What Causes Osteomyelitis
Osteomyelitis is primarily caused by bacterial infection of bone tissue, most commonly through hematogenous spread of bacteria, with Staphylococcus aureus being the predominant pathogen in approximately 50% of cases. 1, 2
Primary Mechanisms of Infection
Osteomyelitis develops through three distinct pathways:
1. Hematogenous Spread (Most Common in Children)
- Bacteria disseminate through the bloodstream to bone, typically seeding in the metaphysis of long bones due to looping nutrient vessels 1
- In children ≤18 months, transphyseal vessels allow infection to spread to the epiphysis 1
- Accounts for the majority of pediatric cases and occurs twice as frequently as septic arthritis 1
2. Contiguous Spread from Adjacent Soft Tissue
- Infection extends directly from overlying wounds or soft tissue infections into bone 2
- Most common mechanism in adults, particularly in diabetic foot osteomyelitis (77.8% of cases) 3
- Develops through penetration of cortical bone into the medullary cavity 2
- Can occur from dental infections spreading to mandible/maxilla, or sinus infections to frontal/mastoid bones 4
3. Direct Inoculation
- Trauma, surgical procedures, or penetrating injuries introduce bacteria directly into bone 4
- Includes puncture wounds, open fractures, internal fixation hardware, and diagnostic procedures 4
Causative Organisms
The bacterial profile varies by patient population:
- All ages: Staphylococcus aureus (50% of cases) 2, 3
- Neonates: Group B streptococcus 1
- Children <4 years: Kingella kingae 1
- Sickle cell disease: Salmonella species 1
- Polymicrobial infections: Common in diabetic foot osteomyelitis, though many cases remain monomicrobial 2
- Other organisms: Coagulase-negative staphylococci (25%), aerobic streptococci (30%), Enterobacteriaceae (40%) 2
Major Risk Factors
Medical History Factors
- History of foot ulcers/foot disease (OR = 2.52) 5
- Previous wounds (RR = 2.2) 6
- Diabetes mellitus - increases overall risk of acute and chronic osteomyelitis 7
- Sickle cell anemia - unusual susceptibility to skeletal infections 4
- Chronic granulomatous disease 4
- Cardiovascular disease and vascular insufficiency 7, 3
Clinical Features
- Deep wounds extending to bone or joint (RR = 23.1) - strongest predictor 6
- Ulcer size >4-5 cm² (OR = 2.74) 5
- Inflamed ulcer (OR = 6.20) 5
- Recurrent or multiple wounds (RR = 1.9) 6
- Exposed bone (positive LR = 9.2) 2
- Fever (OR = 2.20) 5
Surgical/Procedural Factors
- Operation duration >3 hours (OR = 1.74) 5
- Incision length >10 cm (OR = 6.53) 5
- Indwelling intravascular catheters 4
- Internal fixation of fractures 4
Laboratory Findings
- Elevated HbA1c (OR = 1.14) 5
- Hypoalbuminemia/malnutrition (OR = 1.74) 5, 8
- Anemia (OR = 1.54) 5
- Polymicrobial infections (OR = 2.58) 5
Behavioral Risk Factors
- Intravenous drug abuse - predisposes to bacteremia 4
- Alcohol consumption - associated with major complications 8
- Smoking - associated with major complications 8
Important Clinical Pitfalls
The classic triad of fever, pain, and diminished mobility occurs in only slightly over 50% of acute hematogenous osteomyelitis cases 1, making diagnosis challenging. Osteomyelitis can occur without overlying signs of inflammation 2.
Inadequate or delayed management of acute osteomyelitis is the major risk factor for chronic bone infection 4, emphasizing the critical importance of early recognition and treatment.
In diabetic foot infections, wounds that probe to bone substantially increase the likelihood of osteomyelitis (positive LR = 7.2) 2, making this a crucial bedside diagnostic tool.