A 65-year-old man who underwent right total knee replacement 6 months ago comes to the emergency department due to a 3-week history of dull pain in his right knee. He has had no recent respiratory illness, diarrhea, or urinary symptoms. Medical history is notable for osteoarthritis, hypertension, and hyperlipidemia. The patient drinks a beer every night and does not use tobacco or illicit drugs. Temperature is 37.3 C (99.1 F), blood pressure is 120/60 mm Hg, and pulse is 90/min. Examination shows mild swelling and tenderness at the knee with full range of motion. The skin over the joint is intact. The synovial fluid is sampled. Laboratory results are as follows: Which of the following is the most likely cause of this patient's current condition?

Question

A 65-year-old man who underwent right total knee replacement 6 months ago comes to the emergency department due to a 3-week history of dull pain in his right knee.  He has had no recent respiratory illness, diarrhea, or urinary symptoms.  Medical history is notable for osteoarthritis, hypertension, and hyperlipidemia.  The patient drinks a beer every night and does not use tobacco or illicit drugs.  Temperature is 37.3 C (99.1 F), blood pressure is 120/60 mm Hg, and pulse is 90/min.  Examination shows mild swelling and tenderness at the knee with full range of motion.  The skin over the joint is intact.  The synovial fluid is sampled.  Laboratory results are as follows:  Which of the following is the most likely cause of this patient's current condition?

Quizplus · Accepted Answer

Explanation: 11ec25c8_d46c_0b4c_bc81_0fefb07c8b84_MD0003_00 This patient has subacute pain in his prosthetic knee 6 months after arthroplasty. The synovial fluid analysis shows a mildly elevated leukocyte count with a predominance of neutrophils. This is consistent with an inflammatory process, most likely a prosthetic joint infection (PJI). The leukocyte count in the synovial fluid in PJI is usually elevated to >1000/mm³ but is often lower than in septic native joints (usually >50,000/mm³). ³ ³ PJI can be acquired by perioperative contamination of the joint or by extension from an overlying wound infection: Infections due to virulent organisms (eg, Staphylococcus aureus, Pseudomonas aeruginosa) typically present within the first 3 months after surgery (early onset infection) with acute pain, fever, leukocytosis, and overt local signs of infection (eg, erythema, purulent drainage), not seen in this patient (Choices E and G). Infections due to less virulent organisms (eg, coagulase-negative staphylococci, Propionibacterium species), as in this patient, are likely to have a delayed onset (3-12 months) and present with chronic pain, implant loosening, gait impairment, or sinus tract formation. Fever and leukocytosis are usually absent. Staphylococcus epidermidis is a coagulase-negative staphylococcus commonly implicated in delayed-onset PJI. Late-onset infections presenting >12 months after surgery are unlikely to have been acquired perioperatively and are usually due to hematogenous spread of a distant infection (eg, urinary tract infection). (Choice A) Atypical mycobacteria are a rare cause of PJI. They usually cause late-onset infections and are most commonly seen in patients with immunosuppressive disorders (eg, HIV) or local trauma. (Choice B) Borrelia burgdorferi causes Lyme disease. Late Lyme disease can present with acute intermittent attacks of monoarticular or oligoarticular arthritis, but PJI with chronic, persistent joint pain is rare. (Choices C and F) Chlamydia trachomatis and Salmonella species can be associated with reactive arthritis following an initial genitourinary or gastrointestinal infection. Reactive arthritis is characterized by asymmetric oligoarthritis with urethritis/enteritis and conjunctivitis. Subacute PJI is an uncommon presentation. (Choice D) Disseminated Neisseria gonorrhoeae infection most commonly causes a syndrome characterized by migratory polyarthritis, tenosynovitis, and rash. Acute septic monoarthritis can also be seen, but synovial fluid cell counts are usually higher (>50,000/mm³). ³ Educational objective: Prosthetic joint infections can be acquired by perioperative contamination or by extension from an overlying wound infection. Infections due to virulent organisms (eg, Staphylococcus aureus) present within the first 3 months with acute pain, fever, and local signs of infection. Infections due to less virulent organisms (eg, coagulase-negative staphylococci) have a delayed onset and present with chronic pain, implant loosening, gait impairment, or sinus tract formation.

A 65-Year-Old Man Who Underwent Right Total Knee Replacement 6