting surfaces of the tibia and femur, and between the interacting surfaces of the tibia and patella.

Your knee also contains nerves, blood vessels, ligaments, tendons (tissue that connects bone to muscle), cartilage and bursae. Bursae are fluid-filled sacs that act as cushions to reduce friction when the joint moves.

Injury to any of these tissues in your knee joint can cause swelling, pain, discomfort and clicking sounds when you move.

Importance of the meniscus in knee health

There are 2 main types of cartilage in the knee: articular cartilage, which covers the bony interacting surfaces of the joint to reduce friction, and C-shaped cushions of cartilage called menisci.

Each meniscus acts as a shock absorber, which spreads the impact of any force passing through your knee. This reduces strain on your knee joint.

These cushions also cover most of the surface of the tibia, which forms the knee joint. This protects the articular cartilage on this bone and reduces friction on movement.

The menisci also help the femur fit more securely into the tibia, stabilising the joint.

Role of the ACL in knee stability

There are 4 main ligaments – tissue that connects bone to bone – in the knee, which help stabilise the joint.

These include 2 collateral ligaments that prevent excessive sideways movement: the medial collateral ligament (MCL) and lateral collateral ligament (LCL).

The other 2 main ligaments are cruciate ligaments: the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). These prevent the tibia from moving too far forward, in the case of the ACL, or too far backwards, in the case of the PCL. They also prevent too much rotation of the knee.

Depending on the extent of an ACL injury, you may find it hard to bear weight on your knee and may feel as if it is going to give out. You may also struggle to run, pivot or suddenly change direction.

Understanding knee injuries: meniscus tears vs. ACL injuries

Common causes of meniscus tears and ACL injuries

Of all knee ligament tears, the most common torn ligament is the ACL. It’s often injured during sports when landing awkwardly from a jump, twisting, pivoting or suddenly stopping or changing direction.

Less commonly, the ACL is injured due to a direct impact to the knee. ACL injuries can range from sprains to partial or full tears.

Meniscus tears usually occur when you twist or rotate your knee with force, most often while playing sports eg when pivoting or suddenly changing direction. However, with age, less intensive activities can lead to a torn meniscus. Meniscal tears can be partial or full tears.

Key differences between meniscus tear symptoms and ACL tear signs

You’ll usually hear a popping sound and instantly feel intense pain in your knee when an ACL tear happens. Your knee will swell up soon after, and you may feel your knee give way when you try to bear weight on it.

In contrast, a meniscus tear doesn’t produce any obvious sound when it happens. The resulting pain is usually specific to a particular region within the knee. Swelling can take a few hours or days to develop. You may be able to bear weight on your knee, but you might find that your knee catches, clicks and/or locks when you bend and straighten it.

When is a knee MRI needed for injury assessment?

Typical scenarios requiring an MRI scan for knee pain

Mild to moderate knee injuries often get better with at-home treatments such as rest, physiotherapy exercises and taking over-the-counter anti-inflammatory medication.

If, after 4-6 weeks of these treatments, your knee pain is not getting better, your doctor may recommend an MRI (magnetic resonance imaging).

Recognising red flags: when to seek immediate imaging

You may benefit from a knee MRI after a knee injury if:

You can’t bear weight on your knee
You develop immediate swelling
You experience immediate, severe pain
You heard a popping sound when your knee was injured
Your knee gives way, catches or locks up
Your knee looks deformed

How does MRI work?

Magnetic resonance imaging (MRI) is an advanced medical imaging technique that produces highly detailed images of the inside of the body, particularly soft tissues, organs and bones. Unlike X-rays or CT scans, MRI does not use ionising radiation. Instead, it uses powerful magnets and radio waves to create images.

During an MRI scan, a magnetic field temporarily affects tiny particles called protons within the body’s tissues. Radio wave pulses then alter their position, and as they return to normal, they release energy. This energy is detected by the scanner and converted into detailed cross-sectional images, sometimes in 3D.

What does a knee MRI show that X rays or other scans may miss?

Magnetic resonance imaging of the knee provides high quality images of the structures within your knee, including its soft tissues. This is unlike X rays, which are best at detecting bone fractures and cannot create clear images of soft tissues.

A knee MRI can detect damaged cartilage, ligaments, muscles and tendons, which will not show on an X ray. MRI of the knee, therefore, helps to get an accurate diagnosis of your knee injury.

An accurate diagnosis means you can get the right treatment sooner and may return to your usual activity levels.

What to expect during a knee MRI scan in the UK

How to prepare for a knee MRI scan

You can eat, drink and take any medications as normal before a knee MRI scan.

On the day of your scan, you’ll be asked to remove any metal objects from your body, such as piercings, jewellery, clothing with metal zippers, buttons or clasps, and removable metal medical devices.

This is because metal may be attracted to the magent, heat up or interfere with the clarity of the scan images. If you have metal implants that aren’t compatible with MRI, you may need to have a different type of scan. Our Radiographers will ask you some questions to establish if it is safe for you to have an MRI.

If you feel anxious about being in the enclosed space of an MRI machine, tell your care team so they can better support you. Most knee MRI scans are performed with your feet inside the scanner tube so that your knee is in the middle of the machine, and your upper body and head on the outside.

The procedure: step-by-step experience for patients

When you arrive for your scan, your care team will explain what will happen before, during and after your scan, and you’ll be asked to answer some safety questions and sign a consent form.

You’ll be asked to remove all metal objects from your body and may be asked to change into a hospital gown.

You will then lie flat on the patient table of the MRI machine with your head on a pillow. You’ll be slid into the tube of the MRI machine feet-first until your knee is in the centre of the machine.

When your scan begins, you’ll need to lie still to ensure clear images are created. You’ll hear loud banding, clicking and knocking noises – these are the normal sounds of the MRI machine working. You will be given special earplugs and headphones to protect your hearing and reduce the noise. If the noises bother you, your care team can provide you with music to listen to.

Once your scan is complete, the MRI signals detected are processed by a computer into highly detailed pictures of the inside of your knee. The final images of the inside of your knee allow your doctor to view your knee tissues from many angles.

Once your scan is complete, you can get back to your day.

Your knee MRI results

At Vista Health, we aim to send you a report of your knee MRI results within 3 working days. Your report will be written by an expert Consultant Radiologist (a doctor trained in interpreting medical imaging).

Your report can be sent to your NHS GP too. You can then follow up with them, or you can book a follow up appointment with one of our experienced Vista Health GPs. They’ll explain what your results mean for you and any next steps needed, such as treatment with physiotherapy, steroid injections or knee surgery.

Get to the bottom of your knee pain

Have you been putting up with knee pain for weeks? Don’t wait any longer to get a diagnosis. Get a private MRI scan of your knee with Vista Health at one of our clinics nationwide.

Simply book online with a self referral – you don’t need your GP or other healthcare professional to refer you.