High Yield Overview

SEPTIC ARTHRITIS

Orthopaedic Emergency | Urgent Washout | Staph aureus

EmergencySurgical urgency

Staph aureusMost common organism

WCC greater than 50kSuggestive aspirate

WashoutTreatment mainstay

Key Features

Native joint

PatternAspiration + washout + IV antibiotics

TreatmentEmergency

Prosthetic joint

PatternComplex - DAIR, 1-stage, 2-stage

TreatmentDepends on acuity

Gonococcal

PatternYoung, sexually active

TreatmentAntibiotics often sufficient

Critical Must-Knows

Orthopaedic emergency - joint destruction within 24-48 hours
Aspirate joint before antibiotics if possible
Staph aureus is most common organism
WCC greater than 50,000 highly suggestive (greater than 90% sensitivity)
Surgical washout + IV antibiotics is standard treatment

Examiner's Pearls

"
Aspirate: WCC, Gram stain, crystals, culture
"
Kocher criteria for paediatric hip septic arthritis
"
Risk factors: RA, DM, immunosuppression, recent joint procedure
"
Gonococcal: Young, migratory arthralgia, skin lesions

Critical Septic Arthritis Points

Aspiration

Joint aspiration is essential. Send for: WCC and differential, Gram stain, Culture, Crystals (rule out gout). WCC greater than 50,000 with greater than 90% PMNs highly suggestive.

Staph aureus

Most common organism in adults. Consider MRSA in at-risk patients. Gonococcus in young sexually active. Strep, GNBs less common. Consider IV drug users (unusual organisms).

Surgical Emergency

Joint destruction occurs rapidly (24-48 hours). Cartilage damage from enzymes and inflammatory response. Do not delay washout. Repeated washouts may be needed.

Treatment

Surgical washout (arthroscopic or open) + IV antibiotics (4-6 weeks). Choice until cultures: Flucloxacillin (or vancomycin if MRSA risk). Consult microbiology.

Mnemonic

WGCCAspirate Analysis

WCC and differential

Greater than 50k = likely septic

Gram stain

May show organisms

Culture

Definitive organism ID

Crystals

Rule out gout/pseudogout

Memory Hook:WGCC = Aspirate essentials (WCC, Gram, Culture, Crystals)!

Overview

Septic arthritis is a bacterial infection of a joint. It is an orthopaedic emergency because cartilage destruction occurs rapidly (within 24-48 hours).

Pathophysiology

Bacteria enter the joint via haematogenous spread (most common), direct inoculation (injection, surgery), or spread from adjacent osteomyelitis.

Inflammatory response and bacterial enzymes cause rapid cartilage destruction.

Risk Factors

Rheumatoid arthritis
Diabetes mellitus
Immunosuppression
Recent joint injection or surgery
IV drug use
Prosthetic joint

Pathophysiology

Routes of Infection

Bacteria reach the synovial space through three main routes:

Haematogenous spread (most common, 70-80%)
- Bacteraemia from distant focus (skin, UTI, pneumonia)
- Synovium is highly vascular with no basement membrane
- Bacteria lodge and proliferate rapidly
Direct inoculation (15-20%)
- Joint injection or aspiration
- Arthroscopy or open surgery
- Penetrating trauma
Contiguous spread (5-10%)
- Adjacent osteomyelitis
- Soft tissue infection spreading to joint

Pathological Cascade

Within hours:

Bacterial proliferation triggers inflammatory response
Neutrophil infiltration releases proteolytic enzymes (collagenase, elastase)
Cytokines (IL-1, TNF-alpha) amplify destruction

Within 24-48 hours:

Cartilage matrix degradation begins
Proteoglycan loss impairs load-bearing capacity
Chondrocyte death from hypoxia and enzyme damage

Beyond 48 hours:

Irreversible cartilage damage
Pannus formation
Bone erosion at joint margins

Microbiology

Staphylococcus aureus - Most common overall (60-70%)

Both MSSA and MRSA
Produces adhesins, toxins, biofilm

Streptococci - Second most common (15-20%)

Group A, B, and viridans streptococci
Group B common in diabetics, elderly

Gram-negative bacilli (10-15%)

E. coli, Pseudomonas, Klebsiella
More common in elderly, immunocompromised, IVDU

Consider organism based on patient risk factors and presentation.

Mnemonic

HIDRoutes of Joint Infection

Haematogenous

Most common (70-80%), bacteraemia seeds joint

Inoculation

Direct entry via injection, surgery, trauma

Direct spread

From adjacent osteomyelitis or soft tissue

Memory Hook:Bacteria HID in joints via blood, injection, or direct spread!

Mnemonic

PRISMSeptic Arthritis Risk Factors

Prosthetic joint

Foreign material, biofilm risk

Rheumatoid arthritis

Immunosuppression, damaged joint

Immunocompromised

DM, steroids, HIV, malignancy

Skin breakdown

Portal of entry for organisms

IVDU/Recent procedure

Direct inoculation risk

Memory Hook:Think of PRISM to identify high-risk patients for septic arthritis!

Clinical Features and Diagnosis

Clinical Features

Painful, swollen joint - acute onset over hours to days
Unable to bear weight (if lower limb)
Limited ROM (held in position of comfort - flexion for knee, abduction/ER for hip)
Warmth and erythema over joint
Fever (may be absent in elderly, immunocompromised)
Usually monoarticular (knee most common, then hip, shoulder)

Joint Distribution

Joint	Frequency	Key Points
Knee	40-50%	Most common, easily aspirated
Hip	15-20%	Children especially, can be missed
Shoulder	10-15%	May present as pseudoparalysis
Ankle	5-10%	Must exclude osteomyelitis
Wrist/Hand	5%	Consider gonococcal, IVDU

Differential Diagnosis

Crystal arthropathy (gout, pseudogout) - can coexist with sepsis
Reactive arthritis - recent GI/GU infection, HLA-B27
Rheumatoid flare - polyarticular, known RA
Haemarthrosis - trauma, coagulopathy
Transient synovitis (children) - afebrile, mild symptoms

Investigations

Joint Aspiration (Gold Standard)

Aspiration technique:

Aseptic technique essential
Mark anatomical landmarks
Aspirate BEFORE antibiotics if possible (but do not delay treatment)

Synovial fluid analysis:

Test	Septic Arthritis	Normal	Inflammatory
WCC (cells/mcL)	greater than 50,000	less than 200	2,000-50,000
PMN (%)	greater than 90%	less than 25%	50-75%
Gram stain	Positive 50-75%	Negative	Negative
Culture	Positive 80-90%	Negative	Negative

Important: WCC greater than 50,000 has 90% sensitivity but crystals do NOT exclude infection - can coexist.

Blood Tests

WCC - elevated in 50-60%
CRP - elevated in greater than 90% (most sensitive)
ESR - elevated but slow to change
Blood cultures - positive in 40-50%
Procalcitonin - may help differentiate from crystal arthropathy

Imaging

X-ray:

Often normal early
Soft tissue swelling, joint effusion
Late: joint space narrowing, erosions, destruction

Ultrasound:

Detects effusion (especially hip, shoulder)
Guides aspiration
Cannot differentiate septic from sterile effusion

MRI:

Most sensitive for early changes
Shows bone marrow oedema, soft tissue involvement
Useful for deep joints (hip, SI joint)

Clinical presentation of bilateral sternoclavicular joint septic arthritis — Clinical photograph demonstrating bilateral sternoclavicular joint septic arthritis: visible erythema and swelling over both SC joints with black marker line outlining the extent of cellulitis. This patient developed SC joint infection secondary to central venous catheter contamination. SC joint septic arthritis is uncommon but important - risk factors include IV drug use, central line infection, and immunosuppression.Credit: Pradhan C et al., J Med Case Rep - CC BY 4.0

CT imaging of bilateral sternoclavicular joint septic arthritis — Two-panel CT imaging of bilateral SC joint septic arthritis: (A) Coronal CT of chest. (B) Axial CT at SC joint level with red arrows showing bilateral joint destruction and effusion (left greater than right). Note the joint space widening and bony erosion characteristic of septic arthritis. CT is the imaging modality of choice for SC joint infection to assess extent of destruction and guide surgical planning.Credit: Pradhan C et al., J Med Case Rep - CC BY 4.0

Management

Principles: Urgent washout + IV antibiotics.

Surgical Washout

Arthroscopic approach (preferred for accessible joints):

Knee, shoulder, ankle, wrist
Advantages: Less soft tissue trauma, better visualisation, shorter recovery
Technique: Thorough lavage with 9+ litres saline, debridement of infected tissue
Remove fibrin clots and debris

Open approach:

Hip (difficult arthroscopic access)
Failed arthroscopic washout
Complex cases with extensive infection

Key surgical principles:

Tissue samples for culture (at least 3-5 samples)
Copious lavage (minimum 9 litres)
May need repeated washouts every 48-72 hours if ongoing sepsis
Consider leaving drain in situ

Antibiotic Therapy

Empiric antibiotics:

Start after aspiration (do not delay for culture results)
Flucloxacillin 2g IV QID - first-line for most cases
Vancomycin if MRSA risk (recent hospitalisation, IVDU, diabetes)
Add gentamicin or ceftriaxone if Gram-negative suspected

Definitive therapy:

Adjust based on cultures and sensitivities
Involve infectious diseases/microbiology team
Duration: IV 2-4 weeks then oral step-down to complete 4-6 weeks total

Oral step-down options:

Flucloxacillin 1g QID
Cephalexin 1g TDS
Clindamycin (if penicillin allergy)

Post-operative Management

Rehabilitation:

Early active and passive ROM exercises
Weight bearing as tolerated
Physical therapy involvement
Splinting in position of function if needed

Monitoring:

Repeat inflammatory markers (CRP, WCC) every 2-3 days
Clinical assessment for resolution
Repeat aspiration if persistent effusion

Post-operative care crucial for functional outcome.

Australian Context

Antibiotic guidelines (eTG recommendations):

Empiric: Flucloxacillin 2g IV 6-hourly
Penicillin allergy: Vancomycin + consider ceftriaxone
MRSA risk: Vancomycin 25-30mg/kg loading then 15-20mg/kg 12-hourly

Indigenous populations:

Higher rates of S. aureus bacteraemia
Consider community-acquired MRSA
Ensure adequate follow-up in remote settings

Prosthetic joint infections:

Refer to AOANJRR data for revision rates
Multidisciplinary involvement (orthopaedics, ID, microbiology)
Antibiotic cement spacers manufactured locally available

Do Not Delay Treatment

Septic arthritis is an orthopaedic emergency. Joint cartilage is destroyed within 24-48 hours. Aspirate the joint, start antibiotics, and proceed to surgical washout urgently.

Complications

Clinical photograph of iatrogenic septic arthritis at acromioclavicular joint — Iatrogenic septic arthritis: Clinical photograph of left shoulder showing erythema around the acromioclavicular joint following prior corticosteroid injection (black arrow indicates injection site). Inset shows close-up of crusted skin lesion from Mycobacterium avium-intracellulare infection. This case demonstrates the risk of septic arthritis following joint injection procedures - strict aseptic technique and appropriate patient selection are essential to minimize this complication.Credit: Murdoch DM et al., BMC Infect Dis - CC BY 4.0

Early Complications

Persistent infection:

Inadequate debridement - failure to remove all infected tissue
Resistant organism - MRSA, multi-drug resistant GNB
Biofilm formation (especially PJI) - bacteria protected from antibiotics
May require repeated washouts (every 48-72 hours until resolved)
Consider changing antibiotic regimen based on sensitivities

Systemic sepsis:

Can progress to septicaemia with bacteraemia
Multi-organ failure in severe cases (ARDS, AKI, DIC)
Mortality 10-15% overall
Higher mortality in elderly (up to 30%) and immunocompromised patients
Requires ICU admission and aggressive resuscitation

Wound complications:

Wound dehiscence after open washout
Sinus tract formation with chronic drainage
Skin necrosis requiring plastic surgery input

Late Complications

Joint destruction:

Cartilage loss is irreversible once proteoglycans depleted
Occurs within 24-48 hours without treatment
Results in secondary osteoarthritis requiring arthroplasty
Worse outcomes in weight-bearing joints (hip, knee)

Osteonecrosis:

Particularly hip joint in children
Septic arthritis can damage blood supply to femoral head
May develop Perthes-like changes
Long-term surveillance required

Ankylosis:

Fibrous or bony fusion of joint surfaces
More common with delayed treatment beyond 7 days
May require arthrodesis for pain relief
Consider arthroplasty if bone stock adequate

Growth disturbance (children):

Physeal damage if infection crosses growth plate
Limb length discrepancy (can be several centimetres)
Angular deformity requiring corrective osteotomy
Growth arrest lines visible on X-ray

Chronic pain and stiffness:

Post-infectious arthritis even after eradication
Reduced range of motion from fibrosis
May require prolonged rehabilitation

Outcomes and Prognosis

Functional outcomes:

Good to excellent outcome in 70-80% if treated promptly
Poor outcome associated with delayed diagnosis
Hip and shoulder have worse functional prognosis

Prognostic Factors

Poor prognosis associated with:

Delay in treatment more than 7 days (single most important factor)
Age more than 65 years
Pre-existing joint disease (RA, OA)
Polyarticular involvement (often haematogenous)
Virulent organisms (S. aureus worse than streptococci)
Prosthetic joint involvement
Immunocompromised state (diabetes, HIV, malignancy, steroids)
Axial joint involvement (hip, shoulder)

Good prognosis associated with:

Treatment within 24-48 hours
Single joint involvement
Streptococcal or gonococcal infection
Young healthy patient
Peripheral joint (knee, ankle)

Evidence Base

Kocher Criteria for Paediatric Hip (1999)

Level III

Key Findings:

4 independent predictors: fever, non-weight bearing, WCC elevated, ESR elevated
0 predictors = very low probability of septic arthritis
4 predictors = 99.6% probability of septic arthritis
Validated in subsequent studies with moderate accuracy

Clinical Implication: Use Kocher criteria to stratify risk in children with hip pain and effusion. 4/4 criteria mandates urgent aspiration.

Arthroscopic vs Open Washout - Systematic Review (2016)

Level III

Key Findings:

No RCTs available - only retrospective comparative studies
Similar infection resolution rates (85-95%)
Arthroscopic associated with shorter hospital stay
Lower complication rates with arthroscopic approach
Open still preferred for hip, complex cases

Clinical Implication: Arthroscopic washout preferred for accessible joints (knee, shoulder). Open approach for hip or when arthroscopic fails.

Antibiotic Duration in Native Joint Septic Arthritis (2021)

Level II

Key Findings:

Non-inferiority of 3 weeks demonstrated
Remission at 2 years: 91% (3 weeks) vs 93% (6 weeks)
Shorter treatment reduces adverse effects and costs
Oral step-down therapy effective after initial IV

Clinical Implication: Consider shorter antibiotic courses (3-4 weeks) in uncomplicated native joint septic arthritis with good source control.

Procalcitonin in Septic Arthritis Diagnosis (2019)

Level III

Key Findings:

Pooled sensitivity 59%, specificity 85%
Useful to differentiate septic from crystal arthritis
CRP remains more sensitive for screening
Procalcitonin may help in equivocal cases

Clinical Implication: Procalcitonin can supplement but not replace synovial fluid analysis. More specific but less sensitive than CRP.

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Septic Knee

EXAMINER

"A 60-year-old diabetic presents with a hot, swollen, painful knee. He cannot bear weight. Temperature is 38.5°C. How do you manage?"

EXCEPTIONAL ANSWER

This presentation is highly concerning for **septic arthritis** - a hot, swollen, painful joint in a patient with risk factors (diabetes) and systemic features (fever). Septic arthritis is an orthopaedic emergency. My immediate management: I would aspirate the knee under aseptic technique. The aspirate would be sent for **WCC and differential** (greater than 50,000 with greater than 90% neutrophils is highly suggestive), **Gram stain**, **Culture**, and **Crystals** (to rule out gout, which can coexist). I would also take blood cultures and send bloods for WCC, CRP, ESR. If the aspirate is turbid and the clinical picture is consistent, I would treat this as septic arthritis. The most common organism is **Staphylococcus aureus**. I would start empiric **IV antibiotics** (flucloxacillin, or vancomycin if MRSA risk, which is possible in a diabetic) and proceed urgently to **surgical washout** of the knee, preferably arthroscopic. The washout is critical to remove pus and reduce bacterial load. I would plan for possible repeat washouts if ongoing sepsis. Post-operatively, the patient would continue IV antibiotics for 2-4 weeks then oral to complete 4-6 weeks total, guided by cultures and sensitivities. Early mobilization of the knee is important to prevent stiffness.

KEY POINTS TO SCORE

Aspirate before antibiotics if possible

WCC greater than 50k, Gram stain, culture, crystals

Staph aureus most common

Urgent surgical washout + IV antibiotics

COMMON TRAPS

✗Delaying washout

✗Not aspirating before antibiotics

✗Forgetting to check crystals

LIKELY FOLLOW-UPS

"What if it was a prosthetic knee?"

"What is gonococcal arthritis?"

VIVA SCENARIOAdvanced

Scenario 2: Prosthetic Joint Infection

EXAMINER

"A 72-year-old woman presents 3 weeks after total knee replacement with increasing pain, wound drainage, and low-grade fever. Her wound looks erythematous with some purulent discharge. How would you assess and manage this?"

EXCEPTIONAL ANSWER

This clinical picture is concerning for **acute prosthetic joint infection (PJI)** - occurring within 3 months of surgery with wound symptoms and systemic features. My assessment: I would take a thorough history including the primary surgery (cemented vs uncemented, any intraoperative concerns), antibiotic prophylaxis used, and any recent infections elsewhere. Examination would assess wound healing, effusion, range of motion, and signs of systemic sepsis. **Investigations**: Bloods (WCC, CRP, ESR - CRP most useful for monitoring), blood cultures, and crucially **joint aspiration** under aseptic technique for WCC, differential, Gram stain, and culture. For PJI, WCC more than 3000 or PMN more than 80% is concerning. I would also consider holding antibiotics until cultures obtained if the patient is stable. **Management**: Given this is **acute PJI** (within 3 weeks, symptoms less than 3 weeks), the patient is a candidate for **DAIR - Debridement, Antibiotics, Irrigation, and Retention**. This involves urgent return to theatre for thorough debridement of infected tissue, copious lavage (9+ litres), and exchange of modular polyethylene components. If the prosthesis is well-fixed and the organism is susceptible, DAIR success rates are 60-80%. Post-operatively, IV antibiotics for 2-6 weeks followed by prolonged oral suppression, guided by microbiology input. If DAIR fails or the infection is chronic, **two-stage revision** would be required.

KEY POINTS TO SCORE

Acute PJI = within 3 weeks, symptoms less than 3 weeks

DAIR for acute PJI with stable implant

Exchange modular components during DAIR

Two-stage revision for chronic PJI or DAIR failure

COMMON TRAPS

✗Attempting DAIR for chronic PJI

✗Not exchanging polyethylene

✗Forgetting prolonged antibiotics post-DAIR

✗Missing the surgical emergency window

LIKELY FOLLOW-UPS

"What organisms cause late PJI?"

"What are the criteria for successful DAIR?"

"How do you perform two-stage revision?"