osteomyelitis
OSTEOMYELITIS
Osteomyelitis is an acute or chronic inflammation of the bone due to an infection resulting from hematogenous spread, contiguous spread from soft tissues and joints to bone, or direct inoculation into bone from surgery or trauma.
The infection is generally due to a single microorganism but polymicrobial infections may also occur.
Staphylococcus aureus is a major cause of infection.
Signs and symptoms include fever; inflammatory findings of erythema, warmth, pain and swelling over the involved area; draining sinus tracts over affected bone; limited movement of affected extremity; pain in the chest, back, abdomen or leg, and tenderness over involved vertebrae in patients with vertebral osteomyelitis; anorexia, vomiting and malaise.

History

  • Elicit predisposing factors [eg vasculopathy, diabetes mellitus (DM), invasive procedures, etc]

Physical Examination

Physical Exam Findings:

  • Warmth, erythema & tenderness over the involved part
    • Tenderness disproportionate to soft tissue findings favors osteomyelitis over soft tissue infection
  • Tissue ulceration & draining sinus tracts over the affected bone
  • Exposed bone on ulcer bed or a probe used to examine an ulcer encounters bone
  • Signs of septicemia

Laboratory Tests

Bacterial Culture

  • Isolation of the infecting microorganism is extremely important for the diagnosis of long bone osteomyelitis
  • Blood, bone & joint fluid should ideally all be cultured to increase the yield of the test
  • Sinus tract cultures are reliable for confirming S aureus infection but are not useful for detecting gram-negative pathogens
  • Gram-stained smear, aerobic & anaerobic cultures should be performed

Blood Tests

  • WBC count may be high or normal in acute hematogenous osteomyelitis & is usually normal in chronic osteomyelitis
  • Erythrocyte sedimentation rate (ESR) is usually elevated at the onset & may provide a prognostic guide during therapy
  • C-reactive protein (CRP) level is elevated, w/ higher levels predicting a possibly greater risk of sequelae

Bone Biopsy

  • Bone biopsy samples isolating bacteria w/ findings histologically of osteonecrosis & inflammatory disease are the reference standard for diagnosis of osteomyelitis
  • Recommended if the diagnosis of osteomyelitis is still in doubt after imaging
  • Frequently needed to diagnose vertebral osteomyelitis because blood cultures are usually sterile in this situation
  • Open biopsy is better than needle biopsy & at least 2 samples should be obtained for histopath & for Gram stain & culture

Polymerase Chain Reaction (PCR)

  • Helpful in establishing etiology of osteomyelitis when blood & bone cultures are negative

Imaging

Plain X-rays

  • First imaging procedure in the work up of patients w/ possible osteomyelitis
  • Useful for excluding other diseases & can provide clues for other conditions that may be present
  • Positive x-rays are fairly specific for osteomyelitis (75-83%), but negative x-rays cannot be used to rule out the disease
  • Bone demineralization by 30-75% needs to occur before a change on plain x-ray is seen; therefore, it takes 10 to 21 days for a bone lesion to become apparent
  • In children, early changes may be seen w/in 3 days of symptom onset
    • Focal deep soft tissue swelling in the metaphyseal region may be the 1st sign in children & infants, followed by muscle swelling & loss; tissue planes normally seen around affected bone
    • Soft tissue changes are harder to detect in adults
  • Later changes include bone lysis, cortical lucency, osteopenia, periosteal elevation, periosteal new bone formation, single or multiple abscesses, involucrum, sequestration
  • Findings in vertebral osteomyelitis include narrowing of the intervetebral disk space, bone destruction & new bone formation at the anterior edge of the vertebral disk
  • Radiographs should be repeated at 2-wk intervals in diabetes mellitus patients (DM) who have infected foot wounds that do not resolve & whose initial radiographs are normal

Magnetic Resonance Imaging (MRI)

  • Highly sensitive for detecting osteomyelitis, test of choice for diabetic foot ulcers
  • Useful for differentiating bone infection from soft tissue infection, for confirming the extent of infection in patients w/ established osteomyelitis, & for evaluating intraosseous abscesses
  • Able to detect vertebral osteomyelitis early
  • Helpful in planning surgical management ie drainage & debridement
  • Limitations: Not recommended for whole-body exams; metal implants may produce focal artifacts

Bone Scan

  • May already be positive 24-48 hr after symptoms start & therefore can detect osteomyelitis earlier than plain radiographs
  • Useful for patients in whom multifocal bone involvement is suspected
  • Decreased uptake on bone scan may indicate more aggressive infection that has produced thrombosis or ischemia
  • Limitation: May be positive in other conditions eg malignancy, fracture, bone infarction

Other Imaging Techniques

  • Radionuclide studies
    • Not routinely required for evaluation of possible osteomyelitis, but may provide more information about the extent of bone & soft tissue inflammation
    • A gallium scan obtained together w/ a technetium scan or white blood cell (WBC) scan may be more useful than doing either test alone
  • Computed tomography (CT) scan
    • Useful for defining the extent of bone & soft tissue infection esp in areas of complex anatomy eg the vertebral column & for guiding biopsies & aspiration procedures
    • Less sensitive than MRI except for detecting sequestra & should not be used routinely for evaluating patients w/ possible osteomyelitis
  • Ultrasound
    • Noninvasive technique that may be helpful in detection of fluid collection, abscesses or sinus tracts in soft tissues
    • May be used to guide diagnostic or therapeutic drainage, aspiration or tissue biopsy
  • Positron emission tomography & single photon emission computed tomography
    • Detects increased intracellular glucose metabolism that occurs in infection & inflammation
    • Highly accurate for confirming or excluding the diagnosis of chronic osteomyelitis

Classification

Classification of Osteomyelitis in Children

Acute Hematogenous Osteomyelitis (AHO)

  • Bone infection before formation of sequestra (dead bone)
  • Primary acute hematogenous osteomyelitis occurs mainly in infants & children
    • Most patients w/ acute hematogenous osteomyelitis  present w/ symptoms lasting <2 wk
  • Long bones are most frequently involved, w/ most infections in children localizing in the metaphysis
    • Most frequently affected sites are the distal femur & proximal tibia, followed by the distal humerus, distal radius, proximal femur & proximal humerus
  • Multiple bone involvement is common in infants
  • Abscess formation & extension of infection into surrounding soft tissue may occur
  • Infection in infants tends to be more diffuse because anatomic barriers are not able to efficiently limit infection
  • Vertebral osteomyelitis is uncommon in children & often presents as an indolent infection w/ nonspecific symptoms eg septicemia
    • Often involves infection of the endplates of 2 adjacent vertebrae
  • In patients w/ pelvic osteomyelitis, the ilium & ischium are most often involved & usually presents as gait abnormality or hip pain

Contiguous Nonhematogenous Osteomyelitis

  • Associated w/ open fractures requiring surgical reduction, orthopedic devices, decubitus & neuropathic ulcers, human & animal bites, puncture wounds esp of the foot or knee
  • Presents as an indolent condition often w/o fever & w/ continuous drainage or ulceration over the affected bone
  • Has a high rate of recurrence

Classification of Osteomyelitis in Adults

Acute Hematogenous Osteomyelitis (AHO)

  • Adults usually present in a chronic manner, w/ pain & minimal constitutional symptoms lasting several mth
  • Infections usually start in the diaphysis
  • Secondary hematogenous osteomyelitis is more common in adults & is usually a reactivation of a childhood infection
  • Vertebral osteomyelitis is predominantly a disease of adults, w/ incidence increasing w/ age
    • Condition slowly progresses over wk to mth
    • The lumbar & thoracic spine are most commonly affected
    • Infection may spread from skin & soft tissue, respiratory & genitourinary tract, infected IV sites, endocarditis

Contiguous Nonhematogenous Osteomyelitis

  • Associated w/ fractures needing surgical reduction & internal fixation, open fractures, prosthetic devices, soft tissue infections, trauma
  • Patients w/ DM are susceptible to osteomyelitis because of impairment of tissue perfusion that is a result of vascular insufficiency
    • Neuropathy & diminished neutrophil function also contribute to the risk
    • Predisposing events include perforating foot ulcers, an ingrown toenail, cellulitis or deep space infection
    • Diabetic patients often develop osteomyelitis even before bone is exposed
      • Positive palpation of bone in probing an infected foot ulcer confirms osteomyelitis

Chronic Osteomyelitis

  • Bone infection after sequestra formation
  • Diagnosed in patients w/ a history of osteomyelitis w/ recurrence of symptoms ie pain, swelling, erythema, low-grade fever; a sinus tract is pathognomonic
  • Pathologic features include presence of necrotic bone, exposed bone, chronic wound over a fracture or surgical hardware, formation of new bone (involucrum) & exudation of polymorphonuclear leukocytes w/ lymphocytes, histiocytes, plasma cells
  • An abscess or soft tissue infection may be found, esp if a sinus tract becomes obstructed
  • Hematogenous & contiguous-focus osteomyelitis may become chronic
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