Total Knee Replacement Complication — ABOS Oral Boards Preparation
What examiners expect and how to prepare
Clinical Scenario
A 64-year-old right-hand-dominant male retired postal worker presents to your office 14 months after a right total knee arthroplasty performed at your institution. The primary indication was tricompartmental osteoarthritis. He had an excellent early result — pain-free with 0-125 degrees of motion at 6 weeks. At 6 months, he reported insidious onset of right knee pain, swelling, and stiffness. He denies any recent dental procedures, infections, or trauma. His temperature is 99.1 degrees F. Labs show a CRP of 28 mg/L (normal < 10) and an ESR of 42 mm/hr (normal < 20). On exam, the right knee has a moderate effusion. Range of motion is 5-95 degrees. The wound is healed without erythema or drainage. There is warmth compared to the contralateral side.
Radiographs obtained show a well-fixed cemented total knee replacement without evidence of loosening, periprosthetic fracture, or malposition. There are no lucent lines around the tibial or femoral components.
You performed a right knee aspiration under sterile conditions. Synovial fluid analysis shows a WBC count of 12,400 cells/μL with 89% polymorphonuclear leukocytes. Synovial fluid alpha-defensin is positive. Cultures grew coagulase-negative Staphylococcus after 3 days. Based on these findings, you diagnosed a chronic periprosthetic joint infection (PJI). You performed a two-stage exchange arthroplasty — first stage consisting of implant removal, aggressive debridement, and placement of an antibiotic-loaded cement spacer. Intraoperative tissue cultures confirmed coagulase-negative Staphylococcus. After 6 weeks of IV vancomycin followed by 6 weeks of oral suppressive antibiotics, with normalized inflammatory markers, you performed second-stage reimplantation with an excellent result.
What Examiners Look For
- Data gathering: Timeline is everything — 14 months post-op with insidious pain onset changes the differential and the treatment algorithm.
- Diagnosis: Walk through the diagnostic criteria for PJI — MSIS/ICM criteria, serum markers, synovial fluid analysis, and alpha-defensin. What are the thresholds?
- Treatment plan: One-stage versus two-stage exchange — what are the indications for each? Why did you choose two-stage for this organism?
- Technical skill: Describe the first-stage debridement — what do you remove, how aggressively do you debride, and how do you make the antibiotic spacer?
- Complications: What if cultures are negative but the clinical picture suggests infection? What if the spacer dislocates or fractures?
- Applied knowledge: Discuss the role of biofilm in chronic PJI and why DAIR (debridement, antibiotics, implant retention) is less effective after 4 weeks.
- Ethics: How do you disclose a potential complication to the patient? Informed consent for a complex revision procedure.
Common Pitfalls
- Aspirating the knee while the patient is on antibiotics — antibiotics should be held for at least 2 weeks before aspiration when possible.
- Not knowing the MSIS/ICM diagnostic criteria and thresholds for synovial WBC, PMN percentage, and CRP/ESR.
- Attempting DAIR for a chronic infection (>4 weeks from onset) — the evidence strongly favors staged exchange.
- Blaming the patient for the infection or implying poor hygiene — infection is a complication, not a fault.
- Forgetting to send multiple tissue cultures (minimum 3-5 samples) intraoperatively — one sample is insufficient.
Key Classifications
Related Scenarios
Free — takes 3 minutes