Arthritis on a knee x-ray shows up as a combination of four main changes: narrowing of the space between bones, bright white patches along the bone surface, small bone spurs at the edges of the joint, and occasionally dark hollow spots within the bone itself. These signs tell a clear story about how much cartilage has worn away and how the bone underneath has responded. Here’s what each of those changes actually looks like and what they mean.
The Normal Knee vs. the Arthritic Knee
On a healthy knee x-ray, you’ll see a clear gap between the thighbone (femur) and the shinbone (tibia). That gap isn’t empty space. It represents cartilage, the smooth cushioning layer that covers the ends of both bones. Cartilage doesn’t show up on x-rays, so the gap appears dark. When that dark gap is even and wide, the cartilage is intact.
In an arthritic knee, that gap shrinks. The bones look closer together, sometimes dramatically so. In advanced cases, the gap disappears entirely on one side of the joint, and you’re looking at what doctors call “bone on bone.” A Mayo Clinic image of this stage shows how the cushioning cartilage has worn away completely, allowing the two bone surfaces to touch directly. That contact is what drives much of the pain and stiffness people feel.
Joint Space Narrowing
The earliest and most important sign of knee arthritis on x-ray is a narrowing of the joint space. What makes osteoarthritis distinctive is that this narrowing is uneven. It typically affects one side of the knee more than the other, usually the inner (medial) compartment. If you imagine looking at the knee from the front, the gap on the inner side may be noticeably thinner than the gap on the outer side. This asymmetry is a hallmark that distinguishes wear-and-tear arthritis from other conditions.
In early stages, the narrowing can be subtle. That’s one reason your doctor may ask you to stand during the x-ray rather than lie down. When you’re standing, your body weight compresses the joint and reveals narrowing that wouldn’t be visible if the knee were unloaded. Studies confirm that non-weight-bearing x-rays can significantly underestimate how much cartilage has been lost, particularly in more severe cases where the remaining cartilage compresses further under load.
Bone Spurs at the Joint Edges
As cartilage wears down, the body tries to stabilize the joint by growing extra bone at its margins. These bony outgrowths, called osteophytes or bone spurs, appear on x-rays as small, pointed or shelf-like projections extending from the edges of the thighbone or shinbone. They can also form around the kneecap.
Bone spurs are one of the most recognizable signs of arthritis on an x-ray because they change the smooth contour of the bone into something rougher and more irregular. They form as part of the joint’s attempt to repair itself, but that remodeling process is often haphazard. The spurs themselves don’t always cause symptoms, but large ones can restrict movement or irritate surrounding tissue.
Subchondral Sclerosis: The White Patches
Just beneath the cartilage sits a layer of bone called subchondral bone. When cartilage thins out, this underlying bone absorbs more stress than it was designed for. In response, it thickens and becomes denser. On an x-ray, denser bone absorbs more radiation and appears brighter white. These bright white patches along the joint surface are called subchondral sclerosis.
The severity varies. In mild cases, you might see a subtle brightening or partial thickening along the bone surface. In more advanced arthritis, the sclerosis becomes obvious, with substantial thickening across multiple areas of the joint. Radiologists grade it on a scale from no visible change to significant sclerosis with widespread destruction. If you’re looking at your own knee x-ray, these white zones near where the bones meet are one of the clearest visual indicators of ongoing damage.
Subchondral Cysts: Dark Spots in the Bone
In some cases, small fluid-filled pockets develop within the bone just below the joint surface. On x-rays, these appear as dark, hollowed-out areas within the otherwise bright bone. They can be tricky to spot because they’re often poorly defined on standard x-rays, which is why an MRI is sometimes ordered to get a better look.
These cysts are fairly common. Research on over 800 people with knee osteoarthritis found that roughly 30% had them. They can form before significant cartilage loss is visible, and they may or may not connect to the joint space itself. When they do appear on your x-ray, they’re usually surrounded by a thin border of dense (sclerotic) bone, creating a dark circle with a bright rim.
Rheumatoid Arthritis Looks Different
Not all knee arthritis looks the same on x-ray. Osteoarthritis, the wear-and-tear type, tends to affect one knee more than the other and wears down one side of the joint unevenly. Rheumatoid arthritis, an autoimmune condition, creates a distinctly different pattern. It typically affects both knees symmetrically and causes bone erosion, where the bone surface appears chewed away rather than simply compressed. Joint space narrowing in rheumatoid arthritis tends to be more uniform across the entire joint, and you may also see joint deformity as the disease progresses.
This symmetry is one of the key clues radiologists use to tell the two apart. If both knees show similar damage in similar locations, rheumatoid arthritis is more likely. If one knee is significantly worse, or one side of a single knee bears most of the damage, osteoarthritis is the more probable diagnosis.
Why Standing X-Rays Matter
If your knee x-ray was taken while you were lying on a table, it may not tell the full story. Weight-bearing x-rays, taken while you’re standing, are the standard for evaluating knee arthritis because gravity and your body weight push the bones together the way they actually function during daily life. This compression reveals the true extent of cartilage loss.
The difference can be significant. When you’re lying down, slack ligaments and the absence of load can make the joint space appear wider than it really is. In people with more advanced arthritis, chronic looseness in the ligaments on the outer side of the knee exaggerates this effect even further. Research comparing the two approaches found that weight-bearing images agreed much more closely with full-length alignment measurements, with an agreement score of 0.878 compared to just 0.657 for non-weight-bearing films. If your x-ray was taken lying down and shows only mild changes, a standing x-ray may reveal a more accurate picture.
Some doctors also use a slightly bent-knee view, taken with the knee flexed about 30 to 45 degrees while standing, to catch cartilage loss in the area where wear is most common. A standard straight-leg view can miss narrowing that becomes obvious when the knee is slightly bent.
What X-Rays Can and Can’t Show
X-rays are excellent at showing bone changes: spurs, sclerosis, cysts, and the gap between bones. What they can’t show is the cartilage itself, the surrounding soft tissues, or early inflammation. That’s why someone can have significant knee pain with a relatively normal-looking x-ray, or vice versa. The amount of damage visible on x-ray doesn’t always match how much pain a person feels.
When x-ray findings are minimal but symptoms are significant, an MRI can fill in the gaps. MRI reveals cartilage directly, shows swelling within the bone (called bone marrow edema), and can detect tears in the meniscus or ligaments that contribute to knee pain. But for a straightforward assessment of arthritis severity, a standing x-ray remains the starting point.