Septic Arthritis vs Bursitis

Why Disseminated gonococcal infection is correct

• In a sexually active young adult with recent urethritis and a positive urogenital gonococcal NAAT, a purulent monoarthritis (synovial WBC 90,000/mm3, neutrophil-predominant) is gonococcal septic arthritis until proven otherwise.
• Synovial Gram stain and culture are frequently negative in disseminated gonococcal infection, so the positive urogenital NAAT is the highest-yield confirmatory test.
• Treatment is ceftriaxone plus empiric chlamydia coverage with doxycycline.

Why the others are wrong

• Crystal-induced acute gout — no crystals are reported, and the sexual history with a positive gonococcal NAAT explains the effusion; buzzword-match to a markedly elevated synovial WBC.
• Staphylococcus aureus septic arthritis — the commonest non-gonococcal cause, but the positive gonococcal NAAT and antecedent urethritis point to Neisseria; anchoring on the most common organism.
• Post-infectious reactive arthritis — produces a sterile, lower-grade inflammatory effusion days to weeks after infection, not a purulent 90,000/mm3 effusion with documented active gonococcal infection (premature closure on 'post-infectious').

Why Preserved passive knee motion with mild discomfort is correct

• The single most useful bedside discriminator between an infected or inflamed bursa and an infected joint is the response of the joint to passive motion, because a bursa lies outside the capsule and moving the knee does not stretch it.
• Prepatellar bursitis, often called housemaid or carpet layer knee, arises from repetitive kneeling, produces a discrete fluctuant swelling anterior to the patella, and permits nearly normal passive motion.
• Preserved passive knee motion with mild discomfort — the expected finding in bursitis and the strongest evidence against septic arthritis, in which even small arcs of motion are agonizing.

Why the others are wrong

• Knee held in 20 degrees of flexion while resting — the position of maximal intracapsular volume and lowest intracapsular pressure, which patients with a true joint effusion or septic arthritis adopt spontaneously, so it argues for intra-articular disease.
• Severe pain with minimal passive knee rotation — pain on any passive arc, particularly rotation, is the hallmark of an irritated synovium and is highly suggestive of septic arthritis, which is a surgical emergency requiring arthrocentesis.
• Ballottable fluid deep to the patella on tapping — a patellar tap requires fluid within the joint capsule lifting the patella off the trochlea, so it localizes the fluid intra-articularly rather than to the superficial bursa.

Even when bursitis is likely, a warm erythematous bursa in a worker with skin breakdown may be septic, most often from Staphylococcus aureus, and bursal aspiration with Gram stain, culture, and crystal analysis is appropriate. Aspiration should traverse healthy skin and should never pass through cellulitic tissue into the joint.