Model Answer
0 min readIntroduction
Acute bacterial osteomyelitis is an infection of the bone, typically occurring in children and adolescents, though it can affect individuals of any age. It’s a serious condition that, if left untreated, can lead to chronic osteomyelitis, septic arthritis, and even sepsis. The infection is usually caused by hematogenous spread of bacteria, meaning the bacteria travel through the bloodstream to the bone. Understanding the etiopathogenesis and pathological changes is crucial for prompt diagnosis and effective management. This answer will detail the causative agents, mechanisms of infection, and the resulting gross and microscopic features of acute bacterial osteomyelitis.
Etiopathogenesis
The most common causative agent of acute bacterial osteomyelitis is Staphylococcus aureus (approximately 60-90% of cases). Other organisms include Streptococcus pyogenes, Escherichia coli, Pseudomonas aeruginosa (particularly in cases of open fractures or intravenous drug use), and, less commonly, Haemophilus influenzae (decreasing incidence due to vaccination).
The pathogenesis typically involves:
- Hematogenous Spread: Bacteria gain access to the bone via the bloodstream, often originating from a distant site of infection (e.g., skin infection, respiratory tract infection).
- Adherence and Colonization: Bacteria adhere to the endothelial cells lining the capillaries within the bone marrow.
- Inflammation and Abscess Formation: The bacteria trigger an inflammatory response, leading to edema, increased vascular permeability, and infiltration of neutrophils. This results in the formation of a localized abscess within the bone marrow.
- Bone Destruction: The inflammatory process and bacterial toxins cause bone necrosis and destruction.
- Periosteal Reaction: As the infection progresses, it spreads beneath the periosteum, causing periosteal inflammation and new bone formation (Wolff's law).
Gross Pathology
The gross pathology of acute bacterial osteomyelitis varies depending on the stage of the infection:
- Early Stage (0-48 hours): The bone marrow appears congested and edematous. There may be a localized area of redness and swelling.
- Acute Stage (2-7 days): A localized abscess is formed within the bone marrow, containing pus and necrotic debris. The affected bone appears softened and may be surrounded by a zone of inflammation. The periosteum is elevated and inflamed.
- Subacute Stage (1-3 weeks): The abscess may extend through the cortex, leading to the formation of a sinus tract that drains to the skin surface. New bone formation (involucrum) begins to surround the infected area. The periosteum shows significant new bone apposition.
Microscopic Pathology
Microscopic examination reveals the following features:
- Early Stage: Inflammation with predominantly neutrophils infiltrating the bone marrow. Bacteria can be identified using Gram stain or culture.
- Acute Stage: Extensive necrosis of bone and bone marrow. A large number of neutrophils are present, along with bacterial colonies. There is evidence of vascular congestion and thrombosis.
- Subacute Stage: Granulation tissue formation within the abscess cavity. Fibrosis and new bone formation (woven bone) are observed. Chronic inflammatory cells (lymphocytes, macrophages) become more prominent. The involucrum, a sheath of new bone, is formed around the infected area.
Specific Microscopic Findings
| Feature | Description |
|---|---|
| Inflammatory Cells | Predominantly neutrophils in the acute phase, transitioning to lymphocytes and macrophages in the subacute phase. |
| Bone Necrosis | Areas of dead bone, often surrounded by inflammatory cells. |
| Vascular Changes | Congestion, thrombosis, and new vessel formation. |
| Periosteal Reaction | Inflammation and new bone formation beneath the periosteum. |
| Involucrum | A sheath of new, woven bone formed around the infected area. |
Conclusion
Acute bacterial osteomyelitis is a serious infection requiring prompt diagnosis and treatment. The etiopathogenesis involves hematogenous spread of bacteria, leading to inflammation, abscess formation, and bone destruction. Grossly, the affected bone shows congestion, necrosis, and periosteal reaction. Microscopically, the hallmark features are inflammation, bone necrosis, and new bone formation. Understanding these pathological changes is essential for effective clinical management and prevention of long-term complications like chronic osteomyelitis and growth disturbances.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.