Irritable hip versus septic arthritis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Establish age, onset, fever, recent infection, trauma, immunisation, wounds and ability to walk. Ask parents about night waking, rigors, feeding, urine and bowel symptoms, antibiotics and immunocompromise. A toddler may show only reduced crawling or nappy-change distress. Transient synovitis is a diagnosis made in a well child after dangerous alternatives are actively considered; it is not simply any hip effusion after a cold.

Record observations and overall interaction before analgesia and again afterwards. Inspect both legs, skin and adjacent abdomen, spine, pelvis and knee. A septic hip is often held flexed, abducted and externally rotated to maximise capsular volume. Gentle log roll causes marked pain; stop rather than forcing formal range. Palpate pelvis, femur and muscles for adjacent infection. Assess perfusion, rash, bruising, lymph nodes and hepatosplenomegaly when systemic disease is possible.

Blood results modify but do not dictate risk. Request FBC, CRP and ESR and take blood cultures before antibiotics when possible. CRP changes faster than ESR and is useful for response. Young children with Kingella infection can have modest fever and markers, while early Staphylococcus infection can precede a rise. Anaemia, thrombocytopenia, blasts or atypical cells point toward malignancy and need urgent paediatric review.

AP pelvis and a safe lateral hip view assess SUFE, Perthes, fracture and bone destruction but can be normal in early infection. Ultrasound confirms and guides aspiration of an effusion; fluid depth does not prove aetiology. MRI is the reference study for pelvic or femoral osteomyelitis, pyomyositis, abscess and multifocal disease when symptoms, bacteriology or poor response suggest an extra-articular focus. Imaging must not delay resuscitation or source control.

Aspirate an effusion urgently when septic arthritis remains plausible. Use ultrasound guidance and send volume promptly for Gram stain, cell count and differential, aerobic and anaerobic culture and additional molecular testing according to microbiology. Blood-culture bottles can improve yield. Prior antibiotics lower culture sensitivity, but a septic child must not wait for aspiration availability. Crystals are unusual in children and do not make bacterial culture optional.

Keep the child fasting, non-weight-bearing and under paediatric and orthopaedic care. Resuscitate sepsis, take blood cultures and start locally recommended empirical IV therapy covering Staphylococcus aureus, modified for age, MRSA risk, sickle-cell disease, immunocompromise and local resistance. Drain by arthroscopic or open washout; repeated aspiration is used only in selected pathways with close response. Obtain operative tissue as well as fluid when possible.

Narrow antibiotics to organism and sensitivities with microbiology advice. IV-to-oral switch depends on defervescence, clinical recovery, falling CRP, reliable enteral treatment and adequate drainage rather than a fixed day alone. Total duration varies with uncomplicated joint infection versus adjacent osteomyelitis. Persistent fever, pain, bacteraemia or CRP plateau prompts repeat ultrasound or MRI and assessment for retained pus, resistant organism or another focus.

For a well child with a plausible transient synovitis pattern and low infection concern, prescribe rest from impact and weight-based paracetamol or ibuprofen, allow comfortable mobilisation and arrange active review in 24 to 48 hours. Give immediate return advice for fever, increasing pain or reduced weight bearing. Complete recovery is expected within days and usually two weeks; persistence, recurrence or restricted rotation and abduction triggers repeat radiographs or MRI for Perthes and other disease.

Key points

Transient synovitis usually affects a well child aged about three to eight after viral illness, with mild restriction and ability to bear some weight.
Septic arthritis is more likely with fever, toxic appearance, complete non-weight-bearing, severe passive-movement pain and raised CRP, but none is an absolute rule.
Kocher or Caird criteria support probability and escalation; they must never be used as a discharge rule or substitute for repeated clinical judgement.
First-line tests when infection is plausible are FBC, CRP, ESR, blood cultures, hip radiographs and urgent ultrasound for an effusion.
Ultrasound is sensitive for joint fluid but cannot determine sterility; image-guided aspiration with microscopy and culture is the diagnostic reference step.
If sepsis or serious infection is likely, obtain cultures rapidly, start empirical intravenous antibiotics and arrange urgent arthroscopic or open drainage without waiting for every result.
First-line treatment for a confidently low-risk irritable hip is rest from painful activity, weight-based analgesia and planned review within 24 to 48 hours.
Failure to improve promptly, symptoms beyond seven days or recurrence requires repeat examination and imaging for Perthes, infection and other pathology.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Transient post-viral synovitis

A short-lived sterile hip effusion often follows an upper-respiratory or gastrointestinal illness in a well preschool or early-school child.

Haematogenous bacterial seeding

Bacteraemia deposits organisms in vascular synovium; Staphylococcus aureus predominates, while Kingella kingae is important in younger children.

Contiguous infection

Pelvic or proximal-femoral osteomyelitis and pyomyositis can extend to the joint or mimic intra-articular pain and restriction.

Host and procedural risk

Infancy, immunocompromise, sickle-cell disease, recent surgery, penetrating injury and bacteraemia increase infection probability and broaden likely organisms.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Sterile synovial inflammation
Transient synovitis produces reversible effusion and capsular stretch without bacterial invasion, purulence or progressive cartilage destruction.
2
Rapid bacterial proliferation
Organisms multiply within nutrient-rich synovial fluid, recruiting neutrophils and generating damaging proteolytic enzymes and inflammatory cytokines.
3
Intracapsular pressure
The hip capsule fills under tension, causing severe pain and potentially compromising epiphyseal blood flow in the young child.
4
Cartilage destruction
Uncontrolled pus damages articular cartilage within a short period and can spread into bone, physis and surrounding muscle.
5
Growth and perfusion injury
Infection and pressure can injure proximal-femoral vessels and physis, causing osteonecrosis, angular deformity and progressive leg-length discrepancy.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Well child with mild restriction

Comfortable interaction, low fever burden and some weight bearing after analgesia supports transient synovitis but does not prove it.

Severe passive-movement pain

Pain through tiny log-roll movement indicates tense intra-articular pathology and substantially raises infection concern.

Complete weight-bearing refusal

Inability to stand or walk is a major warning, particularly when new and accompanied by fever or raised inflammation.

Hip flexion posture

Flexed abducted external rotation increases capsular volume and can indicate a pressured septic joint.

Effusion without diagnosis

Sonographic fluid confirms synovitis but appears in both transient and bacterial disease and requires clinical interpretation.

Persistent post-synovitis limp

Symptoms that fail to settle or recur should reopen Perthes, inflammatory, infectious and malignant differentials.

Red flags requiring action

Fever, tachycardia, toxic appearance or reduced perfusion with an irritable hip indicates sepsis risk and needs emergency escalation.
Complete refusal to bear weight or pseudoparalysis is more concerning than a mild antalgic limp and should not be labelled transient synovitis casually.
Severe pain through tiny passive movements, a hip held flexed and externally rotated or progressive night pain suggests intra-articular pressure or infection.
Normal early CRP, white count or temperature does not exclude infection in an infant, immunocompromised child or early presentation.
An ultrasound effusion confirms fluid but cannot distinguish sterile synovitis from pus; aspiration and culture answer the microbiological question.
Symptoms lasting beyond a week, recurrence or persistent restricted abduction requires reassessment for Perthes disease, inflammatory arthritis, tumour or osteomyelitis.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

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 FBC, CRP, ESR and blood culturesFirst stepFirst line
Why
Estimate inflammatory burden, detect bacteraemia and screen for cytopenia or malignant alternatives.
Interpretation and limitations
Raised markers support infection, but normal early results do not safely exclude it in infants or indolent organisms.
02
First-line AP and lateral hip radiographsFirst line
Why
Find SUFE, Perthes, fracture, destructive bone change and late joint-space abnormality.
Interpretation and limitations
Normal radiographs neither exclude septic arthritis nor prove transient synovitis.
03
Urgent hip ultrasound
Why
Detect an effusion and guide sterile diagnostic aspiration.
Interpretation and limitations
Fluid presence and volume cannot distinguish bacterial from sterile synovitis; absent effusion redirects assessment to bone, muscle or referred sites.
04
Reference synovial aspiration
Why
Obtain cells, Gram stain, culture and molecular microbiology before definitive drainage when feasible.
Interpretation and limitations
Purulence, high neutrophils or positive culture supports infection, but prior antibiotics and Kingella can yield negative conventional culture.
05
MRI pelvis and proximal femur
Why
Identify occult osteomyelitis, pyomyositis, abscess, tumour or early Perthes when the course is atypical.
Interpretation and limitations
Use for diagnostic discordance or poor response; it should not delay urgent washout of a clearly septic joint.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Perthes disease

Persistent or recurrent limp with loss of abduction and evolving epiphyseal sclerosis or fragmentation suggests femoral-head osteonecrosis.

Slipped upper femoral epiphysis

An adolescent with external rotation and referred knee pain needs immediate non-weight-bearing and bilateral hip radiographs.

Osteomyelitis or pyomyositis

Focal bone or muscle tenderness, bacteraemia and MRI abnormality can cause hip irritability with little or no joint effusion.

Juvenile idiopathic arthritis

Prolonged stiffness, several joints and recurring synovitis suggests inflammatory disease, although infection must be excluded before immunosuppression.

Malignancy

Night pain, bruising, pallor, weight loss, organomegaly or cytopenia requires leukaemia and bone-tumour investigation rather than NSAID-only treatment.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01High riskTreat possible septic arthritisFirst stepThe child is febrile, unwell, unable to bear weight or has severe passive hip pain or raised inflammation.

1Assess ABCDE and sepsis risk, obtain IV access, blood cultures and urgent paediatric, orthopaedic and microbiology input.
2Keep fasting and non-weight-bearing and obtain radiographs and ultrasound without delaying treatment.
3Aspirate the hip urgently when feasible and send complete microbiology, then drain arthroscopically or openly as indicated.
4Start empirical IV antibiotics promptly in sepsis and tailor to culture, age, host and local resistance.

02Low riskReview an irritable hip activelyA well child retains some weight bearing, has mild restriction and infection probability remains low after assessment.

1Provide rest from painful impact and correctly prescribed paracetamol or ibuprofen.
2Explain uncertainty and arrange named review within 24 to 48 hours rather than open-ended return as needed.
3EscalationEscalate immediately for fever, worsening pain, complete non-weight-bearing or systemic change.
4Repeat examination and image for Perthes or other disease when recovery is incomplete by the expected timeframe.

03Confirmed infectionDrain and target treatmentPurulence, culture, operative findings or combined evidence confirms septic arthritis.

1Achieve source control and send fluid and tissue for culture and susceptibility before closure.
2Continue IV therapy until physiology, pain and CRP response permit an agreed oral switch.
3Investigate persistent bacteraemia or inflammation for retained collection, osteomyelitis or metastatic focus.
4Restore range and loading gradually after infection control and follow proximal-femoral growth.

04Non-resolvingReopen the diagnosisThe presumed transient synovitis persists, recurs or develops atypical features.

1Repeat full systemic, spine, abdomen, hip and knee examination and review the original observations and images.
2Recheck blood count and inflammatory markers when infection or malignancy remains plausible.
3Obtain repeat radiographs or MRI for early Perthes, osteomyelitis, tumour or stress injury.
4Refer to paediatric orthopaedics, rheumatology, oncology or infection services according to the emerging pattern.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Provides initial antistaphylococcal cover after cultures while urgent drainage and organism identification proceed.

Empirical intravenous flucloxacillin for presumed MSSA

For a child older than one month, a commonly used severe bone-and-joint infection regimen is 50 mg/kg intravenously every six hours, up to 2 g per dose, verified against current BNFC and local microbiology policy.

Modify for immediate penicillin allergy, MRSA risk, age, renal or hepatic impairment and local epidemiology; broaden cover for young or vulnerable hosts only with specialist advice.

Reduces sterile synovial pain and helps restore comfortable movement while the child remains under active review.

Ibuprofen for low-risk transient synovitis

Choose the current BNFC oral dose band from the child's age and weight, prescribe briefly, and record formulation strength, volume and interval.

Avoid use with dehydration, kidney disease, gastrointestinal bleeding risk or NSAID-sensitive asthma. Improvement after one dose does not exclude infection; escalate fever, worsening pain or complete non-weight-bearing immediately.

Supports comfort during investigation, fasting and postoperative recovery without replacing source control.

Paracetamol for paediatric hip pain

Prescribe the exact BNFC age- and weight-based milligram dose and formulation volume with the correct interval and daily maximum.

Check weight, liquid concentration, liver risk and all other paracetamol-containing products; persistent severe pain demands reassessment.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Articular cartilage loss

Delayed drainage permits irreversible chondral damage, painful stiffness, deformity and premature secondary osteoarthritis during later life.

Femoral-head osteonecrosis

Raised pressure and vascular injury can collapse the epiphysis and permanently alter proximal-femoral growth and hip congruity.

Physeal growth disturbance

Infection crossing the proximal femoral physis causes coxa vara, progressive shortening, gait asymmetry and leg-length discrepancy.

Adjacent osteomyelitis

Uncontrolled joint infection can extend into the femur or pelvis, requiring longer intravenous antibiotics and further operative debridement.

Sepsis and metastatic infection

Persistent bacteraemia can cause circulatory shock, endocardial infection or distant skeletal seeding, especially in clinically vulnerable children.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat observations, perfusion, pain and weight-bearing status during initial assessment because children can deteriorate quickly.
Trend CRP alongside clinical recovery after drainage; a plateau or secondary rise prompts a search for residual infection.
Check culture and molecular results daily and document antimicrobial narrowing, route and planned duration.
After washout, inspect wound, restore hip movement progressively and monitor adjacent osteomyelitis and thrombosis risk.
For transient synovitis, complete the planned 24-to-48-hour review and confirm full recovery rather than assuming it.
Follow growth, hip shape, leg length and range after septic arthritis, particularly in infants with proximal physeal risk.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Effusion is not an organism

Ultrasound shows fluid but cannot distinguish transient synovitis from bacterial pus or predict cartilage safety.

Scores support not discharge

Kocher and Caird variables estimate probability in populations but cannot rule out atypical, early or partially treated infection.

Young children can look subtle

Kingella and infant infection may produce modest fever and markers despite a joint that still requires treatment.

Drainage is disease-modifying

Antibiotics kill organisms, but removing pressurised purulent fluid protects cartilage and femoral-head perfusion.

Transient means demonstrably transient

A diagnosis gains confidence only when symptoms resolve promptly and examination returns to normal on active follow-up.

The joint may be innocent

Pelvic osteomyelitis or pyomyositis can mimic hip sepsis, especially when ultrasound shows no explanatory effusion.

11Common pitfallsFrequent interpretation and management errors.

01
Using a low Kocher score to discharge a clinically concerning child.
02
Calling any ultrasound effusion transient synovitis without integrating fever, weight bearing and inflammatory markers.
03
Delaying antibiotics in sepsis because aspiration or MRI is not immediately available.
04
Giving antibiotics alone without urgent orthopaedic source-control assessment.
05
Failing to culture blood and operative tissue or to look for adjacent osteomyelitis when response is poor.
06
Leaving a presumed irritable hip without a timed review and a plan for persistent or recurrent limp.