Osteomyelitis is one of those Step 1 topics that feels “simple” until a question stem forces you to pick between hematogenous spread vs contiguous infection vs sickle cell vs diabetic foot—and suddenly the details matter. The good news: osteomyelitis has a few high-yield patterns that show up over and over. If you can match the patient + risk factor + organism + imaging/labs, you’ll consistently get these right.
What Is Osteomyelitis?
Osteomyelitis = infection of bone (and bone marrow) leading to inflammation, necrosis, and sometimes chronic structural damage.
Boards framing: “bone pain + fever + elevated inflammatory markers” in the right risk context.
Pathophysiology (How Infection Gets Into Bone)
There are three major routes, and Step questions love distinguishing them:
1) Hematogenous spread (most common in kids)
- Bacteria seed bone via bloodstream
- Classically affects metaphysis of long bones in children (rich blood supply; sluggish flow)
Common sites
- Femur, tibia, humerus metaphyses (kids)
- Vertebrae in adults (esp. older adults, IVDU)
2) Contiguous spread (trauma/surgery/adjacent soft-tissue infection)
- Infection spreads from:
- Open fractures
- Orthopedic hardware
- Pressure ulcers
- Diabetic foot ulcers
3) Direct inoculation
- Penetrating trauma, bites, puncture wounds (e.g., nail through shoe)
- Iatrogenic (post-op)
The Core Pathogenesis: Why Bone Is Hard to “Clear”
Once bacteria are in bone:
- Inflammation increases intramedullary pressure
- Vascular compromise → ischemia → bone necrosis
- Necrotic bone can separate forming a sequestrum (dead bone fragment)
- New bone may form around it: involucrum
- Chronic infection often persists because antibiotics penetrate poorly into devitalized bone and biofilms can form (especially with hardware)
High-yield vocab
- Sequestrum = dead bone
- Involucrum = new bone surrounding sequestrum
- Sinus tract (chronic) can drain to skin surface
Microbiology: Match the Bug to the Patient
Most common overall
- Staphylococcus aureus (MSSA/MRSA) = #1 cause in most settings
High-yield associations table
| Clinical setting / risk factor | Most likely organism(s) | Step-style clue |
|---|---|---|
| Child with hematogenous osteomyelitis | S. aureus | Metaphyseal pain, fever, refusal to bear weight |
| Sickle cell disease | Salmonella (also S. aureus) | Dactylitis history, HbS, bone pain + fever |
| Puncture wound through rubber-soled shoe | Pseudomonas aeruginosa | Nail-through-sneaker → osteomyelitis (often calcaneus/metatarsals) |
| Diabetic foot ulcer / chronic contiguous infection | Polymicrobial (incl. S. aureus, streptococci, gram-neg rods, anaerobes) | Longstanding ulcer, poor perfusion, neuropathy |
| IV drug use (esp. vertebral) | S. aureus, Pseudomonas | Back pain + fever, risk of epidural abscess |
| Prosthetic joint / hardware | S. epidermidis (biofilm), S. aureus | Persistent pain, loosening, low-grade symptoms |
| Animal/human bites with bone involvement | Pasteurella (animal), Eikenella (human) | Bite wound + infection extending deeper |
Clinical Presentation (Acute vs Chronic)
Acute osteomyelitis
Classic triad-ish
- Bone pain (localized tenderness)
- Fever
- Decreased use of limb / refusal to bear weight (kids)
Other clues:
- Warmth, swelling over bone
- Limited range of motion if adjacent joint irritated
Chronic osteomyelitis
- More indolent: persistent pain, intermittent drainage
- Sinus tract to skin can occur
- Systemic symptoms may be minimal
- Think: prior trauma/surgery, poor vascular supply, diabetic foot, hardware
Testable complication
- Chronic osteomyelitis with draining sinus tracts can predispose to squamous cell carcinoma in the tract (classically described).
Diagnosis: Labs, Cultures, and Imaging (What Step 1 Expects)
Labs (supportive, not definitive)
- ↑ ESR and ↑ CRP (often earliest/most useful to trend)
- WBC may be elevated (variable)
Cultures (gold standard concept)
- Bone biopsy with culture is the most specific way to identify the pathogen.
- Blood cultures may be positive in hematogenous cases.
Imaging: Know the timeline
High-yield imaging progression
- X-ray: may be normal early (first ~7–10 days). Later shows lytic changes, periosteal reaction.
- MRI: most sensitive early; best for marrow edema, soft tissue extension, epidural abscess.
- CT: helpful for cortical bone detail, sequestra, surgical planning (less sensitive early than MRI).
- Bone scan (nuclear medicine): sensitive but less specific; useful when MRI isn’t possible.
Quick comparison table
| Modality | Best for | Step takeaway |
|---|---|---|
| X-ray | Initial screen, later bony changes | Can be normal early |
| MRI | Early detection; soft tissue | Most sensitive early |
| Bone scan | Screening when MRI not available | Sensitive, not specific |
| Bone biopsy | Definitive organism ID | Guides targeted therapy |
Treatment (Principles + Board-Friendly Choices)
Core management steps
- Obtain cultures first if patient is stable (blood ± bone biopsy)
- Start empiric IV antibiotics (then tailor to culture results)
- Surgical debridement when needed (necrotic bone, abscess, hardware involvement, chronic disease)
Empiric coverage patterns (conceptual)
- Cover S. aureus (including MRSA if risk/high prevalence)
- Add gram-negative coverage when indicated (diabetic foot, puncture wound, IVDU, trauma)
- Consider anaerobes in polymicrobial contiguous infections (diabetic foot, ischemic tissue)
High-yield “when to operate”
- Chronic osteomyelitis (sequestrum)
- Abscess formation
- Failure of antibiotics
- Infected prosthetic material often requires removal/exchange + prolonged antibiotics (biofilm problem)
Duration (typical board-level expectation)
- Often 4–6 weeks of antibiotics (longer in chronic cases or hardware-associated infections)
Osteomyelitis vs Septic Arthritis (Classic Differentiator)
Step stems sometimes blur these. A simple way to split them:
- Osteomyelitis: focal bone tenderness, sometimes more gradual; imaging (MRI) shows marrow changes.
- Septic arthritis: painful swollen joint, severely limited passive ROM; synovial fluid analysis is key.
High-Yield Step 1 Associations & “If You See This, Think That”
Pediatric metaphysis
- Hematogenous spread → metaphyseal infection (long bones).
- Think of the vascular anatomy: sluggish flow + fenestrated capillaries predispose to seeding.
Sickle cell = Salmonella (but don’t forget S. aureus)
- If stem screams sickle cell, Salmonella is the “test writer’s favorite.”
- Still, real-world and board answers sometimes include S. aureus as a close second—choose Salmonella when sickle cell is explicit.
Puncture wound through shoe
- Nail-through-sneaker → Pseudomonas osteomyelitis.
Diabetic foot osteomyelitis
- Often polymicrobial, contiguous spread, poor healing.
- Probe-to-bone concept: deep ulcer with bone exposure is a big clue.
Biofilms on hardware
- S. epidermidis (and S. aureus) can form biofilms, increasing persistence and antibiotic resistance functionally (reduced penetration/immune clearance).
First Aid Cross-References (Where This Lives in FA)
In First Aid (MSK: Bone disorders / Infections) you’ll typically see osteomyelitis emphasized with:
- S. aureus as the most common cause
- Salmonella in sickle cell
- Pseudomonas in puncture wounds
- General clinical features: bone pain, fever, ↑ ESR
Use FA as your association map, then layer in the details above (metaphysis, imaging timeline, chronic sequelae) to handle longer stems.
Rapid Review (Exam-Day Checklist)
- #1 organism overall: S. aureus
- Sickle cell: Salmonella
- Nail through shoe: Pseudomonas
- Early best imaging: MRI
- Supportive labs: ↑ ESR/CRP
- Definitive diagnosis: bone biopsy culture
- Chronic signs: sequestrum, involucrum, sinus tract ± SCC risk
- Treatment principles: culture → IV antibiotics → debride if necrotic/abscess/hardware