Arthritis in the knee changes how the joint looks both on the outside and on the inside. Externally, you might notice swelling, changes in leg alignment, and muscle wasting around the knee. On imaging, the joint shows cartilage loss, bone spurs, and other structural damage that explains the pain. What you see depends on the type of arthritis and how far it has progressed.
Visible Changes on the Outside
The most obvious external sign is swelling. An arthritic knee often looks puffy or enlarged compared to the other knee, especially after activity. In osteoarthritis, the swelling tends to feel firm and knobby because the bone itself is enlarging around the joint. In rheumatoid arthritis, the swelling has a different quality: it feels doughy or spongy to the touch because inflamed tissue and excess fluid are filling the joint capsule rather than bone growing outward.
Redness and warmth are more common in inflammatory types like rheumatoid arthritis, where the immune system is actively attacking the joint lining. Osteoarthritis can also cause occasional warmth during flare-ups, but it’s less consistent. Rheumatoid arthritis also tends to affect both knees symmetrically, so you may notice matching swelling on each side, while osteoarthritis often hits one knee harder than the other.
Some people develop a visible bulge at the back of the knee called a Baker’s cyst. This happens when the membrane lining the joint capsule tears slightly, allowing joint fluid to pool into a pouch behind the knee. It looks like a soft, egg-sized lump in the crease behind the joint.
Changes in Leg Alignment
As arthritis progresses, it wears down cartilage unevenly. The inner side of the knee usually loses cartilage faster, which gradually shifts the leg into a bow-legged position (called varus deformity). Less commonly, the outer side wears down first, pushing the knees inward into a knock-kneed alignment. These changes happen slowly over years, but in advanced arthritis they become noticeable just from looking at someone standing. The misalignment creates a cycle: the uneven weight distribution accelerates cartilage loss on the already-damaged side, making the deformity worse over time.
Muscle Wasting Around the Knee
One of the subtler visual signs is the thigh muscle on the affected side looking noticeably thinner than the other. The quadriceps, the large muscle group on the front of the thigh, tends to weaken and shrink when knee arthritis limits how much you use the leg. You might notice that your pants fit differently on one side, or that the muscle above the kneecap looks flatter and less defined. This muscle loss isn’t just cosmetic: weakened quadriceps provide less support to the joint, which can accelerate the arthritis itself.
How It Changes the Way You Walk
Arthritis in the knee produces visible changes in gait that other people often notice before you do. People with knee osteoarthritis take shorter, slower strides, walking roughly 11 fewer steps per minute compared to people without the condition. You may see someone lean their upper body over the affected knee during each step, a compensatory pattern called trunk lean that shifts weight to reduce joint stress.
Other movement changes include turning the toes more inward or outward, and the knee drifting slightly inward with each step. Because the knee, hip, and ankle work as a chain, arthritis in the knee also reduces range of motion at the other two joints. The overall effect is a stiffer, more guarded walking pattern that’s often visible from across a room.
What X-Rays Show
On an X-ray, four features define knee arthritis. The first is joint space narrowing: the gap between the thighbone and shinbone gets smaller as the cartilage cushion thins out. In early stages the narrowing is subtle and uneven, typically worse on the inner side of the knee. As it progresses, the gap can nearly disappear.
The second feature is bone spurs (osteophytes), which are bony growths that form around the edges of the joint. They appear most prominently along the inner side of the shinbone and thighbone, and behind the kneecap. Third, the bone just beneath the cartilage becomes denser and brighter on the X-ray, a sign called subchondral sclerosis. This happens because the bone thickens in response to absorbing forces that healthy cartilage would normally cushion. Fourth, small cysts can form within the bone near the joint surface.
In advanced cases, X-rays also show the shinbone shifting slightly outward relative to the thighbone, reflecting the bow-legged deformity visible on the outside.
What MRI Reveals That X-Rays Miss
MRI captures soft tissue detail that X-rays cannot. One important finding is bone marrow lesions: areas of swelling within the bone itself that light up on fluid-sensitive MRI sequences. These lesions are significant because their specific location within the knee can predict how the disease will progress. Research using a detailed scoring system that maps 28 separate locations across the thighbone, shinbone, and kneecap has shown that bone marrow lesions in certain spots are strong predictors of eventually needing a knee replacement.
MRI also shows cartilage damage directly. Healthy cartilage appears as a smooth, consistent layer covering the bone surfaces. In arthritis, it shows softening and discoloration first, then partial-thickness defects where chunks of cartilage have worn through partway. In the most advanced stage, the cartilage is gone entirely and the underlying bone is exposed and eroding. MRI can also reveal inflammation of the joint lining, tears in the meniscus, and ligament damage that often accompany arthritis.
Osteoarthritis vs. Rheumatoid Arthritis Appearance
The two most common types of knee arthritis look different in several ways. Osteoarthritis produces hard, bony enlargement of the joint. The swelling is firm. It typically affects one knee more than the other, and deformity develops gradually from cartilage wear. The skin over the joint usually looks normal in color.
Rheumatoid arthritis produces soft, spongy swelling from inflamed joint tissue and fluid buildup. It often affects both knees. The skin may appear reddened and feel warm. On imaging, rheumatoid arthritis shows more uniform joint space loss (cartilage is destroyed evenly across the joint rather than on one side) and erosions in the bone itself, while osteoarthritis shows the asymmetric narrowing and prominent bone spurs described above. Over time, both types can lead to significant deformity, but they get there through different mechanisms: one through mechanical wear, the other through immune-driven inflammation.