Knee Osteoarthritis (KOA)

Default Category Icon

Overview

Knee osteoarthritis is a chronic, whole-joint disorder characterized by pain, stiffness, functional limitation and progressive structural change affecting cartilage, subchondral bone, synovium, menisci, ligaments and peri-articular muscles. Prevalence rises with age and obesity and contributes substantially to global disability; recent GBD analyses and reviews highlight knee OA as a leading cause of pain and years lived with disability worldwide. Clinically, diagnosis is often made without imaging using validated rules, and core first-line care is education + exercise + weight management.

Definition

Osteoarthritis: a symptomatic, heterogeneous, whole-joint disease with mechanical, inflammatory and metabolic drivers rather than “wear-and-tear” alone.
Kellgren–Lawrence (K-L) grade: radiographic severity scale (0–4) based on osteophytes, joint-space narrowing, sclerosis and deformity (classification, not required for diagnosis).
Bone marrow lesion (BML): MRI high-signal subchondral change linked to pain and progression in KOA.

Anatomy & Physiology

  • Joint surfaces: femorotibial (medial/lateral) and patellofemoral compartments lined by hyaline cartilage; menisci distribute load and enhance stability; collateral and cruciate ligaments constrain translation/rotation.
  • Synovium: regulates nutrient exchange and produces hyaluronan-rich fluid for lubrication; can mount inflammatory responses.
  • Subchondral bone: shock absorption and load transfer; communicates with cartilage via osteochondral channels.
  • Neuromuscular control: quadriceps, hip abductors and calf musculature modulate joint loading; alignment and gait mechanics determine compartmental stresses.

Remember

Lubrication combines fluid-film and boundary layers—movement is protective when dosing is gradual and controlled.

Aetiology and Risk Factors

Aetiology (multifactorial, interacting “whole-joint” mechanisms)

  • Mechanical: abnormal joint loading (varus/valgus malalignment, meniscal loss) drives focal degeneration.
  • Biological: low-grade synovitis, subchondral remodeling and BMLs; nociceptive and nociplastic pain contributions.
  • Metabolic: obesity/adipokines and insulin resistance increase risk and symptom burden.

Risk factors

  • Non-modifiable: age; female sex; genetics/family history.
  • Modifiable: overweight/obesity; prior knee injury (e.g., ACL/meniscus); high occupational kneeling/squatting/lifting; malalignment; low muscle strength; physical inactivity.

Treat prior-injury knees as “secondary prevention” targets (strength, alignment, load).

Pathophysiology

  • Joint load mismatch (demand > capacity) → focal cartilage matrix damage + meniscal stress.
  • Synovial activation (cytokines, neuropeptides) → pain and effusion; central sensitization in some.
  • Subchondral bone remodeling and BMLs → nociceptor ingrowth/angiogenesis → pain and structural progression.
  • Malalignment/deconditioning perpetuate compartmental overload → osteophytes, joint-space loss, functional decline. 

Remember

BMLs and synovitis correlate best with weight-bearing pain—guide both rehab and injection decisions.

Clinical Manifestations

  • Activity-related knee pain (walking, stairs, squatting)
  • Short morning stiffness (<30 min)
  • “giving way”
  • Crepitus and swelling
  • Reduced function and confidence
  • Flares after load spikes
  • Nocturnal pain in advanced disease

Functional features

  • Slower timed-up-and-go
  • Difficulty rising from chair
  • Reduced walking endurance
  • Reduced balance.

Diagnosis

Diagnose clinically without imaging when all present—age ≥45, activity-related pain, no morning stiffness or stiffness ≤30 minutes; image if atypical features or red flags.

Remember

Label early and start core rehab—do not delay for x-ray. 

Imaging 

  • Weight-bearing AP/lateral and skyline views to classify and plan
    • Radiograph severity by K-L grade
  • MRI not routinely required—consider if alternative pathology suspected
  • US can detect effusion/synovitis and guide injections

Imaging severity (K-L) ≠ symptom severity; target impairments and goals, not x-ray grade. 

Classification 

  • By compartment: medial/lateral tibiofemoral; patellofemoral; mixed.
  • By radiograph: Kellgren–Lawrence 0–4 (0 normal → 4 severe with marked joint-space loss and deformity).
  • By phenotype (useful clinically): biomechanical–malalignment/instability; inflammatory/synovitis-dominant; metabolic–obesity; bone-driven (BML-dominant); pain-sensitization predominant.

Treatment

  • Education
  • Structured exercise (strength—esp. quadriceps/hip; aerobic; neuromuscular/balance; tai chi)
  • Weight loss if overweight/obese (goal ≥5–10%)
  • Pacing and flare plan

Devices/adjuncts

  • Cane in contralateral hand
  • Knee taping
  • Tibiofemoral unloader bracing for unicompartmental OA 

Pharmacologic (use lowest effective dose, shortest duration, alongside exercise)

  • Topical NSAIDs (first-line)
  • Oral NSAIDs + PPI if needed; acetaminophen limited benefit
  • Duloxetine conditional for persistent pain
  • Topical capsaicin optional
  • Avoid routine opioids

Intra-articular therapies

  • Corticosteroid: short-term relief for flares; avoid frequent repeat courses (signals of cartilage loss with repeated triamcinolone).
  • Hyaluronic acid (no good evidence)
  • PRP/stem cells: evidence inconsistent; many guidelines advise against outside research/specialist settings.

Interventional pain

  • Radiofrequency ablation (genicular nerves): conditional option in refractory knee OA

Surgery

  • Do not perform arthroscopy for primary degenerative knee disease (no meaningful long-term benefit).
  • Consider realignment osteotomy (younger, unicompartmental malalignment) or unicompartmental/total knee arthroplasty for severe, function-limiting disease after optimal non-surgical care.

Remember

 Infections offer a short-term role; avoid frequent repeats (possible cartilage loss signal with repeated steroids).

Remember

No single exercise is superior—the best plan is the one they’ll do consistently.

Complications and Prognosis

Complications

  • Functional decline
  • Falls/deconditioning
  • Analgesic adverse effects (GI/CV with NSAIDs; dependence with opioids)
  • Post-injection flare
  • Accelerated progression in some with repeated IA steroids
  • Surgical risks after arthroplasty

Prognosis

  • Variable course; many stabilize/improve with sustained exercise and weight loss
  • Poor prognostic factors
    • Obesity
    • Varus/valgus malalignment
    • Prior major knee injury
    • Severe baseline radiographic/structural damage
    • Depression/low self-efficacy

Remember

The strongest modifiable levers are weight loss and ongoing exercise.

Acronym

  • KNEE-OA = Kilos down (weight loss) + NSAIDs (topical → oral) + Exercise (strength/aerobic/balance) + Education/self-management + Offload (brace/tape/cane) + Adjuvants (IA steroid short-term; duloxetine) — escalate to surgery if refractory. (NICE, Arthritis Foundation)

References

  1. National Institute for Health and Care Excellence (NICE). Osteoarthritis in over 16s: diagnosis and management (NG226). London: NICE; 2022. (NICE)
  2. Kolasinski SL, Neogi T, Hochberg MC, et al. 2019 American College of Rheumatology/Arthritis Foundation Guideline for the Management of OA of the Hand, Hip, and Knee. Arthritis Care Res (Hoboken). 2020;72(2):149-162. (Arthritis Foundation)
  3. Bannuru RR, Osani MC, Vaysbrot EE, et al. OARSI guidelines for the non-surgical management of knee, hip, and polyarticular OA. Osteoarthritis Cartilage. 2019;27:1578-1589. (ScienceDirect)
  4. American Academy of Orthopaedic Surgeons (AAOS). Management of Osteoarthritis of the Knee (Non-Arthroplasty). 3rd ed. 2021. (aaos.org)
  5. EULAR. Evidence-based recommendations for the diagnosis of knee OA. Ann Rheum Dis. 2024. (EULAR)
  6. Hunter DJ, Bierma-Zeinstra S. Osteoarthritis. Lancet. 2019;393:1745-1759. (acpacprogram.ca)
  7. Oo WM, Hunter DJ, et al. Bone marrow lesions in osteoarthritis: recent advances. Skeletal Radiol. 2024;53:—. (SpringerLink)
  8. Hall M, et al. Risk factors for incident knee OA: systematic review. Osteoarthritis Cartilage. 2025;—. (oarsijournal.com)
  9. Filbay SR, et al. Risk of knee OA after knee injury in youth. Br J Sports Med. 2020;54(12):725-—. (bjsm.bmj.com)
  10. Siemieniuk RAC, et al. Arthroscopic surgery for degenerative knee arthritis and meniscal tears: guideline. BMJ. 2017;357:j1982. (BMJ)
  11. EULAR 2024 update—non-pharmacological core management of hip/knee OA. Ann Rheum Dis. 2024;—. (ard.bmj.com)
  12. AAFP. Osteoarthritis Management: Updated ACR Guidelines. Am Fam Physician. 2021;103(2):120-122. (AAFP)
  13. Radiopaedia. Kellgren and Lawrence system for classification of OA. 2025. (Radiopaedia)
  14. EULAR recommendations for imaging in OA clinical management. Ann Rheum Dis. 2017;76:1484-1494. (eprints.whiterose.ac.uk)
  15. The Knee Journal. K-L grade and morphological parameters. 2019. (thekneejournal.com)
  16. Kolasinski SL, et al. ACR/AF guideline—hyaluronic acid conditional against. Arthritis Care Res (Hoboken). 2020;72:149-162. (acrjournals.onlinelibrary.wiley.com)
  17. NICE NG226 Visual Summary. 2022. (NICE)
  18. McAlindon TE, et al. Intra-articular triamcinolone vs saline and cartilage volume. JAMA. 2017;317:1967-1975. (JAMA Network)
  19. GuidelineCentral summary of AAOS: lavage/debridement not recommended. 2021. (Guideline Central)

Related Content

Discussion

0 Comments
Most Voted
Newest Oldest
Inline Feedbacks
View all comments

Table Of Contents

The AH Community Platform is coming
Get access to member features
Early access launches soon
Armando hasudungan brain logo
Armando Hasudungan
Medical & Biology Tutorials
© 2025 Visualising Medicine. All rights reserved.
Become a member to access note taking
Orangise your medical learning
This is just one of the many AH community member perks
Become a member to access quizzes
Strengthen your medical knowledge
This is just one of the many AH community member perks
0
Would love your thoughts, please comment.x
()
x