Chapter 7

Osteomyelitis

Overview

• Bone, joint, and soft tissue infections are a significant cause of morbidity in pediatric populations.

• Clinical diagnosis may be challenging or delayed when classic symptoms such as chills, fever, pain, and swelling are absent, or in the very young child.

• Principles of effective treatment are generally well understood; however, changing antimicrobial susceptibility has altered clinical decision-making.

• Magnetic resonance imaging (MRI) has also allowed for more accurate diagnosis.

• Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has led to an increased incidence of musculoskeletal infections in children.

• Hematogenous spread from skin or a distant site of infection is the most common mechanism of bone/joint infection in children.

• Local spread from a traumatized contiguous site, known as direct inoculation, is a less common mechanism of infection in children.

— Loss of skin integrity is caused by penetrating trauma, open fractures, surgery, burns, skin ulcerations, or local soft tissue infections.

• Secondary osteomyelitis typically refers to infection related to predisposing conditions such as vascular insufficiency from diabetes mellitus or peripheral vascular disease.

• Classification of acute, subacute, and chronic infection is based on clinical course and duration.

— Acute osteomyelitis, up to 2 weeks

— Subacute osteomyelitis, 2 weeks or longer

— Chronic osteomyelitis (presence of findings such as Brodie abscess or necrotic bone), the duration of symptoms is variable

— Likelihood of surgical indications, such as periosteal abscess, is greater with subacute compared to acute osteomyelitis.

— Some patients do not recall an initial period of acute symptoms.

Acute and Subacute Hematogenous Osteomyelitis

INTRODUCTION/ETIOLOGY/EPIDEMIOLOGY

• Characteristics of blood vessels at the metaphyses of bone (Figure 7-1) and distribution of hematogenous osteomyelitis

— Eighty percent of hematogenous osteomyelitis in children occurs at the metaphyses of long bones; less frequent are hands and feet, pelvis, and vertebrae. Hematogenous osteomyelitis rarely involves the clavicle, ribs, sternum, or skull.

Figure 7-1. Relation of blood supply to the proximal femur and spread of infection (bold arrows). The blood vessels essentially make a U-turn at the physis, leading to a low flow state within the metaphysis and predisposing to infection.

From Dormans JP, Drummond DS. Pediatric hematogenous osteomyelitis: new trends in presentation, diagnosis, and treatment. J Am Acad Orthop Surg. 1994;2(6):333–341. Reprinted with permission.

— In infants, blood vessels from the metaphysis can pass into the epiphysis.

■Osteomyelitis in infants typically involves both the metaphysis and epiphysis.

■Infants are more prone to complicated osteomyelitis, including extension of infection into the joint.

■Depending on growth plate location, by age 18 months to 3 years, vessels from the metaphysis no longer extend through the growth plate into the epiphysis.

— At all ages, the metaphyses are intracapsular within the shoulder, elbow, ankle, and hip joints. Osteomyelitis extends infection into these joints without traversing the growth plate and epiphysis.

• Annual incidence of hematogenous osteomyelitis in the pediatric age group is approximately 1 per 5,000, with more than half of the cases involving infants and children younger than 5 years; twice as many boys as girls are affected.

• Etiology

— S aureus causes 70% to 90% of hematogenous osteomyelitis at all ages (see Table 7-1).

— CA-MRSA is due to strains that carry virulence factors that promote abscess and venous thrombosis. One virulence factor of CA-MRSA strains is Panton-Valentine leukocidin, and this exotoxin is a marker for CA-MRSA strains.

— Most areas of North America have rates of CA-MRSA (10%–60% of all S aureus cultures) that affect choice of antibiotics for hematogenous osteomyelitis.

Table 7-1. Organisms Commonly Associated With Osteomyelitis by Age Group

Abbreviation: MRSA, methicillin-resistant Staphylococcus aureus.

— Kingella kingae is a fastidious gram-negative rod increasingly recognized in infections in young children aged several months to 5 years (see Table 7-1).

■A proportion of culture-negative osteomyelitis and septic arthritis cases are caused by K kingae.

■Polymerase chain reaction (PCR) may detect K kingae in culture-negative cases, and direct inoculation of large volumes of bone and joint specimens directly into blood culture bottles recovers K kingae more frequently than other culture procedures.

■K kingae is not sensitive to clindamycin and vancomycin, which are often chosen to cover CA-MRSA.

— Many other microorganisms have been implicated in osteomyelitis.

■Group A streptococcus and Streptococcus pneumonia are the next most common after S aureus and K kingae.

■For abnormal hosts (eg, sickle cell disease [Salmonella is a common pathogen], chronic granulomatous disease, HIV infection), unusual presentations (eg, granulomatous osteomyelitis suggestive of tuberculosis or fungal infection), or specific exposure history (eg, bite wound), additional consultation with a pediatric infectious diseases specialist or reference to more detailed publications is necessary.

SIGNS AND SYMPTOMS

• Typical presentation is sudden onset of focal limb pain that does not completely resolve with rest and persists at night. The pain may be episodic; refusal to bear weight or use the affected extremity are important signs.

• At least half of children with hematogenous osteomyelitis have elevated temperature or fever higher than 38°C at some point in the clinical course.

• Physical examination findings may include pain, warmth, and swelling at the metaphyseal location of bones, and may include cellulitis.

• Atypical presentations are common. Questionable cases should be scheduled for re-examination the next day after testing including radiography, complete blood cell count, C-reactive protein (CRP) level, and erythrocyte sedimentation rate (ESR).

DIFFERENTIAL DIAGNOSIS

• See Box 7–1.

DIAGNOSTIC CONSIDERATIONS

• Radiographs are the initial imaging study of choice in all cases. Since pain can be referred, it is important to image the entire bone in question. Radiographs may be sufficient imaging when a suspected diagnosis of osteomyelitis is confirmed on the images.

— During the first 5 to 7 days of symptoms in children and the first 10 to 14 days in skeletally mature individuals, radiographs do not detect changes of hematogenous osteomyelitis, and therefore may appear normal.

— Earliest findings are soft tissue swelling with obliteration of tissue planes, periosteal thickening or elevation, and focal osteopenia.

— Involucrum refers to a periosteal sheath of new bone formation that usually requires 3 weeks to develop (Figure 7-2).

— Lytic changes are a later finding because at least 50% to 75% of the bone matrix must be destroyed to result in radiographic abnormalities (Figure 7-3).

— Sequestrum refers to discrete lucency due to osteonecrosis.

— Brodie abscess is a finding of chronic osteomyelitis. A central lucency is walled off by a fibrous capsule with sclerosis in the surrounding bone (Figure 7-4).

• MRI, when available, is the study of choice when radiographs have not defined osteomyelitis and symptoms are localized (Figure 7-5).

— Intravenous contrast (gadolinium) demonstrates bone or soft tissue abscesses in detail with enhancement about the rim of the abscess.

Box 7-1. Differential Diagnosis for Osteomyelitis

Figure 7-2. Radiograph of chronic osteomyelitis showing necrosis in the entire shaft of the radius (essentially, the entire bone has become a sequestrum) and a developing involucrum.

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