Purulent Anthritis of Knee joint
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Purulent Arthritis of the Knee Joint (Septic Arthritis)
Purulent (septic) arthritis of the knee is an acute, destructive joint infection characterized by the presence of purulent (pus-containing) fluid within the joint space. The knee is the most commonly affected joint, accounting for ~50% of all cases.
Etiology & Pathogenesis
| Route | Description |
|---|---|
| Hematogenous (most common) | Bacteremia seeds the highly vascular synovium |
| Direct inoculation | Trauma, surgery, intra-articular injection |
| Contiguous spread | Adjacent osteomyelitis or soft tissue infection |
Common Causative Organisms
| Population | Organisms |
|---|---|
| Adults (non-STI) | Staphylococcus aureus (most common overall), Streptococcus spp., Gram-negatives |
| Sexually active adults | Neisseria gonorrhoeae (most common in young adults) |
| Elderly / immunocompromised | S. aureus, Gram-negatives (E. coli, Pseudomonas) |
| Children (< 5 yrs) | Kingella kingae, S. aureus, Group B Strep |
| IV drug users | Pseudomonas aeruginosa, S. aureus |
| Prosthetic joint | S. epidermidis, S. aureus, Cutibacterium acnes |
Clinical Presentation
- Acute monoarthritis — pain, swelling, warmth, erythema of the knee
- Restricted range of motion — patient holds knee in slight flexion (position of comfort)
- Fever, rigors, systemic toxicity
- Massive joint effusion — circumferential knee swelling (see image below)
- In gonococcal arthritis: may have preceding migratory polyarthralgia, skin pustules, tenosynovitis (the triad)
Diagnostic Imaging & Clinical Findings
This composite figure illustrates diagnostic findings in a confirmed case of purulent knee arthritis. Row A (sagittal MRI): significant joint effusion, bone marrow edema in distal femur and proximal tibia, posterior horn meniscal tears. Row B (post-operative MRI + AP radiograph): reduced joint swelling, improved marrow signal, soft tissue swelling, and periarticular osteopenia. Row C (clinical photographs): severe knee swelling with 29 cm longitudinal and 47.4 cm circumferential measurements, and bedside arthrocentesis yielding ~6 mL of yellow, purulent synovial fluid.
Investigations
Laboratory
| Test | Finding in Septic Arthritis |
|---|---|
| WBC | Elevated (non-specific) |
| ESR / CRP | Elevated (non-specific) |
| Blood cultures | Positive in ~50% — collect during febrile episodes |
| Procalcitonin | May support bacterial infection |
Synovial Fluid Analysis (Arthrocentesis — KEY diagnostic step)
| Parameter | Normal | Septic Arthritis |
|---|---|---|
| Appearance | Clear, straw-colored | Turbid, yellow-green, purulent |
| Viscosity | High | Low |
| WBC (cells/mm³) | < 200 | > 50,000 (often > 100,000); predominantly PMNs > 75% |
| Glucose | Equal to serum | Reduced |
| Protein | Low | Elevated |
| Gram stain | Negative | Positive in 50–70% |
| Culture | Negative | Positive in 70–90% (send in blood culture bottles) |
Per the Guide to Utilization of the Microbiology Laboratory (p. 82): "Traditionally, a synovial fluid leukocyte count more than 50,000 cells/mm³ was considered to suggest septic arthritis; however, lower counts do not exclude the diagnosis." If Gram stain and culture are negative, multiplex PCR or 16S rRNA gene PCR/sequencing should be considered. K. kingae and N. gonorrhoeae are included in recently FDA-cleared multiplex PCR panels for synovial fluid.
Imaging
| Modality | Findings |
|---|---|
| X-ray | Early: soft tissue swelling, joint space widening; Late: osteopenia, joint space narrowing, bony destruction |
| Ultrasound | Joint effusion (guides aspiration), synovial thickening |
| MRI (most sensitive) | Effusion, synovial enhancement, bone marrow edema, cartilage destruction, adjacent soft tissue involvement |
Differential Diagnosis
- Crystal arthropathies (gout, pseudogout) — synovial fluid microscopy distinguishes
- Reactive arthritis (Reiter's syndrome)
- Rheumatoid arthritis flare
- Hemarthrosis (trauma, coagulopathy)
- Transient synovitis (children)
- Lyme arthritis (Borrelia burgdorferi)
- Osteoarthritis exacerbation
Management
1. Drainage (Surgical or Needle)
Prompt and complete drainage is mandatory to prevent cartilage destruction.
| Method | Indication |
|---|---|
| Serial needle arthrocentesis | Early, accessible effusions; initial management |
| Arthroscopic lavage & debridement | Preferred when feasible; allows direct visualization |
| Open surgical drainage | Failed arthroscopy, loculated collections, adjacent osteomyelitis, prosthetic joint infection |
2. Empiric Antibiotic Therapy
Started immediately after cultures are obtained (do not wait for results).
| Clinical Scenario | Empiric Regimen |
|---|---|
| Healthy adult (non-STI) | Flucloxacillin or Nafcillin (anti-staphylococcal) IV |
| MRSA risk or penicillin allergy | Vancomycin IV |
| Gram-negative concern (elderly, immunocompromised, IVDU) | Add 3rd-gen cephalosporin or Piperacillin-tazobactam |
| Gonococcal arthritis | Ceftriaxone 1g IV/IM daily |
| Children | Flucloxacillin ± cefotaxime (cover K. kingae) |
3. Duration of Therapy
- Native joint: typically 2–4 weeks IV, followed by oral step-down (total 4–6 weeks)
- Gonococcal: 7–14 days (1 week IV then oral)
- Guided by clinical response, inflammatory markers, and organism sensitivities
4. Supportive Care
- Joint immobilization initially, then early mobilization and physiotherapy once infection controlled
- Analgesia (NSAIDs used cautiously — may mask fever)
- Treat underlying source (e.g., skin infection, endocarditis screening if bacteremia)
Complications
| Complication | Details |
|---|---|
| Cartilage destruction | From proteolytic enzymes; begins within hours–days |
| Osteomyelitis | Contiguous spread to bone |
| Avascular necrosis | Especially femoral head if hip involved |
| Joint fibrosis / ankylosis | Chronic stiffness, loss of function |
| Chronic osteomyelitis | In inadequately treated cases |
| Septicemia / sepsis | Life-threatening systemic spread |
| Growth disturbance | In children (epiphyseal involvement) |
Prognosis
- Excellent if treated within 24–48 hours of onset
- Delayed treatment (> 5–7 days) leads to significant cartilage and bone destruction
- Poor prognostic factors: extremes of age, S. aureus infection, pre-existing joint disease (RA), immunosuppression, prosthetic joint, delayed diagnosis
Key Clinical Pearl: The combination of acute monoarthritis + fever + purulent joint fluid must be treated as septic arthritis until proven otherwise. Do not delay arthrocentesis and empiric antibiotics waiting for imaging or confirmatory cultures.
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