Knee Osteoarthritis

Outerbridge grades cartilage damage directly, during arthroscopy, typically at the time of meniscal surgery or a diagnostic arthroscopy. It is more sensitive than radiographs for early OA and does not correlate 1:1 with Kellgren-Lawrence, because radiographs can underestimate early disease. It has prognostic value in young patients with focal chondral defects.

Ahlbäck's system, developed in 1968, grades joint-space narrowing and bone attrition. It has been largely superseded by Kellgren-Lawrence but may still appear in exam questions.

Weight loss is the most effective non-operative intervention. It reduces the biomechanical load and decreases inflammatory adipokines such as leptin. A 5% weight loss improves pain and function scores (NEJM 2013), and combined with exercise the benefit is additive to either alone. The target is a BMI under 30, and a loss of 5-10% of body weight is effective.

Prevention, not treatment. In the Framingham cohort, women who lost about 5 kg over a decade had roughly half the odds of developing symptomatic knee OA (OR 0.46). That is a prevention finding, not a demonstration that weight loss slows established disease.

Exercise and physiotherapy give sustained benefit with no adverse effects and are recommended for every patient. Strengthening of the quadriceps, hamstrings and hip abductors increases muscle force to offload the joint, and quadriceps strengthening reduces pain by 20-30%. The programme:

• Quadriceps strengthening: straight leg raises, isometric holds, eccentric loading (step-downs)
• Low-impact aerobic exercise: swimming, cycling, walking
• Flexibility: hamstring and IT band stretches
• Balance training, which reduces fall risk
• Aerobic and flexibility work 3-5 times a week, for 30-45 minutes

Major guidelines (NICE, OARSI, ACR/AAOS) consistently recommend topical NSAIDs and exercise as core first-line therapy, with oral NSAIDs at the lowest effective dose for those who can tolerate them. The analgesic ladder:

1. Paracetamol 1g four times daily (4g maximum). Safe, with minimal side effects, but of modest efficacy (NNT 10-15). Consider a topical NSAID (diclofenac gel) for additional benefit.
2. Oral NSAIDs if that is inadequate: ibuprofen 400mg TDS or naproxen 500mg BD, or celecoxib (COX-2 selective) if there is GI risk. Use the lowest effective dose for the shortest duration, monitor renal function and blood pressure, and co-prescribe a PPI if the patient is older than 60 or has a GI history.
3. Adjuncts for a neuropathic component: duloxetine 60mg daily (an SNRI with evidence in OA pain), pregabalin or gabapentin if there are neuropathic features, and capsaicin cream for localised pain.
4. Opioids, a limited role in severe, refractory pain: tramadol 50-100mg (a weak opioid), only as a bridge to definitive surgery. Avoid long-term strong opioids (addiction, tolerance); guidelines do not recommend opioids long-term.

Corticosteroid. Triamcinolone 40mg or methylprednisolone 40mg, given intra-articularly with aseptic technique, for an acute flare, an effusion or failed oral analgesia. It relieves symptoms for 4-8 weeks in 60-70% of patients, and the Cochrane review found short-term benefit only. Give no more than 3-4 injections a year, because of concern for the cartilage: repeated corticosteroid may accelerate cartilage loss.

Hyaluronic acid (viscosupplementation). A series of 3-5 weekly injections of high molecular weight hyaluronic acid, which acts as a lubricant and is theoretically chondroprotective. Some patients get 3-6 months of relief. The evidence is mixed: some trials are positive, but AAOS and NICE do not recommend it routinely, and it is often expensive and not publicly funded in many health systems.

Platelet-rich plasma. Autologous PRP has attracted growing interest but has limited high-quality evidence. Small trials suggest benefit over hyaluronic acid, but they are heterogeneous and large RCTs are needed. Preparations are non-standardised, and PRP remains experimental, not standard of care and not routinely funded.

What injections cannot do. Intra-articular therapies provide temporary relief only and do not alter disease progression.

Unloader braces. A valgus-producing brace for medial OA shifts load to the lateral compartment. The pain reduction is modest, compliance is a problem, and the braces are bulky and expensive, with limited long-term use.

Lateral-wedge insoles. Used for medial OA, they aim to reduce the knee adduction moment. The evidence is conflicting and the clinical benefit minimal, but they are cheap and low-harm, so a trial is reasonable.

Nutraceuticals.

• Glucosamine and chondroitin: widely used, evidence mixed. AAOS does not recommend them (no proven benefit in large trials); they are safe and may be used on patient preference
• Fish oil and omega-3: theoretically anti-inflammatory, weak evidence
• Vitamin D: correct a deficiency for general health; no proven OA-specific benefit

Acupuncture. Small trials suggest a modest short-term benefit, by an unclear mechanism (placebo, endorphins), and NICE guidelines say it may be considered as an adjunct.

These modalities have limited evidence but low harm, and can be considered on patient preference.

What it does. High tibial osteotomy (HTO) is a realignment procedure for young patients (under 60) with isolated medial OA and varus malalignment. It shifts load from the medial to the lateral compartment and buys time, delaying arthroplasty by 10-15 years, without compromising the outcome of a future TKA.

Indications. All must be met:

• Age under 60, biological age mattering more than chronological
• An active, high-demand patient
• Isolated medial OA: K-L grade 3-4 medially, grade 0-1 laterally
• Varus deformity, with the mechanical axis passing through the medial compartment
• Flexion greater than 110° and an extension lag less than 10°
• Minimal to no patellofemoral OA
• A stable knee, ACL and PCL intact
• BMI ideally under 30, since obesity reduces survival

Contraindications.

• Inflammatory arthritis
• Tricompartmental OA
• Severe patellofemoral OA (Outerbridge grade 3-4)
• Fixed flexion deformity greater than 15°
• Lateral compartment cartilage worse than Outerbridge grade 0-1
• Medial bone loss, where correction cannot be achieved

Outcomes. Survivorship is 80% good to excellent at 10 years and 60-70% at 15 years, and satisfaction is high in appropriately selected patients. The operation itself is described under Surgical Technique.

What it does. Unicompartmental knee arthroplasty (UKA) is a partial knee replacement of a single compartment, most commonly the medial. It preserves bone stock, recovers faster and gives more physiological kinematics than TKA. UKA is excellent for carefully selected patients but requires strict adherence to the indications.

Oxford criteria. The indications are strict:

• Medial or lateral OA only, not tricompartmental
• Intact ACL, which is mandatory because UKA relies on it for stability
• Opposite compartment and patellofemoral joint Outerbridge grade 0-2 at most
• Deformity passively correctable to neutral, not fixed
• Flexion greater than 90° and an extension lag less than 10°
• Full-thickness cartilage loss in the single compartment, not partial
• Age over 55 preferred, since revision is lower in older patients

Contraindications.

• ACL deficiency (absolute)
• Inflammatory arthritis
• Tricompartmental OA
• Patellofemoral OA (grade 3-4)
• Fixed varus or valgus deformity greater than 10°
• BMI greater than 35 (relative, higher revision)

Advantages over TKA. UKA resects minimal bone, retains the ACL, PCL and collaterals, and gives more normal knee motion with better proprioception. It is less invasive, with a smaller incision and less soft-tissue trauma, and recovery is faster: walking on day 1, return to function at 6 weeks, a lower transfusion rate and a shorter hospital stay.

Disadvantages and complications.

• Higher revision: 10-15% at 15 years, against 5% for TKA
• Progression of OA in the opposite compartment or patellofemoral joint (5-10%)
• Loosening of the tibial component, especially with overcorrection
• Bearing dislocation in mobile-bearing UKA (1-2%)
• Persistent pain in 5-10%, from technical error or patient selection

What the registries show. Pooled national registry data (AOANJRR, NJR, NZJR) consistently show higher revision after UKA than TKA: about 10-13% at 10 years, against about 4-5%. Lateral UKA is performed far less often than medial and has a higher revision rate. High-volume surgeons (over 12-13 UKA a year) achieve markedly lower revision rates, and older patients have lower revision; younger patients and lower-volume practice drive the registry gap with TKA. OA progression, aseptic loosening and unexplained pain are the predominant reasons for revision.

What it does. Total knee arthroplasty is the gold standard for end-stage tricompartmental knee OA failing conservative management. It replaces all three compartments with metal and polyethylene components, and provides excellent pain relief and function in appropriately selected patients.

Indications.

• Pain refractory to 6 months or more of non-operative treatment
• Significant ADL limitation: walking under 500m, unable to climb stairs, night pain that prevents sleep
• K-L grade 4, bone-on-bone (or grade 3 with severe symptoms)
• Tricompartmental OA: medial, lateral and/or patellofemoral
• A major impact on quality of life, in a patient willing to accept the risks of surgery
• No medical contraindication: fit for anaesthesia, with comorbidities optimised

Pre-operative optimisation.

• Cardiac: optimise blood pressure, manage AF, stress test if indicated
• Respiratory: smoking cessation (4-6 weeks), treat COPD
• Diabetes: HbA1c under 7.5%, because of the infection risk
• Anticoagulation: manage warfarin or a DOAC perisurgically
• BMI: target under 35 (infection, wound and mechanical risk)
• Dental: clear infection 3 months before surgery
• Skin: treat psoriasis and eczema
• Nutrition: albumin greater than 35, with supplementation if malnourished

Implant choices. The registries inform each choice. Cemented fixation remains the most common worldwide.

Surgical principles.

1. Alignment: mechanical axis neutral (hip-knee-ankle 180°)
2. Gap balancing: equal flexion and extension gaps
3. Ligament balance: rectangular gaps, medial to lateral
4. Patellar tracking: central through the range, with no tilt
5. Component position: femur 5-7° valgus, tibia perpendicular

Registry key points (NJR, AJRR, AOANJRR, Nordic registries). Younger patients (under 55) have consistently higher revision rates across registries, and OA has better implant survival than inflammatory arthritis. Aseptic loosening, infection, instability and unexplained pain are the predominant reasons for revision.

Opening-wedge HTO. Optimal alignment requires precise planning and meticulous execution.

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Medial UKA. Durable results depend on precise component positioning and alignment.

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TKA by measured resection. Component position, alignment and soft-tissue balance all have to be right.

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Aseptic loosening is progressive loss of fixation between bone and implant without infection. It is multifactorial: malalignment, polyethylene wear, osteolysis and poor initial fixation.

• Clinical: progressive, activity-related start-up pain, loss of function, and mechanical symptoms such as catching and clicking
• Radiographs: progressive lucent lines greater than 2mm, subsidence, component migration, periprosthetic fracture
• Investigation: aspirate to rule out infection, and CT to assess bone stock for revision planning
• Management: revision TKA, bone grafting of defects (morselised or structural), and stems if there is metaphyseal bone loss

Prevention is optimal surgical technique: good alignment, appropriate component sizing, sound cement technique and avoiding notching of the femur.

Arthrofibrosis is excessive scarring that limits range of motion, typically flexion less than 90° at 6-12 weeks. Risk factors are pre-operative stiffness, infection, haematoma, prolonged immobilisation, poor physiotherapy compliance and complex revision. Prevention is immediate mobilisation, continuous passive motion (CPM) for the first 24-48 hours, aggressive physiotherapy and pain control.

Management depends on time from surgery:

• At 6 weeks: intensive physiotherapy, and address the pain
• 6-12 weeks: manipulation under anaesthesia (MUA) if range is under 90°
• Over 12 weeks: arthroscopic lysis of adhesions or open arthrolysis, with higher complication rates
• Over 1 year: revision TKA rarely improves range

Global Epidemiology

Knee OA affects an estimated 250-350 million people worldwide and is a leading cause of years lived with disability (Global Burden of Disease studies). Burden is rising fastest in low- and middle-income countries, driven by ageing and the obesity epidemic. The medial compartment is most commonly affected, reflecting the physiological varus tendency and higher medial joint load.

Side-by-Side Guideline Comparison