Osteoarthritis: what really happens inside a joint?
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For a long time, osteoarthritis was described in simple terms: as we get older, cartilage gradually wears away until the bones begin to rub together. That picture contains a grain of truth, but it leaves out much of what researchers now know.
Osteoarthritis affects the whole joint. Cartilage changes, but so does the bone beneath it. The synovial membrane may become inflamed, bony outgrowths can develop and surrounding muscles may weaken. The way forces are distributed through the joint also changes. This helps explain why severe changes on an X-ray do not always mean severe pain, why symptoms can fluctuate and why two people with similar scans may feel very different. To understand osteoarthritis, we need to look inside the joint.
What does a normally functioning joint look like?
A joint enables two bones to move relative to each other. Their ends are covered with smooth, water-rich articular cartilage, which helps withstand the forces of movement. A capsule surrounds the joint; its inner lining is the synovial membrane, which produces synovial fluid. This fluid helps lubricate the joint and supports exchanges needed by cartilage. Just beneath the cartilage lies the subchondral bone.
These tissues do not operate independently. They communicate and react to one another, which is why osteoarthritis can no longer be understood as a problem affecting cartilage alone.
Is osteoarthritis simply cartilage “wear and tear”?
No. “Wear and tear” makes cartilage sound like the sole of a shoe that inevitably disappears the more it is used. The biology is considerably more complex. Cartilage is living tissue containing cells called chondrocytes, which help maintain its extracellular matrix. In a healthy joint, production and breakdown of the matrix remain relatively balanced.
In osteoarthritis, that balance is disturbed. Mechanical stresses, cellular ageing, previous injuries, metabolic factors or individual predispositions may cause breakdown processes to become more prominent. Cartilage can lose some of its properties, thin or develop fissures — but that is only part of the story.
What happens when cartilage begins to deteriorate?
Fragments of the cartilage matrix may enter the joint environment. The synovial membrane can respond by releasing substances involved in inflammation, while those signals may stimulate cartilage cells to produce more enzymes that break down the matrix. Interactions between cartilage, the synovial membrane and subchondral bone can create a cycle of biological disruption. Inserm describes these mechanisms in its research and patient information on osteoarthritis.
Osteoarthritis is therefore not merely mechanical abrasion. It is a gradual change in how several joint tissues function and interact.
Is osteoarthritis an inflammatory disease?
Inflammation can be part of osteoarthritis, but it is not the same condition as rheumatoid arthritis or another inflammatory rheumatic disease. In osteoarthritis, inflammation is usually more localised and variable. The synovial membrane may become irritated and release inflammatory mediators, contributing to phases when a joint suddenly becomes much more painful.
These episodes are sometimes described as congestive or inflammatory flares. In the knee, they may involve increased pain, swelling from an effusion and pronounced stiffness. Between flares, the joint may be much quieter. This is one reason osteoarthritis does not necessarily progress in a straight line.
Why can a joint with osteoarthritis swell?
Swelling may result from a reaction in the synovial membrane. When irritated, it may produce more synovial fluid, which accumulates inside the joint. This is called an effusion and is particularly noticeable in the knee.
A knee that suddenly looks swollen does not mean that the cartilage disappeared overnight; the joint may be going through a more active inflammatory phase. Sudden severe swelling, warmth, redness or other concerning symptoms need medical assessment, however, because not every swollen joint is caused by osteoarthritis.
If cartilage is damaged, why doesn't the pain simply come from the cartilage?
One of the most surprising facts about osteoarthritis is that healthy articular cartilage has virtually no nerve fibres capable of transmitting pain. It is therefore not the joint's main pain “alarm”.
By contrast, the synovial membrane, capsule, ligaments, subchondral bone and other surrounding tissues do have nerves. They can respond to inflammation, mechanical changes and bone remodelling and contribute to pain. This explains why the amount of cartilage remaining cannot, by itself, tell us how much pain a person will experience.
Why can an X-ray show severe osteoarthritis without much pain?
X-rays and symptoms measure different things. Imaging reveals structural signs such as narrowing of the joint space, changes in the subchondral bone and osteophytes. It does not directly show how the nervous system interprets signals from the joint or all the factors involved in pain.
Two people with similar X-rays can therefore have very different symptoms. French National Health Insurance describes this as a radiographic–clinical mismatch: osteoarthritis visible on an X-ray may cause no pain and may not require specific treatment. NICE also highlights the importance of symptoms and clinical assessment rather than relying solely on images. An X-ray report is not a verdict on your future mobility.
What are osteophytes, sometimes called “parrot beaks”?
Bone also responds as an osteoarthritic joint changes. Bony outgrowths known as osteophytes may develop around the joint; in French they are sometimes nicknamed “parrot beaks”. They are a classic feature on some osteoarthritis X-rays, but they are not simply cartilage turning into bone.
The subchondral bone may become denser in places — called sclerosis — and small cyst-like cavities may form. Radiologists therefore assess several structural signs, not just the apparent thickness of cartilage.
Why is the bone beneath cartilage important?
Subchondral bone receives and distributes forces transmitted through the joint. As osteoarthritis progresses, its structure can remodel. Unlike articular cartilage, this bone is innervated, so changes within it may contribute to pain.
Researchers are increasingly interested in the relationship between cartilage and subchondral bone when investigating both disease progression and symptoms. This is another reason that “cartilage wear” is an incomplete explanation.
Why are some periods so much more painful than others?
Osteoarthritis can fluctuate. A joint may remain relatively settled for a time and then become more sensitive following a change in activity, a synovial flare or other mechanical changes. Symptom intensity is not an instantaneous measure of the condition of the cartilage.
Inserm describes the progression of osteoarthritis as variable and difficult to predict. A bad week does not automatically mean several years' worth of joint deterioration has occurred at once. Equally, it is important to avoid two unhelpful extremes: “it's just wear and tear, so nothing can be done” and “there must be a product that makes the joint brand-new”. Between these lies a more practical reality: strength, activity, function, pain and loading can all be addressed, even when a joint cannot be restored to its exact state at age twenty.
Can cartilage regenerate?
Articular cartilage has limited natural repair capacity. That is why researchers are so interested in treatments capable of restoring or regenerating cartilage. For now, routine osteoarthritis care focuses mainly on symptoms and physical function. Strategies to modify joint structure are being investigated, but there is no simple established way to reverse cartilage deterioration.
That does not prevent meaningful improvement. Someone may increase strength, regain confidence, become more active and reduce the impact of osteoarthritis on everyday life without their X-ray having to look “normal” again. The distinction between joint structure and functional ability matters.
Does movement damage an osteoarthritic joint further?
Many people worry that they should use an affected joint as little as possible. In most stable cases of osteoarthritis, the opposite is true: adapted therapeutic exercise is a central part of recommended care. It helps develop strength and physical function. NICE recommends exercise tailored to the individual and notes that regular participation can improve pain, function and quality of life.
The aim is not to choose between movement and joint protection, but to find a level of activity the joint can tolerate. Our guide to exercises for osteoarthritis covers different areas and levels of discomfort; the walking guide discusses duration, pace, terrain and symptoms during activity.
Why do muscles matter so much?
Muscles are part of the system that enables a joint to function. They generate movement, control weight-bearing and help stabilise the joint. When a joint hurts for months, people often begin to use it less. The surrounding muscles may weaken, making movement harder and encouraging even more inactivity.
Osteoarthritis management aims to interrupt this cycle. Looking after mobility means supporting muscles as well as considering cartilage. See our article on maintaining joint health as we age for the connection between strength, activity and independence.
Is knee osteoarthritis the same as hand osteoarthritis?
The broad mechanisms overlap, but the practical consequences differ. For knees, weight-bearing, walking, stairs and lower-limb strength matter particularly. Hip osteoarthritis may affect walking and pelvic movement. In the hand, changes in a very small joint can disrupt fine tasks such as holding a pen or opening a jar.
That is why different locations deserve specific guidance. Read about gonarthrosis, or knee osteoarthritis, and our guide to thumb-base osteoarthritis (rhizarthrosis).
Does joint pain always mean osteoarthritis?
No. Pain can come from tendons, ligaments, trauma, inflammatory conditions, certain infections or tissues around the joint. A painful knee, hip or hand does not establish an osteoarthritis diagnosis.
Our joint pain guide explains the possibilities. A joint that is hot or very swollen, sudden pain, prolonged marked stiffness or rapid deterioration should be assessed for other causes. NICE regards such presentations as atypical for straightforward osteoarthritis and recommends further evaluation.
Where does nutrition fit into the joint picture?
Understanding that osteoarthritis involves the whole joint also encourages a wider view of nutrition: cartilage, movement, muscles and bones are all relevant. Some nutrients have established functions. Vitamin C contributes to normal collagen formation for the normal function of cartilage; magnesium contributes to normal muscle function; and calcium and vitamin D support the maintenance of normal bones.
Other ingredients, including glucosamine, chondroitin and selected plants, have long appeared in joint food supplements. At Laboratoire Vital Santé, we have developed two approaches that share plant-derived glucosamine: Cartilamine Phyto, featuring Boswellia serrata, and Cartilamine Chondro, built around a different combination. Our joint supplement guide explains these approaches without confusing nutrition with treatment of osteoarthritis.
Do anti-inflammatory medicines have a place in osteoarthritis?
Yes, although they are not needed in every situation and are not the only response to osteoarthritis. During more painful episodes, non-steroidal anti-inflammatory drugs (NSAIDs) may be recommended to reduce pain and inflammatory symptoms temporarily. Depending on the joint and the individual, a topical preparation such as a gel may be considered before an oral medicine.
NSAIDs can have digestive, cardiovascular and kidney-related risks and can interact with other medicines. They should be used according to healthcare advice, generally at the lowest effective dose for the shortest necessary period. Medication may help during a painful phase, while adapted movement, muscle strengthening and activity modification remain important for preserving function.
So what is osteoarthritis, really?
It is not simply “worn-out cartilage”. Osteoarthritis involves progressive changes across the joint, with interactions between cartilage, subchondral bone and the synovial membrane. Cartilage changes, bone responds, the synovial membrane may become inflamed and joint mechanics evolve. Pain depends on several tissues and mechanisms, so its intensity is not always proportional to an X-ray.
Understanding this can change the way people live with the condition. An X-ray does not determine what you will be able to do five years from now. One painful day does not necessarily mean sudden structural damage. And protecting mobility does not mean rationing every movement as though a joint had only a fixed number of steps left. The practical goal is to maintain strength, movement, function and useful daily habits while adapting care to actual symptoms.
Sources
- Inserm — Osteoarthritis: the tissues involved, biological mechanisms and variability of progression.
- French National Health Insurance — Knee osteoarthritis: symptoms, imaging, effusions and differences between X-rays and pain.
- French National Health Insurance — Hip osteoarthritis: definition and the role of several joint tissues.
- NICE — Osteoarthritis in over 16s: diagnosis and management: clinical assessment and the role of exercise.
- PubMed Central — Osteoarthritis Pain: a review of the mechanisms of osteoarthritis-related pain.