Total Knee Replacement
Orthopaedics · Arthroplasty · lean revision notes
Total Knee Replacement
Total knee replacement (TKR), or total knee arthroplasty (TKA), is the surgical resurfacing of the femoral, tibial and (usually) patellar articular surfaces with prosthetic components to relieve pain and restore function in an arthritic, deformed or destroyed knee. For NEET PG, the high-yield zones are indications, component design philosophy (PCL-retaining vs PCL-substituting), the goal of restoring a neutral mechanical axis, the unicompartmental (Oxford) alternative, and complications — especially periprosthetic joint infection (PJI) and the Zimmerli/MSIS diagnostic criteria.
Definition & basic concept
A "total" knee replacement resurfaces all three compartments of the knee:
- Medial tibiofemoral
- Lateral tibiofemoral
- Patellofemoral (patellar resurfacing — sometimes selective)
The native knee is not a simple hinge — it has a screw-home mechanism, rollback, and combined rolling-gliding. Prosthetic design tries to reproduce stability while accepting some loss of native kinematics. The femoral component is typically cobalt-chromium, the tibial tray titanium with an ultra-high-molecular-weight polyethylene (UHMWPE) insert, and the patellar button is all-polyethylene. Most primary TKAs in India are cemented (PMMA bone cement).
High-yield: The articulating bearing surface that wears (and generates osteolysis-inducing debris) is the polyethylene insert, not metal-on-metal. UHMWPE wear debris → macrophage activation → osteolysis → aseptic loosening, the leading cause of long-term failure.
Indications & contraindications
The single most important indication is pain unresponsive to adequate non-operative treatment, with corresponding radiographic disease.
| Indications | Contraindications |
|---|---|
| Tricompartmental osteoarthritis with disabling pain | Active infection (local or systemic) — absolute |
| Rheumatoid / inflammatory arthritis | Extensor mechanism deficiency (non-functional quadriceps) |
| Post-traumatic arthritis | Recurvatum from muscle weakness (e.g., polio) — relative |
| Avascular necrosis of femoral condyle | Neuropathic (Charcot) joint — relative |
| Failed prior surgery (HTO, unicompartmental) | Poor soft-tissue envelope / non-compliance |
High-yield: Active sepsis is the absolute contraindication. A neuropathic (Charcot) joint and significant peripheral vascular disease are relative contraindications.
Non-operative measures that must be exhausted first: weight reduction, physiotherapy/quadriceps strengthening, NSAIDs/paracetamol, intra-articular steroid, and walking aids.
Coronal alignment — the core principle
The mechanical axis of the lower limb runs from the centre of the femoral head → centre of the knee → centre of the ankle. The goal of conventional ("mechanical alignment") TKA is to make the postoperative mechanical axis neutral (0°), i.e., passing through the centre of the knee.
Key angles to remember:
- Femoral component: cut perpendicular to the femoral mechanical axis. Because the anatomical (shaft) axis is ~5–7° valgus to the mechanical axis, the distal femoral cut is made in ~5–7° of valgus relative to the femoral shaft.
- Tibial component: cut perpendicular to the tibial mechanical axis (≈ tibial shaft axis) in the coronal plane.
- Posterior tibial slope: a few degrees of posterior slope (typically 3–7°) is built in to allow flexion and rollback.
Stepwise restoration of the mechanical axis:
Deformity assessment → bone cuts referencing mechanical axis → ligament/soft-tissue balancing (release the concave/tight side) → flexion & extension gap balancing → neutral mechanical axis, balanced gaps, stable knee.
High-yield: The aim of conventional TKA is a neutral mechanical axis (hip–knee–ankle angle ≈ 180°). Persistent malalignment (especially residual varus) accelerates polyethylene wear and early loosening.
Newer philosophies — kinematic alignment (reproduce the patient's pre-arthritic joint lines) — are increasingly discussed but mechanical alignment remains the textbook standard.
Component design: PCL-retaining vs PCL-substituting
This is the most tested design question. The two dominant designs differ in how they handle the posterior cruciate ligament (PCL) and femoral rollback.
| Feature | CR (Cruciate-Retaining) | PS / CS (Posterior-Stabilised / Substituting) |
|---|---|---|
| PCL | Preserved | Excised |
| Rollback mechanism | Native PCL produces femoral rollback | Cam-and-post (tibial spine + femoral cam) substitutes for PCL |
| Bone resection | Less femoral bone removed (no intercondylar box) | More bone removed (box cut for cam) |
| Ideal candidate | Intact, competent PCL; primary OA | Inflammatory arthritis, prior patellectomy, deficient/incompetent PCL, severe deformity |
| Flexion | Good, kinematics more "natural" | Reliable deep flexion via cam engagement |
| Specific complication | Mid-flexion instability if PCL imbalanced | Patellar clunk syndrome; cam-post dislocation/jump |
High-yield: In a PS knee, femoral rollback is produced by the cam-and-post mechanism, which substitutes for the resected PCL. In a CR knee, the native PCL provides rollback.
Increasing constraint hierarchy (examiners love the ladder):
CR → PS → Constrained condylar (CCK / TC3, for collateral insufficiency) → Rotating-hinge (for global instability / large bone loss / tumour). More constraint = more stress transferred to the bone–implant interface = higher loosening risk.
Mnemonic for choosing PS over CR — "PIPS": Prior Patellectomy, Inflammatory arthritis, PCL deficient/incompetent, Severe deformity needing PCL release.
Patellar management
- Patellar resurfacing with an all-poly button is done selectively. Always resurface in inflammatory arthritis. Selective resurfacing in OA is acceptable.
- Restore patellar tracking — avoid lateral maltracking. The "no-thumb test" checks tracking intra-op (patella should track centrally without thumb pressure).
- Component rotation matters: internal rotation of femoral/tibial components causes patellar maltracking and anterior knee pain.
High-yield: Internal malrotation of the femoral or tibial component is a leading cause of post-TKA patellofemoral complications (maltracking, subluxation, anterior knee pain).
Unicompartmental knee arthroplasty (Oxford UKA)
When disease is confined to one compartment (usually medial), a unicompartmental knee replacement resurfaces only that compartment. The Oxford mobile-bearing UKA is the classic prosthesis.
Classic indications (Kozinn & Scott–type criteria):
| Favourable | Unfavourable |
|---|---|
| Isolated anteromedial OA | Inflammatory arthritis |
| Intact ACL & PCL | ACL deficiency (esp. for mobile bearing) |
| Correctable deformity, varus <15° | Fixed/large deformity |
| Flexion contracture <15° | Flexion contracture >15° |
| ROM arc >90° | Patellofemoral inflammatory disease |
Advantages over TKA: smaller incision, bone-preserving, faster recovery, retained cruciates → more natural kinematics, lower blood loss. Drawback: a mobile bearing can dislocate, and progression of OA in other compartments may require revision.
High-yield: A functioning (intact) ACL is essential for Oxford UKA. Inflammatory arthritis is a contraindication — TKA is preferred there.
Complications
1. Periprosthetic joint infection (PJI) — most important
The most feared complication. Classified by timing:
- Early (<3 months): virulent organisms — Staphylococcus aureus, gram-negatives. Managed often by DAIR (Debridement, Antibiotics, Implant Retention) + polyethylene exchange if implant well-fixed.
- Delayed (3–24 months): low-virulence organisms — coagulase-negative staph (S. epidermidis), Cutibacterium — biofilm; usually needs implant removal.
- Late/haematogenous (>24 months): seeding from distant source.
Two-stage revision (resection + antibiotic-impregnated cement spacer → reimplantation) is the gold standard for chronic PJI.
Zimmerli / MSIS criteria
High-yield: Zimmerli's definition — PJI is confirmed by any one of:
- A sinus tract communicating with the prosthesis,
- Purulence around the prosthesis,
- Acute inflammation on histopathology of periprosthetic tissue,
- ≥2 cultures growing the same organism (or one culture of a virulent organism).
The MSIS (Musculoskeletal Infection Society) criteria add major (sinus tract; 2 positive cultures of same organism) and minor criteria (raised ESR & CRP; raised synovial WBC/leukocyte esterase; raised synovial neutrophil %; positive histology; single positive culture). Synovial fluid markers (alpha-defensin) are highly specific modern adjuncts.
Useful screening cut-offs (chronic PJI, knee):
- Serum CRP, ESR elevated (screening — high sensitivity).
- Synovial WBC > ~3000/µL and PMN > ~70–80% suggest infection.
2. Aseptic loosening
Leading cause of late failure overall. Driven by polyethylene wear debris → osteolysis. Radiolucent lines >2 mm, progressive, at the bone–cement interface.
3. Stiffness / arthrofibrosis
Inadequate ROM (target ≥ ~90° flexion for stairs/sitting). Risk factors: poor pre-op ROM, inadequate physiotherapy, component malposition, infection. Treated with manipulation under anaesthesia (MUA) within ~6–12 weeks if progress stalls.
4. Patellofemoral problems
Maltracking, anterior knee pain, patellar clunk syndrome (fibrous nodule catching in the femoral box of a PS implant during flexion–extension), patellar fracture, instability. Often traced to component malrotation.
5. Instability
Flexion instability, mid-flexion instability, or genu recurvatum from gap imbalance.
6. Periprosthetic fracture
Most commonly supracondylar femoral fracture above the femoral component; notching of the anterior femoral cortex is a risk factor.
7. Venous thromboembolism (VTE)
TKA is a high-risk procedure for DVT/PE. Routine mechanical + chemical thromboprophylaxis (LMWH, or oral factor Xa inhibitors / aspirin per protocol).
8. Others
Neurovascular injury (common peroneal nerve — at risk especially when correcting a fixed valgus + flexion deformity), wound problems, extensor mechanism rupture.
High-yield: The common peroneal nerve is most at risk during correction of a fixed valgus deformity with flexion contracture — watch for foot drop postoperatively; release the dressing/flex the knee.
Key differentials & decision-making (what to offer instead of TKA)
| Scenario | Preferred procedure |
|---|---|
| Young, active, isolated medial OA with varus, intact ACL | High tibial osteotomy (HTO) or medial UKA |
| Young, isolated lateral OA with valgus | Distal femoral osteotomy or lateral UKA |
| Isolated single-compartment OA, older patient, intact ligaments | Unicompartmental (Oxford) arthroplasty |
| Tricompartmental OA / inflammatory arthritis | Total knee arthroplasty |
| Severe instability, large bone loss, tumour | Constrained / rotating-hinge TKA |
| End-stage with failed fusion option | Arthrodesis (salvage) |
Rule of thumb: osteotomy for the young/active with unicompartmental disease and deformity; arthroplasty for the older patient or multicompartmental/inflammatory disease.
Investigations & workup
- Standing (weight-bearing) AP & lateral radiographs, skyline (Merchant) view for patellofemoral joint, and full-length hip-to-ankle alignment films to plan the mechanical axis.
- Templating for component sizing.
- Pre-op infection screen if any suspicion: ESR, CRP, joint aspiration with cell count & culture.
- Optimise comorbidities; treat distant infections (dental, urinary) before surgery.
High-yield: Full-length standing scanogram (hip–knee–ankle) is the investigation of choice for assessing limb alignment and planning the mechanical axis in TKA.
Management / "drug & device of choice"
- Procedure of choice for tricompartmental/inflammatory end-stage knee arthritis: cemented TKA with mechanical-axis alignment.
- Bearing/insert material: UHMWPE (often highly cross-linked to reduce wear).
- Thromboprophylaxis of choice: LMWH or oral factor Xa inhibitors per protocol.
- Chronic PJI gold standard: two-stage revision with an antibiotic-loaded cement spacer.
- Early PJI, well-fixed implant: DAIR + poly exchange + targeted antibiotics.
- Persistent stiffness: MUA within the early postoperative window.
Recently asked / exam angle
- PCL-substituting design uses a cam-and-post for rollback — repeatedly tested single-best-answer.
- Indication for choosing PS over CR — prior patellectomy, inflammatory arthritis, PCL deficiency.
- Oxford UKA needs an intact ACL — and inflammatory arthritis is a contraindication.
- Zimmerli criteria for PJI — sinus tract / pus / acute inflammation on histo / ≥2 same-organism cultures.
- Neutral mechanical axis as the alignment goal; full-length scanogram for planning.
- Common peroneal nerve injury with valgus + flexion deformity correction → foot drop.
- Aseptic loosening from polyethylene wear = commonest late cause of failure.
- Patellar clunk syndrome is specific to PS (boxed) designs.
- HTO vs TKA decision in the young active patient with unicompartmental varus OA.
- Supracondylar periprosthetic femur fracture + anterior femoral notching.
Rapid revision
- Active infection is the only absolute contraindication to TKA.
- Goal of conventional TKA = neutral mechanical axis (HKA ≈ 180°); plan with a full-length scanogram.
- Distal femoral cut ≈ 5–7° valgus to the femoral shaft (anatomical) axis.
- CR = PCL kept, native rollback; PS = PCL excised, cam-and-post rollback.
- Choose PS for: prior Patellectomy, Inflammatory arthritis, PCL deficiency, Severe deformity ("PIPS").
- Constraint ladder: CR → PS → CCK → rotating hinge (more constraint = more interface stress).
- Oxford UKA needs an intact ACL; contraindicated in inflammatory arthritis.
- Aseptic loosening from polyethylene wear/osteolysis = commonest cause of late failure.
- Zimmerli PJI: sinus tract, pus, acute inflammation on histology, or ≥2 same-organism cultures.
- Chronic PJI → two-stage revision with antibiotic cement spacer; early PJI well-fixed → DAIR.
- Common peroneal nerve at risk in valgus + flexion-contracture correction → foot drop.
- Patellar clunk syndrome is unique to PS (boxed) designs; component internal malrotation causes patellofemoral pain/maltracking.