Introduction / Definition

Knee osteoarthritis (OA) is a chronic, progressive, disabling disorder of synovial joints characterised by structural breakdown of articular hyaline cartilage accompanied by subchondral bone remodelling, marginal osteophyte formation, synovial inflammation, and capsular fibrosis. Historically conceptualised as passive mechanical 'wear and tear', knee OA is now recognized as an active whole-organ joint disease driven by complex biochemical, inflammatory, and biomechanical factors.

Epidemiology & Aetiology

Epidemiology

• Prevalence: Knee OA is the leading cause of chronic musculoskeletal disability worldwide. Radiographic knee OA affects approximately 30% of adults aged >60 years, with symptomatic disease present in 10–15%.
• Demographics: Females are disproportionately affected compared to males (roughly 1.5:1 to 2:1 ratio), particularly after the menopause, and frequently present with more severe multi-compartmental disease.

Aetiology & Risk Factors

• Primary (Idiopathic): Strongly associated with advanced age, genetic susceptibility (polygenic heritability estimated at 40–60%), female sex, and elevated body mass index (BMI). Obesity acts via excessive joint loading alongside systemic low-grade adipokine-mediated inflammation.
• Secondary: Prior traumatic intra-articular injuries (tibial plateau fractures, high-grade articular chondral shear lesions), anterior cruciate ligament (ACL) rupture, previous meniscectomy (accelerating joint contact stresses), constitutional or acquired malalignment (e.g., severe genu varum or genu valgum), osteonecrosis, chronic crystalline arthropathies, haemochromatosis, and post-infectious arthropathy.

Pathophysiology

• Cellular and Molecular: Chondrocytes alter their metabolic phenotype from quiescent anabolic cells to hypertrophic, catabolic cells. Upregulated cytokines (interleukin-1β [IL-1β], tumour necrosis factor-α [TNF-α]) stimulate the overproduction of proteolytic enzymes, notably matrix metalloproteinases (MMP-1, MMP-13, which degrade type II collagen) and aggrecanases (ADAMTS-4 and ADAMTS-5, which cleave proteoglycans/aggrecan).
• Macroscopic Pathology: Progresses from superficial articular fibrillation and chondral softening to fissuring, eburnation, and subchondral microfractures leading to cystic bone remodelling and peripheral osteophytosis.

Relevant Anatomy & Biomechanics

• Compartmental Distribution: The knee comprises three functional compartments: medial tibiofemoral, lateral tibiofemoral, and patellofemoral. Medial tibiofemoral OA is markedly more prevalent (accounting for ~75% of unicompartmental patterns) due to normal gait mechanics transmitting 60–75% of axial ground-reaction forces through the medial compartment.
• Mechanical vs Anatomical Axis:
  • Mechanical Axis of Lower Extremity: Line drawn from the centre of the femoral head to the centre of the talar dome. In a neutral limb, this intersects within a few millimetres medial to the tibial spine midpoint (neutral mechanical alignment: 0° ± 2°).
  • Anatomical Axis: Formed by intersecting diaphyseal mid-lines of the femur and tibia, exhibiting an anatomical valgus tilt of 5° to 7°.
• Deformity Coronal Profiles:
  • Medial Compartment OA: Typically produces progressive varus deformity, accompanied by attenuation of the posterolateral structures and secondary contracture of the medial collateral ligament (MCL) and posteromedial capsule.
  • Lateral Compartment OA: Produces valgus deformity, lateral capsular/iliotibial band contracture, posterolateral corner (PLC) tightness, and medial soft-tissue laxity.

Classification Systems

Kellgren-Lawrence (KL) Radiographic Grading

Standardised on weight-bearing anteroposterior radiographs:

• Grade 0: Normal appearance; no radiographic features of OA.
• Grade 1: Doubtful joint space narrowing (JSN) with possible osteophytic lipping.
• Grade 2: Definite osteophytes with unimpaired or doubtful JSN.
• Grade 3: Moderate multiple osteophytes, definite joint space narrowing, some subchondral sclerosis, and possible deformity of bone contour.
• Grade 4: Large osteophytes, marked joint space narrowing, severe sclerosis, and definite deformity of bone ends.

Outerbridge Arthroscopic/Macroscopic Cartilage Grading

• Grade 0: Normal hyaline cartilage.
• Grade 1: Softening and swelling of articular cartilage.
• Grade 2: Partial-thickness defect with fissures not exceeding 1.5 cm in diameter or reaching subchondral bone.
• Grade 3: Fissuring down to the level of subchondral bone, with a defect diameter >1.5 cm.
• Grade 4: Exposed subchondral bone.

Ahlbäck Classification

Focused on standing radiographs to evaluate joint space obliteration and bone attrition (Grades I through V, where Grade III reflects bone loss 0–5 mm and Grade V involves severe subluxation).

Clinical Presentation & Examination

Symptoms

• Mechanical, activity-related joint pain exacerbated by weight-bearing, walking, or stairs; relieved by rest.
• In advanced disease: nocturnal pain and resting aches indicating uncontained subchondral bone hyperaemia.
• Morning stiffness lasting <30 minutes (distinguishing OA from inflammatory polyarthritides where stiffness >1 hour is classic).
• Episodic joint effusions, crepitus, and progressive loss of motion.
• Mechanical symptoms (pseudolocking, catching) from degenerative meniscal tears or unstable osteochondral loose bodies.

Examination

• Gait Assessment: Varus/valgus thrust, antalgic gait cadence, and leg length discrepancies.
• Inspection: Fixed coronal malalignment, quadriceps wasting, surgical scars, and joint effusions.
• Palpation: Joint line tenderness (medial or lateral), patellofemoral facet tenderness, anterior retropatellar crepitus, and popliteal (Baker's) cysts.
• Range of Motion: Quantification of fixed flexion contractures (FFC) and terminal flexion limits using a goniometer.
• Stability: Assess coronal stability at full extension and 30° flexion (to evaluate collateral competence and determine if deformity is correctable or fixed) and sagittal translation (cruciate ligaments).

Investigations

• Plain Radiography (Gold Standard):
  • Weight-bearing anteroposterior (AP) and lateral views of the knee.
  • Rosenberg view (45° posteroanterior weight-bearing view) — markedly superior sensitivity for early posteromedial tibiofemoral joint space loss.
  • Skyline/Merchant view (axial patellofemoral projection) — reveals lateral patellar subluxation, tilt, and trochlear joint space narrowing.
  • Full-length weight-bearing mechanical axis view (hip-to-ankle teleoroentgenogram) — mandatory for osteotomy and complex primary arthroplasty planning.
• Magnetic Resonance Imaging (MRI): Not routinely required for classic presentation, but indicated if atypical presentations occur (suspected subchondral insufficiency fracture of the knee [SIFK/SONK], avascular necrosis, or ambiguous red-flag neoplastic pathology).
• Laboratory Investigations: Baseline ESR, CRP, and serum rheumatoid factor/anti-CCP only when required to rule out inflammatory arthritis or periprosthetic/native joint septic arthritis.

Management

Non-Operative Management

In alignment with NICE Guideline NG226 and AAOS clinical practice guidelines:

• Core First-Line Measures:
  • Supervised therapeutic exercise (aerobic conditioning and targeted quadriceps strengthening).
  • Structured weight-loss interventions in individuals with overweight or obesity.
  • Patient education, self-management advice, and walking aids (unloading stick on the contralateral side).
• Pharmacological Therapy:
  • First-line: Topical non-steroidal anti-inflammatory drugs (NSAIDs).
  • Second-line: Oral NSAIDs (with concurrent proton-pump inhibitor [PPI] co-prescription, considering cardiovascular and renal comorbidities).
  • Paracetamol and weak opioids are no longer recommended as routine monotherapy by updated international guidelines due to negligible clinical effect and high dependency risks.
• Injections:
  • Intra-articular corticosteroids: Provide short-term analgesia (2–8 weeks) during severe acute exacerbations; routine frequent repeat dosing should be avoided to limit progressive chondrotoxicity.
  • Viscosupplementation (hyaluronic acid): Not endorsed by NICE guidelines due to lack of robust cost-effectiveness, although conditionally utilised in selective international practices.
  • Orthobiologics (PRP): Emerging data show symptomatic efficacy in mild-to-moderate disease, though evidence heterogeneity precludes uniform guideline mandates.

Operative Management

1. Arthroscopic Debridement

• Guideline Consensus: Strongly NOT recommended for unselected knee OA. No clinically meaningful advantage over sham/placebo surgery (landmark trials: Moseley 2002, Kirkley 2007). Only retained for exceptional cases with acute, definitive mechanical true locking secondary to an unstable loose body.

2. Joint Preserving Realignment Osteotomy

Indicated in young (<60 years), active, non-obese patients with isolated unicompartmental OA and preserved range of motion (>90° flexion, <10° fixed flexion contracture):

• High Tibial Osteotomy (HTO): For medial compartment OA with varus malalignment. Open-wedge (medial) or closed-wedge (lateral) techniques aim to shift the weight-bearing line to the Fujisawa point (62–68% of the tibial plateau width from the medial border), transferring stress onto the healthy lateral compartment.
• Distal Femoral Osteotomy (DFO): For lateral compartment OA with valgus malalignment (>10–12° valgus).

3. Unicompartmental Knee Arthroplasty (UKA)

• Indications (Kozinn & Scott Criteria): Isolated medial or lateral end-stage disease, intact ACL, full-thickness cartilage preserved in remaining compartments, correctable deformity (<10° varus, <5° valgus), fixed flexion contracture <10°, and flexion >90°.
• Benefits vs TKA: More natural kinematics (preserves both cruciates), faster recovery, decreased perioperative morbidity, reduced infection and venous thromboembolism rates.
• Trade-off: Higher lifetime revision rates compared to TKA in registry databases, often reflecting lower thresholds for surgical revision.

4. Patellofemoral Arthroplasty (PFA)

• Indicated for isolated patellofemoral disease refractory to conservative measures, provided tibiofemoral compartments are structurally preserved.

5. Total Knee Arthroplasty (TKA)

• Indications: End-stage multi-compartmental arthritis failing conservative treatment, significantly deteriorating health-related quality of life.
• Surgical Approaches:
  • Medial parapatellar approach (most common; excellent exposure).
  • Midvastus / Subvastus approaches (muscle-sparing variants aiming to preserve extensor apparatus vascularity and improve early quadriceps control).
  • Lateral parapatellar (Keblish) approach (favoured by some for severe fixed valgus deformities).
• Alignment Strategies: Mechanical alignment (neutral coronal cuts) versus kinematic alignment (restoring native pre-arthritic constitutional joint lines).
• Constraint Selection: Unconstrained posterior cruciate-retaining (CR) or posterior-stabilised (PS); constrained condylar knee (CCK) or hinged prostheses reserved for severe ligamentous insufficiency or significant bony defect reconstruction.

Complications

• Infection: Superficial wound dehiscence or periprosthetic joint infection (PJI) occurring in 1–2% of primary arthroplasties. Requires standardised diagnostic algorithms (MSIS/EBJIS criteria).
• Venous Thromboembolism: Deep vein thrombosis (DVT) and pulmonary embolism (PE); mitigated by mechanical prophylaxis and risk-stratified chemical thromboprophylaxis.
• Neurovascular Injury: Peroneal nerve palsy secondary to corrective traction during severe valgus/flexion contracture release; popliteal artery injury during posterior capsular clearance.
• Stiffness / Arthrofibrosis: Flexion <90° or fixed flexion deformity; may necessitate manipulation under anaesthesia (MUA) ideally performed before 6–12 weeks postoperatively.
• Extensor Mechanism Disruption: Patellar tendon avulsion or quadriceps rupture (catastrophic complication often requiring synthetic mesh or allograft reconstruction).
• Aseptic Loosening and Osteolysis: Polyethylene wear-induced osteolysis; late mechanical failure requiring revision.

Exam Pearls

• Fujisawa Point: When performing high tibial osteotomy (HTO) for medial compartment OA, the post-correction mechanical weight-bearing axis should pass through 62% to 68% of the tibial plateau width (measured medial to lateral).
• Kozinn and Scott Indications: Absolute criteria for UKA include intact ACL, non-inflammatory pathology, absence of chondrocalcinosis, flexion ≥90°, correctable deformity, and no significant lateral or patellofemoral facet bone-on-bone disease.
• Arthroscopy in OA: Moseley (NEJM 2002) and Kirkley (NEJM 2008) demonstrated that arthroscopic lavage and debridement provide no benefit over sham/conservative therapy for OA; citing this is essential for FRCS vavas.
• Radiographic Correlation: The Kellgren-Lawrence grade correlates poorly with patient-reported pain scores (VAS); treat the patient's disability and symptoms, not the radiograph.
• Rosenberg View: A 45° weight-bearing posteroanterior (PA) view detects posteromedial tibiofemoral joint space narrowing significantly earlier than extension AP radiographs.