investigations and management of septic arthritis
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Septic Arthritis: Investigations and Management
Definition and Overview
Septic arthritis is an orthopedic emergency with an incidence of 2-10/100,000 in native joints (rising to 30-70/100,000 in rheumatoid arthritis patients). Even with prompt treatment, it causes permanent loss of function in 25-50% of patients. Delay in diagnosis allows neutrophil enzymes (collagenase, elastase, hyaluronidase) to destroy articular cartilage irreversibly.
- Rosen's Emergency Medicine, 9e
Microbiology
| Population | Common Organisms |
|---|---|
| All ages (most common) | S. aureus (MSSA > MRSA) |
| Neonates | Group B Streptococcus, S. aureus, Gram-negative rods |
| Children < 5 years | S. aureus, Group A Streptococcus, S. pneumoniae |
| Sexually active adults | N. gonorrhoeae (most common cause in adults < 40 years) |
| Elderly / immunosuppressed | S. aureus, Gram-negative rods (e.g., Pseudomonas) |
| IV drug users | Pseudomonas, MRSA, sternoclavicular joint involvement |
| Animal/human bites | Pasteurella multocida, Eikenella corrodens, anaerobes |
| Sickle cell disease | Salmonella species |
Investigations
Bedside / Initial
- Joint aspiration (synovial fluid analysis) - the most important investigation
- WBC > 50,000/mm³ with >75% PMNs strongly suggests septic arthritis (though values overlap with crystal arthropathy)
- Glucose: low; Protein: elevated; Gram stain: positive in ~50% of cases
- Culture and sensitivity - positive in 50-70% of cases; mandatory before starting antibiotics if possible
- Blood cultures - taken before antibiotics; bacteremia present in ~50% of septic arthritis cases
Laboratory
- FBC / CBC: leukocytosis (WBC often >10,000), left shift
- ESR and CRP: elevated (CRP more sensitive and faster to respond); useful for monitoring treatment response
- Serum uric acid / calcium pyrophosphate - helps exclude crystal arthritis (though gout and infection can coexist)
- Procalcitonin: elevated in bacterial infection; useful adjunct
- Kocher criteria (for differentiating septic arthritis from transient synovitis in children):
- History of fever >38.5°C
- Non-weight bearing
- ESR >40 mm/hr
- WBC >12,000/mm³
- CRP >20 mg/L (added later as 5th predictor)
- 4 positive predictors = 99% probability of septic arthritis
Imaging
| Modality | Findings | Notes |
|---|---|---|
| Plain X-ray | Soft tissue swelling, joint space widening (early effusion), later: joint space narrowing, erosions, subchondral bone destruction | Not diagnostic in early disease; helps exclude other causes (fracture, osteomyelitis) |
| Ultrasound | Detects joint effusion, synovial thickening, increased vascularity; guides aspiration | Best first-line for deep joints (hip); portable and no radiation |
| MRI (gold standard) | Joint effusion, synovial enhancement with gadolinium, bone marrow edema, adjacent soft tissue involvement | Sensitivity 100%, specificity 77% (with Gd); findings evident within 24 hrs; requires GA in young children |
| CT | Joint effusion, bone erosions; useful when MRI contraindicated; can guide aspiration of deep joints | Less detail than MRI for soft tissue |
| Bone scan (Tc-99m) | Increased uptake; high sensitivity, low specificity | Rarely first-line; useful if multifocal disease suspected |
- Grainger & Allison's Diagnostic Radiology; Rosen's Emergency Medicine, 9e
Synovial Fluid Analysis Summary
| Feature | Normal | Septic | Crystal | Inflammatory |
|---|---|---|---|---|
| Appearance | Clear | Turbid/purulent | Cloudy | Cloudy |
| WBC (/mm³) | <200 | >50,000 | 2,000-50,000 | 2,000-50,000 |
| PMN% | <25% | >75% | >50% | >50% |
| Glucose | Normal | Low (<50% serum) | Normal | Normal |
| Culture | Negative | Positive | Negative | Negative |
Management
Principles
The goals are: (1) remove pus from the joint promptly, (2) administer adequate antibiotics, (3) prevent joint destruction and systemic spread.
1. Joint Drainage (Surgical Emergency)
- Aspiration: Needle aspiration is acceptable for superficial joints (knee, wrist, ankle) - repeated aspiration/irrigation via large-bore cannula supported by recent literature
- Arthroscopic washout: Preferred for most joints; allows direct visualization and irrigation
- Open surgical drainage (arthrotomy): Mandatory for:
- Hip joint (always - due to risk of avascular necrosis from pressure/ischemia on femoral head blood supply)
- Failed aspiration or arthroscopy
- Very young children
- Joints with adjacent osteomyelitis
- Prosthetic joint infections (with debridement or two-stage revision)
Pus confirmed on aspiration = formal washout is mandatory. The joint must be opened, irrigated, and free drainage encouraged via capsulotomy (standard teaching for hip).
- Bailey & Love's Surgery, 28th Ed.
2. Antibiotic Therapy
Start empirically after synovial fluid and blood cultures are taken - do not delay
Empiric regimens (guided by local protocol, patient factors, Gram stain):
| Scenario | First-line Agent |
|---|---|
| Adults (no risk factors, Gram-positive cocci) | Flucloxacillin IV (if MSSA likely) OR Vancomycin (MRSA risk) |
| Gram-negative organisms / elderly / immunosuppressed | Add a third-generation cephalosporin or ciprofloxacin |
| Gonococcal arthritis (sexually active adult) | Ceftriaxone IV/IM |
| IV drug user | Vancomycin + anti-Pseudomonal coverage (pip-tazo or ceftazidime) |
| Neonates | Flucloxacillin + gentamicin |
| Human/animal bites | Co-amoxiclav (amoxicillin-clavulanate) |
Duration:
- IV antibiotics for 2-4 weeks (some sources 1-2 weeks IV then switch to oral guided by sensitivity and clinical response)
- Total course: typically 4-6 weeks
- Monitor CRP for treatment response
3. General Supportive Care
- Analgesia - adequate pain relief is important
- Joint splinting - brief immobilization in position of comfort while inflammation settles; the hip may need splinting if unstable post-drainage
- Physiotherapy - early mobilization once infection controlled to prevent stiffness and ankylosis
- Nutrition - especially in children; optimized general health supports recovery
4. Monitoring
- Serial CRP is the most useful marker of treatment response
- Repeat joint aspiration if symptoms do not improve within 48-72 hours
- Watch for complications: avascular necrosis (especially hip in children), ankylosis, growth disturbance in children, osteomyelitis extension, sepsis/endocarditis
Special Populations
Children: Two-thirds of cases occur in children < 2 years. Use the Kocher criteria to differentiate from transient synovitis. Hip septic arthritis is a surgical emergency due to AVN risk. MRI requires GA in young children.
Neonates: Presentation may be subtle (pseudoparalysis, irritability). High index of suspicion required. Multi-joint involvement more common.
Immunosuppressed / rheumatoid arthritis: Classic signs may be absent or attenuated. Lower threshold for aspiration.
Prosthetic joints: Two-stage revision arthroplasty is often needed. S. aureus is the most common cause in the first 12 weeks post-surgery.
Gonococcal arthritis: Migratory polyarthralgia -> mono/oligoarthritis pattern; associated skin lesions (vesiculopustular rash), tenosynovitis. Responds rapidly to ceftriaxone.
Complications
- Avascular necrosis (especially femoral head in hip septic arthritis)
- Articular cartilage destruction -> osteoarthritis, ankylosis
- Growth plate damage in children -> limb length discrepancy
- Osteomyelitis (contiguous spread)
- Systemic sepsis, septic shock, endocarditis
- Sinus tract formation
Sources:
- Rosen's Emergency Medicine: Concepts and Clinical Practice, 9e
- Bailey & Love's Short Practice of Surgery, 28th Ed.
- Grainger & Allison's Diagnostic Radiology, 7e
- Goldman-Cecil Medicine, International Edition
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