A knee MRI provides detailed images of soft tissue, cartilage, bone marrow, and fluid that X-rays cannot detect. It can reveal meniscus tears, ligament damage, cartilage loss, bone bruising, bursitis, cysts, and early signs of arthritis, giving your doctor a much clearer picture of what’s causing your pain than a physical exam alone.
Meniscus Tears
Meniscus injuries are one of the most common reasons for a knee MRI. Healthy meniscus tissue appears uniformly dark on MRI because of its tightly organized collagen structure. When that structure is disrupted by degeneration or a tear, the damaged area lights up as a brighter signal against the dark background.
Radiologists grade meniscus damage on a scale of 1 to 3. A Grade 1 finding shows a small dot or fleck of bright signal inside the meniscus that doesn’t reach the surface. Grade 2 appears as a horizontal or diagonal line of brightness that extends toward the outer edge (joint capsule) but still doesn’t break through the joint surface. Grade 3 is a true tear: the bright signal reaches the surface of the meniscus. Only Grade 3 findings are considered actual tears, and they’re the ones most likely to cause mechanical symptoms like catching, locking, or giving way.
Ligament Injuries
Like the meniscus, healthy ligaments appear dark on MRI. A torn or stretched ligament shows up as a brighter, irregular signal where the organized collagen fibers have been disrupted. The ACL, PCL, and both collateral ligaments are all clearly visible. A complete tear typically shows a gap or wavy, disorganized fibers where the ligament should be taut. Partial tears appear as thickened, bright areas within an otherwise intact ligament.
Your MRI can also reveal injuries to structures that are harder to examine physically, like the popliteus tendon on the back outer corner of the knee. Some of these injuries are only visible on certain image orientations, which is why radiologists review the knee from multiple angles.
Cartilage Damage and Arthritis
MRI is the best non-surgical way to evaluate articular cartilage, the smooth coating on the ends of your bones that allows them to glide against each other. Cartilage damage is graded from 0 (normal) to IV (bone fully exposed). Grade I is softening and swelling that’s subtle on imaging. Grade II involves partial-thickness cracks or fissures less than half an inch across. Grade III means deeper fissures that reach the underlying bone. Grade IV is complete erosion where bare bone is exposed.
This matters because standard X-rays only show cartilage loss indirectly, by measuring the narrowing of the space between bones. An MRI can detect cartilage problems much earlier, before significant joint space narrowing appears on X-ray. It also picks up bone spurs (osteophytes) and other early osteoarthritis features that may be contributing to your pain.
Bone Marrow Edema and Hidden Fractures
One of the most clinically useful things an MRI reveals is bone marrow edema, essentially swelling inside the bone itself. This shows up as a bright area on certain MRI sequences and is completely invisible on X-rays. Bone marrow edema increases pressure inside the bone, producing pain that typically worsens with weight-bearing and at night.
The pattern of edema tells your doctor a lot about the cause. A diffuse, spread-out pattern suggests an impact injury. An angled pattern points to shearing forces. A linear pattern perpendicular to the bone can indicate a stress fracture or avulsion. MRI is the gold standard for detecting stress fractures, including insufficiency fractures in older adults with weakened bone that may occur without any obvious injury.
In people with osteoarthritis, bone marrow edema is associated with greater pain levels and faster disease progression. Its presence or absence can change treatment decisions.
Fluid, Bursitis, and Baker’s Cysts
Excess fluid inside or around the knee joint is easy to spot on MRI. A general joint effusion (fluid buildup inside the joint capsule) shows up as a bright signal on fluid-sensitive sequences. More importantly, the MRI can pinpoint exactly where fluid is collecting, which helps narrow the diagnosis.
The knee has several small fluid-filled sacs called bursae that reduce friction between tendons, ligaments, and bone. When inflamed, each produces a characteristic pattern. Prepatellar bursitis appears as an oval fluid pocket between the skin and the kneecap. Pes anserine bursitis shows as a multiloculated collection along the inner side of the knee below the joint line. Deep infrapatellar bursitis appears as a triangular pocket behind the patellar tendon. Iliotibial band bursitis shows fluid near the outer side of the knee where the IT band attaches.
A Baker’s cyst, a common cause of pain and tightness behind the knee, appears as a well-defined bright pocket between two specific tendons on the back of the knee. MRI can also show whether the cyst communicates with the joint itself, which it usually does.
What MRI Finds That X-Rays Miss
X-rays show bones clearly but are essentially blind to soft tissue. They cannot detect meniscus tears, ligament injuries, cartilage damage in its early stages, bone marrow edema, bursitis, or synovitis (inflammation of the joint lining). Doctors typically order an MRI when your pain or symptoms don’t match what the X-ray shows, or when soft tissue damage like a torn ligament or meniscus is suspected.
MRI is also better at catching early arthritis. By the time cartilage loss shows up on an X-ray as joint space narrowing, significant damage has already occurred. An MRI can detect cartilage softening, small fissures, and bone marrow changes well before that point.
Not Everything on an MRI Explains Your Pain
This is one of the most important things to understand about knee MRI results. A large meta-analysis in the British Journal of Sports Medicine found that many people with zero knee pain have “abnormal” MRI findings. Among adults under 40 with no symptoms and no history of injury, 11% had cartilage defects and 4% had meniscus tears. In adults 40 and older, those numbers jumped to 43% for cartilage defects and 19% for meniscus tears. About 25% of all asymptomatic adults had bone spurs, and 18% had bone marrow lesions.
This means that if your MRI shows a meniscus tear or cartilage wear, it may or may not be the source of your pain. Your doctor will correlate the MRI findings with your symptoms, physical exam, and the location of your pain to determine what’s actually relevant. A finding on MRI alone doesn’t automatically mean you need surgery or any specific treatment.
When Contrast Dye Is Used
Most knee MRIs are done without contrast. Contrast (a gadolinium-based dye injected through an IV) is reserved for specific situations: suspected infection such as septic arthritis or osteomyelitis, post-surgical evaluation to distinguish scar tissue from new problems, and characterizing collections of fluid to tell an abscess apart from surrounding inflammation. Contrast also helps differentiate joint inflammation from a simple effusion by highlighting thickened, inflamed synovial tissue. If your doctor orders contrast, it’s because they’re looking for something specific that a standard MRI can’t fully clarify.
What to Expect During the Scan
A knee MRI takes about 45 minutes. You’ll need to remove all jewelry and metal objects and change into a hospital gown. Your knee will be positioned inside a coil (a frame that sits around the knee to capture images), and you’ll need to stay still throughout the scan. The machine is loud, producing rhythmic banging and buzzing sounds, so you’ll typically be offered earplugs or headphones. The scan itself is painless, though lying still with an already-painful knee can be uncomfortable. No special preparation is needed unless contrast is ordered, in which case an IV will be placed beforehand.