Osteoarthritis changes how joints look both on the outside and on the inside. Externally, affected joints can appear swollen, bony, and misshapen. On X-rays and MRI, the hallmarks are narrowing of the space between bones, bony growths along joint edges, and thickened bone beneath the cartilage surface. What you see depends on which joint is affected and how far the disease has progressed.
What It Looks Like on the Outside
In the hands, osteoarthritis produces some of the most recognizable visible changes. Hard, bony bumps called Heberden’s nodes form on the joints closest to the fingertips, while Bouchard’s nodes appear on the middle finger joints. These nodes sit on the top and sides of the joint and can push the fingertip sideways, giving the fingers a crooked appearance. They develop gradually over months to years and feel firm to the touch, unlike the soft, squishy swelling of inflammatory arthritis.
The base of the thumb is another common spot. When osteoarthritis affects this joint, the metacarpal bone shifts outward while the surrounding muscles waste away. The combination creates a squared-off look at the base of the thumb that’s distinctive enough for doctors to spot across a room.
In the knees, advanced osteoarthritis can change the alignment of the entire leg. When cartilage wears down more on the inner side of the knee, the leg bows outward, creating a bow-legged stance. When the outer side wears faster, the knees angle inward, producing a knock-kneed appearance. This happens because uneven cartilage loss shifts how weight travels through the joint. In a healthy knee, slightly more load passes through the inner side. As that imbalance worsens, the alignment change becomes visible and accelerates further damage on the overloaded side.
Regardless of location, affected joints often look larger than normal due to bony enlargement around the joint margins. The skin over the joint typically looks normal in color, which is one way to distinguish osteoarthritis from conditions that cause redness and warmth.
What You Can Feel but Not See
One of the most telling physical signs is crepitus: a grinding, crunching, or crackling sensation when the joint moves. In a knee with osteoarthritis, a clinician can feel this by placing a hand over the kneecap during bending and straightening. That continuous grinding sensation reflects roughened cartilage surfaces rubbing against each other, or bone contacting bone in more advanced cases. Studies from the Osteoarthritis Initiative found that crepitus correlates with cartilage damage visible on MRI, particularly in the area behind the kneecap.
Affected joints also feel stiff, especially first thing in the morning. The stiffness typically lasts under 30 minutes, unlike rheumatoid arthritis, where morning stiffness can persist for an hour or more. Range of motion shrinks over time. A knee that once bent fully may stop well short, or a hip may resist rotating inward.
What It Looks Like on X-Ray
Doctors grade osteoarthritis severity on X-rays using the Kellgren-Lawrence scale, which runs from 0 to 4. Each grade reflects a progressively more damaged joint.
- Grade 0: Normal. No narrowing of the joint space and no reactive bone changes.
- Grade 1: Doubtful. The joint space might be slightly narrower, with possible tiny bone spurs forming at the edges.
- Grade 2: Mild. Definite bone spurs are visible, and the joint space may be starting to narrow.
- Grade 3: Moderate. The joint space is clearly narrower, bone spurs are more prominent, the bone beneath the cartilage appears denser and whiter (a sign of thickening called sclerosis), and the bone ends may be starting to change shape.
- Grade 4: Severe. Large bone spurs, marked joint space narrowing, pronounced bone thickening, and definite deformity of the bone ends. At this stage, the bones may nearly touch or actually contact each other.
Bone spurs (osteophytes) are one of the earliest and most consistent findings. They form at the edges of the joint where cartilage meets bone, growing outward like a shelf or lip. In the knee, they can appear in three compartments: the inner side, the outer side, and behind the kneecap. Even small bone spurs can be clinically significant, and they tend to grow larger as the disease progresses.
What Happens Inside the Joint
If you could look directly at an osteoarthritic joint during surgery, the cartilage changes follow a predictable sequence. Healthy cartilage is smooth, white, and glistening. The first visible sign of damage is surface roughening, where the normally glassy surface becomes frayed and velvety. This happens because the collagen fibers that give cartilage its structure begin to separate and disorganize, allowing water to seep in and vertical cracks to form between cell columns.
As the disease advances, the cartilage thins unevenly. Some areas wear down to expose raw bone, which then becomes polished and dense from repeated contact. In late stages, the cartilage surface can develop nodular or cauliflower-like thickenings in areas where the body attempts a disorganized repair. These patches of fibrous cartilage are structurally inferior to the original smooth surface and wear away more quickly.
How It Looks Different From Rheumatoid Arthritis
The visual differences between osteoarthritis and rheumatoid arthritis are significant. Rheumatoid arthritis is symmetrical, affecting the same joints on both sides of the body simultaneously. Osteoarthritis typically starts in a single joint and, when it does affect both sides, one side is usually worse. Rheumatoid arthritis favors the small joints of the hands and feet early on. Osteoarthritis gravitates toward the knees, hips, spine, and the finger joints closest to the nails.
The swelling looks different too. Rheumatoid arthritis produces soft, boggy swelling from inflamed joint lining, often with visible redness and warmth. Osteoarthritis produces hard, bony enlargement. In advanced rheumatoid arthritis, fingers can drift sideways at the knuckles or develop swan-neck and boutonniere deformities that force them into fixed bent positions. Osteoarthritis nodes are lumpy but less dramatically deforming, and the angular changes tend to be milder.
Both conditions can cause morning stiffness and limited range of motion. The key distinguishing features are the pattern of joints involved, the type of swelling, and whether the joint feels hot and inflamed or simply enlarged and stiff.