01OverviewDefinition, clinical context and the essential points that orientate the chapter.
History estimates both probability and organism. Define onset, joint, recent bacteraemia, skin or urinary infection, surgery, injection, penetrating injury, animal or human bite, sexual exposure, travel and previous antibiotics. Ask about diabetes, renal or liver disease, inflammatory arthritis, immunosuppression, intravenous drug use and anticoagulants. Examine skin, heart, spine and every joint for source or metastatic infection rather than treating the swollen joint as an isolated cavity.
Examine active and passive movement gently. Global restriction and pain through a small arc suggest intra-articular inflammation; superficial cellulitis or bursitis may preserve some painless passive range. Look for effusion, warmth, fixed posture and adjacent abscess. Hip infection can present as groin, thigh or knee pain without visible swelling; shoulder infection may mimic rotator-cuff disease. Record neurovascular status before aspiration, splintage or anaesthesia.
Sampling depends on stability. Obtain blood cultures promptly because they can be positive when synovial culture is not. If the patient has sepsis or shock, draw cultures without meaningful delay and start broad-spectrum intravenous therapy immediately. If systemically stable, aspirate first to preserve microbiological yield. Do not inject local anaesthetic or corticosteroid into the joint. Ultrasound guidance improves access to the hip and small or technically difficult effusions.
Send enough fluid for microscopy and Gram stain, cell count and differential, aerobic and anaerobic culture and crystals. Tell the laboratory about antibiotics, immunosuppression and suspected gonococcus or unusual exposure; extended incubation or molecular testing may be needed after negative culture with persistent suspicion. A purulent aspirate is highly concerning, but appearance alone cannot separate crystal from infection. Crystals, low cell count and negative Gram stain each modify probability without excluding disease.
Synovial white-cell thresholds are contextual. Counts above fifty thousand per microlitre support infection, particularly with a high neutrophil percentage, but inflammatory disease can exceed this and immunosuppression or partially treated infection can remain below it. Serum CRP, FBC, renal function and lactate support severity and a baseline but cannot decide alone. Plain radiographs show joint-space, fracture, gas or chronic damage; ultrasound finds fluid, and MRI maps osteomyelitis, deep abscess or an inaccessible joint when it will not delay drainage.
Source control protects cartilage. An infected large native joint usually needs arthroscopic or open irrigation and synovectomy, selected by joint, stage, adhesions, host and surgical expertise. Obtain multiple operative tissue samples with separate instruments. Current BOASt directs native-joint infection toward acute PJI emergency standards because chondral injury is time critical; in an acutely unwell patient, drainage should occur as soon as safe and within six hours unless a documented reason prevents this.
Empiric antibiotics follow local microbiology and the Gram stain, age, immune status, acquisition and allergy. Staphylococcal coverage is usually necessary; add Gram-negative or resistant-organism cover when risk supports it. Narrow promptly to organism and susceptibilities. Route and total duration depend on drainage adequacy, organism, adjacent osteomyelitis, endocarditis and response rather than a fixed universal course. Gonococcal disease requires genital and extragenital nucleic-acid testing, sexual-health input, partner notification and regimen selection under current BASHH guidance.
Review response daily. Pain, fever, range, wound, CRP trajectory and organ function should improve after effective drainage. Persistent fever, positive cultures, rising CRP, continued effusion or worsening movement requires repeat imaging, aspiration or washout and a search for endocarditis, abscess and osteomyelitis. Begin range and weight bearing when the surgeon permits and sepsis settles, avoiding unnecessary prolonged immobilisation. Communicate organism, antibiotic plan, monitoring, access care and return warnings at discharge.
Key points
- Suspect septic arthritis with acute severe joint pain, restricted active and passive movement and an effusion, but recognise that fever and inflammatory markers may be absent.
- Run the sepsis pathway and obtain at least two blood-culture sets. In shock or high-risk sepsis, start intravenous antibiotics immediately after cultures and never delay for aspiration.
- In a stable patient, aspirate before antibiotics using sterile technique and image guidance for a deep or difficult joint; record volume, appearance and any preceding antimicrobial exposure.
- Send synovial fluid urgently for Gram stain, aerobic and anaerobic culture, white-cell count with differential and crystal analysis; inoculate blood-culture bottles when local laboratory policy supports it.
- A synovial white-cell count above 50,000 cells per microlitre increases probability but is not diagnostic; lower counts, negative Gram stain and identified crystals cannot exclude infection.
- Plain radiographs provide a baseline and find fracture or advanced joint damage; ultrasound confirms and guides access to effusion, while MRI assesses deep joints, abscess and adjacent osteomyelitis.
- Drain a large infected native joint urgently by arthroscopic or open washout. An acutely unwell patient needs experienced emergency surgery as soon as safe and generally within six hours.
- Tailor antibiotics to culture, host and source, follow pain, fever, CRP and function, and repeat aspiration or washout when clinical response is inadequate.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Haematogenous seeding
Transient or sustained bacteraemia enters vascular synovium, with Staphylococcus aureus the leading concern and streptococci, Gram-negatives or unusual organisms shaped by host.
Direct inoculation
Injection, aspiration, arthroscopy, penetrating trauma, bite or adjacent wound introduces organisms directly and changes likely microbiology and surgical anatomy.
Contiguous spread
Osteomyelitis, abscess, cellulitis or an infected bursa can extend into a joint, especially in the hip, shoulder, foot and diabetic limb.
Gonococcal dissemination
Sexually transmitted Neisseria gonorrhoeae can cause migratory symptoms, tenosynovitis, skin lesions or purulent monoarthritis and needs site-specific testing and contact care.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Synovial bacterial growth
The vascular synovium lacks a limiting basement membrane, allowing organisms and neutrophils to enter the joint rapidly during bacteraemia.
- 2Cartilage enzyme injury
Neutrophil proteases, cytokines and bacterial products degrade proteoglycan and collagen while pressure impairs subchondral perfusion, creating irreversible chondral loss.
- 3Effusion pressure
Purulent fluid distends the capsule, causes extreme movement pain and can compromise femoral-head perfusion in the hip or spread along communicating bursae.
- 4Systemic dissemination
An infected joint can maintain bacteraemia and sepsis and may reveal endocarditis, skin infection or another primary source.
- 5Adjacent bone involvement
Delayed or aggressive infection extends into subchondral bone, producing osteomyelitis, structural collapse and a longer antimicrobial and reconstructive course.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Acute monoarthritis with effusion, warmth and severe restriction of both active and passive movement makes bacterial infection a priority diagnosis.
Hip or shoulder pain, fixed posture, referred pain and bacteraemia can indicate a concealed effusion requiring image-guided access.
Muted fever, modest CRP and low synovial leukocytes do not reassure when host defence is suppressed and functional loss is acute.
Migratory arthralgia, pustular skin lesions and tenosynovitis may precede monoarthritis and require genital, pharyngeal and rectal testing according to exposure.
Urate or calcium-pyrophosphate crystals explain inflammation but do not exclude bacterial growth, especially with systemic illness or bacteraemia.
Persistent pain, effusion, fever, bacteraemia or inflammatory-marker rise after washout suggests retained pus, loculation, osteomyelitis or another septic focus.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
First-line urgent joint aspirationFirst stepFirst line - Why
- Obtain direct microbiological and inflammatory evidence before antibiotics when stable.
- Interpretation and limitations
- Send Gram stain, cell count and differential, aerobic and anaerobic culture and crystals; use image guidance for deep joints and never let aspiration delay antibiotics in sepsis.
- 02
Blood cultures - Why
- Detect bacteraemia and organisms missed after local antibiotic exposure.
- Interpretation and limitations
- Obtain at least two sets before antibiotics if non-delaying; persistent S. aureus bacteraemia requires an endocarditis and metastatic-focus pathway.
- 03
Synovial white-cell count - Why
- Support probability and provide a baseline for an inflammatory effusion.
- Interpretation and limitations
- Above 50,000 cells per microlitre increases concern, but no universal cutoff confirms or excludes infection across immune states and partially treated disease.
- 04
Crystal microscopy - Why
- Identify gout or calcium-pyrophosphate deposition as a concurrent or alternative cause.
- Interpretation and limitations
- A positive crystal result does not stop culture, antibiotic or drainage decisions when clinical infection remains plausible.
- 05
First-line plain radiographsFirst line - Why
- Identify fracture, foreign body, gas, chronic joint destruction and a baseline for follow-up.
- Interpretation and limitations
- Early infection may show only effusion or normal bone, so a reassuring film cannot replace aspiration.
- 06
Ultrasound - Why
- Confirm effusion and guide safe aspiration or drain placement.
- Interpretation and limitations
- Particularly useful for hip, shoulder and small joints, but cannot establish sterility or exclude adjacent bone infection.
- 07
MRI - Why
- Map osteomyelitis, deep abscess, synovitis and soft-tissue spread.
- Interpretation and limitations
- Use when it changes source control and can be obtained without delaying aspiration or emergency washout.
- 08
Operative tissue cultures - Why
- Improve yield and identify spatial infection at washout.
- Interpretation and limitations
- Take multiple deep samples before irrigation using separate sterile instruments and report prior antibiotics and exact sites.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Crystal arthritis
Urate or calcium-pyrophosphate crystals cause acute neutrophilic inflammation, but their presence does not exclude simultaneous bacterial infection.
Inflammatory flare
Rheumatoid, psoriatic or reactive arthritis can mimic infection, while immunosuppressive treatment increases the probability and mutes signs of sepsis.
Haemarthrosis
Trauma, coagulopathy or anticoagulation causes painful swelling and blood-stained aspirate, which may still become secondarily infected or conceal fracture.
Cellulitis or bursitis
Superficial erythema and focal bursal swelling can spare passive joint movement, although direct spread and dual pathology remain possible.
Osteomyelitis
Deep bone pain and marrow infection may produce a reactive effusion or enter the joint and requires MRI and longer source-control planning.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01UnstableTreat sepsis and drainFirst stepA suspected infected joint accompanies shock, organ dysfunction or high-risk sepsis.+
- 1Obtain blood cultures without delaying treatment, start sepsis resuscitation and protocol broad-spectrum intravenous antibiotics immediately.
- 2Call orthopaedics, anaesthesia and microbiology together and plan emergency drainage by an experienced team within six hours when safe.
- 3Aspirate before antibiotics only if it is immediately available and does not defer either drug delivery or theatre.
- 4Search concurrently for endocarditis, skin, spine, urinary and other joint sources guided by examination and bloodstream organism.
02StablePreserve culture then control sourceSeptic arthritis is suspected without current sepsis or organ compromise.+
- 1Obtain blood cultures and sterile image-guided or landmark-guided aspiration before starting antimicrobial therapy.
- 2Send cell count, Gram stain, aerobic and anaerobic culture and crystals and notify microbiology of special exposures.
- 3Begin local empiric intravenous therapy after sampling and arrange urgent arthroscopic or open washout rather than awaiting final culture.
- 4Narrow to susceptibilities and define route and duration through microbiology, source control and any bone or cardiac involvement.
03ReviewVerify cartilage-saving responseAspiration or washout and antimicrobial treatment have begun.+
- 1Track pain, temperature, movement, wound, CRP, blood cultures and organ function daily and document the expected response.
- 2Repeat aspiration, imaging or operative washout promptly when effusion, bacteraemia, fever or functional restriction persists.
- 3Start guided range and loading after adequate drainage and physiological improvement, protecting any associated bone procedure.
- 4Complete organism-specific follow-up and investigate recurrence for osteomyelitis, endocarditis, immune deficit or inadequate source control.
Key medicines and prescribing safety1 treatment · regimens, roles and cautions+
Empiric intravenous therapy after diagnostic cultures
In a stable adult, start the current trust septic-arthritis regimen immediately after blood and synovial cultures; in sepsis, give full protocol doses after blood cultures without waiting for aspiration, then narrow as soon as organism and susceptibilities permit.Choice and exact dose depend on Gram stain, allergy, renal and hepatic function, MRSA and Gram-negative risk, bites, sexual exposure and previous antibiotics; obtain specialist advice in pregnancy, immune suppression and suspected endocarditis.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Rapid cartilage destruction
Delayed drainage causes joint-space loss, pain, stiffness and secondary osteoarthritis that may ultimately require fusion or arthroplasty.
Sepsis and metastatic infection
Ongoing bacteraemia produces haemodynamic shock, endocarditis, vertebral infection and additional seeded joints or deep abscesses throughout the body.
Osteomyelitis
Subchondral extension creates necrotic bone and persistent infection requiring prolonged organism-directed therapy, repeat imaging and sometimes further operative debridement.
Growth and vascular injury
In children, physeal damage disturbs growth; hip-capsule pressure can impair femoral-head perfusion and cause avascular necrosis.
Stiffness and functional loss
Inflammation, pain and immobilisation cause adhesions, weakness and contracture even after microbiological cure, requiring early guided rehabilitation.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Record temperature, haemodynamics, mental state, urine output and lactate during systemic illness and escalate organ dysfunction through the sepsis pathway.
- Review joint pain, effusion, passive range, wound and neurovascular status daily and after every aspiration or washout.
- Trend CRP and blood count as trajectories rather than clearance tests and repeat blood cultures for persistent fever or S. aureus bacteraemia.
- Review synovial and operative cultures, susceptibilities, antimicrobial dose, renal and liver function and intravenous access with microbiology.
- Use repeat ultrasound, MRI or radiographs when response is inadequate or adjacent osteomyelitis, abscess or structural damage is suspected.
- Follow range, strength, gait or upper-limb use, pain and return to function and address later chondral loss or osteoarthritis.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Crystals are not an exclusion
Bacterial infection and crystal deposition can coexist in the same aspirate, particularly in older or systemically ill patients.
Thresholds move with the host
Immune suppression and pre-treatment can lower synovial leukocytes, while sterile inflammatory disease can raise them above traditional cutoffs.
Blood cultures rescue yield
Haematogenous infection may remain detectable in blood after synovial cultures become negative following prior antibiotics.
Hip pain can appear at the knee
Obturator or femoral nerve referral makes unexplained knee pain and refusal to move a reason to examine and image the hip.
Washout is both treatment and test
Multiple separately obtained synovial and tissue samples at surgery improve diagnosis while removal of pus protects cartilage.
A falling CRP is not enough
Persistent effusion, severe movement pain or bacteraemia can signal retained infection despite an initially favourable blood-marker trend.
11Common pitfallsFrequent interpretation and management errors.
- 01
Withholding antibiotics from a shocked patient while waiting for an aspiration service or MRI.
- 02
Giving antibiotics before aspiration in a stable patient without a reason and losing microbiological yield.
- 03
Stopping the infection pathway because crystals are present, Gram stain is negative or synovial cells are below fifty thousand.
- 04
Using repeated needle aspiration alone for a deteriorating large-joint infection that requires formal washout.
- 05
Choosing a fixed antibiotic duration without integrating organism, drainage, endocarditis and adjacent osteomyelitis.
- 06
Ending review after fever settles without restoring joint movement and investigating persistent pain or structural damage.