Chapter 120. Osteomyelitis (Part 3) docx

Hence a mixture of staphylococci, streptococci, enteric organisms, and anaerobic bacteria may be isolated from a diabetic foot infection or pelvic osteomyelitis underlying a decubitus ul

Chapter 120 Osteomyelitis

(Part 3)

Microbiology

S aureus is a pathogen in more than half of cases of contiguous-focus

osteomyelitis However, in contrast to hematogenous osteomyelitis, these infections are often polymicrobial and are more likely to involve gram-negative and anaerobic bacteria Hence a mixture of staphylococci, streptococci, enteric organisms, and anaerobic bacteria may be isolated from a diabetic foot infection or pelvic osteomyelitis underlying a decubitus ulcer Aerobic and anaerobic bacteria cause osteomyelitis following surgery or soft tissue infection of the oropharynx, paranasal sinuses, gastrointestinal tract, or female genital tract A human bite may result in mixed infection of the hand, with anaerobes included among the etiologic

agents S aureus is the principal cause of postoperative infections;

coagulase-negative staphylococci are common pathogens after implantation of orthopedic appliances; and these organisms as well as gram-negative enteric bacilli, atypical

mycobacteria, and Mycoplasma may cause sternal osteomyelitis after cardiac surgery Infection with P aeruginosa is frequently associated with puncture

wounds of the foot, especially when a nail passes through a sneaker, and

Pasteurella multocida infection commonly follows cat bites

Chronic Osteomyelitis

With prompt treatment, <5% of cases of acute hematogenous osteomyelitis progress to chronic osteomyelitis Chronic infection is more likely to develop in contiguous-focus than in hematogenous osteomyelitis The presence of a foreign body makes establishment of chronic infection especially likely

A protracted clinical course, long periods of quiescence, and recurrent exacerbations are characteristic of chronic osteomyelitis Sinus tracts between bone and skin may drain purulent material and occasionally pieces of necrotic bone An increase in drainage, pain, or swelling signals an exacerbation, which is usually accompanied by increases in CRP level and ESR Fever is unusual except when obstruction of a sinus tract leads to soft tissue infection Rare late complications include pathologic fractures, squamous cell carcinoma of the sinus tract, and amyloidosis

Diagnosis

Early diagnosis of acute osteomyelitis is critical because prompt antibiotic therapy may prevent necrosis of bone The ESR and the CRP level are elevated in most cases of active osteomyelitis, including those in which constitutional symptoms and leukocytosis are lacking These findings are not specific to osteomyelitis, however, and the ESR is occasionally normal in early infections Baseline values are often useful in monitoring the efficacy of treatment

A variety of radiologic tests are available for evaluation of osteomyelitis (Table 120-1) Evaluation usually begins with plain radiographs because of their ready availability, although they typically show no abnormalities during early infection Three-phase bone scans (99Tc-monodiphosphonate) offer high sensitivity but are often of low specificity, especially in the presence of underlying bone abnormalities There is a lack of consensus over the optimal use of other radionuclide studies, and there is considerable variation between institutions in their use Use of MRI (Fig 120-1) is expanding because of its high sensitivity and specificity as well as its ability to demonstrate associated soft tissue abnormalities, but this modality is not available at all institutions

Table 120-1 Diagnostic Imaging Studies for Osteomyelitis

Type of

Study

Comments

Plain

radiographs

Insensitive, especially in early osteomyelitis May show periosteal elevation after 10 days, lytic changes after 2–6 weeks Useful to look for anatomic abnormalities (e.g., fractures, bony variants, or deformities), foreign bodies, and soft tissue gas

Three-phase

bone scan (99m

Tc-MDP)

Characteristic finding in osteomyelitis: increased uptake in all three phases of scan Highly sensitive (~95%)

in acute infection; somewhat less sensitive if blood flow to bone is poor Specificity moderate if plain films are normal, but poor in presence of neuropathic arthropathy, fractures, tumor, infarction

Other

radionuclide scans

Examples: 67Ga-citrate, 111In-labeled WBCs 111 In-WBCs more specific than gallium but not always available Often used in conjunction with bone scan because its greater specificity for inflammation than 99mTc-MDP helps to distinguish infectious from noninfectious processes Lack of

consensus over role; often supplanted by MRI when the latter is available

Ultrasound May detect subperiosteal fluid collection or soft

tissue abscess adjacent to bone, but largely supplanted by

CT and MRI

CT Limited role in acute osteomyelitis In chronic

osteomyelitis, excellent for detection of sequestra, cortical destruction, soft tissue abscesses, and sinus tracts Use limited in the presence of a metallic foreign body

MRI As sensitive as 99mTc-MDP bone scan for acute

osteomyelitis (~95%); detects changes in water content of marrow before disruption of cortical bone High specificity (~87%), with better anatomic detail than nuclear studies Procedure of choice for vertebral osteomyelitis because of high sensitivity for epidural abscess Use may be limited by

a metallic foreign body

Abbreviations: MDP, monodiphosphonate; WBCs, white blood cells