STEPTRACKERMUSCULOSKELETAL | PATHOLOGY + MICROBIOLOGY | HIGH-YIELD REVIEW

Osteomyelitis & Osteonecrosis

Infected bone and dead bone — the organism depends on the host; the infarct depends on a fragile blood supply.

FOCUS ON
S aureus is the most common cause of osteomyelitis. Clues: sickle cell → Salmonella; IV drug use → Pseudomonas, S aureus (vertebrae); prosthesis → S epidermidis; nail through a sneaker → Pseudomonas; diabetic foot → polymicrobial. Children seed the metaphysis. Avascular necrosis: steroids, alcohol, sickle cell, trauma — femoral head, scaphoid.
DISORDER · HOST
MECHANISM · ORGANISM
FEATURES
CLUE
Hematogenous osteomyelitis
Bacteremia seeds slow-flowing metaphyseal vessels; S aureus most common
Children: metaphysis of long bones · adults: vertebrae; fever, bone pain, ↑ ESR/CRP
MRI best early · biopsy and culture
Contiguous osteomyelitis
Spread from ulcer, wound, fracture, surgery
Diabetic foot ulcer (polymicrobial); probe-to-bone ⁺
Vascular disease impairs healing
Sickle cell disease
Bone infarcts + functional asplenia
Salmonella (and S aureus) osteomyelitis
Distinguish from vaso-occlusive crisis
IV drug use
Injected organisms seed bone
Pseudomonas, S aureus, Candida; vertebral and sternoclavicular osteomyelitis
Also endocarditis
Puncture through a shoe
Organism from foam sole
Pseudomonas osteomyelitis of the foot
Nail through sneaker
Prosthetic joint / hardware
Biofilm on foreign material
S epidermidis, S aureus
Often requires removal
Vertebral TB (Pott disease)
Hematogenous M tuberculosis
Thoracic vertebrae, disc destruction, psoas abscess, kyphosis (gibbus)
Endemic area, back pain, weight loss
Pathology & complications
Pus ↑ pressure → ischemia → dead bone
Sequestrum (dead bone), involucrum (new bone around it), Brodie abscess; chronic infection, draining sinus → SCC (Marjolin), amyloidosis, pathologic fracture
X-ray lags ~2 weeks
Avascular necrosis (osteonecrosis)
Bone infarction: glucocorticoids, alcohol, sickle cell, trauma, SLE (and steroids), Gaucher, decompression sickness, radiation
Femoral head (medial circumflex femoral artery) most common; scaphoid proximal pole, talus
Crescent sign; MRI detects early
Scaphoid fracture
Retrograde blood supply enters distally
FOOSH, anatomic snuffbox tenderness; X-ray may be normal early
Proximal pole AVN, nonunion · immobilize
Medication-related jaw osteonecrosis
Antiresorptive drugs impair remodeling
Bisphosphonates, denosumab; exposed jawbone after dental extraction
High-dose IV use for cancer

Mechanism — from bacteremia to sequestrum

Bacteremia
→
Metaphyseal capillary loopsSlow flow
→
Neutrophils, pus↑ intraosseous pressure
→
Ischemic dead boneSequestrum
→
Periosteal new boneInvolucrum

Antibiotics can't reach avascular sequestrum — chronic osteomyelitis often needs surgical debridement.

Organism by host

HOST / SETTING
ORGANISM
Most people
S aureus
Neonates
GBS, E coli, S aureus
Sickle cell
Salmonella, S aureus
IV drug use
Pseudomonas, S aureus, Candida
Prosthetic joint
S epidermidis, S aureus
Diabetic foot, decubitus ulcer
Polymicrobial (anaerobes)
Cat or dog bite
Pasteurella multocida
Vertebral, endemic area
M tuberculosis

How it's tested

COMMON CONFUSIONIn sickle cell disease, bone pain with fever may be a vaso-occlusive crisis or osteomyelitis. Salmonella is the classic exam answer for the infection, though S aureus is also common.
THE TRAPNormal X-rays in the first two weeks do not rule out osteomyelitis — MRI is the sensitive test.
YOUR APPROACHHost (age, sickle cell, IVDU, diabetes, prosthesis) → route (blood, contiguous) → site (metaphysis, vertebra, foot) → organism → imaging and culture.

If you see… think…

Child with fever and tender distal femur metaphysis → hematogenous S aureus osteomyelitis
Sickle cell patient with osteomyelitis → Salmonella
Nail through sneaker, then foot osteomyelitis → Pseudomonas
Heroin user with back pain and fever → vertebral osteomyelitis
Painful hip replacement months later, low-virulence organism → S epidermidis
Thoracic spine destruction with psoas abscess, immigrant → Pott disease (TB)
Draining sinus over chronic osteomyelitis, new ulcerating mass → SCC (Marjolin)
Lupus patient on steroids with groin pain → femoral head AVN
FOOSH with snuffbox tenderness, normal X-ray → occult scaphoid fracture
Exposed jawbone after extraction on IV zoledronate → medication-related jaw osteonecrosis