Osteoarthritis in the knee shows up in two ways: what you can see and feel on the outside, and what doctors see on imaging. On the surface, a knee with osteoarthritis often looks swollen, sometimes visibly larger than the other knee, and may gradually develop a bowed or knock-kneed alignment as the disease progresses. On an X-ray, the hallmark signs are a narrowing gap between the bones, small bony growths called osteophytes at the joint edges, and a whitening of the bone just below the cartilage.
What You Can See and Feel From the Outside
The most noticeable visible change is swelling around the knee joint. This comes from soft tissue inflammation and, in many cases, extra fluid building up inside the joint capsule. The swelling tends to be firm and localized rather than the hot, red, puffy swelling you’d see with an autoimmune condition like rheumatoid arthritis. In osteoarthritis, redness and warmth are less prominent, and the swelling is usually limited to the affected knee rather than showing up symmetrically in both knees at once.
Over time, the joint itself can look wider or bonier than it used to. That’s partly because osteophytes, small spurs of new bone, grow along the joint margins and enlarge the overall shape of the knee. The muscles around the knee, particularly the quadriceps on the front of the thigh, often shrink from disuse and pain-related guarding. This combination of bony enlargement and muscle wasting gives the knee a more angular, knobby appearance.
As cartilage wears unevenly, the knee can shift out of its normal alignment. The most common pattern is a bowlegged (varus) deformity, where cartilage loss on the inner side of the knee causes the leg to angle outward. Less commonly, cartilage loss on the outer compartment produces a knock-kneed (valgus) alignment. These changes develop gradually over years and become more obvious when standing or walking.
One sign you’ll feel more than see is crepitus: a grinding, crunching, or crackling sensation when you bend or straighten the knee. In people with even mild to moderate X-ray changes, crepitus raises the likelihood of an osteoarthritis diagnosis to about 80%. It’s a sign that the smooth cartilage surfaces have roughened, so the bones no longer glide silently past each other. Crepitus in the front of the knee often points to damage in the patellofemoral joint, where the kneecap meets the thighbone.
What an X-Ray Reveals
A standard X-ray is the first-line tool for diagnosing knee osteoarthritis. It’s inexpensive, widely available, and highly specific for the key structural changes. Four features stand out on a typical osteoarthritis X-ray:
- Joint space narrowing. Healthy knees show a clear gap between the thighbone and shinbone, filled by cartilage that doesn’t show up on X-ray. As cartilage wears away, that gap shrinks. The narrowing is often uneven, usually worse on the inner (medial) side of the knee.
- Osteophytes. These are small bony projections that grow at the edges of the joint, the body’s attempt to stabilize a deteriorating surface. They appear as white, pointed bumps along the bone margins.
- Subchondral sclerosis. The bone directly beneath the cartilage becomes denser and appears whiter on X-ray. This happens because the bone remodels under abnormal stress once cartilage protection is lost.
- Bone deformity. In advanced cases, the ends of the bones flatten, develop cysts, or change shape entirely. Loose bony fragments may also float inside the joint.
The damage can affect one, two, or all three compartments of the knee. The inner compartment between the thighbone and shinbone is the most commonly involved. X-rays are excellent at showing these later structural changes but have a significant blind spot: they can’t detect early cartilage softening, inflammation in the joint lining, or damage to ligaments and menisci.
How Doctors Grade the Severity
Most doctors use the Kellgren-Lawrence scale, a five-point grading system based on X-ray appearance, to classify how far the disease has progressed:
- Grade 0: Normal. No visible changes on X-ray.
- Grade 1: Doubtful. The joint space might be slightly narrowed, and there may be a tiny hint of bone spurs. Many people at this stage have no symptoms at all.
- Grade 2: Mild. Definite osteophytes are visible, with possible early narrowing of the joint space. This is typically the first grade where a clear diagnosis is made.
- Grade 3: Moderate. Multiple osteophytes, obvious joint space narrowing, some bone thickening, and early changes to the shape of the bone ends. Most people at this stage have consistent pain and stiffness.
- Grade 4: Severe. Large osteophytes, dramatically narrowed or completely absent joint space, significant bone hardening, and clear deformity of the bone surfaces. This is often the stage where joint replacement is discussed.
It’s worth knowing that X-ray severity and symptom severity don’t always match. Some people with Grade 2 changes on imaging have significant daily pain, while others with Grade 3 changes manage reasonably well. The grade helps guide treatment decisions, but it’s only part of the picture.
What’s Happening Inside the Joint
The visible and X-ray changes reflect a deeper process unfolding in the cartilage and bone. In early osteoarthritis, the smooth cartilage covering the bone ends softens and begins to fray. At the same time, the thin layer of bone just beneath the cartilage (the subchondral bone plate) actually becomes thinner and more porous, almost the opposite of what you’d expect. This early bone loss appears to accelerate cartilage breakdown above it.
As the disease advances, the pattern reverses. The subchondral bone thickens and hardens, producing the white “sclerotic” appearance on X-ray. But this denser bone is paradoxically weaker than normal. Despite its increased volume, it’s under-mineralized, meaning it has less calcium per unit of bone than healthy tissue or even osteoporotic bone. The cartilage continues to thin, and a calcified layer at the boundary between bone and cartilage creeps upward, further reducing the functional cartilage that cushions the joint.
Small fluid-filled pockets called subchondral cysts can form within the bone, and areas of bone marrow swelling (visible on MRI but not X-ray) are common in symptomatic knees. The joint lining often becomes inflamed, producing excess fluid that contributes to the visible swelling.
Baker’s Cysts and Other Visible Complications
Between 20% and 40% of people with knee osteoarthritis develop a Baker’s cyst, a fluid-filled pouch that forms at the back of the knee in a natural pocket between two tendons. It feels like a smooth, firm lump behind the knee and can range from grape-sized to golf ball-sized. Baker’s cysts become more common as osteoarthritis worsens. In one study, more than half of patients with Baker’s cysts had Kellgren-Lawrence Grade 3 disease, compared to under 30% of those without cysts.
A Baker’s cyst can sometimes rupture, sending fluid down into the calf and causing sudden pain and swelling that mimics a blood clot. If you notice a new lump behind your knee or sudden calf swelling, that’s worth getting checked promptly.
How It Looks Different From Rheumatoid Arthritis
Osteoarthritis and rheumatoid arthritis can both cause knee swelling, but they look and behave differently. Rheumatoid arthritis is an autoimmune condition that typically causes warm, red, visibly inflamed joints, often affecting both knees symmetrically along with smaller joints like those in the hands and feet. Early MRI in rheumatoid arthritis often shows synovial thickening and joint effusion driven by immune system activity rather than mechanical wear.
Osteoarthritis swelling tends to be less dramatic in color and temperature. The bony enlargement and angular deformity of osteoarthritis develop over years, while rheumatoid arthritis can cause rapid joint damage during flares. On X-ray, rheumatoid arthritis erodes bone inward (creating holes in the bone surface), while osteoarthritis builds bone outward in the form of osteophytes. These differences help doctors distinguish the two conditions even when symptoms overlap.