Arthritis Septik

01Pathophysiology

Septic arthritis is a joint space infection that constitutes an orthopedic emergency because untreated infection can destroy articular cartilage within hours to days. The most common route of infection is hematogenous seeding. The synovial membrane is highly vascularized and lacks a limiting basement membrane, which means circulating bacteria can easily penetrate the joint space during transient or sustained bacteremia. Less commonly, organisms reach the joint by direct inoculation (penetrating trauma, arthroscopy, intra-articular injection) or by contiguous spread from adjacent osteomyelitis or a periarticular soft tissue abscess.

Once bacteria colonize the synovial fluid, they trigger an intense neutrophilic inflammatory response. Massive neutrophil infiltration into the joint produces a purulent effusion. These activated neutrophils release proteolytic enzymes (collagenases, elastases) and reactive oxygen species that directly degrade the articular cartilage matrix. Bacterial toxins compound this destruction. The cartilage, which is avascular and relies on synovial fluid for nutrition, has essentially no capacity for self-repair once damaged. This is why the clinical urgency is so high: every hour of untreated infection translates to irreversible cartilage loss.

The patient holds the joint in flexion because this position maximizes the intra-articular volume, thereby reducing intracapsular pressure and minimizing pain. Any attempt at passive movement compresses the inflamed, distended capsule and produces severe pain, which is why both active and passive range of motion are restricted (unlike tendinitis or bursitis, where passive motion is relatively preserved).

In gonococcal arthritis, the pathophysiology follows a biphasic pattern. Initially, N. gonorrhoeae disseminates hematogenously, causing the classic triad of migratory polyarthralgias, tenosynovitis, and vesiculopustular skin lesions (the bacteremic or disseminated phase). If untreated, the organism then localizes to a single joint, producing a frank purulent arthritis identical in character to non-gonococcal septic arthritis. This two-phase presentation is a frequent vignette construct.

02Classification and Clinical Manifestation

03Diagnostic Workup

Arthrocentesis is the single most important step whenever a patient presents with an acute, hot, swollen joint. It should be performed before initiating antibiotics unless the patient is hemodynamically unstable. The synovial fluid is sent for cell count with differential, Gram stain, culture, and crystal analysis. The cell count is the most useful initial discriminator: a WBC above 50,000/microL with a PMN predominance above 75% is highly suggestive of bacterial infection, though counts can occasionally overlap with severe crystal arthropathy.

Gram stain provides a rapid result but has limited sensitivity, particularly for gonococcal arthritis. Synovial fluid culture remains the confirmatory test because it identifies the pathogen and guides targeted antibiotic therapy. Always order blood cultures in parallel, as a positive blood culture in the setting of a compatible joint presentation can establish the diagnosis even if the joint fluid culture is negative (as sometimes occurs with prior antibiotic use).

For suspected gonococcal arthritis, the standard synovial fluid culture has poor sensitivity (below 50%). You must also obtain nucleic acid amplification testing (NAAT) from the cervix or urethra, pharynx, and rectum, as well as cultures of the blood and any skin lesions. A positive NAAT from a mucosal site combined with a compatible clinical syndrome is often how the diagnosis is confirmed.

In children, when the hip joint is involved, ultrasound is the initial imaging of choice to confirm the presence of an effusion before proceeding to aspiration. The Kocher criteria (fever >38.5C, non-weight-bearing, ESR >40 mm/hr, WBC >12,000/microL) help stratify the probability of septic arthritis of the hip in pediatric patients, but they do not replace arthrocentesis.

04Management and Treatment

Acute stabilization begins with arthrocentesis for diagnosis followed by immediate empiric IV antibiotics. Do not wait for culture results before starting treatment. The standard empiric regimen covers S. aureus (including MRSA) and common Gram-negative organisms. Vancomycin provides MRSA coverage, while a third-generation cephalosporin covers Gram-negatives and streptococci. Narrow the regimen once culture and sensitivity data return.

Joint drainage is equally critical and must accompany antibiotic therapy. For most native joints (especially the knee), serial needle arthrocentesis performed daily or as needed is effective. The aspirate should be sent for repeat cell counts to monitor treatment response. Indications for surgical drainage (arthroscopic or open) include involvement of the hip or shoulder (joints that are difficult to drain by needle), failure to improve after 72 hours of appropriate antibiotics plus needle drainage, loculated effusion, or prosthetic joint infection.

For gonococcal arthritis, response to ceftriaxone is typically rapid (within 24 to 48 hours). Once the patient demonstrates clinical improvement (reduced pain, declining inflammatory markers), transition to an oral agent to complete a shorter total course of 7 to 14 days. Always treat for possible concurrent chlamydial infection with a single dose of azithromycin.

Contraindications and adjustments: In patients with severe penicillin or cephalosporin allergy, use vancomycin combined with aztreonam or a fluoroquinolone for Gram-negative coverage. In renal insufficiency, adjust vancomycin dosing based on trough levels and consider alternatives like daptomycin. In pregnancy with gonococcal arthritis, ceftriaxone remains safe, but azithromycin is preferred over doxycycline for chlamydial coverage.

Long-term management includes physical therapy to restore range of motion once the infection is controlled, monitoring inflammatory markers (CRP is the most useful marker for treatment response), and completion of the full antibiotic course. Premature discontinuation of antibiotics is a common cause of relapse.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most tested trap with septic arthritis is the coexistence of crystals and infection. Finding monosodium urate or CPPD crystals in the synovial fluid does not rule out a concurrent bacterial infection. If the clinical picture is concerning for septic arthritis (high fever, WBC >50,000, acutely ill patient), you must still send the fluid for Gram stain and culture regardless of whether crystals are present. Choosing "gout flare, start colchicine" when the correct answer is "send fluid cultures and start empiric antibiotics" is the classic wrong answer on this topic.

The second common pitfall involves gonococcal arthritis presenting as the disseminated syndrome rather than a straightforward septic joint. The vignette will describe a young, sexually active patient with migratory joint pains, tenosynovitis of the wrists or hands, and scattered vesiculopustular skin lesions. Students who are anchored on "septic arthritis = one hot swollen joint" may miss this presentation entirely and select reactive arthritis or a viral syndrome instead. The key is recognizing the skin-joint combination in a sexually active patient as disseminated gonococcal infection.

A third trap concerns the hip joint in children. A limping child with hip pain, refusal to bear weight, and fever can represent either septic arthritis or transient synovitis. The vignette will use the Kocher criteria to push you toward one diagnosis or the other. A child who looks toxic, has a high fever, an elevated WBC, and a markedly elevated ESR meets multiple Kocher criteria, and the next best step is ultrasound-guided hip aspiration, not observation or empirical NSAIDs.

Finally, remember that the next best step for any acute, atraumatic, hot, swollen joint is always arthrocentesis. The question is testing whether you know to aspirate first and treat second. The only exception is the hemodynamically unstable septic patient, where empiric antibiotics and resuscitation take priority, with arthrocentesis performed as soon as the patient is stabilized. The core competency being tested is your ability to recognize an orthopedic emergency, perform the correct diagnostic procedure without delay, and initiate both antibiotic therapy and joint drainage simultaneously.