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Knee osteoarthritis

Also called knee OA, osteoarthritis of the knee

A long-term disease of the knee joint, in which the cartilage lining the joint wears away. How guidelines define it, who it affects, how it changes over time, and what trials of exercise, weight loss and surgery found.

Region
Knee
Updated
October 22, 2026
Reviewed
October 22, 2026 · how entries are checked

What it is

How guidelines and reviews define it

The 2022 NICE (UK National Institute for Health and Care Excellence) guideline describes osteoarthritis as the most common form of arthritis1. It says the condition typically shows as joint pain and stiffness, mostly in the knee, hip, hand and foot1. A 2016 review notes that thinking has shifted, from a disease limited to cartilage to one that affects the whole joint5.

A draft definition from OARSI (the Osteoarthritis Research Society International) describes a disorder of movable joints6. In it, small and large injuries trigger repair that goes wrong, with stress on cells and breakdown of the tissue around them6. Later signs are cartilage breakdown, bone remodelling, osteophytes (bone spurs at the edge of a joint), inflamed joints, and joints that stop working normally6.

What changes in the joint

An anatomy textbook describes the articular cartilage, the layer covering the bone ends, thinning gradually in osteoarthritis7. The joint may swell with extra synovial fluid, the slippery fluid that oils its moving surfaces7. The bone beneath the damaged cartilage thickens and roughens7.

Two labelled drawings of the right knee. On the left, a side-on cut through the joint shows the thigh bone resting on the shin bone, with the kneecap in front, the cruciate ligaments and menisci between the bones, and the joint capsule around them. On the right, the top of the shin bone seen from above shows two grey oval patches labelled articular cartilage on the medial and lateral tibial condyles, ringed by the C-shaped menisci.
Articular cartilage in the kneeBetts JG, Young KA, Wise JA, et al. Anatomy and Physiology, Figure 9.19. OpenStax, 2013. Access for free at openstax.org. (source, CC BY 4.0). Cropped to panels (a) and (b), removing panel (c), the front view of the knee; resized to 1600 pixels wide.

How it is identified

The NICE guideline recommends identifying osteoarthritis clinically, without imaging, in people aged 45 or over1. Its criteria are joint pain linked to activity, and either no morning stiffness or stiffness lasting 30 minutes or less1.

Osteoarthritis can also affect the joint behind the patella (kneecap)8. A 2016 consensus statement (an expert panel’s agreed report) found it there alone in 11 to 24% of older people, by population and definition8. It occurred together with osteoarthritis of the main knee joint in 4 to 40%8.

Who it affects

How common it is

The NICE guideline says osteoarthritis is more common in women, in people aged 45 and over, and in people living with obesity1. It is also more common in people in deprived areas, and its prevalence is increasing1.

What was measuredWhat was found
Prevalence (share with it), worldwide16.0% of people aged 15 and over, 22.9% of those aged 40 and over, in 88 population studies2
Incidence (new cases) in people 20 and over203 per 10,000 person-years (people multiplied by years followed), rising with age2
Women against menPrevalence 1.69 times and incidence 1.39 times higher in women2
Age of peakKnee osteoarthritis peaked at ages 80 to 84 in 2020 estimates9

Body weight

A meta-analysis pooled 12 observational studies, which watch people without assigning groups10. Risk rose with body mass index (a measure of weight for height), though not in a straight line10. Against an index of 22.5, risk was 1.59 times higher at 25, 3.55 at 30 and 7.45 at 3510.

A 2023 review of 23 trials and cohort studies (which follow people over time) linked adult weight gain with more symptoms and joint change11. Each 5 kg gained went with more knee replacement11. The hazard ratio (how many times higher the rate was) was 1.34 in women and 1.25 in men11.

Past knee injury

In a Swedish cohort of 149,288 people aged 25 to 34, a recorded knee injury went with more osteoarthritis recorded later12. The adjusted hazard ratio was 5.7 over the first 11 years and 3.4 over the next 812. Cruciate ligament injury, meniscus tears and fractures of the shinbone top or kneecap carried the largest rises in risk12.

A 2022 review of people injured at a mean age of 30 or under found moderate-certainty evidence on many injury types13. Ligament, meniscus, cartilage, dislocation, fracture and multi-part injuries each raised the odds of osteoarthritis with symptoms13. A 2019 review took 41 studies of people with an ACL tear, a tear of the front ligament crossing inside the knee14. Rates on X-ray more than 10 years later ranged from 0% to 100%14. Over half those studies were of low quality, and meniscectomy (removing meniscus tissue) was the only consistent risk factor14.

Sport and running

Reviews of athletes report a wide spread of findings15,16:

  • Former athletes: a 2018 meta-analysis of 15 studies (3,100 people) estimated prevalence at 30%, on low-quality evidence15.
  • Former professional footballers: a UK study compared 1,207 of them with 4,085 men from the general population17. Knee pain, X-ray signs and knee replacement were two to three times as common in the footballers17.
  • By sport: a 2022 review of 32 studies, excluding osteoarthritis after injury, linked soccer, handball, ice hockey and rugby with early knee and hip osteoarthritis16. Runners and ballet dancers showed no significant increase16.

In the footballers, knee injury was the main risk factor17. Even after adjusting for it, risk of knee pain and X-ray signs stayed more than doubled17. A meta-analysis of 15 studies of running found mixed results and concluded that its role could not be determined18.

What studies found

How it changes over time

The NICE guideline says symptoms range from mild and on-and-off to more lasting or severe1. The guideline adds that the condition does not inevitably get worse, though symptoms go up and down and flares are common1. A 2020 review of seven studies (7,747 people, mainly early or moderate disease) found pain stayed steady over the mid-term in 85%19. Pain fell in 8% and worsened in 7%, while day-to-day pain was unstable in almost half19.

That review linked low education, other illnesses and depression with severe or worsening pain19. Evidence on joint factors such as X-ray severity was conflicting or absent19. A 2016 meta-analysis of 58 studies found results varied too much to describe a typical course of pain and function20. It found strong evidence that worse X-ray changes, rising pain, weaker knee straightening and slower walking predicted declining function20.

Exercise

A 2024 Cochrane review included 139 trials (12,468 people), with land-based programmes lasting 2 to 104 weeks3. Its findings varied by comparison group3:

  • Against no exercise, usual care (the care people would get anyway) or limited education: pain may improve by 13.1 points out of 1003. That evidence was of low certainty3. Function likely improves by 12.5 points3.
  • Against attention control or placebo: pain may improve slightly, by 8.7 points3. Function likely improves by 11.3 points3.
  • Harms: against no exercise, exercise may raise adverse events (unwanted effects seen during a trial), on low-certainty evidence3. Against attention control or placebo, it may not increase them3.

The review judged these short-term gains of uncertain clinical importance, and noted most participants knew their group3. It rated most trials at unclear or high risk of bias, meaning flaws may have skewed results3. It found no differences between types of exercise, and no link with the number of sessions3.

Guidelines from NICE, OARSI, EULAR (a European rheumatology body) and the American College of Rheumatology include exercise1,21,22,23. NICE notes that joint pain may increase when exercise starts1.

Weight loss

In 142 overweight and obese older adults with the condition, each 1 kg lost went with about a 4-fold larger drop in walking knee force24. A 2007 meta-analysis of four trials (454 overweight people with the condition) found disability improved after moderate weight loss25. A further analysis in it suggested a loss of more than 5% of body weight25. NICE says any weight loss is likely to help people with osteoarthritis and overweight or obesity1. It adds that a 10% loss is likely better than a 5% loss1.

An 18-month trial took 454 overweight and obese adults aged 55 or over with the condition26. This trial, called IDEA, was funded by US National Institutes of Health grants and by General Nutrition Centers26. The diet-plus-exercise group lost 11.4% of body weight on average26. That group had less pain and better function than groups given diet or exercise alone26. In overweight or obese people with the condition, a 2019 meta-analysis found diet alone did not reduce pain, while diet with exercise reduced it moderately27.

Keyhole surgery

Studies of arthroscopy (keyhole surgery inside a joint) in worn knees found little or no lasting gain4,28,29:

  • Against a placebo operation: in a trial of 180 people, patients and assessors were not told who had which operation4. Washing out or trimming gave no less pain or better function over 24 months4.
  • Added to physical and medical care: in moderate-to-severe disease, one trial found washing out and trimming added no knee-score benefit at 2 years28.
  • Pooled trials: these covered middle-aged and older people with degenerative (worn) knee disease29. A small pain gain at 3 and 6 months was gone by 1 to 2 years29.

NICE recommends against offering keyhole washing out or trimming for osteoarthritis1.

Knee replacement

A 2026 Cochrane review found one trial (100 adults, Denmark) comparing knee replacement with a non-surgical programme30. It gave low-certainty evidence that replacement may reduce pain by a clinically important amount at one year30. Evidence on serious adverse events, more common after surgery, was very uncertain30.

A Danish trial named MEDIC took 100 people with moderate-to-severe disease who were eligible for surgery31,32. Its funders were the Obel Family Foundation, among others31. Replacement plus a 12-week programme raised a knee score 32.5 points, against 16.0 for the programme alone31. The surgery group had 24 serious adverse events against 631. Within 12 months, 13 of those given the programme alone (26%) had a knee replacement31.

A 2018 report gave 2-year results from the same MEDIC trial, under the same registration number31,32,33. It reported them beside a parallel trial of 100 people not eligible for surgery31,33. By 2 years, 16 of the 50 eligible people given the programme (32%) had had a replacement33. So about two in three put off surgery for at least 2 years33.

Medicines

A 2016 review states that drugs for osteoarthritis mostly relieve symptoms5. At that time, no drug that also slows or stops the disease had been approved by regulators5.

Connections

Knee osteoarthritis involves the whole knee joint, its cartilage and the bone beneath5,7. The menisci are pads of cartilage in the knee. A European consensus states it is unclear whether their degenerative (worn) tears cause osteoarthritis or result from it35. In a trial of 321 people aged 45 to 70 with such meniscus tears, X-ray change progressed at comparable rates after surgery or exercise36.

A meta-analysis of 17 studies found hip and knee osteoarthritis in 3.5% of recreational runners and 13.3% of competitive ones37. In non-running controls it was 10.2%37. Its authors could not tell whether these links were causal37. In 28 people aged 50 or older with inner-knee osteoarthritis and bowed legs, the lunge raised muscle forces above walking38. Peak inner-knee contact force did not differ significantly from walking38.

Knee osteoarthritis involves

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Knee osteoarthritis has studies of runners in

Knee osteoarthritis has lab studies of joint forces during

Sources

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Every source the library cites

Changes

  • First published.