Paediatric Bone and Joint Infection

Paediatric bone and joint infection is time-sensitive because bacteria, pus and inflammatory pressure can damage cartilage, physis and bone quickly. The two core diagnoses are acute haematogenous osteomyelitis and septic arthritis, but many children sit between these categories: metaphyseal infection may spread to an adjacent joint, a subperiosteal abscess may mimic a joint problem, or pyomyositis may present as a painful limb with fever.

Acute haematogenous osteomyelitis often starts in metaphyseal bone around the knee, hip, ankle and shoulder. Septic arthritis is an orthopaedic emergency because pus within a joint can rapidly damage cartilage and because joint drainage is a source-control decision, not merely an antibiotic decision.

Presentation varies with age. Neonates and infants may present with irritability, poor feeding or pseudoparalysis rather than a classic fever. Toddlers with Kingella kingae infection may have a subtler systemic response than children with Staphylococcus aureus. Children with sickle cell disease, immunosuppression, penetrating wounds or unusual exposures need organism-specific thinking.

The key management question is not "which antibiotic first?" It is "does this child have pus that needs drainage, and have we obtained cultures without delaying treatment?" Antibiotics treat bacteria, but undrained pus can keep the child febrile, painful and systemically unwell.

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Bacteria seed metaphyseal vessels where sluggish flow and vascular loops favour bacterial deposition. Infection raises intramedullary pressure, compromises perfusion and can break through cortex into a subperiosteal abscess. If the infection is near a joint, the adjacent joint may become involved, especially in infants where transphyseal vessels can connect metaphysis, epiphysis and joint.

In septic arthritis, bacteria and inflammatory mediators enter the joint space. Pus under pressure damages cartilage by mechanical pressure, inflammatory enzymes and impaired nutrition. This is why painful passive motion of a joint is treated as a major warning sign.

Organism thinking is clinical, not academic. Staphylococcus aureus is a common cause. MRSA risk depends on local epidemiology. Kingella kingae is important in toddlers and young children and may have less dramatic fever or blood tests. Salmonella is classically considered in sickle cell disease. Group A Streptococcus can be aggressive, especially with toxic soft-tissue features.

Delayed or inadequate source control can lead to abscess expansion, pathological fracture, chronic osteomyelitis, sinus formation, growth disturbance, angular deformity, limb-length discrepancy, joint stiffness, chondrolysis, dislocation or avascular necrosis after septic hip.

History

The history should establish severity, likely source, organism clues and whether cultures may already be compromised.

• Onset: acute severe pain, progressive limp, refusal to walk, refusal to use the limb, or pseudoparalysis.
• Systemic symptoms: fever, rigors, malaise, poor feeding, irritability, lethargy or sepsis features.
• Portal or exposure: skin infection, varicella, puncture wound, penetrating trauma, surgery, bite, farm, water or footwear puncture.
• Prior treatment: antibiotics already given, prior aspiration, previous admission or immunisation status when relevant.
• Risk factors: sickle cell disease, immunosuppression, diabetes, indwelling lines, renal disease, malignancy or neonatal history.
• Location clues: back pain, abdominal pain, psoas symptoms, refusal to sit, refusal to crawl or multifocal pain.
• Functional change: weight-bearing status, ability to move the joint, night pain and response to analgesia.

Examination

Examine the whole child first. Tachycardia, fever, toxicity, dehydration and reduced interaction change urgency. Then localise the problem.

Look for pseudoparalysis, refusal to bear weight, a joint held still, swelling, warmth, erythema or a limb position of comfort. Palpate metaphyses, joints and adjacent soft tissues. Pain with passive joint motion suggests septic arthritis and should not be dismissed as a sore bone. Focal metaphyseal tenderness with less dramatic passive joint pain may suggest osteomyelitis, but the two can coexist.

Always assess adjacent joints and the spine when the child will not walk. Document neurovascular status and be alert to compartment syndrome in severe swelling or aggressive infection. Look for skin portals, puncture wounds, cellulitis, surgical scars, bites and pressure areas.

Blood tests and cultures

FBC, ESR and CRP support diagnosis and trend response. CRP is particularly useful for monitoring because it changes faster than ESR. Obtain blood cultures before antibiotics when safe, but do not leave a septic child untreated for ideal culture timing. Joint aspirate, bone aspirate or operative tissue should be sent for Gram stain, culture and sensitivity when drainage or biopsy occurs.

Imaging

Plain radiographs are useful first-line imaging because they may show alternative diagnoses, advanced bone change, fracture or tumour, but they can be normal early in osteomyelitis. Ultrasound is useful for joint effusion and superficial collection. MRI is the best escalation test for marrow oedema, subperiosteal abscess, intraosseous abscess, pyomyositis, soft-tissue spread, multifocal disease and adjacent joint involvement.

Response monitoring

Clinical response should be visible: falling fever, decreasing pain, improved limb use and falling CRP. Failure to improve should trigger a search for undrained pus, resistant organism, wrong diagnosis, adjacent septic joint, multifocal infection or inadequate drug delivery.

• Transient synovitis: well child with hip effusion but improving symptoms.
• Trauma or toddler fracture: focal tenderness and mechanism may dominate.
• Malignancy or leukaemia: night pain, bruising, pallor or systemic symptoms.
• Inflammatory arthritis: morning stiffness, multiple joints or recurrent swelling.
• CRMO: recurrent sterile inflammatory bone lesions.
• Discitis: young child with refusal to sit or walk and back stiffness.

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The treatment plan has three linked goals: stabilise the child, identify and treat the organism, and obtain source control when pus is present. Antibiotics without source control may fail; surgery without microbiology may make definitive treatment harder.

Early

• Sepsis and clinical deterioration.
• Abscess formation or spread to adjacent joint.
• Pathological fracture in severe osteomyelitis.
• Joint dislocation or cartilage injury in septic arthritis.
• Need for repeat drainage.

Late

• Growth arrest or angular deformity.
• Avascular necrosis after septic hip.
• Chronic osteomyelitis or sinus formation.
• Joint stiffness, chondrolysis or degenerative change.
• Limb length discrepancy after physeal injury.

Paediatric bone and joint infection management is source-control thinking. Antibiotics are essential, but the child improves only when the organism, anatomical source and any pus under pressure are addressed.

The most important fork is septic arthritis versus osteomyelitis without joint sepsis. Septic arthritis is a cartilage emergency. Osteomyelitis may be managed medically if the child is stable and there is no collection requiring drainage, but abscess, adjacent joint spread, toxic sepsis or failure of inflammatory markers to fall should trigger repeat imaging and source control.

Kingella kingae has changed the interpretation of the well-looking toddler with osteoarticular infection. Fever and blood tests may be less dramatic, so age, joint symptoms, PCR where available and clinical trajectory matter. Staphylococcus aureus remains central, and local MRSA prevalence should guide empiric therapy.

• Name whether the problem is bone, joint, both or soft tissue.
• The first question is not which antibiotic; it is whether there is pus needing drainage.
• A normal early X-ray should not end the assessment if the child remains clinically infected.
• Infection around the hip and shoulder in infants is especially dangerous because of joint spread.
• CRP should move in the right direction after treatment; if not, search for pus or another diagnosis.
• Document passive joint motion because it helps separate septic arthritis from isolated osteomyelitis.
• Culture results matter, but a septic child gets treated while cultures are pending.
• Families need clear return advice for fever, increasing pain or refusal to move after discharge.
• Long-term complications may be growth-related, not just infection-related.
• Multidisciplinary care is standard, not a sign that the case is unusual.

Common pitfalls

• Calling it transient synovitis in a febrile non-weight-bearing child.
• Using normal X-ray to exclude early osteomyelitis.
• Forgetting Kingella in toddlers.
• Not imaging the adjacent joint.
• Continuing antibiotics without looking for undrained abscess when CRP rises.
• Stopping follow-up before growth risk is clear.