Introduction / Definition

Total knee arthroplasty (TKA) is one of the most successful and cost-effective operations in modern orthopaedic surgery. It involves resecting diseased articular cartilage and subchondral bone from the distal femur, proximal tibia, and selectively the patella, replacing them with prosthetic components designed to restore functional alignment, joint stability, and painless mobility. Despite excellent implant survivorship exceeding 90–95% at 15–20 years in national joint registries, up to 15–20% of patients report some degree of residual dissatisfaction, highlighting the demanding technical and patient-selection nuances inherent to the procedure.

Epidemiology & Indications

• Epidemiology: The annual volume of primary TKAs continues to grow exponentially, driven by an ageing demographic and increasing rates of obesity. Registry data (e.g., the National Joint Registry [NJR] of England, Wales, and Northern Ireland) demonstrates that primary osteoarthritis (OA) accounts for >95% of cases.
• Indications:
  • End-stage symptomatic tricompartmental or bicompartmental knee osteoarthritis.
  • Inflammatory arthropathies (e.g., rheumatoid arthritis, psoriatic arthritis).
  • Post-traumatic arthritis with joint space obliteration.
  • Osteonecrosis of the femoral condyle (SPONK/SONK) with structural collapse.
  • Failure of joint-preserving interventions (e.g., osteotomy, unicompartmental arthroplasty).
• Contraindications:
  • Absolute: Active local or systemic infection; absent or non-functioning extensor mechanism; recurvatum secondary to muscular weakness; severe peripheral arterial disease causing critical limb ischaemia.
  • Relative: Morbid obesity (BMI >40 kg/m²), poorly controlled diabetes mellitus (HbA1c >8% or 64 mmol/mol), severe peripheral vascular disease, active neuropathic arthropathy (Charcot joint), and poor patient compliance or profound neurocognitive impairment.

Relevant Anatomy & Surgical Approaches

Vascular Anatomy

• The popliteal artery lies directly posterior to the posterior cruciate ligament (PCL) and joint capsule, separated only by thin pericapsular fat and the popliteus tendon. It is closest to the posterior joint line in full extension; flexion displaces the vessel posteriorly, providing a safety margin during tibial resection and PCL recession.
• The patellar blood supply relies on an anastomotic peripatellar ring contributed by the superior and inferior medial and lateral genicular arteries, and the anterior tibial recurrent artery. Extensive lateral retinacular release endangers the lateral superior genicular artery, predisposing to patellar osteonecrosis and anterior knee pain when combined with a medial arthrotomy.

Standard & Alternative Approaches

• Medial Parapatellar (Insall / von Langenbeck): The workhorse approach. Straight anterior midline skin incision followed by an arthrotomy coursing through the medial border of the quadriceps tendon, peripatellar retinaculum, and medial to the tibial tubercle. Offers versatile, reproducible exposure but disrupts medial soft tissues.
• Midvastus Approach: Traverses obliquely through the vastus medialis oblique (VMO) muscle fibres (sparing the quadriceps tendon). Aims to preserve extensor continuity, theoretically accelerating quadriceps recovery, but risks denervation of inferior VMO fibres.
• Subvastus (Southern) Approach: Elevates the lower border of the VMO off the medial intermuscular septum without splitting muscle. Preserves the extensor mechanism entirely and avoids disruption of patellar blood supply, but exposure can be challenging in obese or muscular patients and severe fixed deformities.
• Lateral Parapatellar (Keblish): Preferred in severe fixed valgus and external rotation deformities (>15–20°). Facilitates direct release of contracted lateral structures (iliotibial band, popliteus, posterolateral capsule) and avoids medial devascularisation.
• Extensor Mechanism Augmenting Exposures (Stiff or Ankylosed Knee):
  • Quadriceps Snip: Proximal oblique division of the quadriceps tendon at 45° across the rectus femoris; heals reliably without altering rehabilitation protocols.
  • V-Y Turndown (Coonse-Adams): Inverted V incision through the quadriceps tendon; requires protected postoperative rehabilitation and carries a high risk of extensor lag.
  • Tibial Tubercle Osteotomy (TTO): A 6–8 cm bone block elevated from medial to lateral, preserving the lateral periosteal hinge; fixed rigidly with screws/wires.

Classification Systems Relevant to TKA

• Kellgren-Lawrence Grading of Osteoarthritis: Radiographic staging from Grade 0 (none) to Grade 4 (large osteophytes, marked joint space narrowing, severe sclerosis, definitive bony attrition).
• Anderson Orthopaedic Research Institute (AORI) Classification: Guides management of bone defects:
  • Type 1: Intact metaphysis, minor cancellous defects not compromising implant stability (managed with morcellised bone graft or cement fill).
  • Type 2: Damaged metaphysis involving one (2A) or both (2B) condyles/hemi-plateaus; cancellous bone compromised (managed with modular metal augments, cement with screws, or small bone graft).
  • Type 3: Deficient metaphysis with massive cortical and cancellous bone loss affecting collateral ligament insertions (requires metaphyseal sleeves, cones, bulk allografts, or hinged implants).
• Insall-Salvati & Caton-Deschamps Indices: Quantify patellar height to avoid patella baja (joint line elevation) or alta.

Clinical Presentation & Examination

• Symptoms: Progressive, weight-bearing mechanical pain localised to specific compartments; inflammatory rest pain or night pain; stiffness, loss of terminal extension, instability, and functional limitation (difficulty with stairs, car egress).
• Inspection: Scars, gait (antalgic, thrust), fixed coronal alignment (varus vs valgus), recurvatum or fixed flexion deformity.
• Palpation: Joint effusion, warmth (exclude occult infection), joint line tenderness, pes anserine bursitis, popliteal fossa (Baker's cyst).
• Range of Motion: Document fixed flexion deformities (FFD) and maximal flexion.
• Stability & Ligamentous Laxity: Assess collateral competency (MCL and LCL at 0° and 30° flexion). Differentiate correctable (flexible) deformities from fixed contractures.
• Neurovascular Status: Palpate distal pulses (dorsalis pedis, posterior tibial) and examine common peroneal nerve function, particularly in valgus deformities.
• Adjacent Joints: Systematically examine the ipsilateral hip (referred pain from hip OA) and lumbar spine (radiculopathy).

Investigations

• Radiographs:
  • Weight-bearing bilateral anteroposterior (AP) and lateral views of the knee.
  • Weight-bearing 45° flexion posteroanterior view (Rosenberg view) to identify subtle posterior cartilage loss.
  • Axial patellofemoral view (Skyline / Merchant view) to assess patellofemoral tracking, joint space, and dysplasia.
  • Long-leg weight-bearing mechanical axis scan (hip-to-ankle radiograph) to measure the mechanical axis, hip-knee-ankle (HKA) angle, and identify extra-articular femoral or tibial deformities.
• Laboratory Investigations: Baseline ESR and CRP in patients with suspected inflammatory arthropathy or previous joint interventions to exclude infection.
• Diagnostic Aspiration: Mandatory if any suspicion of crystalline arthropathy or septic arthritis exists before arthroplasty.

Management & Surgical Techniques

Non-Operative Management

NICE and AAOS guidelines advocate structured conservative therapy prior to surgery:

• Structured exercise (strengthening and aerobic conditioning).
• Weight management (aiming for BMI reduction).
• Analgesia (paracetamol, topical/oral NSAIDs, weak opioids as adjunctive).
• Walking aids, biomechanical unloaders (in isolated unicompartmental disease).
• Intra-articular corticosteroids (short-term symptom relief); intra-articular hyaluronic acid is generally not endorsed by NICE due to cost-benefit ratios.

Alignment Philosophies

• Mechanical Alignment (MA): Cuts the distal femur at 5–7° of valgus (relative to anatomical axis) and proximal tibia perpendicular (90°) to its mechanical axis. Creates a straight HKA angle (180° ± 3°). Requires soft-tissue releases to balance asymmetric gaps.
• Kinematic Alignment (KA): Aims to restore the patient's constitutional, pre-arthritic joint line orientation and natural laxity by co-planar resection matching implant thickness to native bone cuts, avoiding ligamentous releases.

Surgical Resection Strategies

• Measured Resection: Bone cuts correspond strictly to prosthetic thickness. Femoral rotation is referenced from anatomical axes:
  • Whiteside’s line (anteroposterior axis).
  • Transepicondylar axis (TEA).
  • Posterior condylar line (commonly 3° external rotation to compensate for 3° of tibial varus).
• Gap Balancing: Tibial resection is performed first perpendicular to its mechanical axis. The extension gap is balanced symmetrically with appropriate soft-tissue releases. The knee is then flexed to 90°, and femoral rotation is set parallel to the resected tibial surface under ligamentous tension.

Soft-Tissue Balancing

• Varus Deformity (Tight Medially):
  1. Resect medial osteophytes (tibia and femur).
  2. Release deep medial collateral ligament (MCL).
  3. Release posteromedial capsule.
  4. Subperiosteal release of superficial MCL from the tibia.
  5. Release semimembranosus insertion if persistent tightness.
• Valgus Deformity (Tight Laterally):
  1. Resect lateral osteophytes.
  2. Release posterolateral capsule (tight in extension).
  3. Release iliotibial band (tight in extension; perform pie-crusting or subperiosteal detachment).
  4. Release popliteus tendon (tight in flexion; preserve if possible to prevent subluxation).
  5. Release lateral collateral ligament (LCL) off the epicondyle if severe fixed deformity.

Gap Dynamics Rule of Thumb

• Tight in Extension Only: Distal femoral recut (resects more bone, opens extension space) or release posterior capsule.
• Tight in Flexion Only: Downsize femoral component (increases AP space) or increase posterior tibial slope.
• Tight in Flexion and Extension: Resect more proximal tibia or reduce polyethylene insert thickness.
• Loose in Flexion, Balanced in Extension: Upsize femoral component with posterior augments, or elevate joint line.

Prosthetic Design & Constraint Choices

• Cruciate-Retaining (CR): Preserves the PCL. Requires an intact PCL and minimal deformity; preserves bone stock and proprioception, but carries risk of paradoxical anterior femoral slide if PCL is loose, or excessive rollback and flexion tightness if PCL is tight.
• Posterior-Stabilised (PS): PCL is sacrificed; a tibial spine/cam mechanism provides femoral rollback. Indicated in PCL insufficiency, prior patellectomy, severe fixed deformities, or inflammatory arthropathy. Risks include cam-spine jump or patellar clunk syndrome.
• Constrained Non-Hinged (Condylar Constrained Knee - CCK): Tall tibial post with tight femoral box constraining varus-valgus and rotational forces. Indicated in moderate collateral laxity, MCL incompetence, or revision surgery.
• Rotating Hinge Knee: Provides complete mechanical constraint while permitting axial rotation. Indicated in massive bone loss, complete collateral absence, gross recurvatum, or severe flexion-extension gap mismatch.

Complications

• Periprosthetic Joint Infection (PJI): Occurs in 1–2% of primary cases. Managed according to International Consensus Meeting (ICM) criteria with DAIR (Debridement, Antibiotics, and Implant Retention) for acute presentations (<3–4 weeks from index surgery or <3 weeks of acute hematogenous symptoms with well-fixed components), or 1-stage vs 2-stage revision for chronic infection.
• Aseptic Loosening: Long-term failure mediated by ultra-high-molecular-weight polyethylene (UHMWPE) wear, macrophage activation, and osteolysis. Tibial loosening is more prevalent than femoral.
• Periprosthetic Fractures:
  • Supracondylar Femoral: Classified using the Rorabeck-Taylor system:
    • Type I: Undisplaced, stable prosthesis (non-operative or retrograde intramedullary nailing/plating).
    • Type II: Displaced, stable prosthesis (open reduction and internal fixation [ORIF] with locking plates or retrograde nail).
    • Type III: Unstable or loose prosthesis (requires revision arthroplasty with stemmed/modular components).
  • Tibial: Classified using the Felix classification based on anatomical location and component stability.
• Extensor Mechanism Complications: Patellar maltracking, subluxation/dislocation, patellar fracture (Goldberg classification), patellar clunk syndrome (fibrous nodule at the superior patellar pole catching in the femoral box during extension), and patellar tendon avulsion.
• Neurovascular Injury: Common peroneal nerve neuropraxia occurs in up to 3% of severe valgus or fixed flexion corrections. Treat immediately by releasing tight bandages, flexing the knee to 30°, and relieving nerve tension.
• Arthrofibrosis: Flexion <90° or FFD >10°. Addressed by early manipulation under anaesthesia (MUA) within 6–12 weeks of surgery; open/arthroscopic arthrolysis carries variable outcomes thereafter.
• Deep Vein Thrombosis (DVT) & Pulmonary Embolism (PE): Routine chemical and mechanical thromboprophylaxis is mandated per national protocols (e.g., NICE NG89).

Exam Pearls for the FRCS (Orth) / Exit Exams

• Whiteside's Line: Runs from the deepest point of the trochlear groove to the highest point of the intercondylar notch; perpendicular to the true transepicondylar axis. In severe trochlear dysplasia, Whiteside's line can be inaccurate; rely on the surgical transepicondylar axis.
• Joint Line Elevation: Excessive distal femoral resection combined with a thick polyethylene insert elevates the joint line. Consequences: mid-flexion instability, patella baja, impingement, and decreased range of flexion.
• Valgus Knee Correction Sequence: Remember the tight structures in extension (ITB, posterolateral capsule) versus flexion (popliteus). Resecting popliteus can destabilise the knee in flexion; preserve it if possible.
• Common Peroneal Nerve Palsy Post-Op: Immediately remove compressive dressings, splint the knee in flexion, and check for an expanding haematoma. Do not wait for electrodiagnostic studies.