Key Facts

Knee pain is the most common large joint complaint in primary care. In adults over 45, osteoarthritis is the most common cause. In younger patients, ligament and meniscal injuries predominate. Systematic assessment helps distinguish the common causes.

Epidemiology

• 25% of adults >55 have significant knee pain
• OA of the knee: affects >4.7 million in the UK
• ACL injuries: approximately 20,000/year in the UK
• Patellofemoral pain syndrome: most common cause of anterior knee pain in adolescents/young adults

Aetiology

• Osteoarthritis: degenerative, age-related, obesity, previous injury, genetic predisposition
• Ligament injuries: ACL (pivot/twist), MCL (valgus force), PCL (dashboard injury), LCL (varus force)
• Meniscal tears: twisting on weight-bearing knee; medial more common than lateral
• Inflammatory arthritis: RA, gout, pseudogout, reactive arthritis, psoriatic arthritis
• Patellofemoral pain: anterior knee pain in young, active patients; overuse, malalignment
• Referred pain: hip OA, L3/4 radiculopathy
• Other: prepatellar bursitis (housemaid's knee), Baker cyst, iliotibial band syndrome

Pathophysiology

• OA: cartilage degeneration, subchondral bone sclerosis, osteophyte formation, synovial inflammation
• ACL: pivot/twisting mechanism ruptures anterolateral band; often combined with meniscal and MCL injury (unhappy triad)
• Meniscal tears: degenerative (>40 years, minimal trauma) or traumatic (young, sport)
• Gout: monosodium urate crystal deposition; pseudogout: calcium pyrophosphate deposition

Osteoarthritis

• Pain worse with activity, better with rest
• Morning stiffness <30 minutes
• Crepitus on movement
• Bony enlargement
• Reduced range of motion
• Gradually progressive over months/years

ACL Tear

• Acute injury: pivot/twist, often with audible 'pop'
• Immediate swelling (haemarthrosis within 2 hours)
• Feeling of instability/giving way
• Positive Lachman test (most sensitive), anterior drawer test, pivot shift

Meniscal Tear

• Twisting injury or insidious onset (degenerative)
• Delayed swelling (24-48 hours)
• Mechanical symptoms: locking, catching, giving way
• Joint line tenderness
• Positive McMurray test

Inflammatory Arthritis (Gout/Pseudogout)

• Acute hot, swollen, red, exquisitely tender joint
• Gout: typically first MTP but can affect knee
• Pseudogout: knee is the most commonly affected joint
• Rapid onset (hours)

Red Flags

• Hot, swollen joint with fever (septic arthritis — emergency joint aspiration)
• Acute locked knee (meniscal fragment, loose body)
• Significant trauma with inability to weight-bear (fracture)
• Rapidly progressive deformity (malignancy, infection)

Bedside

• Full knee examination: inspection, palpation, range of motion, stability testing
• Lachman test (ACL — most sensitive), anterior/posterior drawer
• McMurray test (meniscus)
• Valgus/varus stress (MCL/LCL)
• Patellar apprehension test, Clarke test (patellofemoral)
• Assess for effusion: cross fluctuance, patellar tap, sweep test

Bloods

• Not required for OA diagnosis
• Serum urate: for gout (may be normal in acute attack)
• ESR, CRP: inflammatory or infectious causes
• RF, anti-CCP: if RA suspected

Imaging

• X-ray (weight-bearing AP, lateral, skyline): for OA (joint space narrowing, osteophytes, subchondral sclerosis, cysts), fracture, chondrocalcinosis
• NOT required to diagnose OA if clinical criteria met (NICE NG226)
• MRI: gold standard for soft tissue pathology (ACL, meniscus, cartilage)
• USS: Baker cyst, effusion assessment

Special Tests

• Joint aspiration: if effusion present (especially if septic arthritis or crystal arthropathy suspected)
  • Septic: turbid, WCC >50,000, positive culture
  • Gout: negatively birefringent needle-shaped crystals
  • Pseudogout: positively birefringent rhomboid crystals
  • OA: clear/straw-coloured, low WCC

Osteoarthritis (NICE NG226)

• Non-pharmacological (first-line):
  • Exercise: quadriceps strengthening, aerobic exercise, flexibility; supervised or home-based
  • Weight loss: if overweight; every 1kg loss reduces knee load by 4kg
  • Education and self-management: Arthritis Action, Versus Arthritis resources
  • Walking aids if needed
• Pharmacological:
  • Topical NSAID (diclofenac gel): first-line before oral NSAIDs
  • Oral NSAID (ibuprofen/naproxen): if topical insufficient; lowest dose, shortest duration; PPI cover
  • Paracetamol: adjunct (limited evidence for OA — Cochrane)
  • Intra-articular corticosteroid injection: for flares; short-term benefit (6-8 weeks)
  • Do NOT offer glucosamine, chondroitin, hyaluronic acid injections (NICE NG226)
• Surgical: total knee replacement for severe OA with significant impact on QoL

Ligament Injuries

• ACL: physiotherapy (rehabilitation) for low-demand patients; ACL reconstruction (hamstring/patellar tendon graft) for young, active, unstable knee
• MCL: usually conservative (bracing, physiotherapy)

Meniscal Tears

• Degenerative: physiotherapy first-line; arthroscopic partial meniscectomy NOT superior to physiotherapy (FIDELITY trial)
• Traumatic with locking: arthroscopic meniscal repair or partial meniscectomy

Crystal Arthropathy

• Acute gout: NSAID (naproxen 750mg stat then 250mg TDS), or colchicine 500mcg BD-TDS, or short-course prednisolone 30mg/day
• Urate-lowering: allopurinol 100mg (start 2 weeks after acute attack), titrate to target urate <360μmol/L

Referral Criteria

• Emergency: suspected septic arthritis (joint aspiration), acute locked knee
• Orthopaedic: ACL reconstruction candidate, refractory meniscal symptoms, severe OA for arthroplasty
• Rheumatology: inflammatory arthritis, recurrent crystal arthropathy
• Physiotherapy: first-line for all knee pain

• OA: progressive but variable; exercise and weight loss significantly reduce pain and disability; >90% of knee replacements last >15 years
• ACL tear: without reconstruction, risk of further meniscal damage and early OA; post-reconstruction, >85% return to sport
• Meniscal tear: degenerative tears — physiotherapy as effective as surgery (FIDELITY trial); traumatic tears in young patients often require surgery
• Gout: excellent prognosis with urate-lowering therapy; chronic tophaceous gout is preventable
• Septic arthritis: requires emergency treatment; delay increases risk of joint destruction and mortality (10-15%)

NICE NG226 — OA Clinical Diagnosis Criteria (No Imaging Needed)

Knee Examination Special Tests

Key Trial: FIDELITY (Meniscal Tears)