Osteomyelitis & Septic Arthritis
A board-focused MSK/rheumatology lesson on osteomyelitis and septic arthritis, moving from pathophysiology and organism-by-scenario buzzwords through synovial fluid analysis, imaging, and next-best-step management. Emphasizes arthrocentesis as the pivotal test, empiric vancomycin ± ceftriaxone, and crystal/reactive-arthritis mimics.
Overview & Pathophysiology
Osteomyelitis is infection of bone; septic (infectious) arthritis is infection of the joint space. Both most often arise from hematogenous seeding, but also from contiguous spread (diabetic foot ulcer, adjacent soft tissue) or direct inoculation (trauma, surgery, prosthesis). *Staphylococcus aureus* is the leading organism for both.
In children, hematogenous osteomyelitis targets the metaphysis of long bones (sluggish capillary flow); in adults it favors the vertebrae (Batson venous plexus). Chronic disease produces a sequestrum (necrotic bone), involucrum (new periosteal bone), and a draining sinus tract.
Septic arthritis is a medical/surgical emergency — bacterial enzymes and inflammation destroy cartilage within days. It is usually acute and monoarticular (knee most common). Prompt arthrocentesis and joint drainage prevent permanent damage. Because both conditions can smolder or be masked by pre-existing arthritis, a high index of suspicion plus cultures before antibiotics (when feasible) drives every board answer.
Match the buzzword to the bug:
- Overall (all ages): S. aureus
- Sickle cell disease: *Salmonella (classic board association, over-represented vs. the general population; S. aureus* is still the single most common organism overall)
- Puncture wound through a sneaker / plantar foot: *Pseudomonas aeruginosa*
- IV drug use: S. aureus, Pseudomonas — vertebral, sacroiliac, sternoclavicular joints
- Prosthetic joint / hardware: *S. epidermidis (coagulase-negative staph), S. aureus*
- Vertebral, indolent + Pott disease: *Mycobacterium tuberculosis*
- Cat / dog bite: *Pasteurella multocida*
- Human bite / fist-to-mouth: Eikenella corrodens
- Diabetic foot / decubitus ulcer: polymicrobial
- Neonate: S. aureus, group B Streptococcus, E. coli
Imaging pearl: plain X-ray is first but normal for ~1–2 weeks (earliest sign = soft-tissue swelling, then lytic lucency/periosteal reaction). MRI is the most sensitive early test; bone biopsy + culture is definitive and guides therapy.
Non-gonococcal: S. aureus is most common → acute monoarticular hot, swollen, exquisitely tender joint (knee), fever, refusal to bear weight. Risk: damaged/prosthetic joint, RA, elderly, immunocompromised, IVDU.
Gonococcal / disseminated gonococcal infection (DGI) — young, sexually active. Two classic forms:
- Arthritis–dermatitis triad: migratory polyarthralgia + tenosynovitis + pustular/vesicular skin lesions; blood/synovial cultures often negative.
- Purulent monoarthritis — usually no skin lesions.
- Culture urethra/cervix/pharynx/rectum (not just blood) on Thayer–Martin; NAAT.
- Associated with menses/pregnancy and terminal complement (C5–C9) deficiency (recurrent Neisseria).
Children — hip: distinguish septic arthritis from transient synovitis using the Kocher criteria (non–weight-bearing, fever >38.5°C, ESR >40 mm/hr, serum WBC >12,000/µL; CRP >20 mg/L is added in modified versions) — the more criteria met, the higher the probability of septic arthritis.
Synovial Fluid Analysis
| Parameter | Normal | Non-inflammatory (OA) | Inflammatory (RA/gout) | Septic |
|---|---|---|---|---|
| Appearance | clear | clear/straw | cloudy, yellow | purulent/opaque |
| WBC/µL | <200 | 200–2,000 | 2,000–50,000 | >50,000 (often >100,000) |
| % PMNs | <25% | <25% | ~50% | >90% |
| Gram/culture | neg | neg | neg | often positive |
| Crystals | none | none | urate / CPPD | none |
Gonococcal vs Non-Gonococcal Septic Arthritis
| Feature | Gonococcal (DGI) | Non-gonococcal |
|---|---|---|
| Typical patient | Young, sexually active, healthy | Elderly/comorbid; damaged or prosthetic joint |
| Organism | N. gonorrhoeae | S. aureus (most), strep, gram-negatives |
| Joint pattern | Migratory polyarthralgia → mono/oligoarthritis; tenosynovitis | Monoarticular (knee) |
| Skin | Pustular/vesicular rash | Usually none |
| Cultures | Blood/synovial often negative; culture mucosa + NAAT | Blood/synovial frequently positive |
| Treatment | Ceftriaxone (+ doxycycline if chlamydia not excluded) | Vancomycin ± ceftriaxone + joint drainage |
| Joint outcome | Good; rarely destroyed | Can rapidly destroy cartilage |
Vignette: A 68-year-old woman with rheumatoid arthritis develops a single hot, swollen, exquisitely tender knee over 2 days with fever (38.9°C) and inability to bear weight.
Best next step → ARTHROCENTESIS for cell count/differential, Gram stain, culture, and crystal exam — obtain blood cultures too. Aspirate before antibiotics when feasible, but never delay antibiotics in an unstable/septic patient.
Fluid returns: WBC 90,000/µL, 95% PMNs, no crystals, Gram stain gram-positive cocci in clusters → septic arthritis (S. aureus).
Management:
- Joint drainage — serial aspiration or arthroscopic/surgical washout (a prosthetic joint usually needs surgery).
- Empiric IV vancomycin (MRSA); add ceftriaxone if gonococcal or gram-negative risk. Narrow after cultures.
Pitfall: Pre-existing RA or gout does not exclude infection — a flare and sepsis look alike, so always tap the joint.
Vignette: A 14-year-old boy with sickle cell disease has 5 days of fever and focal tibial pain, warmth, and point tenderness. X-ray of the leg is initially unremarkable.
Likely diagnosis: osteomyelitis — in sickle cell, think *Salmonella (though S. aureus* remains the most common organism overall).
Next-best-step diagnosis:
- Plain X-ray first — but often normal for 1–2 weeks (earliest change is soft-tissue swelling; later lytic lucency and periosteal reaction).
- MRI = most sensitive early test (marrow edema) and best to define extent.
- Bone biopsy + culture = definitive; identifies organism for targeted therapy.
- Labs: ESR and CRP elevated (CRP best for tracking response); blood cultures.
Management: prolonged (~4–6 weeks) organism-directed IV (then oral) antibiotics; surgical debridement for necrotic bone, abscess, or chronic disease. Distinguish from bone infarct/vaso-occlusive crisis, which can mimic this presentation.
Crystal arthritis is the great mimic of septic arthritis — memorize the two real classics:
- "Needle = Negative" → Gout: needle-shaped, negatively birefringent monosodium urate crystals. (Precipitated by alcohol, red meat/seafood, diuretics, chemotherapy/tumor lysis.)
- Pseudogout (CPPD): rhomboid crystals, positively birefringent — the opposite of gout. Look for chondrocalcinosis on X-ray; associated with hemochromatosis and hyperparathyroidism.
- Color under polarized light — "Parallel Yellow = gout" (negative); parallel Blue = CPPD (positive).
Bottom line: crystals confirm gout/pseudogout but do not exclude infection — send Gram stain and culture on every aspirate.
Decision points boards love:
- Suspected septic arthritis → arthrocentesis is the single best next step (before antibiotics when feasible; do not delay antibiotics in a septic/unstable patient).
- Empiric antibiotics: vancomycin (MRSA) ± ceftriaxone (gonococcal/gram-negative) or antipseudomonal coverage by risk; narrow after culture.
- Diabetic foot: a positive probe-to-bone test or a deep/large ulcer raises osteomyelitis probability → MRI; bone biopsy for culture-directed therapy (superficial swab cultures are unreliable).
- Vertebral osteomyelitis/discitis: subacute back pain + focal point tenderness ± fever, ↑ESR/CRP → MRI; screen for IVDU and endocarditis (blood cultures, echocardiography).
- Mimic — reactive arthritis: post-GU/GI, HLA-B27, sterile synovial fluid, "can't see, can't pee, can't bend the knee." Tapping the joint distinguishes it.
- Chronic osteomyelitis: almost always needs surgical debridement plus antibiotics.
Practice MSK now
Board-style questions, spaced-repetition flashcards, and a Socratic AI tutor — free to start.