Paediatric musculoskeletal infection is acute bacterial infection of a child's bones, joints, muscles or intervertebral discs — principally septic arthritis, osteomyelitis, pyomyositis and discitis. It is an orthopaedic emergency: the British Orthopaedic Association's May 2022 BOAST, The Management of Children with Acute Musculoskeletal Infection, warns that these infections "can be life threatening and lead to long-term disability". Core management is early recognition, prompt cultures and empirical intravenous antibiotics, urgent surgical drainage of septic arthritis (ideally within 24 hours of diagnosis), and antibiotic-first treatment of uncomplicated osteomyelitis.
How does paediatric musculoskeletal infection present?
The typical child presents with a limp or refusal to weight-bear, pseudoparalysis of a limb, fever and localised pain. Neonates and infants may show only irritability, poor feeding or reluctance to move a limb, without fever. The May 2022 BOAST requires a full, documented systematic assessment of all extremities, the spine and potential sources of infection — infection is easily missed at a second site, and cellulitis may be the surface marker of a deeper collection. Every child with suspected musculoskeletal infection should be admitted under the joint care of orthopaedic and paediatric teams, per the BOAST.
How do you distinguish septic arthritis from transient synovitis?
This is the classic diagnostic dilemma in the child with an acutely irritable hip. Transient synovitis is a self-limiting inflammatory effusion, typically in a well child aged 3–8 years, often following a viral illness; septic arthritis destroys articular cartilage within hours to days. Kocher and colleagues (JBJS Am, 1999) derived four independent predictors of septic arthritis of the hip:
| Kocher predictor | Threshold |
|---|---|
| Fever | > 38.5 °C |
| Weight-bearing | Unable to weight-bear |
| ESR | > 40 mm/hr |
| White cell count | > 12,000 cells/mm³ |
In the original series, a child with all four predictors had a predicted probability of septic arthritis of 99.6%. Caird and colleagues (JBJS Am, 2006) prospectively added CRP > 20 mg/L as a fifth predictor, reporting probabilities of roughly 83% with three, 93% with four and 98% with all five factors present; in their cohort fever was the single best predictor.
Use these scores as a structured framework, not a rule-out test. The BOAST is explicit that "no single investigation algorithm is completely reliable" — a Kocher score of 1–2 in an unwell child still mandates senior review, imaging and often aspiration. Kingella kingae infection in toddlers frequently produces a mild clinical picture with near-normal inflammatory markers, so a low score is falsely reassuring in the under-4s. Where genuine doubt persists, ultrasound-guided or open aspiration of the joint under senior supervision is the definitive discriminator (good practice, not a numbered BOAST standard).
Which organisms cause infection at each age?
Staphylococcus aureus is the commonest organism at every age, but the differential shifts with age and host, and this drives empirical antibiotic choice (agreed with local microbiology):
| Age / host | Typical organisms |
|---|---|
| Neonates (< 3 months) | S. aureus, Group B Streptococcus, Gram-negative coliforms (e.g. E. coli) |
| 3 months – 4 years | S. aureus, Kingella kingae, Streptococcus pneumoniae |
| Older children / adolescents | S. aureus (including PVL-producing and MRSA strains), Group A Streptococcus; consider Neisseria gonorrhoeae in sexually active adolescents |
| Sickle cell disease | Salmonella species in addition to S. aureus |
Kingella kingae is now recognised as a leading cause of osteoarticular infection in children under 4 years (Yagupsky, Lancet Infectious Diseases, 2004). It is fastidious on conventional culture — inoculating aspirates into blood-culture bottles and requesting PCR substantially improves detection, which is why deep tissue and fluid sampling before antibiotics matters so much in this age group. Panton–Valentine leukocidin (PVL)-producing S. aureus is a recognised cause of severe, multifocal disease with subperiosteal abscesses, deep venous thrombosis and septic emboli; a child who is disproportionately unwell should prompt discussion with microbiology and screening for multifocal disease and DVT (good practice).
What investigations and imaging should you request?
The May 2022 BOAST sets the baseline investigation set for every child with suspected musculoskeletal infection:
- Full blood count, CRP and ESR;
- Blood cultures — a minimum of 2 ml of blood, taken before antibiotics wherever possible;
- Plain radiographs of the affected region.
Radiographs are frequently normal in early osteomyelitis — classical teaching is that bony changes lag 10–14 days behind symptoms — so their main early role is excluding fracture, effusion signs and tumour. For second-line imaging the BOAST names MRI as the preferred modality, ideally "within 48 hours", with ultrasound available in the same timeframe; clinical urgency (an unwell child with an obvious hip effusion) should shortcut straight to aspiration or theatre rather than waiting for MRI. Ultrasound is the practical first test for the irritable hip: it detects effusion reliably but cannot distinguish septic from reactive fluid — that distinction is made by aspiration and the clinical picture.
What is the initial management?
Initial management follows a sepsis-first logic, anchored to the BOAST standards:
- Recognise and treat sepsis. Children meeting high-risk sepsis criteria receive empirical intravenous antibiotics immediately, as per the May 2022 BOAST; screen using the paediatric sepsis pathway (NICE guideline NG254, Suspected sepsis in under 16s, which superseded NG51 for children).
- In the stable child, cultures before antibiotics. The BOAST permits deferring antibiotics when the child is clinically stable and surgery is planned imminently, so that deep tissue and fluid samples are taken first — this maximises the chance of identifying the organism, including K. kingae.
- Admit under joint orthopaedic and paediatric care, with a documented systematic assessment.
- Escalate complex cases early. Local treatment is appropriate where competencies exist, supported by agreed network pathways; complex cases (multifocal disease, neonates, spinal or pelvic infection, failure to respond) should be transferred to the specialist centre within 24 hours of referral, per the BOAST.
What are the surgical thresholds and definitive management?
The BOAST draws a clear line between the two main diagnoses:
- Septic arthritis is a surgical diagnosis. Confirmed septic arthritis requires surgical drainage — arthrotomy (or arthroscopic washout in appropriate joints) with copious lavage and synovial sampling — "as soon as possible, ideally within 24 hours of diagnosis". Pus is chondrolytic, and in the hip a tense effusion also threatens the femoral head blood supply.
- Uncomplicated osteomyelitis is a medical diagnosis. First-line treatment of acute osteomyelitis, pyomyositis and discitis is intravenous antibiotics; surgery is reserved for abscess-associated disease (subperiosteal, intraosseous or soft-tissue collections), failure to respond, or the need for diagnostic tissue.
Response is tracked with temperature and serial CRP, ESR and white cell trends, which the BOAST states should guide decisions on further intervention, intravenous-to-oral switch and total antibiotic duration, agreed with paediatric infectious diseases or microbiology. Where a prolonged intravenous course is unavoidable, consider peripherally inserted central venous access early and use outpatient parenteral antibiotic (OPAT) pathways rather than keeping the child in hospital, per the BOAST. A child who fails to settle should trigger repeat imaging for an undrained collection or a second site.
What are the complications, and how long should follow-up continue?
Complications are the reason this topic carries weight at interview: growth arrest and limb-length discrepancy or angular deformity from physeal damage, avascular necrosis of the femoral head after hip sepsis, chondrolysis and secondary arthritis, chronic osteomyelitis with sequestrum formation, pathological fracture, and — particularly with PVL-positive S. aureus — DVT and septic pulmonary emboli. Because growth-plate injury may only declare itself as the child grows, the BOAST mandates clinical and radiographic follow-up "for a minimum of 12 months" by clinicians trained to recognise these late sequelae.
Key points
- Paediatric MSK infection spans septic arthritis, osteomyelitis, pyomyositis and discitis; manage per the May 2022 BOAST, The Management of Children with Acute Musculoskeletal Infection.
- Kocher's four predictors (fever > 38.5 °C, non-weight-bearing, ESR > 40, WCC > 12,000) — plus Caird's CRP > 20 mg/L — stratify the irritable hip, but "no single investigation algorithm is completely reliable".
- S. aureus dominates at all ages; think Group B strep and coliforms in neonates, Kingella kingae under 4 years, Salmonella in sickle cell disease.
- Baseline work-up: FBC, CRP, ESR, blood cultures (≥ 2 ml), plain radiographs; MRI is the preferred second-line imaging, ideally within 48 hours.
- Septic children get immediate IV antibiotics; stable children can have antibiotics deferred until deep samples are taken at planned surgery.
- Septic arthritis: surgical drainage as soon as possible, ideally within 24 hours of diagnosis. Osteomyelitis: IV antibiotics first, surgery for abscess or failure to respond.
- Follow up clinically and radiographically for at least 12 months to detect growth arrest, AVN and chronic infection.