Osteoarthritis does far more than wear down your joints. It reshapes bone, rewires pain signaling in your nerves, triggers chronic low-grade inflammation, and can limit your mobility enough to affect your cardiovascular and metabolic health. Globally, it ranks as the 14th leading cause of years lived with disability, and for adults over 70, it climbs to seventh. Understanding what’s actually happening inside your body helps explain why the condition feels the way it does and why it tends to get worse over time.
How Cartilage Breaks Down
Healthy cartilage is a smooth, rubbery tissue that caps the ends of your bones and lets them glide past each other with almost no friction. It’s maintained by specialized cells called chondrocytes, which produce and repair the surrounding matrix of collagen and water that gives cartilage its structure and bounce.
In osteoarthritis, those cells start dying off through a process called apoptosis, or programmed cell death. Research published in Osteoarthritis and Cartilage found a direct, linear relationship between the severity of osteoarthritis and the rate of chondrocyte death: the worse the disease, the more cells are lost. With fewer cells maintaining the matrix, the cartilage thins, cracks, and eventually wears away entirely in some areas. Once cartilage is gone, bone grinds directly against bone, which is where much of the deep, aching pain comes from.
Unlike bone or skin, cartilage has almost no blood supply. That means it heals extremely slowly, if at all. The damage is largely a one-way street, which is why osteoarthritis is considered a progressive condition.
Bone Spurs and Bone Thickening
As cartilage deteriorates, the bone underneath it begins to change. The layer of bone just below the cartilage (called subchondral bone) thickens and hardens, a process known as sclerosis. This is your body’s attempt to stabilize the joint under increased mechanical stress, but the result is a stiffer, less shock-absorbent surface that accelerates further damage.
At the same time, small bony growths called osteophytes, or bone spurs, develop at the edges of the joint. These form from precursor cells in the tissue lining the bone and are driven by growth factors your body releases in response to the damage. Osteophytes are strongly associated with cartilage loss, though they can sometimes appear even before visible cartilage damage on imaging. They can restrict your range of motion, create a grinding or catching sensation, and press on nearby soft tissues or nerves, adding new sources of pain beyond the joint surface itself.
On X-rays, doctors grade osteoarthritis on a 0-to-4 scale. Grade 2 shows definite bone spurs and possible joint space narrowing. By Grade 4, there are large bone spurs, severe bone thickening, and visible deformity at the ends of the bones.
Inflammation Inside the Joint
Osteoarthritis was long considered a purely “wear and tear” disease, but inflammation plays a significant role. The synovium, a thin membrane lining the inside of the joint capsule, becomes inflamed in many people with osteoarthritis. This inflammation produces signaling molecules that accelerate cartilage breakdown and increase pain sensitivity.
The two most commonly detected inflammatory molecules in osteoarthritic joints are the same ones involved in many inflammatory diseases. Their levels in osteoarthritic tissue are consistently higher than in healthy joints, though lower than in autoimmune conditions like rheumatoid arthritis. Immune cells, particularly macrophages and T cells, infiltrate the joint lining and sustain this cycle. The inflammation tends to be lower-grade and more persistent than what you’d see in an acute injury, which is part of why osteoarthritis pain often feels like a constant, dull ache rather than a sharp, sudden flare.
This inflammation also contributes to the swelling, warmth, and morning stiffness that many people notice, especially in weight-bearing joints like the knees and hips.
How Your Nervous System Changes
One of the more surprising effects of osteoarthritis is how profoundly it alters your nervous system. The pain you feel isn’t just a passive signal from a damaged joint. Over time, the nerves around and within the joint physically change in ways that amplify and distort pain.
In damaged areas like the fat pad behind the kneecap and inflamed synovial tissue, normal nerve fibers can nearly disappear. But in other locations, nerves sprout in abnormal patterns, sometimes forming dense clusters resembling neuromas (tangled nerve bundles that fire pain signals excessively). Sensory and sympathetic nerve fibers have been found growing into vascular channels that break through the boundary between subchondral bone and cartilage, bringing pain-sensing nerves into areas where they don’t normally exist.
The changes extend beyond the joint itself. In the nerve cell clusters along the spinal cord that relay signals from the knee, researchers have found markers of nerve injury, infiltration of immune cells, and shifts in chemical signaling that mirror what happens after direct nerve damage. This pattern, sometimes called peripheral sensitization, means the nervous system begins interpreting normal joint movement as painful. It helps explain why pain severity in osteoarthritis often doesn’t match up neatly with the amount of visible cartilage loss on an X-ray. Some people with severe structural damage report moderate pain, while others with milder damage are in significant discomfort, because the nervous system’s response varies.
Effects on Movement and Daily Life
The combined effect of cartilage loss, bone changes, inflammation, and nerve rewiring is a progressive decline in joint function. Stiffness after rest, reduced range of motion, and pain during weight-bearing activities are the hallmarks. Knees and hips are the most commonly affected weight-bearing joints, but osteoarthritis also frequently hits the hands, spine, and feet.
The disability burden is substantial. The global rate of years lived with disability from osteoarthritis rose nearly 10% between 1990 and 2020. For most people, the condition doesn’t just make one activity harder. It creates a cascade: pain during walking leads to less physical activity, which leads to muscle weakening around the joint, which leads to less joint stability, which leads to more pain. Breaking this cycle with targeted exercise is one of the most effective strategies for slowing functional decline, even though it feels counterintuitive when your joints hurt.
Grip strength, the ability to open jars, and fine motor tasks are commonly affected when osteoarthritis involves the hands. In the spine, it can narrow the spaces where nerves exit, contributing to radiating pain, numbness, or tingling in the arms or legs.
Connections to Metabolic and Heart Health
Osteoarthritis doesn’t exist in isolation. The chronic low-grade inflammation present in osteoarthritic joints overlaps with the same inflammatory pathways involved in metabolic syndrome, a cluster of conditions that includes high blood pressure, elevated blood sugar, excess body fat around the waist, and abnormal cholesterol levels. Researchers have identified shared mechanisms of inflammation, oxidative stress, and blood vessel dysfunction between the two conditions, with some proposing that metabolic osteoarthritis should be considered a component of metabolic syndrome itself.
The relationship runs in both directions. Excess body weight increases mechanical load on joints, but the metabolic changes associated with obesity, particularly the inflammatory molecules released by fat tissue, also directly damage cartilage. This is why osteoarthritis affects hand joints too, where weight-bearing isn’t a factor. Meanwhile, the reduced physical activity caused by joint pain contributes to weight gain, insulin resistance, and cardiovascular deconditioning, creating a feedback loop that worsens both joint and systemic health over time.