Progress
Introduction
Overview & Learning Objectives
Why This Matters
Core Content
Pathophysiology
Who Is At Risk?
Clinical Presentation
Investigations
Joint Aspiration
Fluid Interpretation
Management
Clinical Application
Scenario 1 — Hot Swollen Knee
Scenario 2 — Knocked Knee
Assessment
Knowledge Check
References
Completion
Progress
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Orthopaedic Emergency Module

Septic Arthritis

Learning Objectives

By the end of this module, you should be able to confidently recognise, investigate, and initiate management for patients with suspected septic arthritis.

Recognise
Identify septic arthritis early
Recognise the typical history, examination findings, and red flags that should raise suspicion.
Investigate
Plan the initial work-up
Outline the key blood tests, imaging, and microbiological investigations required.
Aspirate
Understand joint aspiration
Describe the role of aspiration, the samples required, and the importance of prompt processing.
Interpret
Make sense of synovial fluid results
Interpret aspiration findings and understand how synovial fluid analysis supports diagnosis.
Manage
Start urgent treatment
Explain the principles of early management, including escalation, antibiotics, and surgical washout.
Avoid pitfalls
Recognise common errors
Identify diagnostic pitfalls and understand why delay can lead to irreversible joint damage.
Orthopaedic Emergency Module

Septic Arthritis

Why This Matters

Septic arthritis is an important orthopaedic emergency. A delay in diagnosis can lead to rapid cartilage destruction, permanent joint damage, systemic sepsis, and even death.

Because early symptoms can resemble other conditions such as gout, inflammatory arthritis, or traumatic effusions, the diagnosis is sometimes missed or delayed. Clinicians must therefore maintain a high index of suspicion.

Rapid Joint Destruction
Bacterial infection can destroy cartilage within hours to days due to inflammatory enzymes and increased intra-articular pressure.
Time Critical Diagnosis
Early aspiration and antibiotic treatment significantly improve outcomes and reduce the risk of permanent joint damage.
High Morbidity
Septic arthritis may lead to joint destruction, osteomyelitis, prolonged hospital stay, and long-term disability.
Take-home point: A hot swollen joint is septic arthritis until proven otherwise.
Orthopaedic Emergency Module

Septic Arthritis

Pathophysiology

Septic arthritis occurs when microorganisms enter the joint space and trigger a rapid inflammatory response. Because articular cartilage has limited ability to regenerate, infection can quickly lead to irreversible joint damage.

Understanding the mechanisms by which bacteria reach the joint helps clinicians recognise high-risk patients and initiate prompt investigation and treatment.

Routes of Infection to the Joint
Routes of joint infection diagram

• Haematogenous spread
• Direct inoculation (surgery or trauma)
• Spread from adjacent osteomyelitis or soft tissue infection

Click each route of infection to learn more

Haematogenous Spread (Most Common)
Bacteria circulating in the bloodstream seed the synovial membrane. Because the synovium is highly vascular and lacks a protective basement membrane, microorganisms can easily enter the joint space.

Common sources include:
  • Skin infections
  • Urinary tract infections
  • Respiratory infections
  • Intravenous lines or bacteraemia
This is the most common mechanism of septic arthritis.
Direct Inoculation
Infection may occur when bacteria are introduced directly into the joint.

This may happen following:
  • Joint aspiration
  • Intra-articular injections
  • Arthroscopy or open surgery
  • Penetrating trauma
Strict aseptic technique (ANTT) is therefore essential when performing joint procedures.
Contiguous Spread
Infection may spread into the joint from nearby tissues.

Examples include:
  • Adjacent osteomyelitis
  • Soft tissue abscess
  • Septic bursitis
  • Deep wound infection
This mechanism is more common in patients with immunosuppression or extensive local infection.
What Happens Inside the Joint?
1. Bacteria enter the synovial fluid and rapidly begin to multiply.
2. The immune system responds with neutrophil infiltration and inflammatory cytokine release.
3. Proteolytic enzymes and inflammatory mediators begin to damage articular cartilage.
4. Increased intra-articular pressure reduces blood supply to cartilage.
5. Rapid cartilage destruction can occur within days if infection is untreated.
Orthopaedic Emergency Module

Septic Arthritis

Who Is At Risk?

Although septic arthritis can occur in any patient, certain groups have a significantly higher risk of joint infection. Identifying these risk factors helps clinicians maintain a higher index of suspicion.

Patients with prosthetic joints are particularly important to recognise, as infections involving joint replacements can lead to severe complications and often require surgical management.

Knee surgery image
Prosthetic Joints
Joint replacements are vulnerable because bacteria can adhere to implant surfaces and form biofilms that are difficult to eradicate.
Immunosuppression
Patients receiving steroids, chemotherapy, or immunosuppressive medications are more susceptible to infection.
Chronic Disease
Conditions such as diabetes, chronic kidney disease, and liver disease increase the risk of bloodstream infections that may seed the joint.
Pre-existing Joint Disease
Rheumatoid arthritis and other inflammatory joint diseases damage the joint and increase susceptibility to infection.
Recent Joint Procedure
Joint injections, arthroscopy, and aspiration procedures may introduce bacteria if aseptic technique is not maintained.
Intravenous Drug Use
IV drug use increases the risk of bacteraemia, which may lead to haematogenous spread of infection to the joint.
Orthopaedic Emergency Module

Septic Arthritis

Clinical Presentation

Patients with septic arthritis typically present with a painful swollen joint and reduced ability to move the joint. Symptoms may develop rapidly over hours to days.

The presentation can vary depending on the patient's age, immune status, and underlying joint disease. Clinicians must therefore maintain a high index of suspicion.

Common Symptoms
😣
Severe Joint Pain
Pain is usually intense and worsens with movement or weight bearing.
🎈
Joint Swelling
Effusion causes visible swelling and a sensation of tightness or fullness in the joint.
🔥
Warmth & Erythema
The overlying skin may appear red and feel warm compared to the opposite joint.
🚫
Restricted Movement
Movement of the affected joint becomes extremely painful and limited.
🩼
Difficulty Weight Bearing
Patients may be unable to stand or walk due to severe pain.
🤒
Systemic Symptoms
Patients may feel generally unwell with fatigue, fever, chills, or malaise.

Examination

Examination should be focused, systematic, and gentle. Compare with the opposite side where possible, and avoid repeatedly manipulating a very painful joint.

Knee Examination Video

Watch the video below for a demonstration of how to examine for fluid within the knee.

Examine the Knee: Look · Feel · Move
👁️
Look
  • Inspect for swelling or a visible effusion
  • Look for erythema over the knee
  • Assess resting position
  • Check for scars from previous surgery
  • Observe reluctance to move
✋
Feel
  • Compare temperature with opposite knee
  • Palpate gently for tenderness
  • Assess for effusion
  • Identify focal tenderness or cellulitis
  • Consider if joint feels tense
🔄
Move
  • Assess active range of movement
  • Assess passive movement carefully
  • Note if small movements cause marked pain
  • Check straight leg raise or weight bearing
  • Painful passive movement is a red flag
Orthopaedic Emergency Module

Septic Arthritis

Investigations

When septic arthritis is suspected, blood tests and imaging may support the diagnosis, but the key diagnostic investigation is joint aspiration.

🩺
Observations
  • Temperature
  • Heart rate
  • Blood pressure
  • Respiratory rate
  • Oxygen saturation

Fever and tachycardia may suggest systemic infection, but normal observations do not exclude septic arthritis.

🧪
Blood Tests
  • Full blood count
  • CRP
  • ESR
  • Urea and electrolytes
  • Blood cultures

Inflammatory markers are often raised but are not specific for septic arthritis.

🩻
Imaging
  • X-ray of the affected joint
  • Ultrasound to detect effusion
  • MRI if diagnosis uncertain

Imaging may demonstrate an effusion or alternative pathology, but cannot exclude septic arthritis.

Imaging in Suspected Septic Arthritis

Why get an X-ray before aspiration?
It helps confirm the joint being assessed, identifies metalwork or a prosthesis, and may show other explanations for pain such as fracture, osteoarthritis, osteomyelitis, chondrocalcinosis, or loosening around a prosthesis.
X-ray of knee showing effusion

X-ray of the knee showing a knee effusion, marked by black arrows. It displaces the patella anteriorly and extends into the suprapatellar bursa.

Key findings to comment on
  • Soft tissue fullness suggesting an effusion
  • Presence of prosthesis or metalwork
  • Degenerative change or osteoarthritis
  • Chondrocalcinosis suggesting crystal arthropathy
  • Fracture, dislocation, or other structural abnormality
Orthopaedic Emergency Module

Septic Arthritis

Joint Aspiration

Joint aspiration is the gold standard investigation in suspected septic arthritis. It should be performed urgently to obtain fluid for microbiological analysis and crystal assessment.

If the patient is clinically stable, aspiration should ideally be performed before antibiotics are started to maximise diagnostic yield.

Demonstration Video

Watch the video below for an example of joint aspiration technique.

Equipment Checklist

Prepare everything before starting the procedure so that samples can be collected and sent without delay.

🧤
Sterile Preparation
  • Sterile gloves
  • ANTT wound pack
  • Chlorhexidine spray or prep sticks
  • Sterile gauze
💉
Aspiration Equipment
  • 10–20ml syringes
  • White or green needles
  • Local anaesthetic if required
  • Dressing (mepilex border)
  • Sharps bin
🧪
Sample Bottles
  • Blood culture bottles
  • EDTA bottle (red top)
  • Sterile universal containers (white top)
  • Request forms / labels
Processing the Samples

Once synovial fluid is obtained, it should be labelled clearly and sent promptly. Follow the flowchart below — start by selecting whether the aspiration was performed in or out of hours.

🧪
Synovial fluid obtained
Label all samples clearly before sending
💻
Request on eCare
'Joint aspirate MCS' → white top containers
'Sterile fluid in blood culture bottle' → culture bottles
🏷️
Print labels & microbiology forms
Print labels and forms for all samples
⚠️
State on request form
"Suspected septic arthritis" — this must be written clearly so microbiology can prioritise processing
What time is
the aspiration?
In hours
9am – 5pm Out of hours
5pm – 9am
🏥
In Hours
Drop samples immediately to the WSH laboratory

No further steps required — the lab will process directly
✅
Done
Samples delivered to WSH lab
Await results
🌙
Out of Hours
Samples processed at Addenbrookes Hospital

Note: you will need 2 universal white top containers (instead of 1)
📋
Follow overnight micro guidance
Refer to the overnight microbiology PDF — scroll to the Joint Aspirate section and follow the flowchart for sending samples to Addenbrookes
🔬
Retain one sample at WSH
Keep one sample at WSH where possible
Leave in a blue box in the biochemistry lab, or hand to the lab in the morning
✅
Done
Samples dispatched to Addenbrookes
Retain one at WSH · Await results
📄 Overnight Micro Samples PDF
Scroll to the Joint Aspirate section for the full flowchart covering the Addenbrookes sample dispatch process.
Orthopaedic Emergency Module

Septic Arthritis

Interpreting Synovial Fluid

Aspiration findings should always be interpreted alongside the clinical picture, observations, blood tests, and imaging.

Synovial fluid interpretation

Condition Appearance WCC (cells/µL) PMN % Crystals Bacteria
Non-inflammatory
Osteoarthritis Clear / yellow < 1,000 < 25% None None
Traumatic arthritis Straw / slightly cloudy < 2,000 < 25% None None
Inflammatory
Pseudogout Cloudy yellow 10,000–20,000 ~70% Calcium pyrophosphate None
Gout Cloudy / milky 10,000–50,000 ~70% Urate None
Rheumatoid arthritis Cloudy yellow 10,000–50,000 ~70% None None
Infection
Septic arthritis Turbid / purulent > 50,000 (often >100,000) > 90% None Present
⚠️ Important: WCC and PMN% thresholds overlap between conditions. Gram stain sensitivity in septic arthritis is only approximately 50–70%. Always interpret results in the context of the full clinical picture.

Synovial fluid appearance

The image below shows synovial fluid from different conditions. Click each appearance type below to learn more about what it indicates.

Synovial fluid appearance samples
Normal
Clear, colourless
▼

Normal synovial fluid is clear and colourless or pale straw, with high viscosity. It should look similar to egg white.

  • WCC < 200 cells/µL
  • PMN < 25%
  • No crystals, no bacteria
Seen in: healthy joints with no significant pathology.
Non-inflammatory
Straw coloured, transparent
▼

Straw-coloured, transparent fluid with good viscosity is typical of non-inflammatory conditions. The fluid is clear enough to read newsprint through it.

  • WCC < 2,000 cells/µL
  • PMN < 25%
  • No crystals, no bacteria
Seen in: osteoarthritis, traumatic effusion, early degenerative disease.
Inflammatory
Cloudy, yellow
▼

Cloudy or turbid yellow fluid with reduced viscosity indicates significant inflammation. The cloudiness is caused by a high white cell count and cellular debris.

  • WCC 10,000–50,000 cells/µL
  • PMN ~70%
  • Crystals may be present (gout, pseudogout)
  • No bacteria
Seen in: gout, pseudogout, rheumatoid arthritis, reactive arthritis. Septic arthritis must always be excluded.
Septic
Turbid, thick, purulent
▼

Purulent or frankly thick fluid is strongly suggestive of joint infection. It may be yellow-green and thick, and cannot be read through.

  • WCC > 50,000 cells/µL (often > 100,000)
  • PMN > 90%
  • Gram stain may be positive — but sensitivity is only ~50–70%
  • Culture and sensitivity essential
⚠️ Treat as septic arthritis until proven otherwise. Purulent appearance alone is sufficient to escalate urgently, regardless of Gram stain result.
Haemorrhagic
Red / bloody
▼

Bloody or blood-stained fluid indicates haemarthrosis. This is distinct from a traumatic tap, which typically clears as aspiration continues.

  • RBC predominant
  • WCC variable depending on cause
  • Look for fat globules — lipohemarthrosis suggests an intra-articular fracture
Seen in: trauma, fracture into the joint, coagulopathy, haemophilia, pigmented villonodular synovitis (PVNS).
Key point: Appearance alone is not diagnostic — always send synovial fluid for microscopy, culture, and crystal analysis, regardless of how it looks macroscopically.
Key Takeaways
  • Joint aspiration is the key diagnostic investigation in suspected septic arthritis
  • Perform aspiration urgently and ideally before antibiotics if the patient is stable
  • Prepare equipment and sample bottles before starting
  • Follow the correct in-hours or out-of-hours sample pathway
  • Interpret results in the context of the whole clinical picture
Orthopaedic Emergency Module

Septic Arthritis

Management

Management overview image
Immediate Management Steps
1. Assess the patient
  • Check for systemic upset or sepsis
  • Document examination findings carefully
  • Assess whether the patient is stable for aspiration before antibiotics
2. Take investigations
  • Blood tests including inflammatory markers
  • Blood cultures
  • X-ray of the joint
  • Urgent joint aspiration where feasible
3. Provide supportive care
  • Analgesia
  • Intravenous fluids if indicated
  • Limb support and rest
  • Sepsis management if clinically required
4. Make a definitive plan
  • Discuss with senior orthopaedic decision-maker
  • Contact microbiology if needed
  • Arrange theatre if washout is indicated
  • Document timing clearly
Aspiration Before Antibiotics

In a stable patient, aspiration should usually be performed before antibiotics are given. This maximises microbiological yield and supports accurate diagnosis.

When to Start Antibiotics Immediately

If the patient is systemically unwell, septic, or deteriorating, treatment should not be delayed. Obtain blood cultures and start IV antibiotics as part of the sepsis response.

Clinical judgement matters
Septic shock takes priority over ideal sequencing of samples.
Surgical Management
Surgical management image

Many patients with septic arthritis will require operative washout. The goal is to reduce the bacterial load, remove purulent material, and protect articular cartilage from ongoing damage.

Common indications
  • Frank pus on aspiration
  • Strong clinical suspicion
  • Failure to improve
  • Large symptomatic effusion
Possible approaches
  • Arthroscopic washout
  • Open washout
  • Repeat washout if required
In theatre
  • Take multiple samples
  • Send specimens correctly
  • Record joint appearance
  • Document the procedure
Antibiotic Therapy

Empirical antibiotic choice should follow local policy and microbiology advice. Treatment is later tailored according to Gram stain, culture results, sensitivities, and clinical response.

Key points
  • Use local guidance
  • Seek microbiology input early
  • Tailor treatment to culture results
  • Monitor response clinically and biochemically
Monitoring during treatment
  • Temperature and observations
  • Pain and joint symptoms
  • CRP and white cell count trend
  • Clinical assessment if post-op
Clinical Scenario

The Hot Swollen Knee

Clinical scenario image — hot swollen knee
⏱️ Time: 14:20

You are the orthopaedic SHO covering ED. A 68-year-old man presents with:

  • 24-hour history of a painful swollen left knee
  • Increasing pain and inability to weight-bear
  • Feeling generally unwell

He has a background of type 2 diabetes and hypertension.

On examination:

  • The knee is hot, swollen, and held in slight flexion
  • Severely restricted range of movement
"I can't put any weight on it… it's getting worse."

Temperature: 38.2°C

He appears systemically unwell.

⏸️ Pause and Think
Before continuing, consider what your next step would be in assessing this patient.
Scenario Questions
Q1. What is the single most important initial investigation in this patient?
AMRI of the left knee
BUrgent joint aspiration
CBlood cultures alone
DX-ray of the knee
Q2. This patient has type 2 diabetes. How does this affect your level of suspicion for septic arthritis?
AIt reduces your suspicion — diabetics commonly get inflammatory arthritis
BIt increases your suspicion — diabetes is a recognised risk factor for septic arthritis
CIt has no effect — septic arthritis risk is unrelated to comorbidities
DIt makes septic arthritis impossible to diagnose without a positive culture
Q3. His temperature is 38.2°C but his CRP is only mildly elevated. Which statement is most accurate?
AA normal CRP rules out septic arthritis
BSeptic arthritis can still be present with a mildly elevated CRP — clinical findings take priority
CCRP is the gold standard investigation for septic arthritis
DA temperature above 38°C is required to diagnose septic arthritis
Clinical Scenario

A Knocked Knee

Clinical scenario image — knocked knee
⏱️ Time: 18:45

You are the orthopaedic SHO covering ED. A 67-year-old woman presents with:

  • 2-day history of worsening left knee pain, after her grandson ran into her knee
  • Increasing pain and inability to weight-bear
  • Pain has been keeping her up at night
  • Feeling generally unwell with low-grade fevers

She has a background of rheumatoid arthritis and is currently taking:

  • Adalimumab
  • Prednisolone

On examination:

  • The knee is hot, swollen, and tense with a large effusion
  • Severely restricted active and passive range of movement
  • She is unable to tolerate any movement of the joint
"My joints flare sometimes… but this feels different — I can't move it at all."

Temperature: 37.8°C

She appears mildly systemically unwell.

Scenario Questions
Q1. This patient has rheumatoid arthritis and is taking adalimumab and prednisolone. Why is septic arthritis still the primary concern despite her background of inflammatory arthritis?
ARheumatoid arthritis and septic arthritis cannot occur in the same joint
BImmunosuppression significantly increases her risk of joint infection, and the two conditions can coexist
CHer history of trauma to the knee makes gout more likely
DA temperature below 38°C makes infection unlikely
Q2. She reports this feels "different" from her usual RA flares. Which feature is most suggestive of septic arthritis rather than an inflammatory flare?
AInvolvement of a large joint such as the knee
BThe presence of rheumatoid arthritis in her history
CInability to tolerate any passive movement of the joint
DThe fact that the effusion developed over 2 days
Q3. It is 18:45. Aspiration is performed and the fluid is turbid. Gram stain shows no organisms. What is the correct next step?
AReassure the patient — a negative Gram stain rules out infection
BTreat as a rheumatoid flare and withhold antibiotics
CDiscuss urgently with the on-call senior; do not be falsely reassured by a negative Gram stain
DDischarge with oral antibiotics and outpatient review
Q4. Crystal analysis of the fluid shows calcium pyrophosphate crystals. What is the most appropriate interpretation?
ACrystal-positive fluid confirms pseudogout and excludes septic arthritis
BThe presence of crystals does not exclude septic arthritis — both can coexist
CCrystal-positive fluid means antibiotics are not needed
DCalcium pyrophosphate crystals indicate rheumatoid arthritis, not pseudogout
Assessment

Knowledge Check

Test your understanding

Complete the following 6 questions to test your understanding of septic arthritis. Select the best answer for each question.

References

References & Further Reading

Evidence base for this module

Clinical Guidelines

  • 1.National Institute for Health and Care Excellence (NICE). Septic arthritis. Clinical Knowledge Summary. London: NICE; updated 2023. Available at: cks.nice.org.uk
  • 2.British Society for Rheumatology (BSR). Guidelines for the management of the hot swollen joint in adults. London: BSR; 2006. (Endorsed by BHPR.)
  • 3.Mathews CJ, Weston VC, Jones A, Field M, Coakley G. Bacterial septic arthritis in adults. Lancet. 2010;375(9717):846–855.

Key Texts

  • 4.Coakley G, Mathews C, Field M, et al. BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults. Rheumatology (Oxford). 2006;45(8):1039–1041.
  • 5.Roberts S, Holloway A. Septic arthritis — management. BMJ Best Practice. London: BMJ; 2023. Available via BMJ Best Practice.
  • 6.Shirtliff ME, Mader JT. Acute septic arthritis. Clin Microbiol Rev. 2002;15(4):527–544.

Synovial Fluid Analysis

  • 7.Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis? JAMA. 2007;297(13):1478–1488.
  • 8.McGillicuddy DC, Shah KH, Friedberg RP, Nathanson LA, Edlow JA. How sensitive is the synovial fluid white blood cell count in diagnosing septic arthritis? Am J Emerg Med. 2007;25(7):749–752.

Surgical Management

  • 9.Luhmann SJ, Luhmann JD. Etiology of septic arthritis in children: an update for the 1990s. Pediatr Emerg Care. 1999;15(1):40–42.
  • 10.Skedros JG, Hunt KJ, Pitts TC. Variations in corticosteroid/anesthetic injections for painful shoulder conditions: comparisons among orthopaedic surgeons, rheumatologists, and physical medicine and primary-care physicians. BMC Musculoskelet Disord. 2007;8:63.

Imaging

  • 11.Knee effusion X-ray image: Häggström M. Knee effusion, annotated. Wikimedia Commons; 2016. CC0 licence. Available at: commons.wikimedia.org
  • 12.Routes of joint infection diagram: Radiologykey.com. C9-FF1.gif. Available at: radiologykey.com

Video Resources

  • 13.Knee examination video. YouTube. Available at: youtube.com
  • 14.Joint aspiration demonstration video. YouTube. Available at: youtube.com
🏆

Module Complete!

Congratulations — you have completed the Septic Arthritis orthopaedic emergency module. You are now better equipped to recognise, investigate, and manage this joint-threatening emergency.

13
Sections completed
2
Clinical scenarios
—
Knowledge check score
Remember: Septic arthritis is a joint-threatening emergency. A hot swollen joint is septic arthritis until proven otherwise.

What you should now be able to do

✓Maintain a high index of suspicion for septic arthritis in any patient with an acutely painful swollen joint
✓Identify risk factors including immunosuppression, prosthetic joints, and chronic disease
✓Plan an appropriate investigation pathway including urgent joint aspiration
✓Handle and process synovial fluid samples correctly via the in-hours and out-of-hours pathways
✓Interpret synovial fluid findings in the context of the clinical picture
✓Initiate immediate management and escalate appropriately to senior colleagues
✓Avoid common diagnostic pitfalls including false reassurance from negative investigations
Take-home point:

A hot swollen joint is septic arthritis until proven otherwise. Do not delay aspiration, escalation, or antibiotics in a deteriorating patient.