Medial parapatellar approach (the workhorse exposure) with gap balancing for end-stage knee arthritis · advanced arthroplasty procedure
- Indication is end-stage knee arthritis (osteoarthritis, rheumatoid, post-traumatic, avascular necrosis) with bone-on-bone radiographic changes that has failed conservative management (six months of physiotherapy, weight loss, NSAIDs and injections) and is limiting activities of daily living.
- The TIBIAL CUT is the most critical cut for overall limb alignment. Set the extramedullary guide to the centre of the ankle (second ray) and the tibial tubercle anteriorly, cut perpendicular to the tibial mechanical axis (0° varus/valgus), apply 3-7° posterior slope to match the implant, and resect 8-10mm from the LEAST worn side.
- FEMORAL ROTATION uses three references that should agree — the transepicondylar axis (gold standard), Whiteside's line (perpendicular to the AP axis) and the posterior condylar axis plus 3° external rotation. If they disagree, prioritise the transepicondylar axis. Internal rotation causes patellar maltracking and medial-side flexion instability.
- GAP BALANCING demands extension and flexion gaps that are EQUAL (9-10mm, matching the polyethylene) and RECTANGULAR (equal medial and lateral opening). Balance with sequential medial release for a varus knee (80% of cases) and sequential lateral release for a valgus knee (20%); never complete a collateral release.
- Popliteal vessels lie only 10-15mm posterior to the posterior tibial cortex — stay anterior with the saw during the tibial cut. Anterior femoral notching increases periprosthetic fracture risk roughly ten-fold, so always leave 3-5mm of bone anterior to the cut.
When & Why
Indication. Symptomatic end-stage knee arthritis that has failed a fair trial of non-operative care. Absolute indications are bone-on-bone radiographic changes (joint-space narrowing, osteophytes, subchondral sclerosis, bone-onbone articulation) after at least six months of conservative management (physiotherapy, weight loss, NSAIDs, intra-articular injections), with pain limiting activities of daily living, and a patient who understands the procedure and will comply with rehabilitation. Relative indications include inflammatory arthritis (rheumatoid, psoriatic) with joint destruction, post-traumatic arthritis with malunion or nonunion, osteonecrosis with collapse and secondary arthritis, and neuropathic arthropathy (Charcot knee), where a higher-constraint implant is considered. Contraindications. Absolute: active infection (knee or systemic bacteraemia), insufficient bone stock for fixation, an absent extensor mechanism (consider arthrodesis or megaprosthesis), and severe peripheral vascular disease (risk of limb loss). Relative: poor skin condition (scarring, ulceration, stasis dermatitis), a neuropathic joint needing higher constraint, morbid obesity (BMI greater than 40) with higher complication rates, young age (less than 55 years) with higher revision rates, active smoking (wound complications roughly three times higher), and poorly controlled diabetes (HbA1c greater than 8%, an infection risk). Pre-operative assessment. A full history (pain pattern, functional limitation, previous treatments and injections) and examination (deformity in varus or valgus, fixed flexion contracture, range of motion, ligament stability, extensor mechanism integrity, skin condition, pulses and neurology). Templating determines component sizes, assesses bone quality and plans for defects. Medically optimise the patient — cardiology clearance if indicated, diabetes to HbA1c less than 8%, smoking cessation, dental clearance, and nutritional status (albumin and pre-albumin). Radiographic assessment. Weight-bearing AP and lateral knees (joint-space narrowing, osteophytes, subchondral sclerosis or cysts, bone defects), a long-leg hip-knee-ankle alignment film (mechanical axis deviation, constitutional varus or valgus), and a skyline patella view (patellofemoral arthritis, dysplasia, tilt or subluxation). Assess deformity magnitude, bone defects needing grafting, any extra-articular deformity, and ligament competency. Implant selection — the one decision that shapes the operation. Every primary TKR uses the same exposure and the same bone cuts; what varies is the constraint and fixation:
The workhorse when the PCL is intact. Less constraint, allows physiological femoral rollback via the posterior cruciate ligament. Usually needs slightly more posterior tibial slope (5-7°).
Chosen when the PCL is deficient, in inflammatory arthritis, after previous patellectomy, or for a fixed flexion contracture greater than 15°. A cam-and-post mechanism provides rollback. Less posterior slope required (3-5°).
Semi-constrained (CCK) for ligament incompetence; rotating-hinge for severe instability, gross deformity or revision. Sacrifices collaterals and transfers load to the stem-cement interface — higher failure rate.
Fixation. Cemented fixation is the gold standard for primary TKR (the best long-term registry survival data). Cementless fixation is considered in younger, active patients but remains controversial; hybrid fixation (cementless femur, cemented tibia) is rarely used. Patellar resurfacing. Always resurface in inflammatory arthritis, with significant patellar wear or eburnation, and with patellofemoral symptoms or maltracking. Selective resurfacing (surgeon preference) is defensible in osteoarthritis with minimal wear, good tracking and a low-demand patient; very thin patellae (less than 15mm) are rarely resurfaced as the fracture risk is prohibitive. Consent and counselling. Set realistic expectations: pain relief in 90-95%, improved function but NOT a normal knee, target range of motion 0-120° (may not achieve full flexion), activity modification to avoid high-impact sports, implant survival around 90-95% at 10 years, and the risks — infection 1-2%, venous thromboembolism 2-4%, stiffness 5-10%, and revision risk 5-10% at 10 years. Setup. Supine on the operating table with the operative leg free to flex through full range of motion (drop the foot of the table or use a leg holder). A lateral post at hip level stops the leg falling off the table; a small bump under the ipsilateral hip (15-20°) externally rotates the leg. Apply the tourniquet as high on the thigh as possible to maximise working space. Inflating to 300mmHg or limb occlusion pressure plus 100mmHg after exsanguination (elevation or Esmarch), keep tourniquet time under two hours, and deflate before closure for haemostasis.
The Operation
The goal: expose the knee through the medial parapatellar approach, remove all osteophytes, make the tibial and femoral cuts in correct alignment and rotation, balance equal rectangular flexion and extension gaps, resurface the patella when indicated, and cement the components in the correct rotation and alignment. The exposure is laid out in full as the opening steps (and in depth on the medial parapatellar approach to the knee page).
Operative sequence
- Supine, leg free, lateral post at the hip, tourniquet high on the thigh, bump under the hip for external rotation.
- Exsanguinate and inflate the tourniquet (300mmHg or LOP plus 100mmHg); keep total tourniquet time under two hours.
- Drape the leg free including the foot so the whole limb can be manipulated and overall alignment assessed through full range of motion.
- Confirm the full tray is available: cutting guides, trials, spacer blocks, third-generation antibiotic cement, pulsatile lavage and fresh saw blades.
- Palpate and mark the superior and inferior poles of the patella, the tibial tubercle, the joint line (about 1cm below the inferior pole of the patella), the medial femoral epicondyle and the fibular head.
- Make a midline longitudinal incision of 12-15cm — from 5cm above the superior pole of the patella to 2-3cm below the tibial tubercle — deepening straight down to capsule in full-thickness flaps.
- A midline incision is critical: it allows extension for future revision, gives access to either side, and preserves blood supply. If previous incisions exist, always use the most LATERAL one to preserve the medial blood supply.
- Extend the arthrotomy proximally 4-5cm into the quadriceps tendon, 2-3mm medial to the patella, cutting in line with the vastus medialis fibres and avoiding the muscle belly.
- Continue along the medial border of the patella, staying ON BONE to protect the medial blood supply, down to the inferior pole.
- At the inferior pole curve slightly medially and run 2-3mm medial to the patellar tendon; at the tibial tubercle STAY DIRECTLY ON BONE to protect the tendon insertion — avulsion here is a catastrophic complication.
- Extend the arthrotomy posteriorly along the proximal medial tibia, deep to the MCL fibres, to allow patellar eversion.
- Evert the patella laterally with gentle traction (never force it) and flex the knee to 90°; release the medial gutter to the fat pad to create a pocket for the everted patella. If eversion is difficult, do NOT force it — sublux instead, or consider a lateral retinacular release (cautiously), a tibial tubercle osteotomy (stiff or revision cases), or converting to a subvastus approach if still early.
- Remove loose bodies, assess the PCL (if considering a CR design), the menisci and cartilage, and identify all osteophytes.
- Remove ALL osteophytes before the bone cuts (they distort soft-tissue balance): femoral (anterior, medial, lateral, posterior with a curved osteotome, intercondylar), tibial (peripheral, intercondylar eminence preserving the PCL if CR), and patellar (inferior pole, peripheral margin). Clear meniscal remnants, remove the ACL remnant, and preserve or sacrifice the PCL according to design.
- Identify the rotational landmarks now (see Step 7): the transepicondylar axis, Whiteside's line and the posterior condylar axis.
- Set up the extramedullary guide: distally aim at the centre of the ankle (the second ray, just medial to the tibialis anterior tendon); anteriorly align to the tibial tubercle (medial third in varus knees); in the coronal plane the guide must be perpendicular to the mechanical axis (0° varus/valgus).
- Set the posterior slope to 3-7° to match the implant — more slope for CR designs (5-7°, for rollback) and less for PS designs (3-5°, as the cam provides rollback).
- Measure the resection from the LEAST worn side (the lateral plateau in a varus knee, the medial in a valgus knee) and resect 8-10mm, restoring the joint line. Pin the guide (2-3 pins) and recheck the distal alignment after pinning.
- Cut with the oscillating saw from anterior to posterior, STAYING ANTERIOR — the popliteal vessels are only 10-15mm posterior to the posterior cortex. Verify the cut is perpendicular in the coronal plane and carries the correct posterior slope, with no residual bone bridge.
- Entry point for the intramedullary guide is 1cm anterior to the PCL insertion, centred in the intercondylar notch and in line with the canal; in a bowed femur use an anterolateral entry to avoid perforating the anterior cortex.
- Set the valgus cut angle to 5-7° (5-6° in most systems) to compensate for the 6° anatomic valgus of the distal femur and produce a cut perpendicular to the femoral mechanical axis.
- Resect 9-10mm from the prominent condyle (lateral in varus, medial in valgus) aiming for equal resection from both condyles; pin the guide and cut both condyles smoothly, protecting the collateral origins 1cm proximal to the joint line.
- Verify both condyles were resected equally and the cut is smooth and flat.
- Size the femur anteroposteriorly to avoid anterior overhang (patellofemoral overstuffing, stiffness) and undersizing (flexion instability, posterior impingement). Leave 3-5mm of bone anterior to the anterior flange and NO notching.
- Set rotation from three references that should agree: (1) the transepicondylar axis — the component parallel to a line from the medial epicondylar sulcus to the lateral epicondylar prominence (gold standard, unaffected by condylar wear); (2) Whiteside's line — the component perpendicular to the AP axis from the deepest trochlear groove to the centre of the intercondylar notch; (3) the posterior condylar axis plus 3° external rotation.
- If the references disagree (dysplastic or worn condyles, common in valgus knees) PRIORITISE the transepicondylar axis. Internal rotation causes patellar maltracking, anterior knee pain and a tight medial flexion gap with a loose lateral one; excessive external rotation reverses that imbalance.
- Seat the 4-in-1 cutting block at the correct AP size and rotation and pin it to the distal femoral cut.
- The block determines the anterior cut, both posterior condyle cuts and the anterior and posterior chamfers.
- Before the anterior cut confirm 3-5mm of bone remains anterior (no notching); if notching threatens, downsize the femur or accept slight anterior overhang.
- Complete both posterior condyle cuts — equal resection creates a rectangular flexion gap, asymmetric resection a trapezoidal one — then the chamfers. Trial the femoral component: it should sit flush without rocking, with no notching and correct rotation.
- Insert a spacer block (usually 9-10mm) between the distal femur and proximal tibia, extend the knee fully and apply varus and valgus stress. The goal is a RECTANGULAR extension gap — equal medial and lateral space, tight enough to be stable and loose enough to extend fully.
- For a VARUS knee (80% of cases): release sequentially from the tibial side — deep MCL fibres, then staged superficial MCL, pes anserinus tendons, semimembranosus, and finally the posteromedial capsule (dangerous — popliteal vessels posterior). Reassess the gap after each release and never completely release the MCL.
- For a VALGUS knee (20%): release sequentially laterally — lateral capsule and synovium, popliteus tendon, posterior fibres of the iliotibial band (PIE), the LCL very carefully, and the lateral head of gastrocnemius if needed. Consider staged correction for deformity greater than 20° and protect the common peroneal nerve (2-3cm distal to the fibular head).
- For a fixed flexion contracture: release the posterior capsule from its femoral insertion and excise posterior osteophytes (often the major contributor); if a CR knee remains tight, pie-crust the PCL or convert to a PS design.
- Flex the knee to 90°, insert the same spacer block and apply varus and valgus stress. The flexion gap must EQUAL the extension gap and be rectangular.
- Flexion gap TIGHT: downsize the femoral component (most common), resect more posterior femur (check anterior notching first), release the posterior capsule, or use a thinner polyethylene.
- Flexion gap LOOSE: upsize the femoral component, use a thicker polyethylene, or move to higher constraint.
- In a CR knee assess PCL tension — snug but not tight at 90°. Too tight limits flexion and raises contact stress (recess or convert to PS); too loose allows paradoxical anterior slide and instability (thicker poly or convert to PS).
- Medial-lateral flexion imbalance usually reflects femoral rotation — a tight medial gap with a loose lateral gap means internal rotation; revise the rotation before reaching for collateral releases.
- Trial the femoral component (correct size and rotation, flush, no overhang or notching), the tibial baseplate (covers the cut with under 2mm overhang, keel on axis) and the polyethylene insert (thickness from the gap balancing).
- Check all seven parameters before cementing: (1) ALIGNMENT — an alignment rod from the femoral head to the ankle centre passes through the knee centre; (2) STABILITY — stable in extension, mid-flexion (30-60°) and at 90° (small laxity acceptable, no gross instability); (3) RANGE OF MOTION — full extension and at least 120° flexion, smooth with no catching; (4) PATELLAR TRACKING — a positive "no-thumb test", the patella centred through the range; (5) JOINT LINE — within 8mm of anatomic (elevation over 8mm gives patella baja and extensor lag); (6) FLEXION-EXTENSION GAP BALANCE — equal and rectangular throughout, no opening over 2mm with stress; (7) SOFT-TISSUE BALANCE — equal tension medial and lateral. Revise anything unsatisfactory before final implantation.
- Resurface when indicated (inflammatory arthritis, significant wear or eburnation, patellofemoral symptoms, maltracking, young high-demand patients). Measure native thickness (typically 24-26mm) with a caliper.
- Clamp the patella and resect to leave 12-15mm of residual bone (minimum 12mm — thinner risks fracture); cut parallel to the anterior surface. Drill the three peg holes (or central peg) with depth stops to avoid anterior perforation.
- Trial the button: the median ridge aligns with the trochlear groove, the lateral facet with the lateral trochlea, no overhang. Restore native thickness (bone plus button equal to 24-26mm, within 2mm of native) — overstuffing causes pain, stiffness and reduced motion.
- Prepare all bone surfaces with a minimum of 3L pulsatile lavage and dry them. Mix third-generation antibiotic cement (gentamicin, about 40mg per 40g) under vacuum; work in the doughy phase (3-5 minutes of working time) with all components ready.
- Cement the TIBIAL component first (better penetration supine with the knee flexed): apply cement to the cut surface and the baseplate, pressurise into cancellous bone, insert in correct rotation and hold under firm pressure.
- Cement the FEMORAL component second: apply cement to all cut surfaces and the component, pressurise, insert in correct rotation and hold.
- Cement the PATELLAR button third if resurfacing, ensuring median-ridge alignment, and clamp it.
- Remove ALL excess cement while still workable — posterior cement is the most critical (impingement, vessel compression, third-body wear). Hold the components under pressure for 8-12 minutes (cure time) with the knee flexed 20-30°.
- After cure, remove the trial poly, confirm cement interdigitation and remove any debris with a final 3L lavage (particulate debris causes third-body wear).
- Insert the final polyethylene at the trialed thickness and lock it (verify with a pull test). Re-check range of motion (0-120°), stability in extension and at 90°, patellar tracking (no-thumb test), alignment, and smooth motion throughout.
- Release the tourniquet (or do so after closure, per preference) and achieve meticulous haemostasis with electrocautery. Tranexamic acid (1-3g in 100mL saline intra-articular) reduces blood loss by 30-50%; avoid it with previous VTE, coronary stents under one year, or stroke.
- Close the medial parapatellar arthrotomy with interrupted number-1 Vicryl — critical for extensor mechanism integrity, with a strong repair at the tibial tubercle (the avulsion site). Ensure a tension-free closure that does not limit flexion; reduce the patella into the trochlea and confirm it sits centrally.
- Close the subcutaneous layer (2-0 Vicryl) to obliterate dead space, then a running subcuticular 3-0 or 4-0 Monocryl for skin, reinforced with glue or Steri-Strips and a waterproof dressing. Drains are not routine (no benefit and higher infection risk with modern TXA use).
- Apply a compression dressing (TED stocking or bandage) and cryotherapy. The knee is stable and the patient mobilises weight-bearing-as-tolerated on day zero.
The popliteal artery and vein lie only 10-15mm posterior to the posterior tibial cortex. Stay anterior with the oscillating saw during the tibial cut, use retractors (not blind posterior release) for the posteromedial capsule, and remove all posterior cement. The common peroneal nerve winds around the fibular neck 2-3cm distal to the fibular head and is most at risk from a tight lateral post, an aggressive valgus correction (consider staging deformity greater than 20°), and lateral releases — pad the post, correct gradually, and stay anterior to the fibular head.
Anterior notching increases the risk of periprosthetic supracondylar fracture roughly ten-fold. Always confirm 3-5mm of bone remains anterior to the anterior flange. If notching threatens — common with a small femur, anterior femoral bowing or an oversized component — downsize the femur or accept slight anterior overhang rather than notch.
The medial parapatellar is the default because it is extensile, revisable and works in almost every knee. The SUBVASTUS approach elevates vastus medialis off the septum without incising it — better patellar blood supply, faster recovery and less anterior knee pain, but exposure is limited in muscular or obese patients. The MIDVASTUS approach splits the vastus medialis obliquus in line with its fibres — a compromise with slightly better quadriceps function. The LATERAL parapatellar is reserved for severe valgus deformity (over 20°) or previous lateral incisions, giving access to lateral structures for release but with higher patellar devascularisation risk.
Aftercare & Complications
Rehabilitation. Cemented TKR allows immediate weight-bearing. Early, aggressive range-of-motion physiotherapy is the single most important factor in avoiding stiffness; combine it with multimodal analgesia (adductor canal block, acetaminophen, NSAIDs, opioids) and VTE prophylaxis. | Phase | Timing | Weight-bearing and ROM target | Focus | |-------|--------|-------------------------------|-------| | Day zero to one | 0-2 days | Weight-bearing as tolerated; ankle pumps, quad sets, straight-leg raises; target 60° flexion, full extension | Multimodal analgesia, VTE prophylaxis, first mobilisation with a frame | | Early discharge | 2-7 days | Walker or frame; stairs with a rail; ROM 0-90° by discharge | Pain control on oral meds, safe mobilisation, dry wound | | Early recovery | 2-6 weeks | Wean from walker to cane; ROM 0-110°; driving at 4-6 weeks (right knee, off narcotics), 2-3 weeks (left knee) | Progressive ROM and strengthening; wound review and suture removal at 10-14 days | | Six-week review | 6 weeks | ROM 0-110° minimum; baseline weight-bearing AP, lateral and skyline radiographs | Confirm healing, stability and alignment | | Intermediate | 6-12 weeks | ROM 0-120° by 12 weeks; walking unlimited, swimming, stationary bike | Proprioception and balance; return to sedentary work at 6-8 weeks, physical work by 12 weeks | | Long-term | over 3 months | Low-impact activities unlimited; defer high-impact running and jumping | Annual radiographs to screen for loosening; infection vigilance; fall prevention | Expected outcomes. About 90-95% achieve significant pain relief, 80-85% are satisfied (lower than hip arthroplasty, hence the emphasis on realistic expectations), average range of motion is 0-120° (dependent on pre-operative ROM), and implant survival is roughly 90-95% at 10 years and 80-85% at 20 years. Complications
- Recognition
- Early (under 3 months): S. aureus, Gram-negatives. Late (over 3 months): coagulase-negative staphylococci, Cutibacterium. Persistent pain, swelling, warmth, erythema, wound drainage, fever; raised CRP/ESR (concerning after six weeks). Aspiration: WBC greater than 3000, PMN over 80%, positive culture (gold standard, 20% false-negative); intra-op purulence or over 5 PMN per high-power field on frozen section
- Prevention
- Screen and eradicate S. aureus (nasal swab, mupirocin), dental clearance, optimise nutrition and diabetes; prophylactic IV cefazolin 2g (3g if over 120kg) or vancomycin/teicoplanin if MRSA risk or beta-lactam allergy, within 60 minutes of incision and re-dosed every 4 hours; chlorhexidine prep, laminar flow, antibiotic cement, meticulous haemostasis, gentle soft-tissue handling
- Management
- Early (under 3 months, stable implants): irrigation and debridement, polyethylene exchange, retain components, six weeks organism-specific IV antibiotics (50-70% success if under three weeks and susceptible). Late (over 3 months or loose): TWO-STAGE revision (gold standard, about 90% success) — explant, debridement, antibiotic spacer, six weeks IV antibiotics, reimplantation once ESR/CRP normalise. Chronic suppression for non-surgical candidates
- Recognition
- DVT: calf pain or swelling, duplex ultrasound (95% sensitive for proximal, 50-70% distal). PE: dyspnoea, tachypnoea, chest pain, hypoxia; CTPA is the gold standard, V/Q if contrast-allergic. Wells score for pre-test probability
- Prevention
- Mechanical for all: early mobilisation on day zero to one (most effective), TED stockings and sequential compression, hydration. Chemical, risk-stratified: aspirin 325mg twice daily for six weeks for standard risk, OR LMWH (enoxaparin 40mg daily) for 2-4 weeks, OR a Factor Xa inhibitor; therapeutic agents for previous VTE, thrombophilia or active malignancy
- Management
- Symptomatic DVT or PE: therapeutic anticoagulation (LMWH, warfarin INR 2-3, or a DOAC) for at least three months. Proximal DVT carries higher PE risk (consider an IVC filter if anticoagulation is contraindicated); distal DVT may be observed with serial ultrasounds if asymptomatic. Massive PE with instability: anticoagulation, haemodynamic support, thrombolysis or embolectomy
- Recognition
- Progressive loss of ROM or failure to regain it; flexion under 90° at three months is arthrofibrosis until proven otherwise. Differentials include component malposition, overstuffing, infection (rule out), undiagnosed instability and CRPS
- Prevention
- Adequate soft-tissue releases, restore the joint line (elevation over 8mm is a risk factor), avoid overstuffing, PCL balancing; post-operatively, early aggressive ROM physiotherapy, adequate analgesia and early mobilisation
- Management
- ROM under 90° at six weeks: intensive physiotherapy and consider manipulation under anaesthesia in the 6-12 week window (gentle, to 120°, avoiding supracondylar fracture). Refractory at 3-6 months despite MUA: arthroscopic or open debridement of scar and the anterior interval. Revision to address the underlying cause (malposition, overstuffing, joint-line elevation) if persistent
- Recognition
- Acute fall or trauma with pain, deformity, inability to weight-bear; insidious progressive pain or mechanical symptoms. Radiograph the fracture line and assess component stability (well-fixed versus loose dictates treatment); CT if occult
- Prevention
- Avoid anterior femoral notching (the most critical factor — roughly ten-fold risk), preserve bone stock, avoid oversized components and IM-rod perforation, treat osteoporosis, and counsel on fall prevention
- Management
- By location and stability: proximal to the component — ORIF with plates and cables; at the component level — ORIF with a locking plate and cables if well-fixed, revision TKR with stems plus or minus allograft if loose; distal to the component — ORIF. Patellar — partial patellectomy if the extensor mechanism is intact, repair if displaced with adequate bone. Use long stems bypassing the fracture by two cortical diameters
- Recognition
- Fracture: trauma or insidious pain, crepitus, extensor lag if disrupted. Maltracking: anterior knee pain, lateral tilt or subluxation on merchant views, a J-sign. Clunk: painful catching on extension from flexion (PS designs) as a fibrous nodule catches the femoral box. AVN: progressive pain with radiographic sclerosis or collapse
- Prevention
- Fracture: avoid over-resection (minimum 12mm residual) and preserve blood supply (no circumferential lateral release). Maltracking: correct femoral rotation (TEA reference) and median-ridge alignment, lateral release only if maltracking persists after balancing. Clunk: meticulous synovectomy around the patellar tendon
- Management
- Non-displaced fracture with an intact extensor mechanism — conservative (brace, non-weight-bearing, gentle ROM). Displaced or disrupted — ORIF (tension-band, screws, cables) or partial patellectomy. Maltracking — physiotherapy and bracing, arthroscopic lateral release if refractory, revision of malposition. Clunk — arthroscopic debridement (about 95% success). AVN — observe if stable, revise if collapse or loosening
- Recognition
- Start-up or activity pain, mechanical symptoms; progressive radiolucent lines (over 2mm at the bone-cement interface is concerning), subsidence or migration, periprosthetic osteolysis on serial radiographs. Aspirate to RULE OUT infection before revision
- Prevention
- Optimal cement technique (pressurisation, third-generation cement, 2-5mm penetration), correct sizing and alignment (HKA 0° plus or minus 3°), highly cross-linked polyethylene, and meticulous gap balancing
- Management
- Painful loose components — revision TKR, assessing bone loss by AORI grade (I minimal, II moderate, III severe with ligament incompetence) and planning stems (bypass defects by four cortical diameters), augments or allografts, and higher constraint (CCK) for ligament incompetence. Osteolysis without loosening — observe versus curettage and grafting
- Recognition
- Subjective giving way or pain; opening with varus or valgus stress (over 5mm is concerning). Flexion instability opens at 90° but is stable in extension; extension instability opens throughout. Assess for component malrotation, malalignment and joint-line elevation (over 8mm gives mid-flexion laxity)
- Prevention
- Meticulous gap balancing (equal rectangular extension and flexion gaps), correct component rotation, restore the joint line within 8mm, avoid complete collateral release, PCL balancing in CR designs, and adequate polyethylene thickness (9-10mm minimum)
- Management
- Mild (under 5mm, asymptomatic) — conservative. Symptomatic mild — thicker poly if space allows. Moderate (5-10mm) — thicker poly or a CCK (allows 2-3° varus/valgus). Severe (over 10mm or dislocation) — a hinged prosthesis. Address the underlying cause (rotation, joint line) at revision
Viva & Exam Focus
CATSTibial cut alignment — 'CATS'
TEWFemoral rotation references — 'TEW'
The structures at risk throughout the operation: the popliteal artery and vein (10-15mm posterior to the posterior cortex — tibial cut, posterior release, posterior cement); the tibial nerve (deep in the popliteal fossa — deep posterior release, tourniquet); the common peroneal nerve (at the fibular neck — lateral post, valgus correction, lateral release); the patellar tendon insertion at the tibial tubercle (arthrotomy, eversion — stay on bone); and the anterior femoral cortex (notching — ten-fold fracture risk).
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“Walk me through femoral component rotation. What references do you use and which matters most?”
“You are balancing a varus knee. Walk me through your sequential medial release — when do you stop and what are the danger points?”
“Explain gap balancing. Why does it matter and what do you do if the flexion and extension gaps do not match?”
Indication
- End-stage knee arthritis (OA, RA, post-traumatic, AVN) with bone-on-bone changes after six months of failed conservative care and pain limiting ADLs
- Contraindicated by active infection, poor bone stock, absent extensor mechanism or severe PVD
Exposure
- Medial parapatellar arthrotomy is the workhorse; stay ON BONE at the tibial tubercle to avoid tendon avulsion
- Alternatives: subvastus (blood supply, recovery) and lateral (severe valgus)
- At risk: popliteal vessels, common peroneal nerve, patellar tendon insertion
The cuts
- Tibial cut (CATS): centre of ankle, anterior to tubercle, 3-7° posterior slope, 8-10mm from least worn side
- Distal femur: IM guide, 5-7° valgus, 9-10mm from the prominent condyle
- Femoral rotation (TEW): TEA gold standard, 3° ER from PCA, Whiteside's perpendicular — prioritise TEA if discrepant
Balancing
- Extension and flexion gaps equal (9-10mm) and rectangular throughout ROM
- Varus (80%): sequential medial release; valgus (20%): sequential lateral release
- Never complete a collateral release — use thicker poly or higher constraint
Finish
- Patellar resurfacing when indicated; minimum 12mm residual bone
- Cement tibia, then femur, then patella; pressurise and remove all posterior cement
- Capsular closure with number-1 Vicryl is critical for the extensor mechanism
Danger zones
- Popliteal vessels 10-15mm posterior to the tibial cortex — stay anterior
- Common peroneal nerve at the fibular neck — pad the post, correct gradually
- Anterior femoral notching — ten-fold supracondylar fracture risk
Background & Evidence
Place of the operation. Total knee replacement is one of the most performed and most studied elective orthopaedic procedures, and the benchmark against which joint-replacement outcomes are measured. It is reserved for end-stage arthritis that has exhausted non-operative care; earlier-stage disease is managed with activity modification, weight loss, physiotherapy, analgesia, intra-articular injections and — for isolated compartment disease — unicompartmental arthroplasty or osteotomy. Alignment philosophy. Restoration of a neutral mechanical axis (hip-knee-ankle angle 0° plus or minus 3°) is the foundation of longevity. The classic "mechanical alignment" school aims for a neutral tibial and femoral cut; contemporary "kinematic" and "restricted kinematic" approaches respect the patient's native constitutional alignment and joint-line obliquity. Both demand the same disciplined bone cuts and gap balancing — the difference is the target alignment. Ritter's large cohort showed failure was lowest (0.2%) when both components were neutrally oriented and rose to 8.7% with a tibial component under 90° to the axis combined with a femoral component in 8° or more valgus, and that BMI independently increased failure. The femoral rotation question. Berger's anatomic study of 75 cadaveric femora defined the surgical transepicondylar axis and showed the posterior condylar angle averaged 3.5° internal rotation in males and 0.3° in females — the basis for the "plus 3° external rotation from the posterior condylar axis" rule, and the reason that rule is sex-dependent and unreliable with asymmetric wear. Cemented versus cementless. Cemented fixation remains the gold standard for primary TKR, supported by the best long-term registry survival data. Cementless fixation is considered in younger, active patients but is controversial — the survival data are less mature, and the trade-off is the risk of osseointegration failure against the theoretical advantage of biologisable fixation. Patellar resurfacing. The selective-versus-routine debate persists. Bourne's randomised trial of 100 osteoarthritic knees found the non-resurfaced group had less pain and better flexor torque at two years, but two non-resurfaced knees needed reoperation for anterior knee pain — establishing that selective resurfacing is defensible in osteoarthritis while resurfacing reduces the anterior-knee-pain reoperation rate and remains mandatory in inflammatory arthritis. Outcomes and registries. Large national joint registries (the National Joint Registry for England, Wales, Northern Ireland and the Isle of Man; the American Joint Replacement Registry; the Australian Orthopaedic Association National Joint Replacement Registry; the Swedish Knee Arthroplasty Register; and the New Zealand Joint Registry) consistently report roughly 95% cumulative survival at 10 years for primary cemented TKR, with infection, aseptic loosening and instability the leading causes of revision. Lombardi's contemporary failure-mode analysis ranked instability (about 21% of revisions), aseptic loosening (16%), infection (16%) and polyethylene wear (10%) as the leading drivers, with technical errors — component malposition, malalignment and soft-tissue imbalance — contributing to the majority.
References
Determining the rotational alignment of the femoral component in total knee arthroplasty using the epicondylar axis
- Anatomic study of 75 cadaveric femora defining the surgical transepicondylar axis from the lateral epicondylar prominence to the medial epicondylar sulcus
- Mean posterior condylar angle (PCA to TEA) was 3.5° internal rotation in males and 0.3° in females — the basis for the 'plus 3° external rotation from the posterior condylar axis' rule
- TEA is a reproducible reference that remains valid when posterior condyles are worn or hypoplastic
The effect of alignment and BMI on failure of total knee replacement
- Retrospective cohort of 6070 knees: failure was lowest (0.2%) when both tibial and femoral components were neutrally oriented
- Failure rose to 8.7% with a tibial component under 90° to the tibial axis combined with a femoral component in 8° or more valgus
- Higher BMI independently increased failure rate (up to 7.1% in valgus knees in patients with BMI 41 or above)
Rationale of the Knee Society clinical rating system
- Introduced the Knee Society Score (KSS), splitting assessment into a Knee Score (pain, stability, range of motion) and a separate Function Score (walking, stairs)
- The dual structure prevents declining patient mobility from artificially lowering the implant-specific score
- Became the most widely used objective outcome instrument for TKR worldwide
Resurfacing versus not resurfacing the patella during total knee replacement
- Randomised controlled trial of 100 osteoarthritic knees comparing patellar resurfacing with retention using the same implant
- At two years the non-resurfaced group had significantly less pain and better knee-flexor torque, but two non-resurfaced knees required reoperation for anterior knee pain
- Selective (rather than universal) resurfacing is defensible in osteoarthritis, while resurfacing reduces the reoperation rate for anterior knee pain
Venous thromboembolism prophylaxis after major lower-limb arthroplasty (AAOS and NICE NG89 / BOA)
- Mechanical prophylaxis and early mobilisation are recommended for all patients undergoing TKR
- Risk-stratified pharmacological prophylaxis: aspirin is an accepted option for standard-risk patients, with LMWH or direct oral Factor Xa inhibitors as alternatives
- Higher-risk patients (previous VTE, thrombophilia, active malignancy) warrant therapeutic-intensity agents
The anteroposterior axis for femoral rotational alignment in valgus total knee arthroplasty (Whiteside's line)
- Described the anteroposterior axis (Whiteside's line) from the deepest trochlear groove to the centre of the intercondylar notch as a reference for femoral component rotation
- Showed that perpendicular alignment to this line produced optimal patellar tracking
Subvastus (Southern) approach for primary total knee arthroplasty
- Described elevating vastus medialis off the intermuscular septum without incising the quadriceps mechanism
- Reported faster recovery, better quadriceps function and less anterior knee pain versus the medial parapatellar approach
- Exposure is limited in muscular or obese patients
Why knee replacements fail in 2013: patient, surgeon, or implant?
- Contemporary analysis of TKR failure modes
- Ranked instability (about 21% of revisions), aseptic loosening (16%), infection (16%) and polyethylene wear (10%) as the leading causes
- Technical errors — component malposition, malalignment and soft-tissue imbalance — contribute to the majority of failures
Global joint-replacement registries — primary cemented TKR survival
- Across these registries primary cemented TKR shows approximately 95% cumulative survival at 10 years
- Infection, aseptic loosening and instability are the leading causes of revision
- Registry data are the highest-level real-world evidence for implant survival and bearing or fixation comparisons