The correct answer is D. Osteomyelitis is a pyogenic infection of the bone. Pathogenesis is similar to septic arthritis, with the origin of infection occurring from hematogenous spread, direct extension of a local infection, or direct inoculation of bone either from trauma or surgical manipulation. In children the most frequent presentation is acute hematogenous spread. The most common location of osteomyelitis is the metaphysis of the distal femur and proximal tibia.
The most prevalent pathogens are the same as those seen in septic arthritis. Staphylococcus aureus is the most common pathogen followed by group A beta-hemolytic streptococci. Neonates are at risk for group B beta-hemolytic streptococci, but risk is dramatically lower since pregnant women are routinely tested and treated for this pathogen prior to labor. Haemophilus influenzae may occur in infants and young children, but it is not seen as frequently as in septic arthritis. In addition, children who have H. influenzae osteomyelitis usually have fever and concomitant joint infection. Patients who have puncture wounds of the foot are susceptible to Pseudomonas aeruginosa osteomyelitis. Patients with sickle cell disease are at risk for infection by Salmonella and other gram-negative bacteria.
Most patients who have osteomyelitis will present with a chief complaint of fever and bone pain. The pain is usually severe, constant, and aggravated by motion. The older the child, the more exquisite the point tenderness. That is because the bone has a thicker metaphyseal cortex with a dense fibrous periosteum. Localized swelling, warmth, and erythema are signs seen late in the infection, as the periosteum becomes more involved. Neonates can present with vague symptoms, consisting only of irritability and poor feeding, or they can show signs of fulminant sepsis. Peripheral WBC count may be normal or elevated with a left shift with a predominance of neutrophils. Erythrocyte sedimentation rate (ESR) is usually elevated and blood cultures are positive in approximately 60% cases. Bone cultures taken either surgically or by needle aspiration result in a culture yield of 80%. Plain radiographs may be normal for up to 2 weeks from the onset of illness, and the earliest signs on plain radiographs are soft-tissue swelling and displacement of muscle plane. Bony changes begin to appear by 7-10 days, starting with a hazy appearance of the metaphysis followed by irregular areas of trabecular necrosis and absorption. Eventually subperiosteal new bone formation occurs as the infection spreads through the cortex. A bone scan usually diagnoses osteomyelitis as early as 24-48 hours from onset.
Treatment at first is empiric parenteral antibiotics. The selection of antibiotic should include coverage of Staphylococcus aureus, as well as other organisms; the agent can probably be based on the patient's age and history of illness. Surgical debridement may be necessary if pus is present on needle aspiration or if evidence of either joint involvement or abscess is present.
PEARL: Osteomyelitis is an infection of bone that is usually bacterial in origin. Microorganisms can be introduced into bone in three ways:
- Hematogenous delivery
- Direct inoculation (usually traumatic, but also surgical)
- Local invasion from a contiguous infection (usually decubitus ulcer or periodontal disease)
In children, acute osteomyelitis is primarily hematogenous in origin. Risk factors for hematogenous osteomyelitis in children include the following:
- Sickle cell disease
- Immunodeficiency disorders, such as chronic granulomatous disease
- Sepsis
- Minor trauma coincident with bacteremia
- Indwelling vascular catheters
- Chronic hemodialysis vascular access
The growing metaphyses of long bones are most commonly involved, particularly the femur, tibia, or fibula. Bacteria, principally Staphylococcus aureus, cause the vast majority of cases of acute osteomyelitis in children.
Staph aureus is the most common pathogen for children with osteomyelitis. Among patients with sickle cell disease (SCD), hematogenous osteomyelitis is most often caused by Salmonella or other gram-negative organisms, such as E. coli.
