High Yield Overview

TWO-STAGE EXCHANGE FOR INFECTED TKA

Gold standard treatment for chronic periprosthetic joint infection | Advanced

Indications for Two-Stage Exchange

Chronic periprosthetic joint infection (greater than 3-4 weeks duration)
Failed DAIR (debridement, antibiotics, implant retention)
Unknown organism or culture-negative infection
Resistant organisms (MRSA, VRE, multi-drug resistant)
Sinus tract present
Poor soft tissue envelope requiring staged approach
Severely immunocompromised patient

MSIS Criteria for PJI Diagnosis (2018 Updated)

Major Criteria (one = definite PJI):

Two positive cultures with same organism
Sinus tract communicating with joint

Minor Criteria (total greater than or equal to 6 = PJI):

CRP greater than 10 mg/L (2 points)
ESR greater than 30 mm/hr (1 point)
Synovial WBC greater than 3000 cells/uL (3 points)
Synovial PMN greater than 80% (2 points)
Alpha-defensin positive (3 points)
Single positive culture (2 points)
Positive histology (3 points)

Exam Pearl

Examiner Question: "How do you distinguish acute from chronic periprosthetic joint infection? Why does this matter?"

Model Answer: "The distinction is based on duration of symptoms: Acute PJI is less than 3-4 weeks from symptom onset (or within 4 weeks of primary surgery for early postoperative infection). Chronic PJI is greater than 3-4 weeks duration. This distinction is CRITICAL because it determines treatment: Acute PJI may be treated with DAIR (debridement, antibiotics, and implant retention) if the implant is well-fixed, soft tissues are viable, and the organism is susceptible. Chronic PJI requires two-stage exchange because the mature biofilm cannot be eradicated with debridement alone - the entire implant and cement must be removed. The biofilm takes 2-4 weeks to mature and become resistant to antibiotic penetration and host immune response."

Indication Selection Pitfalls

Attempting DAIR for chronic infection = high failure rate (50-80%) - use two-stage
Missing the diagnosis - culture-negative PJI is still PJI if MSIS criteria met
Sinus tract = definite PJI regardless of culture result - do not delay treatment
Proceeding with one-stage in resistant organism - high reinfection rate with MRSA, VRE

Critical Danger Structures

Popliteal Artery

Behind posterior capsule. At risk during aggressive cement removal posteriorly. Maintain posterior retractor on bone. Catastrophic if injured - vascular surgery backup essential.

Common Peroneal Nerve

Around fibular neck. May be encased in scar tissue in chronic infection. At risk during lateral releases and exposure. Identify and protect - decompress if needed.

Patellar Tendon

Tibial tubercle insertion. High risk of avulsion in revision setting with scarring. Use extensile exposures (TTO) if tendon at risk. Catastrophic if avulsed.

Collateral Ligaments

Medial and lateral joint line. May be weakened by infection/debridement. Assess carefully - plan constraint level accordingly. MCL at risk during medial exposure.

Skin/Soft Tissue

Compromised in chronic infection. Multiple incisions, sinus tracts, poor vascularity. May need plastic surgery involvement. Tension-free closure essential.

Mnemonic

M-S-I-SMSIS

Memory Hook:MSIS 2018 criteria are the GOLD STANDARD for PJI diagnosis. Major criteria (2 positive cultures OR sinus tract) = definite PJI. Minor criteria are scored - total of 6 or more points = PJI.

Mnemonic

S-P-A-C-E-RSPACER

Memory Hook:The 2-week antibiotic holiday before reimplantation aspiration is CRITICAL - don't proceed to Stage 2 without confirming eradication. False negative aspiration if antibiotics still active.

Stage 1: Explantation and Spacer Insertion

Positioning and Preparation

Patient Position: Supine on radiolucent table. Thigh tourniquet. Leg holder or foot positioner.

Prep and Drape: Prep entire limb including sinus tracts if present. Use antimicrobial-impregnated drapes. Consider wound retraction system for infected cases.

Pre-incision: Stop prophylactic antibiotics until tissue samples obtained (if organism unknown). Have multiple specimen containers ready.

Step 1: Exposure

Use previous surgical incision (most lateral if multiple). Raise full-thickness skin flaps. Medial parapatellar arthrotomy. Excise any sinus tracts completely back to healthy tissue. May require extensile exposure for stiff or scarred knee.

Exam Pearl

Technical Pearl: "I use the previous incision and raise full-thickness flaps. Before giving any antibiotics, I take 5-6 tissue samples from different areas for culture and hold them for 14 days. I excise all sinus tracts back to healthy bleeding tissue. If the knee is stiff, I may need a quadriceps snip or TTO for adequate exposure."

Critical Safety Point

Take tissue samples BEFORE antibiotics if organism unknown
Minimum 5-6 tissue samples from different locations
Hold cultures for 14 days (slow-growing organisms)
Frozen section if available (greater than 5 PMN/HPF = infection)

Step 2: Component Removal

Remove all implants including polyethylene, femoral component, tibial baseplate, and patellar component if present. Use thin osteotomes at implant-bone interface. Remove all cement meticulously using burrs, chisels, or ultrasonic device. Remove any remaining fixation screws or wires.

Exam Pearl

Technical Pearl: "I must remove ALL foreign material - any residual cement or debris harbors biofilm and will cause persistence of infection. I carefully inspect the entire joint, including the posterior recesses. I debride all necrotic and infected tissue until I reach healthy bleeding bone and soft tissue."

Critical Safety Point

Remove ALL cement - residual cement harbors biofilm
Protect posterior structures during cement removal
Preserve bone stock where possible (for Stage 2)
Debride to healthy bleeding tissue

Step 3: Thorough Debridement

Perform radical debridement of all infected and necrotic tissue. Curette any bone cavities. Irrigate copiously (6-9L pulsatile lavage). Consider Betadine solution soak. The joint should appear clean with healthy bleeding surfaces.

Exam Pearl

Technical Pearl: "I debride aggressively - the adequacy of debridement is the most important factor in infection eradication. I irrigate with at least 6-9 liters of normal saline using pulsatile lavage. Some surgeons add dilute Betadine for 3-minute soak, then lavage out."

Critical Safety Point

Inadequate debridement = treatment failure - the most important surgical factor
Posterior capsule debridement - popliteal artery is 5-10mm behind; use posterior retractor on bone
Do not compromise bone stock excessively - need some bone for Stage 2 fixation
If tissue quality poor - consider adding local antibiotic beads for additional elution

Step 4: Antibiotic Spacer Fabrication

Prepare antibiotic-loaded cement spacer:

Static Spacer: Mix antibiotics into cement (vancomycin 2-4g + tobramycin 2-4g per 40g PMMA). Form into block that fills the joint space. May use K-wires or rods for reinforcement.

Articulating Spacer: Use metal femoral trial or mold cemented femoral component. Create cement tibial component. Allows range of motion and easier second-stage surgery.

Exam Pearl

Technical Pearl: "I typically use an articulating spacer because it maintains ROM and makes the second stage easier. I mix vancomycin 4g and tobramycin 4g into each 40g pack of cement - this high-dose elutes therapeutic antibiotic levels locally. I ensure the spacer is stable and will not dislocate."

Critical Safety Point

High antibiotic load acceptable for spacers (4g per 40g cement)
Cement becomes more brittle with high antibiotic load - reinforce if needed
Ensure spacer is stable - dislocation is common complication
Size appropriately to maintain soft tissue tension

Step 5: Closure

Irrigate final time. Achieve meticulous hemostasis. Close in layers - deep capsular closure is critical. Consider negative pressure wound therapy if soft tissue concerning. Apply bulky dressing and knee immobilizer.

Exam Pearl

Technical Pearl: "I close the capsule and retinaculum securely over the spacer. If I'm concerned about the soft tissue envelope, I use negative pressure wound therapy. The patient is kept in a knee immobilizer, especially if I used a static spacer or there's any instability concern."

Critical Safety Point

Tension-free closure is essential - excessive tension leads to wound breakdown
Consider gastrocnemius flap if soft tissue deficient - plastic surgery referral
Drain placement controversial - some surgeons avoid to prevent colonization track
NPWT (negative pressure wound therapy) - consider for high-risk wounds (previous sinus, diabetes, obesity)

Interim Period Between Stages

Antibiotic Regimen

Begin IV antibiotics after cultures obtained (may be empiric until sensitivities available)
Duration: Minimum 6 weeks IV organism-specific antibiotics
Infectious diseases consultation for regimen guidance
Monitor for antibiotic toxicity (renal function, levels for aminoglycosides/vancomycin)

Monitoring

Weekly CRP and ESR initially, then every 2 weeks
Clinical wound inspection
Inflammatory markers should trend down and normalize

Antibiotic Holiday

Stop antibiotics 2 weeks before reimplantation aspiration
Critical to avoid false-negative cultures
Reaspirate at 2-3 weeks off antibiotics

Eradication Criteria for Stage 2

CRP normalized (less than 10 mg/L)
ESR trending down or normalized
Knee aspiration negative (WBC less than 2000, culture negative)
No clinical signs of infection (wound healed, no erythema, no fever)
If any doubt, delay Stage 2 or repeat cultures

Stage 2: Reimplantation

Timing

Minimum 6-8 weeks after Stage 1 (longer if difficult organism or slow response)
Some surgeons use 8-12 week interval
ONLY proceed if eradication criteria met

Step 1: Exposure and Spacer Removal

Use previous incision. May require extensile exposure as knee often stiff. Remove antibiotic spacer carefully - articulating spacers may be well-fixed. Take fresh tissue cultures (5-6 samples).

Exam Pearl

Technical Pearl: "At Stage 2, I take 5-6 fresh tissue cultures before giving antibiotics. If frozen section is available, I send tissue - greater than 5 PMN per high-power field suggests persistent infection and I would consider aborting reimplantation and repeating debridement/spacer exchange."

Critical Safety Point

Take cultures BEFORE antibiotics - just like Stage 1; don't give prophylactic antibiotics until samples obtained
Frozen section >5 PMN/HPF = abort reimplantation - consider repeat debridement and spacer exchange
Articulating spacer may be well-fixed - careful extraction to avoid bone loss
Extensile exposure may be needed - knee often very stiff, especially after static spacer

Step 2: Bone Assessment

Assess bone loss using AORI classification after spacer removal:

Type 1: Intact metaphysis - standard revision
Type 2A/2B: Damaged metaphysis - augments, sleeves/cones, stems
Type 3: Deficient metaphysis - structural allograft, megaprosthesis

Exam Pearl

Technical Pearl: "After spacer removal, I assess bone stock. There is often more bone loss than at Stage 1 due to disuse and the explantation process. I need to plan for stems to bypass the deficient metaphysis and achieve diaphyseal fixation."

Critical Safety Point

Expect MORE bone loss than at Stage 1 - disuse osteopenia, spacer erosion, debridement
AORI Type 3 defects may need megaprosthesis - have backup plan and equipment
Assess ligament integrity - MCL/LCL may be compromised; plan constraint accordingly
Extensor mechanism assessment - if damaged, may need reconstruction or consider alternative (arthrodesis)

Step 3: Revision TKA Reconstruction

Perform revision TKA using standard principles:

Establish tibial and femoral platforms on healthy bone
Address bone defects with augments, sleeves/cones
Add stems for diaphyseal fixation
Select constraint based on ligament competency
May use antibiotic cement for fixation (some surgeons add low-dose)

Exam Pearl

Technical Pearl: "I approach Stage 2 as a complex revision TKA. I expect significant bone loss and plan for stems and augments. I typically use cemented components with consideration of adding low-dose antibiotics to the cement (1g vancomycin per 40g) - though this is controversial."

Critical Safety Point

Use longer stems to bypass metaphyseal defects - achieve diaphyseal fixation and protect deficient bone
Do not undersize constraint - if ligaments compromised, VVC or hinge may be needed
Cement technique - antibiotic-loaded cement for reimplantation is controversial; lower dose (1g/40g) if used
Intraoperative fracture risk - osteopenic bone; handle gently, especially during impaction

Step 4: Closure and Post-op

Irrigate thoroughly. Layered closure. Consider drains. Extended antibiotic prophylaxis (some surgeons give 24-48 hours IV, others extend to 5 days). DVT prophylaxis. Early mobilization.

Exam Pearl

Technical Pearl: "I irrigate copiously, achieve meticulous hemostasis, and close in layers. Some surgeons advocate for extended oral antibiotic suppression after Stage 2 (3-6 months), particularly for resistant organisms or immunocompromised patients. DVT prophylaxis for 35 days per Australian guidelines. Early mobilization with weight bearing as bone stock allows - typically WBAT unless significant bone loss or reconstruction concerns."

Critical Safety Point

Extended antibiotic prophylaxis - controversial; 24-48hrs IV minimum, some extend to 5 days or add oral suppression
DVT risk is HIGH - prolonged immobility between stages; rivaroxaban 35 days post Stage 2 (Australian guidelines)
Wound surveillance - any drainage beyond 5-7 days is concerning for persistent infection
Monitor cultures - Stage 2 cultures may take 14 days; if positive, reinfection or persistence must be addressed

Complications

Complications: Recognition and Management

Post-operative Protocol

Stage 1 Post-op

Weight bearing: Toe-touch or protected WB with walker (depends on spacer stability)
Immobilization: Knee immobilizer if static spacer or unstable
ROM: Early gentle ROM if articulating spacer, restricted if static
Antibiotics: 6 weeks IV organism-specific
Monitoring: Weekly CRP/ESR, clinical assessment, antibiotic levels

Between Stages

Continue ROM exercises (if articulating spacer)
Optimize nutrition (albumin greater than 3.5, total protein greater than 6)
Stop smoking
Optimize glucose control (HbA1c less than 8)
Address any other infections (dental, urinary)
2-week antibiotic holiday before reimplantation aspiration

Stage 2 Post-op

Weight bearing: Per revision TKA protocol (WBAT unless TTO or bone loss concerns)
ROM: CPM or early passive ROM, target 0-90° by 2 weeks
Antibiotics: Variable - some give extended oral suppression (3-6 months)
DVT prophylaxis: 35 days per Australian guidelines
Follow-up: 2 weeks wound check, 6 weeks X-ray, then 3/6/12 months, annual

Long-term Surveillance

Lifelong risk of reinfection - any dental procedures need antibiotic prophylaxis
Monitor for signs of recurrent infection
Annual review with CRP

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

EXAMINER

"A 72-year-old man is 3 years post primary TKA with 6 weeks of worsening knee pain and swelling. His CRP is 45 mg/L and aspiration shows 15,000 WBC with 92% PMN. Cultures grow MSSA. Outline your management."

EXCEPTIONAL ANSWER

This is a CHRONIC periprosthetic joint infection (greater than 4 weeks duration) confirmed by MSIS criteria. His CRP of 45 and synovial fluid showing 15,000 WBC with 92% PMN far exceed the thresholds (CRP greater than 10, WBC greater than 3000, PMN greater than 80%), and he has a positive culture. For CHRONIC PJI, the gold standard treatment is TWO-STAGE EXCHANGE. STAGE 1 would involve: Preoperative planning including discussing with infectious diseases for antibiotic regimen, Previous incision with extensile exposure if needed, taking 5-6 tissue samples before administering antibiotics, removal of all components and ALL cement, radical debridement of infected and necrotic tissue, copious irrigation (6-9L), and insertion of antibiotic-loaded cement spacer. For MSSA, I would use vancomycin 4g + tobramycin 4g per 40g cement (or organism-specific cefazolin). I prefer an articulating spacer to maintain ROM. Post-operatively, he would receive 6 weeks of IV antibiotics - for MSSA, flucloxacillin 2g IV 6-hourly (or cefazolin) would be first-line based on ID advice. I would monitor CRP weekly. ANTIBIOTIC HOLIDAY: 2 weeks off antibiotics, then reaspirate. Eradication criteria: CRP less than 10, aspiration WBC less than 2000, culture negative. STAGE 2 at 8-12 weeks if eradication confirmed: Previous incision, remove spacer, fresh cultures, perform revision TKA addressing bone loss. Some surgeons continue oral suppressive antibiotics for 3-6 months post Stage 2.

VIVA SCENARIOStandard

EXAMINER

"Compare static and articulating antibiotic spacers for infected TKA. What are the advantages and disadvantages of each?"

EXCEPTIONAL ANSWER

This is a common exam question. STATIC SPACER (cement block): ADVANTAGES include simpler fabrication - just form cement into a block, higher antibiotic elution because more surface area and less constraint on cement properties, cheaper (no need for trial components), and easier removal at Stage 2 since it's not well-fixed. DISADVANTAGES include significant ROM loss and stiffness (the main problem), extensor mechanism contracture making Stage 2 exposure difficult, disuse osteopenia and bone loss, patient immobility and functional limitation between stages, and potential leg length discrepancy. ARTICULATING SPACER (formed like a knee with motion): ADVANTAGES include preservation of ROM which prevents contracture, maintains extensor mechanism length and function, easier Stage 2 surgery because of better exposure and less bone loss, better patient function between stages (can mobilize, use walker), may allow limited weight bearing, and potentially maintains better bone quality. DISADVANTAGES include more complex fabrication (need trial components or molds), risk of spacer dislocation (10-20%), risk of spacer fracture especially if high antibiotic load, potentially less antibiotic elution, and higher cost. MY PREFERENCE: I generally use articulating spacers unless there is severe bone loss that precludes stable articulating construct, the patient is non-ambulatory, or cost is a significant concern. The easier second-stage surgery and preserved ROM outweigh the risks in most cases.

VIVA SCENARIOStandard

EXAMINER

"You have completed Stage 1 two-stage exchange for infected TKA. The patient has completed 6 weeks of IV antibiotics. Their CRP is now 8 mg/L. What are your criteria for proceeding to Stage 2?"

EXCEPTIONAL ANSWER

Before proceeding to Stage 2, I must confirm that the infection has been ERADICATED. This is a critical decision point - reimplanting into an infected knee will fail. My ERADICATION CRITERIA are: 1) CLINICAL: Wound healed completely, no sinus tract recurrence, no erythema or warmth, no fever, patient feels well. 2) SEROLOGY: CRP normalized (less than 10 mg/L) - in this case 8 mg/L is acceptable. ESR should be trending down (ESR normalizes more slowly than CRP). 3) ASPIRATION: After a 2-WEEK ANTIBIOTIC HOLIDAY, I would reaspirate the knee. This holiday is essential to avoid false-negative cultures. The aspiration should show: synovial WBC less than 2000 cells/uL, PMN less than 65%, culture negative after 14-day incubation. I would also check alpha-defensin if available. 4) INTRAOPERATIVE: At Stage 2, I take fresh tissue samples (5-6) before giving antibiotics. If frozen section is available, greater than 5 PMN per high-power field suggests persistent infection - I would abort reimplantation, repeat debridement, and insert a new antibiotic spacer. DECISION: If this patient has CRP 8, healed wound, negative aspiration after antibiotic holiday, and no clinical signs of infection, I would proceed to Stage 2. If any criteria are not met, I would delay and continue to monitor, or consider repeat debridement and spacer exchange.

References

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Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR). Annual Report 2023. Hip, Knee and Shoulder Arthroplasty.
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