What Is Knee Osteoarthritis?
Knee osteoarthritis (OA), also called arthrosis or degenerative joint disease, is the most prevalent form of arthritis. It is a chronic progressive condition characterized by deterioration of the articular cartilage of the knee, accompanied by changes in the subchondral bone, synovial membrane, ligaments, and periarticular musculature.
The old idea that OA is simply cartilage "wear and tear" is outdated. Today, OA is understood as a disease of the entire joint, involving low-grade inflammatory processes, bone remodeling, muscle weakening, and neural changes that contribute to pain and disability.
The knee is the joint most often affected by OA, partly because it bears loads up to 3-4 times body weight during everyday activities like climbing stairs. The medial compartment (inner side) is the most frequently affected.
Disease of the Entire Joint
It is not just cartilage wear: bone, synovium, muscles, ligaments, and nerves are all involved.
Relationship with Weight
Each kg of excess body weight adds 4 kg of load on the knee during walking.
Exercise Is Essential
Contrary to popular belief, regular exercise is the most effective treatment, not a worsening factor.
Epidemiology
Knee OA is one of the leading causes of disability in adults over 50 worldwide. Its prevalence increases exponentially with age, and an aging population has made this condition a growing challenge for health systems.
Modifiable risk factors include obesity (the most important factor), prior knee injuries (meniscus, anterior cruciate ligament), occupational activities with repetitive kneeling, and quadriceps weakness. Non-modifiable factors include age, female sex, genetics, and anatomic malalignment (varus or valgus).
Pathophysiology
Articular cartilage is composed of chondrocytes (cells) embedded in a matrix of type II collagen and proteoglycans (aggrecan). This matrix gives cartilage its biomechanical properties of compression resistance and lubrication.
Pathological Cascade
In OA, the synthesis and degradation of the cartilage matrix become unbalanced. Metalloproteinases (MMPs) and aggrecanases, produced by activated chondrocytes and synovial cells, degrade collagen and proteoglycans. Cytokines such as IL-1beta and TNF-alpha amplify this catabolic process.
The subchondral bone undergoes remodeling: in early phases, there is increased bone resorption; in advanced phases, sclerosis (hardening) and cyst formation. Osteophytes (bone spurs) form at the joint margins as an attempt at stabilization.
The synovial membrane shows low-grade inflammation (synovitis), with infiltration of macrophages and lymphocytes that perpetuate the production of cytokines and degradative enzymes. This synovitis contributes significantly to pain and joint effusion.
Symptoms
Symptoms of knee OA develop gradually over months or years. Pain is the most common symptom, followed by stiffness and functional limitation.
Symptoms of Knee Osteoarthritis
- 01
Mechanical pain
Pain that worsens with activity (especially going up or down stairs) and improves with rest. Unlike inflammatory pain, which worsens at rest.
- 02
Brief morning stiffness
Morning stiffness that lasts less than 30 minutes (in rheumatoid arthritis, it exceeds 60 minutes).
- 03
Joint crepitus
Sensation of "sand" or popping when moving the knee, caused by irregularity of the joint surfaces.
- 04
Joint swelling
Intermittent joint effusion, especially after more strenuous physical activity.
- 05
Instability and giving way
Sensation that the knee will "give way," caused by muscle weakness and ligament laxity.
- 06
Limitation of movement
Progressive loss of full extension and, later, flexion, making it harder to squat or use stairs.
- 07
Pain on initiating movement
Pain in the first steps after rest ("start-up pain") that improves after a few minutes.
- 08
Progressive deformity
In advanced stages, varus (bow-legs) or valgus (knock-knees) deformity may develop.
Diagnosis
The diagnosis of knee OA is clinical-radiographic. The American College of Rheumatology (ACR) criteria are the most widely used and allow diagnosis with or without additional tests.
🏥ACR Criteria for Knee OA
Fonte: American College of Rheumatology
Clinical Criteria (Presence of Knee Pain +)
Presence of 3 or more criteria- 1.Age greater than 50 years
- 2.Morning stiffness less than 30 minutes
- 3.Crepitus on active movement
- 4.Bone tenderness on palpation
- 5.Bony enlargement at the joint (palpable osteophytes)
- 6.Absence of local warmth on palpation
Radiographic Classification (Kellgren-Lawrence)
- 1.Grade 0: Normal
- 2.Grade 1: Doubtful osteophytes
- 3.Grade 2: Definite osteophytes, preserved joint space
- 4.Grade 3: Moderate joint space narrowing
- 5.Grade 4: Obliterated joint space with subchondral sclerosis
DIAGNÓSTICO DIFERENCIAL
Differential Diagnosis
Patellofemoral Syndrome / Chondromalacia
Read more →- Anterior knee pain
- Worse climbing/descending stairs
- Patellar crepitus
Testes Diagnósticos
- Clarke test
- MRI
Meniscal Pathology
- Pain at the joint line
- Positive McMurray
- Joint effusion
Testes Diagnósticos
- Knee MRI
- McMurray test
Rheumatoid Arthritis
Read more →- Bilateral and symmetric
- Morning stiffness >1h
- Elevated inflammatory markers
Testes Diagnósticos
- RF
- Anti-CCP
- ESR/CRP
Pes Anserine Bursitis
- Pain on the medial face of the knee, below the joint line
- More common in obese patients with OA
- Focal point of pain
Testes Diagnósticos
- Ultrasonography
Ligamentous Injury (ACL/PCL)
- Recent trauma
- Joint instability
- Positive drawer tests
- Severe instability requires immediate evaluation
Testes Diagnósticos
- MRI
- Anterior/posterior drawer tests
Patellofemoral Syndrome and Patellar Chondromalacia
Patellofemoral syndrome is one of the conditions most often confused with knee OA, especially in young and middle-aged adults. Pain is typically anterior, retropatellar, and worsens with stairs, squatting, and prolonged sitting (the movie sign). Patellar crepitus when flexing the knee is characteristic.
The Clarke test (compressing the patella against the femur while the patient contracts the quadriceps) is positive in patellofemoral syndrome. Knee OA, by contrast, predominates in the medial compartment, with pain at the joint line and worsening with weight-bearing activities, but without anterior-compartment predominance. MRI distinguishes patellar chondromalacia (cartilage changes in the patella) from tibiofemoral OA.
Meniscal Pathology
Meniscal injury may coexist with knee OA (meniscal degeneration is part of the OA spectrum) or occur in isolation, especially after trauma in young adults. Meniscal pain is typically at the joint line (medial or lateral), with a positive McMurray sign (pain on rotating the knee under load) and frequent joint effusion.
MRI is the standard test for diagnosing meniscal injuries. An important point: a degenerative meniscal injury on MRI does not necessarily mean it is the cause of pain, since in patients over 50 with OA, meniscal changes are common and not always symptomatic.
Pes Anserine Bursitis
Pes anserine bursitis is often underdiagnosed and may coexist with knee OA (they are distinct but associated conditions). The pes anserine bursa sits on the medial surface of the tibia, about 5 cm below the joint line. Pain is pinpoint at this site, unlike the more diffuse joint-line pain of OA.
Pes anserine bursitis is especially common in obese patients with knee OA and diabetes. Ultrasound confirms the diagnosis by showing bursal distention. Treatment includes local corticosteroid injection and hamstring stretching exercises.
Treatments
Management of knee OA is multimodal, combining non-pharmacologic, pharmacologic, and, when necessary, surgical interventions. Current guidelines emphasize that non-pharmacologic treatments are the foundation of therapy.
Non-Pharmacologic Treatment
Therapeutic exercise is the intervention with the highest level of evidence for knee OA. Programs combining quadriceps strengthening, low-impact aerobic exercise, and stretching demonstrate pain reduction of 30-50% and functional improvement equivalent to analgesics.
Weight loss is crucial in overweight patients. A reduction of just 5-10% of body weight results in clinically significant improvement in pain and function. Each kg lost reduces the load on the knee during gait by 4 kg.
Patient education about the nature of the disease, self-management, and the importance of regular physical activity is fundamental for long-term therapeutic success.
TREATMENTS FOR KNEE OA
| TREATMENT | MECHANISM | EVIDENCE | RECOMMENDATION |
|---|---|---|---|
| Therapeutic exercise | Strengthening, biomechanics improvement | Strong (level A) | First line — all patients |
| Weight loss | Joint load reduction and inflammation | Strong (level A) | BMI > 25, combine with exercise |
| Acetaminophen | Central analgesic | Weak | Modest effect, safe short-term |
| Topical NSAIDs | Local anti-inflammatory | Strong (level A) | Prefer over oral NSAIDs |
| Oral NSAIDs | Systemic anti-inflammatory | Strong | Lowest dose, shortest possible time |
| Acupuncture | Pain modulation, anti-inflammatory | Moderate (level B) | Adjuvant to exercise |
| Hyaluronic acid infiltration | Viscosupplementation | Controversial | Some patients benefit |
| Total arthroplasty | Joint replacement | Strong | Conservative failure, severe OA |
Acupuncture as Treatment
Knee OA is one of the most heavily researched conditions in acupuncture. Trials such as GERAC (Germany) and the NIH studies (US) suggest pain reduction and functional improvement, though much of this effect may come from non-specific factors (expectation, therapeutic ritual) when compared with sham. Guidelines diverge: ACR and OARSI keep acupuncture as a conditional recommendation; whereas NICE NG226 (2022) does not recommend acupuncture as routine treatment for OA, considering the evidence insufficient.
Hypothesized mechanisms — in preclinical models and preliminary human studies — include modulation of endogenous opioid and adenosinergic pathways, effects on inflammatory cytokines, and activation of descending inhibitory pain circuits. These mechanisms remain under investigation and should not be presented as established clinical explanations.
In practice, acupuncture can be considered as an adjunct to therapeutic exercise — the best-supported first-line intervention — especially in patients with contraindications to NSAIDs or who prefer to avoid medication. The decision should be shared with the physician.
Prognosis
Knee OA is a chronic and progressive disease, but how fast it progresses varies enormously between individuals. Many patients maintain adequate function for years or decades with appropriate conservative treatment. Total arthroplasty is reserved for cases with significant functional impairment.
Long-Term Management Strategy
Level 1
PermanentTreatment Base
Education, regular exercise, weight loss if necessary, self-management. These measures should be maintained indefinitely.
Level 2
As neededComplementary Therapies
Add topical NSAIDs, acupuncture, targeted physical therapy, insoles or knee braces when necessary.
Level 3
Intermittent periodsPharmacotherapy
Oral NSAIDs (lowest dose, shortest duration), duloxetine for chronic pain, joint injections.
Level 4
When necessarySurgery
Total arthroplasty when conservative treatment fails and quality of life is significantly impaired.
Myths and Facts
Myth vs. Fact
Arthritis is just an old-age problem — there is no treatment.
OA has multiple effective treatment options. Exercise, weight loss, and education can reduce pain by 30-50% and preserve function for years.
Physical exercise wears the knee more with arthrosis.
Regular exercise is the most effective treatment for OA. Muscle strengthening protects the joint and improves pain. Inactivity worsens the condition.
Supplements like glucosamine and chondroitin regenerate cartilage.
Large clinical trials have not shown significant benefit of these supplements over placebo for pain or cartilage regeneration.
If the radiograph shows arthrosis, I need surgery.
Many people with severe radiographic changes are asymptomatic. Treatment is guided by symptoms, not by images.
Running causes knee arthrosis.
Population studies do not show that recreational running increases the risk of knee OA. Recreational runners actually have a lower prevalence of OA than sedentary individuals.
When to Seek Medical Help
Frequently Asked Questions about Knee Osteoarthritis
Knee osteoarthritis (OA) is the progressive degeneration of articular cartilage of the knee, accompanied by changes in subchondral bone, synovial membrane, and periarticular musculature. It is not simply "wear": it is a disease of the entire joint, with a low-grade inflammatory component. The main risk factors are obesity (the most important), prior injuries, quadriceps weakness, repetitive occupational activities, and genetics.
The classic symptoms are: mechanical pain (worsens with activity, improves with rest), brief morning stiffness (less than 30 minutes), joint crepitus on moving the knee, intermittent swelling, sensation of giving way, and progressive limitation of movement. Pain on initiating gait after rest ("start-up pain") that improves after a few steps is very characteristic. In advanced phases there may be deformity in varus or valgus.
Diagnosis is clinical-radiographic. The American College of Rheumatology criteria combine knee pain with at least 3 of 6 clinical findings (age > 50 years, morning stiffness < 30 min, crepitus, bone tenderness, bony enlargement, absence of warmth). Radiographs confirm the diagnosis and grade severity (Kellgren-Lawrence scale, grades 0 to 4). Laboratory tests are normal in primary OA.
Therapeutic exercise is the best-supported intervention — it reduces pain by 30-50% and improves function. Weight loss is essential in overweight patients. Among medications, topical NSAIDs are preferred over oral ones. For more severe cases, corticosteroid joint injections offer temporary relief. Total arthroplasty is reserved for cases where conservative treatment fails and function is significantly impaired.
Knee OA is one of the most studied conditions in acupuncture. Trials such as GERAC and the NIH studies suggest pain reduction and functional improvement, though the effect size versus sham is modest and of debated clinical relevance. Guidelines diverge: ACR and OARSI keep acupuncture as a conditional recommendation; NICE NG226 (2022) does not recommend it routinely. Proposed mechanisms (endogenous opioid pathways, adenosine, inflammatory modulation) remain under investigation.
Protocols studied in clinical trials generally used 8 to 15 sessions over 6-8 weeks, starting at 2 sessions per week. Effects tend to be durable: studies show benefits last 6-12 months after treatment ends. The acupuncture physician will assess the response and tailor the protocol to each patient.
When performed by an acupuncture physician, the safety profile is generally favorable for knee OA: adverse events reported in large-scale studies are mostly mild and transient (bruising, local discomfort). Serious events (significant bleeding, infection, syncope) are rare, but they do occur. Anticoagulated or immunosuppressed patients should tell the physician. There is no evidence that acupuncture alters cartilage progression — but it does not reverse it either.
Yes, and this is the recommended approach. Acupuncture as an adjunct to therapeutic exercise produces better results than either treatment alone. By controlling pain, acupuncture improves adherence to the exercise program — especially in early stages, when pain limits activity. The physician may include quadriceps strengthening as part of an integrated treatment plan.
Knee OA is chronic and progressive, but how fast it progresses varies widely between individuals. With proper treatment — especially regular exercise and weight control — many patients maintain good function for years or decades. Poor prognostic factors include obesity, varus deformity, bilateral involvement, and severe muscle weakness. When arthroplasty is needed, outcomes are excellent.
Seek immediate care if: the knee becomes acutely swollen, hot, and red (may indicate septic arthritis or a gout flare); you cannot bear weight after trauma (may indicate fracture or severe ligamentous injury); the knee locks in flexion and cannot extend (joint locking from a meniscal fragment or loose body); or you have sudden, intense pain out of proportion to your usual symptoms. In all these cases, see a physician right away.
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