High Yield Overview
Pediatric Acute Osteomyelitis
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).
Examiner's 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
At a Glance: Acute vs Chronic OM
| 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
Memory 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
Memory 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
Memory 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
Memory Hook:Ready for oral when you can go to the CAFE!
Overview and Epidemiology
Definition: Acute infection of the bone, typically typically involving the metaphysis of long bones in children.
Epidemiology:
- More common than Septic Arthritis.
- Age: Boys > Girls. Any age, often rapid growth phases.
- Sites: Distal Femur, Proximal Tibia (Fast growing ends).
- Pathogens: S. aureus (80-90%). Kingella (less than 4yo). GBS (Neonates).
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.
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
📊 Management Algorithm
Click to expand
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
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
MRI Sensitivity
Key Findings:
- MRI sensitivity over 97% for acute OM.
- Can detect marrow changes within 24-48 hours.
- Superior to X-ray and Bone Scan.
Clinical Implication: MRI is imaging modality of choice.
Limitation: Retrospective
Oral vs IV Antibiotics
Key Findings:
- Short IV course (3-4 days) followed by Oral is as effective as Long IV.
- Outcome depends on total duration and sensitivity, not route.
- Allows earlier discharge.
Clinical Implication: Switch to oral early if responding and high bioavailability agent available.
Limitation: RCT
CRP Monitoring
Key Findings:
- CRP is the most sensitive parameter for monitoring response.
- Failure of CRP to drop indicates failure of treatment (wrong Abx or abscess needing drainage).
Clinical Implication: Track the CRP.
Limitation: Prospective
Kingella Kingae
Key Findings:
- Kingella increasingly common in under 4yo.
- May have milder clinical course.
- Requires PCR for detection.
Clinical Implication: Cover for Kingella in toddlers.
Limitation: Review
Intra-articular Metaphyses
Key Findings:
- 4 locations where metaphysis is intracapsular: Proxi Femur, Distal Humerus, Proximal Radius, Distal Fibula.
- OM here causes Septic Arthritis.
Clinical Implication: Check joint in these locations.
Limitation: Anatomic
Viva Scenarios
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
The Normal X-ray
EXAMINER
"How do you manage?"
EXCEPTIONAL ANSWER
**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 POINTS TO SCORE
X-ray insensitive acute
MRI gold standard
CRP elevated
Admit for workup
COMMON TRAPS
✗Trusting the normal X-ray
✗Discharging a septic child
LIKELY FOLLOW-UPS
"What do you see on MRI?"
"When does periosteal reaction appear?"
VIVA SCENARIOStandard
The Treatment Failure
EXAMINER
"What is happening and what do you do?"
EXCEPTIONAL ANSWER
**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 POINTS TO SCORE
48-72 hour rule
Look for abscess
Surgical indication
Think MRSA
COMMON TRAPS
✗Just changing Abx without imaging
✗Wait and see
LIKELY FOLLOW-UPS
"How do you decompress OM?"
"What is a sequestrum?"
VIVA SCENARIOStandard
Intra-articular Metaphyses
EXAMINER
"List them and explain."
EXCEPTIONAL ANSWER
**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 POINTS TO SCORE
Proximal Femur
Distal Humerus
Proximal Radius
Distal Fibula
OM causes Septic Arthritis
COMMON TRAPS
✗Forgetting Distal Fibula
✗Not checking the joint
LIKELY FOLLOW-UPS
"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.
Australian Context
- MRSA: Community-acquired MRSA (CA-MRSA) rates vary by region (higher in some indigenous communities/QLD).
- PICC lines: Hospital-in-the-home (HITH) often manages long-term IV antibiotics.
- Guidelines: ASID (Australasian Society) and Therapeutic Guidelines (Antibiotic).
High-Yield Exam Summary
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