High-yield overview
Metaphyseal Infection | Hematogenous Spread
MetaphysisLocation
S. aureusPathogen
MRIGold Std Imaging
AntibioticsMainstay Rx
Types by Duration
Acute (less than 2 weeks)
PatternInflammation. No sequestrum yet. Antibiotics +/- washout.
TreatmentIV Abx
Subacute (2-6 weeks)
PatternBrodie's Abscess. Indolent.
TreatmentAbx +/- Debridement
Chronic (greater than 6 weeks)
PatternSequestrum (dead bone) & Involucrum (new bone).
TreatmentSurgical Debridement
Critical Must-Knows
- Pathophysiology: Hematogenous seeding of the metaphysis (slow flow in venous sinusoids).
- Pathogen: S. aureus is #1. Kingella kingae in under 4yo. Salmonella in Sickle Cell.
- Diagnosis: MRI is most sensitive/specific early. X-rays normal for 10-14 days.
- Treatment: Antibiotics are mainstay. Surgery (Drilling/Decompression) if abscess or no response.
- Complications: Chronic OM, Growth arrest, Septic arthritis (transphyseal spread in neonates).
Clinical Pearls
- “Infection starts in the METAPHYSIS (vascular loop slowing).
- “X-rays are negative early (need 30-50% bone loss to see lysis).
- “CRP is the best marker for monitoring response.
- “Treatment is 90% Medical (Antibiotics), 10% Surgical (Abscess).
Osteomyelitis Pitfalls
X-ray Sensitivity
X-ray Lags. X-rays are normal for the first 2 weeks. Do not rule out OM based on a normal X-ray. Get MRI.
Septic Arthritis
Intra-articular Metaphysis. Metaphyses inside cartilage (proximal femur, distal humerus, proximal radius, distal fibula) can breach into the joint → Septic Arthritis.
Transphyseal Spread
Neonates. Vessels cross the physis in infants under 18 months. OM can spread to epiphysis and joint easily.
Pathologic Fracture
Weak Bone. Infection weakens bone. Protect weight-bearing during treatment.
At a Glance
| Feature | Acute OM | Chronic OM |
|---|---|---|
| Duration | less than 2 weeks | greater than 6 weeks |
| Bone Viability | Ischemia reversible | Sequestrum (Dead bone) |
| Pathology | Pus / Inflammation | Sequestrum / Involucrum |
| Treatment | Antibiotics (+/- Decompression) | Surgical Debridement essential |
| Biofilm | Immature | Mature / Established |
Mnemonic
SLOWPathophysiology
S
Stasis
Vascular stasis in metaphyseal loops
L
Loops
Hairpin capillary loops
O
Obstruction
Micro-thrombi form
W
Walling off
Abscess formation
Hook:Why metaphysis? Slow flow.
Mnemonic
KINGSPathogens
K
Kingella
less than 4 years old
I
Infants
GBS / E. coli
N
Normal
Staphylococcus aureus (Most Common)
G
Gonorrhea
Adolescents
S
Sickle / Salmonella
Sickle Cell → Salmonella
Hook:Know the bugs.
Mnemonic
S-I-B-CChronic OM Features
S
Sequestrum
Dead bone (nidus)
I
Involucrum
New bone sheath around it
B
Brodie's
Abscess (Subacute)
C
Cloaca
Draining sinus tract
Hook:The terminology of chronic OM.
Mnemonic
CAFEIV to Oral Switch Criteria
C
CRP falling
CRP trending down from peak, not necessarily normal
A
Afebrile
No fever for 24-48 hours
F
Feeding well
Able to tolerate oral medications and food
E
Erythema settling
Local signs improving clinically
Hook:Ready for oral when you can go to the CAFE!
Overview and Epidemiology
Definition: Acute infection of bone, typically involving the metaphysis of long bones in children, usually arising by haematogenous spread.
Epidemiology:
- Incidence: Roughly 1 in 5,000 to 1 in 10,000 children per year in high-income settings; higher in some low-resource regions. More common than septic arthritis.
- Age: Roughly half of cases occur under 5 years. Boys affected more than girls (about 2:1).
- Sites: Lower limb predominates (distal femur, proximal/distal tibia) — fast-growing, well-vascularised metaphyses.
- Pathogens: S. aureus predominant in older children. Kingella kingae is the leading organism in children aged 6 to 48 months when PCR is used. Group B Streptococcus and E. coli in neonates; Salmonella in sickle cell disease.
Pathophysiology (Hematogenous):
- Bacteremia: Transient bacteremia (e.g., from teeth, skin, gut).
- Seeding: Bacteria lodge in the Metaphyseal Vascular Loops.
- Why? Blood flow slows down in hairpin loops, allowing bacteria to settle.
- Why Metaphysis? Reticuloendothelial system is deficient here.
- Proliferation: Bacteria proliferate → Inflammation → Pus.
- Pressure: Intra-osseous pressure rises → Ischemia of bone.
- Spread:
- Subperiosteal abscess: Pus breaks through cortex (children have loose periosteum).
- Joint: If metaphysis is intra-articular (Hip, Shoulder, Elbow, Ankle).
- Medullary canal.
Pathophysiology and Mechanisms
Metaphyseal Anatomy
The metaphysis of pediatric long bones is the primary site of infection due to its unique vascular architecture.
- Vascular Loops: Nutrient arteries terminate in hairpin capillary loops near the physis.
- Slow Flow: Blood flow slows significantly in these loops, allowing bacteria to settle.
- Physis: The growth plate (physis) usually acts as a barrier to the spread of infection into the epiphysis.
- Periosteum: In children, the periosteum is thick but loosely attached. Pus can easily lift it, forming a subperiosteal abscess.
Intra-articular Metaphyses
In four specific locations, the joint capsule inserts distal to the metaphysis (or proximal in femur/humerus terms), meaning the metaphysis is intracapsular.
- Proximal Femur (Hip Joint)
- Proximal Radius (Elbow Joint)
- Distal Humerus (Elbow Joint)
- Distal Fibula (Ankle Joint)
Infection here can rupture directly into the joint, causing Septic Arthritis.
Classification Systems
- Acute (less than 2 weeks): Inflammation. No sequestrum yet. Antibiotics +/- washout.
- Subacute (2-6 weeks): Brodie's Abscess. Indolent.
- Chronic (greater than 6 weeks): Sequestrum (dead bone) & Involucrum (new bone).
Classification guides duration of antibiotic therapy.
Clinical Assessment
History:
- Pain: Focal limb pain.
- Limp/Pseudoparalysis: Refusal to use limb.
- Fever: Often febrile (but not always).
- History: Recent trauma (minor) often reported (red herring or localizes bacteria).
Physical Examination:
- Tenderness: FOCAL bony tenderness (Metaphysis).
- Swelling: Soft tissue swelling / Erythema (late sign).
- ROM: Joint range often preserved (vs Septic Arthritis) unless pararticular or sympathetic effusion.
- Systemic: Signs of sepsis.
Differential Diagnosis
| Condition | Distinguishing Features | Key Investigation |
|---|---|---|
| Septic arthritis | Joint held flexed, marked pain on ANY passive movement, effusion | Joint aspiration (WCC over 50,000), MRI |
| Transient synovitis | Afebrile/low fever, normal-near-normal CRP, recent viral URTI, settles in days | Kocher criteria, normal inflammatory markers |
| Ewing sarcoma / leukaemia | Diaphyseal, lamellated periosteal reaction, night pain, cytopenias, weight loss | MRI, biopsy, blood film, LDH |
| Chronic non-bacterial osteomyelitis (CRMO) | Multifocal, recurrent, sterile cultures, clavicle/metaphyses, no true sepsis | Whole-body MRI, biopsy excluding infection |
| Pyomyositis / cellulitis | Soft-tissue focus, normal underlying bone marrow on MRI | MRI (soft-tissue vs marrow signal) |
| Trauma / occult fracture | Clear mechanism, normal inflammatory markers | Radiograph; MRI if uncertain |
Investigations
Labs:
- WBC: Elevated (neutrophilia).
- CRP/ESR: Elevated. CRP rises first and best for monitoring.
- Blood Cultures: POSITIVE in 40-50%. CRITICAL STEP.
Imaging:
-
X-ray:
- Early (less than 2 weeks): Normal or soft tissue swelling.
- Late: Periosteal reaction, Lytic lesions (need 30-50% bone loss).
-
MRI (Test of Choice):
- High sensitivity (greater than 90%) and specificity.
- Shows Marrow Edema (T2 bright, T1 dark).
- Shows Subperiosteal Abscess.
- Shows associated Septic Arthritis.
-
Ultrasound: Can show subperiosteal abscess (fluid under periosteum).
-
Bone Scan: If MRI unavailable or multifocal suspicion.
Management Algorithm
Surgical Technique
Cortical Window / Drilling
- Locate: Use fluoro to locate metaphyseal focus.
- Incision: Directly over maximal tenderness/abscess.
- Periosteum: Incise periosteum (often releases pus from subperiosteal abscess).
- Drill: 2.0mm or 3.2mm drill holes into metaphysis.
- Window: Create small cortical window if needed to evacuate intramedullary pus.
- Irrigate: Copious washout.
- Culture: Swab pus, bone biopsy.
- Closure: Loose closure over drain.
Decompress the "boil inside the bone".
Complications
| Complication | Mechanism | Management |
|---|---|---|
| Chronic Osteomyelitis | Inadequate Rx, Sequestrum | Debridement + Long Abx |
| Growth Arrest | Physis damage | Epiphysiodesis / Reconstruction |
| Septic Arthritis | Proximity / Transphyseal | Joint Washout |
| Pathologic Fracture | Bone cleaning/weakening | Cast / Fixation |
| DVT | Inflammation + Immobility | Anticoagulation |
Pelvic Osteomyelitis: Specifically tricky. Often presents as deep hip/abdominal pain. MRI essential. May need prolonged antibiotics.
Postoperative Care
- Antibiotics: Guided by culture sensitivities. Monitor CRP weekly.
- Immobilization: Splint/Cast for comfort and to prevent pathologic fracture.
- PICC Line: For long-term IV (if oral not suitable/available).
- Follow-up: X-rays to ensure healing, monitor growth.
Outcomes
- Uncomplicated: Excellent prognosis with antibiotics.
- MRSA: More aggressive, higher DVT risk, often needs surgery.
- Chronic OM: Difficult to eradicate. Recurrence common.
Evidence Base
Evidence
Short-Course Antibiotics (Landmark RCT)
Key Findings:
- 131 culture-positive children randomised to 20 vs 30 days of clindamycin or first-generation cephalosporin (first 2-4 days IV).
- S. aureus caused 89% of cases, all methicillin-susceptible; 24% had no surgery.
- Both groups recovered fully (only 1 mild sequela per arm) when CRP normalised within 7-10 days.
Clinical implication: Uncomplicated AHO in MSSA-predominant settings can be treated for about 20 days with a short IV phase then high-dose oral; extensive surgery is rarely needed.
Limitation: Excluded immunocompromised and MRSA-predominant populations; not generalisable where CA-MRSA is common.
Evidence
Simplified Treatment & CRP Monitoring
Key Findings:
- Prospective RCT of 50 children with acute S. aureus osteomyelitis, early IV-to-oral switch (mostly within 4 days).
- CRP normalised within a median of 9 days; ESR took a median of 29 days.
- No failures or long-term sequelae; serum bactericidal titres added nothing.
Clinical implication: CRP is the preferred marker to track treatment response and time the IV-to-oral switch; a failing CRP signals undrained pus or wrong antibiotic.
Limitation: Small, single-country (Finland), MSSA-only cohort.
Evidence
Kingella kingae Epidemiology (PCR Era)
Key Findings:
- 217 children with suspected osteoarticular infection; organism identified in 63.6% using culture plus PCR.
- K. kingae caused 87.7% of confirmed infections in children aged 6-48 months, exceeding S. aureus.
- S. aureus remained dominant (78.2%) in children over 4 years.
Clinical implication: In toddlers, empirical cover and diagnostics must target K. kingae (often culture-negative); routine PCR markedly improves yield.
Limitation: Retrospective, single-centre Swiss cohort.
Evidence
Oropharyngeal PCR for Kingella
Key Findings:
- Prospective study of 123 children aged 6-48 months with atraumatic osteoarticular complaints.
- Oropharyngeal swab PCR for K. kingae had sensitivity 100% and specificity 90.5% for K. kingae osteoarticular infection.
- A non-invasive swab can predict K. kingae as the cause.
Clinical implication: A simple throat swab PCR can support diagnosis of K. kingae infection in young children when blood and joint cultures are negative.
Limitation: Single-centre; specificity imperfect because of asymptomatic carriage.
Evidence
Mimic: Chronic Non-Bacterial Osteomyelitis (CRMO)
Key Findings:
- CRMO (chronic recurrent multifocal osteomyelitis) is a sterile autoinflammatory mimic of bacterial OM in children.
- Whole-body MRI detects multifocal lesions at typical sites and clinically silent lesions.
- Imaging plus clinical features can avoid biopsy when infection and malignancy are excluded.
Clinical implication: Consider CRMO in recurrent, multifocal, culture-negative bone lesions without true sepsis; whole-body MRI guides diagnosis.
Limitation: Narrative review.
Evidence
PIDS/IDSA Guideline (Diagnosis & Management)
Key Findings:
- First multidisciplinary PIDS/IDSA GRADE-based guideline on acute haematogenous osteomyelitis in children (2021).
- Recommends MRI as the preferred imaging, blood cultures and image-guided/operative sampling, and prompt empirical antibiotics covering local S. aureus resistance.
- Supports early IV-to-oral transition and total courses of about 3-4 weeks in uncomplicated, responding cases.
Clinical implication: Anchor management to local microbiology and resistance, with MRI-led diagnosis and an evidence-based short IV-to-oral pathway.
Limitation: Several recommendations rest on low-certainty evidence.
Viva Scenarios
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
The Normal X-ray
Clinical prompt
“How do you manage?”
Practical approach
**Do NOT discharge based on X-ray.**
1. **Fact**: X-rays are normal in acute OM for the first 10-14 days. Need 30-50% bone loss to visualise lysis.
2. **Workup**:
- Labs: CRP/ESR, WBC, Blood Culture.
- If CRP elevated + Focal tenderness → ADMIT.
3. **Diagnostic Imaging**: MRI is the gold standard (shows marrow edema).
4. **Treatment**: Start Abx after cultures. If MRI shows abscess → Surgery.
Key clinical points
X-ray insensitive acute
MRI gold standard
CRP elevated
Admit for workup
Common pitfalls
Trusting the normal X-ray
Discharging a septic child
Further questions
“What do you see on MRI?”
“When does periosteal reaction appear?”
Viva scenarioStandard
The Treatment Failure
Clinical prompt
“What is happening and what do you do?”
Practical approach
**Failure of Medical Management.**
1. **Differential**:
- *Abscess*: Undrained pus (subperiosteal or intra-osseous).
- *Wrong Bug*: MRSA? Kingella? Unusual pathogen?
- *DVT*: Occurs with OM (especially MRSA).
2. **Management**:
- *Re-image*: MRI (if not done or repeat) to look for abscess.
- *Surgery*: If abscess found or persistent clinical failure → Surgical Decompression/Drilling.
- *Change Abx*: Broaden coverage (consider MRSA).
Key clinical points
48-72 hour rule
Look for abscess
Surgical indication
Think MRSA
Common pitfalls
Just changing Abx without imaging
Wait and see
Further questions
“How do you decompress OM?”
“What is a sequestrum?”
Viva scenarioStandard
Intra-articular Metaphyses
Clinical prompt
“List them and explain.”
Practical approach
**The 4 Locations (Hip, Elbow, Shoulder, Ankle).**
1. **Locations**:
- *Proximal Femur* (Hip).
- *Distal Humerus* (Elbow).
- *Proximal Radius* (Elbow).
- *Distal Fibula* (Ankle).
2. **Significance**:
- The joint capsule attaches *distal* to the physis/metaphysis.
- Therefore, the metaphysis is *inside* the joint capsule.
3. **Result**:
- Acute Metaphyseal Osteomyelitis can break through the cortex directly into the joint space.
- Causes **Septic Arthritis**.
4. **Management**: Requires treating both the OM and the Septic joint (Washout).
Key clinical points
Proximal Femur
Distal Humerus
Proximal Radius
Distal Fibula
OM causes Septic Arthritis
Common pitfalls
Forgetting Distal Fibula
Not checking the joint
Further questions
“What about the proximal tibia?”
“How do neonates spread to joint?”
MCQ Practice Points
Pathogenesis
Q: Where does hematogenous osteomyelitis start? A: In the Metaphysis. Specifically in the slow-flowing venous sinusoidal loops.
Imaging
Q: How long before X-ray changes are visible in acute OM? A: 10-14 days. Need 30-50% bone mineral loss to see changes.
Chronic OM Terms
Q: What is an Involucrum? A: New bone formation (sheath) surrounding the dead bone (sequestrum) in chronic osteomyelitis.
Pathogen Sickle Cell
Q: What unique pathogen causes OM in Sickle Cell patients? A: Salmonella (though S. aureus is still common/more common, Salmonella is the unique association).
Surgical Indication
Q: When is surgery indicated in acute OM? A: 1) Abscess formation (subperiosteal/intra-osseous), 2) Failure to respond to antibiotics (48-72h), 3) Sequestrum, 4) Associated septic arthritis.
Controversies and Areas of Uncertainty
Duration of Therapy
Landmark RCTs support total courses near 3 weeks for uncomplicated, responding disease, yet many units still treat for 4-6 weeks. Optimal duration in MRSA, neonates and complicated cases is unresolved.
IV-to-Oral Timing
Early switch (within days) is safe with high-bioavailability oral agents and a falling CRP, but exact thresholds differ between guidelines and there is no universal CRP cut-off.
Routine MRI vs Selective Imaging
MRI is the most sensitive test but access, cost and the frequent need for sedation/anaesthesia in young children limit universal early use; some centres treat clinically with ultrasound and markers.
Surgery vs Antibiotics Alone
Most acute OM resolves on antibiotics alone. The threshold for drilling/decompression (especially for small intraosseous collections) and the value of "source control" without a drainable abscess remain debated.
Guidelines, Registries & Global Practice
Global epidemiology
- Incidence roughly 1 in 5,000 to 1 in 10,000 children/year in high-income settings; higher burden and later presentation (more chronic OM) in low-resource regions.
- Kingella kingae predominates in children 6-48 months in PCR-equipped centres; S. aureus predominates in older children worldwide. CA-MRSA proportion varies sharply by region and drives empirical choices.
| Body | Imaging | Antibiotic Approach |
|---|---|---|
| PIDS / IDSA (US, 2021) | MRI preferred; culture/PCR sampling stressed | Empirical cover by local resistance; early IV-to-oral; ~3-4 weeks total if responding |
| BSAC / UK practice (BOA-aligned) | MRI gold standard; ultrasound for subperiosteal collections | Short IV then oral; flucloxacillin first-line where MSSA dominant |
| European (ESPID-informed) | MRI; supports short-course strategy (Peltola data) | Total course as short as ~20 days in uncomplicated MSSA disease |
| Limited-resource settings | Radiograph +/- ultrasound; MRI often unavailable | Longer empirical IV courses; higher surgical/source-control role due to late presentation |
Practice variation (high- vs limited-resource)
- High-resource: MRI-led diagnosis, PCR microbiology, early IV-to-oral switch, outpatient/ambulatory IV (e.g. PICC) where oral not feasible, short total courses.
- Limited-resource: later presentation, more chronic OM and sequestra, greater reliance on radiographs/ultrasound and surgical debridement, longer parenteral therapy.
- Registries: no dedicated paediatric OM implant registry; surveillance comes from national paediatric infection networks and antimicrobial-resistance programmes that track local S. aureus/MRSA rates guiding empirical therapy.
Exam day cheat sheet
Key Features
- Metaphyseal start
- Slow flow loops
- S. aureus #1
- Start Abx after CX
Imaging
- X-ray normal early (10-14 days for changes)
- MRI Gold Standard (95% sensitivity)
- Marrow edema = Early sign
- Subperiosteal abscess = Surgical drainage
- US useful for soft tissue/abscess
Chronic Terms
- Sequestrum (Dead)
- Involucrum (New)
- Cloaca (Drain)
- Brodie's (Abscess)
Intra-articular
- Prox Femur
- Dist Humerus
- Prox Radius
- Dist Fibula