After the Extraction

A 68-year-old man with a history of osteoarthritis presents to the emergency department with 10 days of worsening right knee pain, warmth, and swelling. He underwent an uncomplicated total right knee arthroplasty 4 years ago and had been fully functional without any pain or instability until this episode. Three weeks before presentation he underwent a routine molar extraction at his dentist's office; he did not receive prophylactic antibiotics. He has no history of prior joint infections and takes no immunosuppressive medications. His other medical history includes hypertension and type 2 diabetes mellitus, well controlled on metformin.

On examination he is febrile to 38.7°C, blood pressure 138/84 mmHg, heart rate 96 bpm. The right knee is visibly swollen, warm to touch, and exquisitely tender with passive range of motion. There is a moderate effusion. The overlying skin is intact with no erythema tracking along the limb, no sinus tract, and no wound drainage. The knee prosthesis feels stable on clinical examination.

White blood cell count is 13,400/μL with 88% neutrophils. ESR is 94 mm/hr and CRP is 188 mg/L. Plain radiographs of the right knee show the prosthetic components in good position with no radiolucency at the bone-cement interface and no evidence of component loosening or periprosthetic fracture. Knee arthrocentesis yields 65 mL of turbid yellow fluid. Synovial fluid analysis shows 78,000 WBC/μL (94% neutrophils), glucose 18 mg/dL (serum 112 mg/dL), and LDH 890 U/L. Gram stain reveals gram-positive cocci in chains. Synovial fluid culture grows Streptococcus mitis (viridans group streptococcus) susceptible to penicillin (MIC 0.06 μg/mL), amoxicillin, ceftriaxone, and vancomycin. Two sets of blood cultures are also positive for S. mitis.