What Is Knee Osteoarthritis? — The Door Frame That Changes With the Hinge

The Phrase "My Knees Are Worn Out"

Whenever someone over sixty walks down the stairs, their knees ache. When they stand up after sitting for a long time, the first few steps are stiff before soon loosening up. They take an X-ray at the hospital and hear the words, "Your knees are quite worn out." On the way home, their thoughts lean in one direction. The thought is that since what is worn out cannot be reversed, they must use their knees less and spare them.

The name attached to this knee condition is knee osteoarthritis. Osteoarthritis is a disease in which various tissues forming a joint change together over a long period, causing the joint to become painful, stiff, and difficult to move. It is estimated that approximately 654 million people worldwide have this disease in their knees (PMID 37874571).

The phrase "worn out" is an image that has long explained this disease. This disease was once considered the result of the smooth surface of the joint wearing away, but now it is viewed as a complex disease in which the entire joint changes together (PMID 41970229). Following that changed picture, this article looks in turn at the structure of the knee, the reason why scans and pain do not match, the reason why exercise and weight management are the first line of treatment, and the place of injections and surgery.

What Makes Up a Knee

In the knee, the thigh bone and shin bone meet, and the kneecap rests in front of them. A joint is the place where two bones meet and move, consisting of cartilage covering the ends of the bones and a joint capsule surrounding them. Articular cartilage is a smooth, tough tissue covering the bone ends inside a joint, which has no blood vessels and allows the bones to slide smoothly against each other.

Beneath the cartilage lies the bone that supports it. Subchondral bone is the layer of bone directly beneath the articular cartilage that supports the cartilage. The inside of the joint capsule is lined with a thin membrane. The synovium is a thin membrane lining the inside of the joint capsule, which produces joint fluid that lubricates the joint.

The knee also features special components. The meniscus is a tough, crescent-shaped piece of cartilage—one on the inner side and one on the outer side—wedged between the thigh bone and shin bone, which distributes the weight placed on the knee and absorbs shock. Because the meniscus is important for protecting the structure of the knee joint and cutting it away risks early osteoarthritis, the trend in treatment has also shifted toward preserving or repairing the meniscus whenever possible (PMID 30195704).

Bones are connected to other bones by tough cords called ligaments, and the force that moves the knee comes from the surrounding muscles. The front thigh muscle (quadriceps femoris) is a large muscle covering the front of the thigh that generates the force to straighten the knee.

If you compare the knee to a door, the articular cartilage corresponds to the hinge that lets the door swing smoothly, while the bones, membranes, capsules, cords, and muscles that support and hold that hinge in place correspond to the doorframe. Saying it is "worn out" is a picture that looks only at the hinge. The knee is an organ made together by the cartilage, along with the bones, membranes, capsules, cords, and muscles that support and hold it.

A Disease That Changes the Entire Joint

In a knee with osteoarthritis, while the cartilage breaks down, the bone beneath it is remodeled, bone grows out of place at the joint edges, the joint capsule thickens, and the synovial membrane becomes inflamed (PMID 27539668). Osteophytes are bony projections that grow anew at the edges of an osteoarthritic joint. The focus of research has also broadened from cartilage to the subchondral bone, synovial membrane, meniscus, ligaments, and muscles and nerves around the joint, and osteoarthritis is now viewed as a disease affecting the entire joint (PMID 24653755).

Inflammation is a response in which the body summons immune cells against stimuli such as injury, infection, or substances introduced from outside the body, causing the site to swell and reacting to it. The inflammation of osteoarthritis differs from the inflammation of autoimmune diseases like rheumatoid arthritis in that it lasts a long time but is relatively weak and is primarily driven by innate immunity (PMID 27539668). Changes in the synovial membrane can appear before visible cartilage damage, and synovial inflammation is commonly seen at all stages of osteoarthritis (PMID 28148295).

Applied to a door, this is equivalent to the door frame also twisting, being reinforced, and swelling while the hinges wear down. The analogy holds true up to this point. A door frame is made of wood and metal so it does not change on its own, but the bones and membranes of the knee are living tissue, so they rebuild themselves and react with inflammation.

Osteoarthritis is a disease in which the entire joint—including cartilage along with bone, the synovial membrane, the joint capsule, ligaments, and muscles—changes.

What Increases the Risk

Knee osteoarthritis and newly developing knee pain appear more often in people with a history of knee injury or high body weight (PMID 40174718, PMID 25447976). In an analysis pooling 131 studies, a history of knee injury and older age were associated with newly appearing knee osteoarthritis on X-rays, and overweight/obesity and a history of knee injury together explained 14% of newly developed cases (PMID 40174718). In an analysis pooling studies following newly developed knee pain in people aged 50 and older, overweight, obesity, female sex, and a history of knee injury were associated with risk, and about a quarter (24.6%) of newly developed knee pain was calculated to be the share associated with overweight and obesity (PMID 25447976). Because both are pooled analyses of studies observing people, these numbers represent the magnitude of association rather than the magnitude of cause.

In addition to increasing the weight loaded on the knees, body weight is thought to contribute to this condition through systemic inflammation throughout the body accompanying obesity and metabolic syndrome (PMID 37629507). A narrative review considers that the way force is loaded onto the knees, such as leg alignment, is also closely associated with the onset and progression of this condition (PMID 42889631).

Muscles are also on the list of risks. People with weak front thigh muscles had a higher likelihood of newly developing knee osteoarthritis in both women and men, and the quality of this evidence was rated as low grade (PMID 34916210). Exercise prompts worries that it wears out the knees, but in an analysis pooling risk factors in people aged 40 and older, doing more exercise was associated with the lower-risk side (PMID 37641050). Because this is an analysis pooling observational studies, we cannot go so far as to say that exercise lowers the risk.

Age is a true risk factor. 33% of people over the age of 75 have knee osteoarthritis with both symptoms and X-ray changes (PMID 33560326). Flipping that same number, at that age, two out of three still do not fall into this criteria. Risk rises with age, but not all aging knees develop this condition.

The representative modifiable risk factors are body weight and knee injury (PMID 39948092).

Photos and Pain Do Not Match

X-rays show bones well, but they do not show cartilage clearly. Therefore, X-rays read osteoarthritis by the shape of the bone and the gap between the two bone ends. Joint space narrowing is when the gap between the two bone ends looks narrower than normal on an X-ray, which indirectly shows that the cartilage filling that gap has become thinner. Osteophytes at the edges of the joint and joint space narrowing are representative findings pointing to osteoarthritis on an X-ray (PMID 33560326).

However, these photos and pain often tell different stories. In studies surveying the general population, among people with knee pain, the proportion of people in whom osteoarthritis was seen on X-ray ranged from 15% to 76%, and among people in whom osteoarthritis was seen on X-ray, the proportion of people who had knee pain also ranged from 15% to 81% (PMID 18764949). That spread varied depending on which direction the X-rays were taken from, how pain was defined, what grading scale was used for X-ray changes, and what people were gathered. The authors concluded that when evaluating a person with knee pain, X-ray results should not be used in isolation (PMID 18764949).

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One of the reasons lies in where pain starts. The nerve endings that cause osteoarthritis pain are in places like the synovial membrane, the bone and its outer membrane, and tendons, but not in cartilage (PMID 26267003). Pain signals are also affected by changes in pain pathways in the spinal cord and brain, and factors like environment, mind, and constitution change the magnitude of pain. This review even writes that the intensity of osteoarthritis pain has no correlation with the degree to which the joint is damaged (PMID 26267003). Another review writes that as the disease becomes more severe, pain and functional disability also increase (PMID 39250809). The mismatch is common, but it does not mean that the degree of joint damage and pain have no relation at all. Markers of inflammation such as synovial inflammation or fluid accumulated in the joint are also associated with pain, but the strength of that association was between weak and moderate (PMID 34968719). To put it to a door, the hinge itself has no sensation, and the nerves transmitting pain are on the doorframe side. It is difficult to guess pain simply by how worn the hinge looks.

MRIs show more things. In knees after middle age, degenerative tears of the meniscus are commonly seen on MRIs, and these tears are closely linked to tissue aging and are thought to be linked to the process of osteoarthritis as well (PMID 28873348). A degenerative meniscal tear is when the meniscus slowly tears after middle age without a major accident, often splitting horizontally as if opening up into two layers, upper and lower. Such damage has been reported on MRIs even in knees that do not hurt (PMID 36057031). Seeing a tear in a photo and that tear causing current pain are different things.

That does not mean X-rays have nothing to do. Guidelines do not recommend taking X-rays for everyone suspected of having osteoarthritis, and recommend using imaging selectively (PMID 37874571, PMID 30826805). A consensus document by a European knee specialist society (ESSKA) dealing with degenerative meniscal lesions writes to use knee X-rays to support the diagnosis of osteoarthritis or to look for rare diseases such as tumors or fractures (PMID 28210788). When considering surgery that changes the joint surface, it is also checked whether almost no joint space remains (PMID 37874571).

The degree seen on X-rays and the degree to which the knee hurts frequently do not match, so a single X-ray alone cannot read a person's pain.

Diagnosis Relies on Stories and Physical Examinations

If someone aged 45 or older comes to the hospital with a knee that hurts when active, and has no morning stiffness or stiffness lasting less than 30 minutes, knee osteoarthritis is the most likely diagnosis (PMID 37874571). This combination held true in 95% of people with osteoarthritis, and also in 31% of people without osteoarthritis (PMID 37874571). Having clues that fit well is different from reaching a conclusion based on those clues alone.

Typical osteoarthritis pain worsens with use and eases with rest, morning stiffness is relatively short and resolves on its own, and it is not accompanied by fever or whole-body symptoms (PMID 39250809). The European Alliance of Associations for Rheumatology (EULAR) diagnostic recommendations selected three symptoms—persisting knee pain, brief morning stiffness, and reduced function—and three signs—crepitus when moving, reduced range of motion, and bony enlargement palpable on touch—as the most useful clues (PMID 19762361). Assuming that 12.5% of people aged 45 or older have X-ray-confirmed osteoarthritis, the probability of having osteoarthritis on an X-ray when all six features are present rose to 99%. The authors concluded that while a reference standard test is not agreed upon, a meticulous physical examination alone allows for a confident diagnosis (PMID 19762361).

These clues are read together by the person conducting the physical examination. High-quality guidelines for musculoskeletal pain recommend filtering out red flags for other serious diseases first during the examination (PMID 30826805). Confirmation is done through the physical examination and, when necessary, selective tests.

The diagnosis of knee osteoarthritis is made primarily through symptoms and physical examination, with imaging used selectively when needed (PMID 31034380).

The First Line Is Exercise and Weight

The first line of treatment for knee osteoarthritis is education, exercise, and, if necessary, weight management. Guidelines from the Osteoarthritis Research Society International (OARSI) listed education about arthritis, a structured land-based exercise program, and weight management through diet when necessary as the core treatments for knee osteoarthritis (PMID 31278997). Guidelines from the American College of Rheumatology (ACR) also strongly recommended exercise, weight loss for individuals who are overweight or obese, programs to build self-management skills, and cane use (PMID 31908149). An analysis combining high-quality guidelines also consistently recommended education, exercise, and weight management (PMID 37394226).

This order comes from understanding the disease. The understanding of the process by which osteoarthritis develops has guided rehabilitation treatments aimed at reducing the impact of known damage and risk factors (PMID 37423596). Weight and the muscles at the front of the thigh were on the list of risks we saw earlier.

How large are the effects of exercise? A recently updated analysis by Cochrane, an international organization that collects and summarizes medical evidence, gathered 139 randomized trials involving 12,468 people (PMID 39625083). Compared to doing nothing or receiving usual care, those who exercised had pain that was better by about 13 points and function better by about 12.5 points on a 100-point scale immediately after finishing exercise, and compared to those who received attention only or sham treatment, pain was better by about 9 points and function by about 11 points. The certainty of the evidence for pain was low, and for function, it was moderate. The authors concluded that while exercise is likely to improve pain and function in the short term, it is uncertain how clinically important that benefit is when compared against the minimum size that patients can feel as a difference (12 points for pain, 13 points for function). They noted that the fact that most participants knew they were exercising may also have contributed to the results (PMID 39625083).

In the same analysis, no differences were seen among types of exercise, nor was there a relationship between the prescribed number of exercise sessions and the effect (PMID 39625083). Another analysis comparing types of exercise with one another found that aerobic exercise was most likely to be the most beneficial, and the certainty of that evidence was moderate (PMID 41093618). There are also low-certainty results suggesting there may have been more adverse events in the exercise group compared to usual care (PMID 39625083). A recent review notes that new evidence has cast doubt on the effects of exercise programs and weight loss, but managing obesity and maintaining activity remain important (PMID 39755397). There are risks in the opposite direction, too. People with osteoarthritis tend to move less than those without it, and one review notes that this reduced activity links to worsening overall health (PMID 33560326).

The evidence for weight pertains to individuals who are overweight or obese. In an analysis pooling 13 randomized trials involving 2,800 such knee osteoarthritis patients, all weight loss methods reduced weight more than the control groups, but the method that was clearly better than the control group in reducing pain was the combination of dietary modification and exercise (PMID 39233046). Regardless of the method used, pain was predicted to decrease noticeably when weight decreased by 7% or more (PMID 39233046). A dose-response relationship was also reported, showing that greater weight loss led to greater improvement in symptoms (PMID 37423596).

These treatments do not replace the hinge with a new one, either. Whether medication or non-medication treatments, they only improve pain and function and do not fundamentally reverse the pathology and X-ray changes of knee osteoarthritis (PMID 39250809). And fewer than 40% of knee osteoarthritis patients receive this first-line treatment (PMID 33262080).

While exercise and weight management do not reverse the structure of the knee, evidence is accumulating that they improve pain and function to some extent, and just how large that benefit is remains under debate.

The Place of Medications and Injections

Nonsteroidal anti-inflammatory drugs are a group of pain relievers and anti-inflammatory drugs widely used to reduce pain, fever, and inflammation. The OARSI guidelines strongly recommended topical nonsteroidal anti-inflammatory drugs applied to the skin for knee osteoarthritis, did not recommend oral nonsteroidal anti-inflammatory drugs for people with concurrent cardiovascular disease or frailty, and placed strong opioid analgesics in the not-recommended category (PMID 31278997). Guidelines since 2017 have grown more cautious about acetaminophen (paracetamol), a common pain reliever and fever reducer, and strong opioid analgesics (PMID 34411512). Which medication fits a person depends on concurrent illnesses and is decided in clinical care.

Intra-articular injections are shots that deliver medication directly into the joint capsule using a needle. High-quality guidelines consistently recommend steroid injections for the knee (PMID 37394226), and their effect is short-term pain relief (PMID 33560326). In an analysis of 57 large trials pooling more than 100 people per group and 22,795 participants for knee and hip osteoarthritis, one type of steroid injection was most likely to produce a patient-perceptible difference at 2 to 6 weeks post-injection, but the evidence was from a single trial, and the effects of the other 16 injection types were smaller than that minimum threshold and mostly similar to sham injections. Evidence for effects beyond 24 weeks was lacking (PMID 39265924).

Hyaluronic acid injections showed no effect on pain in the same analysis, while discontinuation due to adverse events and serious adverse events were more frequent than with sham injections (PMID 39265924), and evaluations vary across guidelines (PMID 37394226). Platelet-rich plasma (PRP) injections, which concentrate platelets from one's own blood, showed the best results in some analyses (PMID 38037038), but the American Academy of Orthopaedic Surgeons (AAOS) guidelines stated that steroid, hyaluronic acid, and PRP injections all require better research (PMID 35383651), and high-quality guidelines consistently did not recommend stem cell injections (PMID 37394226).

One review noted that contemporary treatments manage symptoms only, with no U.S.-approved treatments to prevent or slow this disease (PMID 27539668), and a recent review notes that the search for drugs to improve both symptoms and structure continues (PMID 39755397).

Two Surgeries with Different Evidence

Trials comparing arthroscopic surgery to a sham surgery asked about its benefits. Arthroscopic surgery is a procedure in which a few small holes are made in the skin and a thin camera and instruments are inserted into the joint. In degenerative knees, it has been used to trim damaged tissue or cut away part of a meniscus (PMID 35238404).

In people with osteoarthritis or degenerative meniscal tears, trials comparing it to sham surgery showed almost no difference in pain and function at three months after surgery, and the certainty of this evidence was high (PMID 35238404). Treatment success and serious adverse events a few years later both leaned slightly toward the arthroscopic side, but this was low-certainty evidence, and whether more knee surgeries like joint surface-altering procedures are received after arthroscopy remains unanswered. Cochrane authors concluded that arthroscopic surgery provides little to no clinically important benefit for pain and function (PMID 35238404).

The general trend in guidelines is against arthroscopy. High-quality guidelines have consistently recommended against arthroscopy (PMID 37394226), and evidence is also growing that older patients with degenerative arthritis who have complex tears should not undergo arthroscopic surgery (PMID 30195704). Still, some point out that there is a group of patients who do not improve with exercise therapy but feel comfortable after surgery (PMID 36057031), and the ESSKA consensus statement noted that while this surgery is not recommended as a first-line treatment, it can be considered after standard evaluation when non-surgical treatments are unsatisfactory (PMID 28210788).

Divergence remains when mechanical symptoms, such as the knee catching or locking, are present. One review reports that even with such symptoms, surgery is not indicated (PMID 37874571), while another review considers that surgery can be considered earlier when mechanical symptoms are substantial (PMID 28873348). The ESSKA consensus statement noted that definitions for important criteria like mechanical symptoms do not yet exist (PMID 28210788).

This evidence is for the degenerative knee. Traumatic tears in young people caused by twisting injuries are viewed differently. Traumatic tears with a largely displaced torn fragment are likely to require surgery (PMID 37874571), and vertical tears in the outer zone with good blood supply have good outcomes from repair surgery (PMID 28873348).

Joint replacement surgery is a procedure where damaged joint surfaces are shaved away and replaced with artificial joint surfaces made of metal and plastic. Knee replacement surgery is a common procedure with a high success rate, and it relieves pain and improves knee function in people with advanced arthritis (PMID 22398175). It targets people with severe symptoms and progressive structural damage (PMID 33560326). One review notes that in end-stage osteoarthritis with almost no joint space left and pain that is hard to bear, surgical referral can be considered after trying all appropriate conservative treatments (PMID 37874571). Because there are no guidelines determining how long conservative treatment should be done before surgery, when to refer for surgery remains unclear (PMID 34411512). To use an analogy, joint replacement is akin to replacing the surfaces of a damaged hinge with brand new ones. The evidence for this surgery differs in nature from that of arthroscopy. The evidence for joint replacement examined in this article consists of reviews and registry data, with no sham-controlled trials included among them, and an analysis pooling high-quality guidelines found no high-quality guidelines addressing joint replacement (PMID 37394226).

The evidence that arthroscopic surgery has little benefit applies to the degenerative knee, whereas joint replacement is a surgery with a different nature of evidence from arthroscopy and is considered in advanced osteoarthritis.

The Knee Before the Stairs, Again

Let us say that person from the beginning is sixty-two years old. It hurts when walking down stairs, and it is stiff for about 10 minutes upon waking in the morning. The X-ray reading mentions narrowed joint space and bone spurs, and an MRI taken at another hospital notes a degenerative meniscus tear. Two thoughts remain in the head: the thought that it is worn out so it must be conserved, and the thought that it is torn so it needs to be fixed.

This combination of symptoms fits the textbook definition of knee osteoarthritis, but the same combination is fairly common even in people without osteoarthritis (PMID 37874571), so sorting out the signals of other diseases is the job of a physical examination (PMID 30826805). X-ray findings support the diagnosis, but the pictures and the pain frequently do not match, so one should not gauge how much pain this person will be in based solely on a single reading report (PMID 18764949). An MRI tear is a finding that appears even in knees that do not hurt (PMID 36057031), and exercise therapy is the first line for degenerative meniscus tears (PMID 37874571). Arthroscopic surgery in this group showed little to no difference in pain and function compared to sham surgery (PMID 35238404), and in knees with concurrent osteoarthritis like this person's, high-quality guidelines consistently do not recommend arthroscopy (PMID 37394226).

Guidelines point the other way regarding the thought that "it must be conserved." The first line is education and exercise, and if this person is overweight, weight management is added (PMID 31278997). Current evidence holds that the benefit is modest over a short period and its magnitude is debated (PMID 39625083). If it still hurts, medications and short-acting steroid injections are placed on top of that (PMID 33560326), and when it reaches the end stage where almost no joint space remains and the pain is hard to bear, joint replacement becomes an option (PMID 37874571). How to apply this sequence to this person is something to decide with a doctor in the clinic.

Where to Look Further

Drugs that improve joint structure along with symptoms are still being searched for (PMID 39755397), and how meaningful the benefits of exercise are in size to patients is also under debate (PMID 39625083). How to define early-stage knee osteoarthritis and find and manage it early has not yet been settled either (PMID 34465902).

The principles by which exercise becomes a treatment and restoring function continue in "Rehabilitation — Starting to Plan a Livable Home as the Fire Is Brought Under Control", and the question of how much exercise should be done continues in "How Much Is Enough — An Introduction to Dose-Response in Rehabilitation". Body weight and weight-loss drugs are covered by "What Are Obesity and GLP-1 Drugs? — A Letter That Lingers, a Weight That Wants to Return".

The fact that bone beneath cartilage is alive and gets repaired starts from "How Bones Stay Alive — A Demolishing and Building Construction Site, a Material Warehouse to Draw From". The mismatch between imaging findings and pain meets us again in a different place in "What Is Back Pain? — Wrinkles of the Spine, Unnamed Pain". Muscle weakening and long-lasting pain are topics that will be covered ahead in this series.

Source Papers

• "Evaluation and Treatment of Knee Pain: A Review." (2023) — PMID 37874571 — An estimated 654 million people worldwide have knee osteoarthritis; if aged 45 or older, with activity pain, and no morning stiffness or morning stiffness lasting less than 30 minutes, it is the most likely diagnosis (sensitivity 95%, specificity 69%), X-rays are not recommended for all suspected patients, first-line management is exercise, weight loss, education, and self-management, total knee arthroplasty referral is considered for end-stage (no joint space, unbearable pain) after conservative treatment, degenerative meniscal tears are managed with exercise first and are not an indication for surgery even with mechanical symptoms, and severe traumatic (bucket-handle) tears may require surgery.
• "Osteoarthritis: Epidemiology, Pathogenesis, and Treatment." (2026) — PMID 41970229 — Once considered the result of cartilage wear, it is now recognized as a complex whole-joint disease involving concurrent changes in cartilage, synovium, and subchondral bone.
• "Treatment of Knee Meniscus Pathology: Rehabilitation, Surgery, and Orthobiologics." (2019) — PMID 30195704 — The meniscus is important for maintaining knee structure, and resection carries a risk of early osteoarthritis, prompting a paradigm shift toward preservation and repair whenever possible, and growing evidence suggests older patients with complex tears in degenerative arthritis should not undergo arthroscopic surgery.
• "Low-grade inflammation as a key mediator of the pathogenesis of osteoarthritis." (2016) — PMID 27539668 — Cartilage destruction is accompanied by subchondral bone remodeling, ectopic bone formation, joint capsule thickening, and synovial inflammation, making it a whole-joint disease; the inflammation is chronic, relatively mild, and innate-immune-mediated, differing from rheumatoid arthritis, and current treatments only manage symptoms, with none FDA-approved to stop or slow progression (anti-inflammatory treatments in animal models are promising).
• "Osteoarthritis pathogenesis - a complex process that involves the entire joint." (2014) — PMID 24653755 — Focus has expanded from cartilage to subchondral bone, synovium, meniscus, ligaments, periarticular muscles, and nerves, marking it as a multifactorial disease affecting the entire joint.
• "Diagnosis and Treatment of Hip and Knee Osteoarthritis: A Review." (2021) — PMID 33560326 — Pathologic changes occur in cartilage, bone, synovium, ligaments, muscle, and periarticular fat; 33% of adults over 75 have symptomatic radiographic knee osteoarthritis; risk factors include age, female sex, obesity, genetics, and major joint injury; it describes that individuals with OA move less than those without, and reduced activity leads to increased age-adjusted mortality (comparison group not in abstract); radiographic markers include marginal osteophytes and joint space narrowing; corticosteroid injections provide short-term pain relief; and advanced symptoms and structural damage are indications for joint replacement, which effectively relieves pain.
• "Synovitis in osteoarthritis: current understanding with therapeutic implications." (2017) — PMID 28148295 — Synovial changes can appear even before visible cartilage degeneration, and synovial inflammation is common at all stages and associated with pain and functional decline.
• "Risk factors for the development of knee osteoarthritis across the lifespan: A systematic review and meta-analysis." (2025) — PMID 40174718 — Across 131 studies, a history of knee injury, older age, and high bone mineral density were associated with the development of radiographic knee osteoarthritis, overweight and obesity together with knee injury accounted for 14% of development, and occupational physical activity also contributed.
• "Current evidence on risk factors for knee osteoarthritis in older adults: a systematic review and meta-analysis." (2015) — PMID 25447976 — Across 46 knee pain incidence cohorts in adults aged 50 and older, associations were found with overweight, obesity, female sex, and history of knee injury, with population-attributable fractions of 24.6% for overweight and obesity, and 5.1% for injury.
• "Obesity-Related Knee Osteoarthritis-Current Concepts." (2023) — PMID 37629507 — Obesity and metabolic syndrome involve not only mechanical load but also systemic inflammation associated with obesity, representing a whole-joint disease, and weight optimization and movement should be included in management.
• "Biomechanics of knee osteoarthritis from local knee loading to whole kinetic chain imbalance: a narrative review." (2026) — PMID 42889631 — The onset and progression of knee osteoarthritis are closely linked to abnormal mechanical loading such as lower-limb malalignment, and local factors alone may not fully explain the mismatch between radiographic changes and pain (narrative review).
• "Knee extensor muscle weakness is a risk factor for the development of knee osteoarthritis: an updated systematic review and meta-analysis including 46 819 men and women." (2022) — PMID 34916210 — Across 11 studies comprising 46,819 individuals, knee extensor muscle weakness was associated with the development of symptomatic and radiographic knee osteoarthritis in both women and men, with low quality of evidence.
• "Evidence on risk factors for knee osteoarthritis in middle-older aged: a systematic review and meta analysis." (2023) — PMID 37641050 — Across 29 cohort and case-control studies comprising 60,354 individuals aged 40 and older, history of trauma, BMI $\ge$ 24, female sex, and age were risk factors, and higher physical activity was associated as a protective factor.
• "Osteoarthritis." (2025) — PMID 39948092 — It is a heterogeneous whole-joint disease, with obesity and joint injury as well-known and modifiable main risk factors, management involves self-management, education, physical activity, exercise, and weight management, and joint replacement is considered for persistent symptoms that do not respond to core treatments.
• "The discordance between clinical and radiographic knee osteoarthritis: a systematic search and summary of the literature." (2008) — PMID 18764949 — Among people with knee pain, radiographic osteoarthritis is present in 15% to 76%, and among those with radiographic osteoarthritis, pain is present in 15% to 81%, varying by radiographic direction, pain definition, grading, and population characteristics, and radiographic results should not be used in isolation for individual assessment.
• "Osteoarthritis pain." (2015) — PMID 26267003 — Pain originates from free nerve endings in the synovium, periosteum, and tendons, not in cartilage; peripheral and central mechanisms are involved, influenced by environmental, psychological, and constitutional factors, and pain intensity does not correlate with the degree of joint damage.
• "Association between knee inflammation and knee pain in patients with knee osteoarthritis: a systematic review." (2022) — PMID 34968719 — Across 37 studies, the correlation between inflammatory markers and pain is weak to moderate ($0.19$ to $0.61$), and the quality of evidence ranges from mixed to moderate.
• "Management of traumatic meniscal tear and degenerative meniscal lesions. Save the meniscus." (2017) — PMID 28873348 — Degenerative meniscal lesions are common on MRIs of middle-aged and older adults and are closely linked to tissue aging and osteoarthritis processes; 7 out of 8 randomized studies show arthroscopy is not superior, conservative treatment is first-line, arthroscopy may be performed earlier upon failure or for "substantial" mechanical symptoms, and traumatic vertical tears in the vascular zone have good repair outcomes.
• "Surgical management of degenerative meniscus lesions: the 2016 ESSKA meniscus consensus." (2017) — PMID 28210788 — For degenerative lesions (defined as age $> 35$, no major trauma, mostly horizontal cleavage), arthroscopic partial meniscectomy is not suggested as a first-line treatment and is considered only when non-surgical treatment is unsatisfactory after standard evaluation, MRI is generally unnecessary for initial evaluation, X-rays are used to support osteoarthritis diagnosis and detect rare conditions like tumors or fractures, and precise algorithms are lacking due to poorly defined critical criteria such as mechanical symptoms.
• "Arthroscopic surgery or exercise therapy for degenerative meniscal lesions: a systematic review of systematic reviews." (2023) — PMID 36057031 — Degenerative meniscal lesions are reported even in asymptomatic knee MRIs, 13 systematic reviews agree that arthroscopy provides no long-term benefit over exercise or sham surgery in middle-aged patients, physical-therapy-based conservative treatment is first-line, but there is a subgroup of patients who find relief with surgery after conservative treatment fails.
• "What does best practice care for musculoskeletal pain look like? Eleven consistent recommendations from high-quality clinical practice guidelines: systematic review." (2020) — PMID 30826805 — Eleven consistent recommendations from high-quality musculoskeletal pain guidelines include: screening for red flags, selective imaging, clinical examination, education, exercise, and high-quality non-surgical care prior to surgery.
• "EULAR evidence-based recommendations for the diagnosis of knee osteoarthritis." (2010) — PMID 19762361 — Three symptoms (persistent pain, brief morning stiffness, functional limitation) and three signs (crepitus, restricted movement, bony enlargement) are most useful; assuming a background prevalence of 12.5% in those aged 45 and older, the presence of all six yields a 99% probability of radiographic osteoarthritis, and although no consensus reference standard exists, a thorough clinical evaluation alone can confirm the diagnosis.
• "Knee Osteoarthritis." (2024) — PMID 39250809 — Pain worsens with use and eases with rest, brief morning stiffness that resolves on its own, no systemic symptoms, greater pain and functional disability with more advanced disease, and overweight and obesity are the most important modifiable risk factors; medical and non-medical treatments improve pain and function but do not fundamentally reverse the pathological or radiographic process.
• "Osteoarthritis." (2019) — PMID 31034380 — Despite the overuse of imaging, diagnosis is clinical, core treatments are self-management, education, exercise, and weight loss, and surgery is reserved for cases unresponsive to less invasive methods.
• "OARSI guidelines for the non-surgical management of knee, hip, and polyarticular osteoarthritis." (2019) — PMID 31278997 — Core treatments for the knee are arthritis education and structured land-based exercise ($\pm$ dietary weight management), topical NSAIDs are strongly recommended, oral NSAIDs are not recommended for patients with cardiovascular comorbidities or frailty, intra-articular corticosteroids or hyaluronic acid are conditional (1B/2) depending on comorbidities, and opioid analgesics are strongly not recommended.
• "2019 American College of Rheumatology/Arthritis Foundation Guideline for the Management of Osteoarthritis of the Hand, Hip, and Knee." (2020) — PMID 31908149 — Strongly recommended are exercise, weight loss for overweight and obesity, self-efficacy and self-management programs, tai chi, cane use, topical and oral NSAIDs, and intra-articular knee corticosteroid injections, emphasizing shared decision-making.
• "Recommendations for the management of hip and knee osteoarthritis: A systematic review of clinical practice guidelines." (2023) — PMID 37394226 — Seven high-quality guidelines consistently recommend education, exercise, weight management, NSAIDs, and intra-articular knee corticosteroids, consistently oppose stem cell injections and arthroscopy, show mixed positions on knee hyaluronic acid, and no high-quality guideline addresses joint replacement.
• "Fundamentals of osteoarthritis. Rehabilitation: Exercise, diet, biomechanics, and physical therapist-delivered interventions." (2023) — PMID 37423596 — Understanding pathogenesis guides rehabilitation treatments that reduce the impact of known injuries and risk factors, exercise plus education plus diet is core, there is a dose-response relationship between the magnitude of weight loss and symptom improvement, and contextual factors such as attention and placebo also contribute to mechanisms.
• "Exercise for osteoarthritis of the knee." (2024) — PMID 39625083 — In a Cochrane review of 139 trials involving 12,468 participants, compared to no treatment or usual care, pain improved by 13.14 points and function by 12.53 points; compared to attention control or placebo, pain improved by 8.70 points (low certainty) and function by 11.27 points (moderate certainty); compared to the minimal important difference (12 for pain, 13 for function), clinical importance is uncertain, and unblinding may have contributed; there are no differences between exercise types or relationships with prescription frequency, and there is a possible increase in adverse events compared to usual care (low certainty).
• "Comparative efficacy and safety of exercise modalities in knee osteoarthritis: systematic review and network meta-analysis." (2025) — PMID 41093618 — Across 217 trials involving 15,684 participants, aerobic exercise is most likely to be beneficial for improving pain, function, gait, and quality of life (moderate certainty), and safety was reported in only some trials with no clear differences.
• "Osteoarthritis." (2025) — PMID 39755397 — New evidence has raised questions about the efficacy of exercise programs and weight loss, but obesity management and maintaining activity remain important, and drugs that simultaneously improve symptoms and structure are being sought.
• "Comparison of weight loss interventions in overweight and obese adults with knee osteoarthritis: A systematic review and network meta-analysis of randomized trials." (2025) — PMID 39233046 — Across 13 randomized trials involving 2,800 participants, all interventions reduced weight, but only diet plus exercise surpassed the control group in pain relief, and weight loss of 7% or more through any intervention is predicted to yield meaningful pain relief.
• "Knee osteoarthritis: key treatments and implications for physical therapy." (2021) — PMID 33262080 — Education, exercise, and weight loss are first-line treatments, yet fewer than 40% of patients receive them.
• "Cross-sectional Analysis of Recommendations for the Treatment of Hip and Knee Osteoarthritis in Clinical Guidelines." (2022) — PMID 34411512 — Across 17 guidelines, those published since 2017 are more cautious about paracetamol (acetaminophen) and strong opioid analgesics, hyaluronic acid and other treatments show inconsistency, and no guidelines specify a minimum duration of conservative treatment before surgery, making the timing of surgical referral unclear.
• "Effectiveness and safety of intra-articular interventions for knee and hip osteoarthritis based on large randomized trials: A systematic review and network meta-analysis." (2025) — PMID 39265924 — For knee and hip osteoarthritis across 57 large trials with 100 or more participants per arm involving 22,795 participants, a single type of steroid injection had the highest probability of reaching the minimal important difference at 2 to 6 weeks (1 trial), the effects of 16 types of injections were smaller than the minimal important difference and mostly matched the placebo effect, hyaluronic acid showed no pain effect alongside increased adverse event dropouts and serious adverse events, and there is no evidence of efficacy beyond 24 weeks.
• "Efficacy and safety of corticosteroids, hyaluronic acid, and PRP and combination therapy for knee osteoarthritis: a systematic review and network meta-analysis." (2023) — PMID 38037038 — Across 35 trials involving 3,104 participants, PRP and PRP plus hyaluronic acid yielded the best outcomes in function and pain at 3, 6, and 12 months, with no increase in treatment-related adverse events.
• "AAOS Clinical Practice Guideline Summary: Management of Osteoarthritis of the Knee (Nonarthroplasty), Third Edition." (2022) — PMID 35383651 — Better studies with subgroups, severity stratification, and control groups are needed regarding intra-articular corticosteroids, hyaluronic acid, and PRP.
• "Arthroscopic surgery for degenerative knee disease (osteoarthritis including degenerative meniscal tears)." (2022) — PMID 35238404 — In a Cochrane review of 16 trials involving 2,105 participants (mean age 46 to 65), compared to sham surgery, the difference at 3 months was 4.6 points for pain and 0.1 points for function, showing virtually no difference (high certainty); success up to 5 years was 74 per 100 versus 82 (low certainty), serious adverse events were 6 versus 8 (low certainty), whether knee surgery (joint replacement, osteotomy) increases after arthroscopy remains unresolved, and the authors concluded there is little to no clinically important benefit.
• "Knee replacement." (2012) — PMID 22398175 — It is a common and successful surgery that relieves pain and improves function in advanced arthritis, is increasingly considered for individuals under 55 requiring improved decision-making, and outcomes are evaluated using registries and patient-reported measures.
• "Early-stage symptomatic osteoarthritis of the knee - time for action." (2021) — PMID 34465902 — Efforts are underway to define early-stage symptomatic knee osteoarthritis and address it in primary care through education, exercise, and weight management, while disease-modifying treatments do not yet exist.