Paediatrics chapters
Septic arthritis and osteomyelitis in children
Bacterial infection of a joint or of bone in children: the usual organisms by age, why the metaphysis and the hip matter, how to tell infection from transient synovitis, which tests to order, and how antibiotics and drainage are used.
- Septic arthritis is a bacterial infection of a joint, usually by blood spread; the knee is commonest, the hip is the most dangerous, and S. aureus is the leading organism after the newborn period.
- Acute haematogenous osteomyelitis starts in the metaphysis of a long bone; X-rays stay normal for 10-14 days, MRI shows it early, and needle aspiration with Gram stain and culture proves the diagnosis.
- Treat with IV anti-staphylococcal antibiotics chosen by age, aspirate or drain the joint (the septic hip needs urgent surgical drainage), and follow fever, pain, ESR and CRP.
On this page
Overview
Septic (suppurative, pyogenic) arthritis is a bacterial infection inside a joint. Osteomyelitis is infection of bone, starting in the medullary cavity and spreading to the cortex. In children both usually arise from bacteria in the blood, both are most often caused by Staphylococcus aureus, and the two often occur together in infants.
They matter because delay destroys cartilage, damages the growth plate and can leave a short or stiff limb. The exam tests the organisms by age and setting, the reason the metaphysis is involved, the timing of X-ray changes, the value of joint aspiration, and why the septic hip must be drained at once.
Part 1: Septic arthritis
How bacteria reach the joint
- Blood spread (haematogenous) is the usual route.
- Contiguous spread from nearby infected soft tissue or bone.
- Direct inoculation by a penetrating injury.
- Spread from adjacent osteomyelitis is commonest in infants, whose vessels still cross the growth plate.
Pus damages articular cartilage by enzymes, by cutting off nutrients carried in synovial fluid, and by raising pressure inside the joint.
Epidemiology
- Most common in children under 5 years and in adolescents.
- Joints involved: knee about 40%, hip about 20%, ankle about 14-15%; then elbow, wrist and shoulder.
- Small joints of the hand are usually infected after a penetrating wound or a closed-fist injury.
Organisms by age
| Age group | Common organisms |
|---|---|
| Newborns and infants under 2 months | Group B streptococcus, S. aureus, enteric Gram-negative rods (E. coli, Klebsiella) |
| Older infants and children | S. aureus (commonest), group A streptococcus, S. pneumoniae, Kingella kingae; Hib in unimmunised children; N. meningitidis |
| Adolescents | Neisseria gonorrhoeae must be considered, as well as S. aureus |
| Sickle cell disease | Salmonella is an important extra cause |
- Hib bone and joint infection has almost disappeared where conjugate Hib vaccine is given routinely.
- Gonococcal arthritis has two stages: early arthritis-dermatitis syndrome (fever, rash, symmetric polyarthritis, sterile joint fluid) and later monoarthritis of a large weight-bearing joint, from which the organism is often grown.
Clinical features
- A single hot, swollen, very tender joint with an effusion and marked loss of movement.
- Fever, irritability, poor feeding in infants.
- Pain may be noticed only when the joint is moved, for example during a nappy change.
- Pseudoparalysis: the child will not move the limb.
- Toddlers limp or refuse to walk.
- The septic hip is held flexed, abducted and externally rotated, which lowers pressure on the capsule; the knee and elbow are held flexed.
- In newborns the only signs may be fever and poorly localised limb pain; a septic hip may show just a limp or a fixed flexed hip.
A febrile child who refuses to move a limb has septic arthritis or osteomyelitis until proved otherwise.
Investigations
| Test | What it shows |
|---|---|
| Full blood count, ESR, CRP | White cell count and acute phase reactants usually raised; ESR and CRP are repeated to follow response |
| Blood culture | Positive in only about half of cases |
| Joint aspiration (arthrocentesis) | Essential: Gram stain, culture, cell count, protein, glucose; also relieves pressure |
| Plain X-ray | Adds little; may show capsular swelling, a widened joint space and displaced fat lines |
| Ultrasound | Detects an effusion; the first test of choice for a suspected septic hip |
| MRI or technetium bone scan | To look for coexisting osteomyelitis or a deep abscess |
| Gonococcal swabs or nucleic acid tests | In adolescents: urethra, cervix, rectum and pharynx |
Synovial fluid in pyogenic arthritis (Nelson table):
| Condition | Appearance | White cells per mm3 (average) | Neutrophils |
|---|---|---|---|
| Normal | Clear, yellow | 0-200 | under 10% |
| Trauma | Clear, turbid or bloody | 50-4,000 | under 30% |
| JIA, reactive arthritis | Turbid | 250-80,000 (about 19,000) | over 70% |
| Pyogenic infection | Turbid | 10,000-250,000 (about 80,000) | over 90% |
| Tuberculosis | Turbid | 2,500-100,000 | over 60% |
- Akbar: a joint fluid white cell count above 100,000/microlitre means definite purulent infection.
- In septic arthritis the synovial glucose is low compared with blood and the mucin clot is poor.
- Culture of pus can still be negative, because synovial fluid itself slows bacterial growth; a Gram stain is often more useful.
A negative joint culture does not rule out septic arthritis when the fluid looks purulent with a high neutrophil count.
Differential diagnosis
- Transient (toxic) synovitis of the hip: the main mimic (table below).
- Osteomyelitis, cellulitis, bursitis, myositis, fasciitis, soft tissue abscess.
- Psoas abscess: fever with pain on hip flexion and rotation.
- Reactive arthritis after enteric infection (Yersinia, Campylobacter, Shigella, Salmonella): symmetric, several large joints.
- Juvenile idiopathic arthritis, Henoch-Schonlein purpura, Kawasaki disease, rheumatic fever (migratory polyarthritis), Crohn disease.
- Tuberculosis of the joint, Lyme disease, trauma.
Involvement of several or symmetric joints usually points away from bacterial infection.
Septic hip versus transient synovitis
Transient synovitis is a common cause of limp, typically at about 3-8 years (mean 6), twice as common in boys, often 1-2 weeks after a viral upper respiratory infection. It is a diagnosis of exclusion.
| Feature | Bacterial infection of the hip | Transient synovitis |
|---|---|---|
| Temperature | Fever above 38.5°C | Afebrile or mild fever |
| Walking | Refuses to walk | Walks with a painful limp |
| White cell count | Raised | Normal |
| ESR, CRP | ESR above 20 mm/hour, CRP raised | Normal or slightly raised |
| Hip position | Held flexed, abducted, externally rotated | Held normally |
| Pain | Severe, marked tenderness | Moderate, mild tenderness |
| Treatment | IV antibiotics and drainage | Rest, NSAID, review |
If an effusion is present and doubt remains, aspirate the hip and send cell count and culture.
"Fever, won't walk, high white count, high ESR": the more of these, the more likely the hip is septic.
Management
- Aspirate the joint before antibiotics if this does not delay treatment; send Gram stain and culture.
- Start IV antibiotics chosen for the child's age and the Gram stain:
| Situation | Suggested empirical cover (Akbar and Nelson) |
|---|---|
| Newborn | Anti-staphylococcal penicillin (oxacillin or nafcillin) plus cefotaxime or an aminoglycoside (covers S. aureus, GBS, Gram-negatives) |
| Older than about 3 months | Anti-staphylococcal cover, e.g. oxacillin, often with ceftriaxone or cefotaxime; add Hib cover if unimmunised |
| MRSA suspected | Clindamycin (if susceptible) or vancomycin |
| Gonococcal arthritis | Ceftriaxone |
| Group A streptococcus | Penicillin G |
- Drain the joint:
- Hip (and shoulder): prompt surgical drainage. The capsule attaches below the epiphysis, so pus under pressure can cut off blood supply to the femoral or humeral head and cause avascular necrosis. The hip is also deep and hard to aspirate repeatedly.
- Knee: repeated needle aspiration with IV antibiotics is often enough.
- Duration: guided by settling of fever and pain and by falling ESR and CRP. S. aureus usually needs at least about 3 weeks in total. Switch to oral antibiotics (for example cephalexin, clindamycin, amoxicillin-clavulanate) once the child is improving, if adherence is assured.
- Rest the joint in the acute phase, then physiotherapy to regain movement.
Septic arthritis of the hip is a surgical emergency. Antibiotics alone are not enough.
Prognosis
- Excellent with early drainage, before the cartilage is harmed.
- If pus remains for more than about 24 hours, there is a risk of cartilage loss, joint fibrosis and growth plate injury (Akbar).
- Worst outcomes: hip and shoulder infections (avascular necrosis) and newborns (frequent coexisting osteomyelitis damaging the growth plate).
Part 2: Osteomyelitis
Types
| Type | Key features |
|---|---|
| Acute haematogenous | The commonest form in children; follows bacteraemia |
| Subacute | Usually after local inoculation (penetrating injury); few systemic symptoms. Brodie abscess is a walled-off abscess inside bone, classically in the distal tibia |
| Chronic | After untreated or poorly treated infection; sequestrum (dead separated bone) and involucrum (thick sheath of new bone) are its hallmarks |
| Chronic recurrent multifocal | Autoinflammatory, not infective; recurrent fever and bone pain, cultures negative, often clavicle or small bones |
Pathogenesis: why the metaphysis
- In children, blood flows slowly through looped vessels in the metaphysis of long bones, so circulating bacteria settle there.
- Minor blunt trauma often comes before the infection and may prepare the site.
- In infancy: vessels cross the growth plate (transphyseal vessels), so infection spreads into the epiphysis and joint, causing septic arthritis and growth disturbance.
- Older children: the growth plate acts as a barrier; pus spreads outward under the periosteum (subperiosteal abscess) and joints are involved less often.
- Where the joint capsule attaches below the growth plate (hip, shoulder, elbow, ankle), a metaphyseal abscess can burst into the joint.
- In sickle cell disease, infection often involves the diaphysis, probably after bone infarction.
Epidemiology
- Peak age 3-12 years; boys twice as often as girls.
- Lower limbs most often; femur, tibia or humerus in about two-thirds; hands or feet about 15%; pelvis about 10%.
Organisms
| Setting | Organism |
|---|---|
| Most children | S. aureus (over 80%); also group A streptococcus, S. pneumoniae |
| Newborns | Add group B streptococcus and enteric Gram-negatives |
| Sickle cell disease and other haemoglobinopathies | Salmonella and S. aureus |
| Nail puncture through a shoe | Pseudomonas aeruginosa |
| Cat or dog bite | Pasteurella multocida |
| Culture-negative cases in young children | Often Kingella kingae (found by PCR) |
| Chronic osteomyelitis | S. aureus |
| Less common | Mycobacterium tuberculosis, anaerobes, Bartonella |
In sickle cell disease S. aureus also causes osteomyelitis, so treatment must cover both; but Salmonella is the organism linked specifically with sickle cell and is the usual exam answer.
Clinical features
- Fever, irritability, lethargy, poor appetite.
- Focal bone pain with exquisite point tenderness over the metaphysis, warmth, redness and swelling.
- Pseudoparalysis: reduced active and passive movement and refusal to bear weight.
- Usually one bone.
- Vertebral osteomyelitis: slow onset, back pain, little fever, sometimes cord compression.
- Pelvic osteomyelitis: fever, limp and vague hip, groin, thigh or abdominal pain.
Investigations
| Test | Finding |
|---|---|
| White cell count | Often raised, but not reliably |
| ESR and CRP | Sensitive but not specific; repeated to monitor treatment |
| Blood culture | Important, though often negative |
| Needle aspiration of bone (subperiosteal or metaphyseal) | Proves the diagnosis; Gram stain can identify bacteria within hours |
| Plain X-ray | Deep soft tissue swelling in the first few days; periosteal elevation and bone destruction only after 10-14 days |
| MRI | Most sensitive early test: shows marrow inflammation from the start |
| Technetium bone scan | Useful for multifocal disease; gallium scan if technetium is negative |
A normal X-ray in the first week does not exclude osteomyelitis. MRI (or a bone scan) is needed early.
Differential diagnosis
Septic arthritis, cellulitis, pyomyositis, fasciitis, discitis, fracture or trauma, juvenile idiopathic arthritis, bone cyst, histiocytosis and bone tumours (and leukaemia). Fever and malaise can make other serious infections and autoimmune diseases look similar at first.
Management
- Start IV antibiotics promptly, chosen by age, Gram stain of the aspirate and underlying disease:
| Organism or setting | Antibiotic (Nelson) |
|---|---|
| S. aureus, methicillin-sensitive | Oxacillin or nafcillin, or cefazolin |
| MRSA suspected | Clindamycin, vancomycin or linezolid |
| Group A streptococcus | Penicillin G |
| S. pneumoniae | Penicillin G, ceftriaxone or cefotaxime, or vancomycin |
| Sickle cell disease | Include a drug active against Salmonella |
| Pseudomonas (puncture wound) | Ceftazidime or piperacillin-tazobactam plus an aminoglycoside |
| Kingella kingae | Cefotaxime, oxacillin or penicillin G |
- Splint the limb to ease pain.
- Response is usually seen within about 48 hours. If there is no improvement (Akbar allows up to 3 days), think of pus needing drainage or an unusual organism.
- Surgical drainage and debridement when: abscess or extensive disease, chronic or atypical infection, hip involvement, a sequestrum, or spinal cord compression.
- Duration: Akbar advises at least 3 weeks; Nelson advises a minimum of 4-6 weeks. After a good clinical response with falling CRP or ESR, treatment can continue orally at home if adherence is assured.
- Puncture wounds of the foot: clean, irrigate, debride, remove foreign material and give tetanus prophylaxis.
Complications and prognosis
- Excellent prognosis if treated early and drained when needed.
- Complications come mainly from delay, inadequate treatment, poor blood supply or trauma.
- About 2-4% recur despite adequate treatment; a minority of these become chronic with sinus tracts, bone loss or, rarely, amputation.
- Growth disturbance and limb shortening are most common after neonatal osteomyelitis (Nelson: roughly 40-50% of affected newborns).
- Worst outcomes: newborns and infants with hip or shoulder involvement.
- Hib vaccination has practically removed Hib bone and joint infection.
"Metaphysis in the middle years, joint in the infant, diaphysis in sickle cell."
Expect: S. aureus as the commonest organism; Salmonella in sickle cell disease; Pseudomonas after a nail through a shoe; group B streptococcus and Gram-negatives in newborns; Kingella in culture-negative toddlers; knee as the commonest septic joint; metaphyseal start of osteomyelitis and spread to the joint in infants; X-ray normal for 10-14 days and MRI as the best early test; aspiration as the key diagnostic step; blood cultures positive in only about half; septic hip versus transient synovitis (fever, refusal to walk, raised white count and ESR); urgent drainage of the septic hip; sequestrum, involucrum and Brodie abscess; growth plate damage in neonates.
Quick revision
- Both infections usually blood-borne; S. aureus is the leading organism after the newborn period.
- Septic joint: knee commonest, hip most dangerous; aspirate for Gram stain, culture and cell count.
- Pyogenic fluid: turbid, tens of thousands of white cells, over 90% neutrophils, low glucose.
- Ultrasound first for a suspected septic hip; drain the septic hip surgically and urgently.
- Osteomyelitis starts in the metaphysis; in infants it spreads into the joint.
- X-ray changes after 10-14 days; MRI is the most sensitive early test.
- Sickle cell: Salmonella; shoe puncture: Pseudomonas; chronic: sequestrum and involucrum.
- IV antibiotics, then oral when improving; follow fever, pain, ESR and CRP.
- Transient synovitis: afebrile, walking, normal markers, recovers with rest.
Written by The House Job from standard references (Basis of Paediatrics by Pervez Akbar; Nelson Essentials of Paediatrics).
FAQ
Why must a septic hip be drained surgically?
Pus under pressure inside the hip capsule can block the blood supply to the femoral head and cause avascular necrosis. The hip is also deep and hard to aspirate repeatedly, so open drainage is preferred.
Can osteomyelitis be excluded with a normal X-ray?
No. Bone changes on X-ray take about 10-14 days to appear. MRI or a bone scan is needed if osteomyelitis is suspected early.
Which organism should be covered in a child with sickle cell disease and bone infection?
Salmonella as well as S. aureus. The infection often affects the shaft of the bone after an area of infarction.
How long are antibiotics given?
Usually several weeks: at least about 3 weeks for septic arthritis due to S. aureus, and 3-6 weeks for osteomyelitis depending on the reference. Treatment can be completed by mouth once the child is clearly improving and ESR and CRP are falling.