Osteomyelitis

Osteomyelitis (plural: osteomyelitides) refers to inflammation of bone that is almost always due to infection, typically bacterial. This article primarily deals with pyogenic osteomyelitis, which may be acute or chronic. 

Osteomyelitis

Other non-pyogenic causes of osteomyelitis are discussed separately:

• fungal osteomyelitis
• skeletal syphilis
• tuberculous osteomyelitis

Epidemiology

Osteomyelitis can occur at any age. In those without specific risk factors, it is particularly common between the ages of 2-12 years and is more common in males (M: F of 3:1) 6.

Pathology

In most instances, osteomyelitis results from haematogenous spread, although direct extension from trauma and/or ulcers is relatively common (especially in the feet of diabetic patients).

In the initial stages of infection, bacteria multiply, triggering a localized inflammatory reaction that results in localized cell death. With time, the infection becomes demarcated by a rim of granulation tissue and new bone deposition.

Although no organisms are recovered in up to 50% of cases 1, when one is isolated, Staphylococcus aureus is by far the most common pathogen. Different organisms are more common in specific clinical scenarios 1,4:

• Staphylococcus aureus: 80-90% of all infections
• Escherichia coli: intravenous drug users (IVDU) and genitourinary tract infection
• Pseudomonas spp.: IVDU and genitourinary tract infection
• Klebsiella spp.: IVDU and genitourinary tract infection
• Salmonella spp.: sickle cell disease
• Haemophilus influenzae: neonates
• group B streptococci: neonates

Other uncommon infective agents include

• fungi - fungal osteomyelitis

Location

Frequency by location, in descending order 18:

• lower limb (most common)
• vertebrae: lumbar > thoracic > cervical
• radial styloid
• sacroiliac joint
• The location of osteomyelitis within a bone varies with age, on account of changes in vascularization of different parts of the bone 1,4:

• neonates: metaphysis and/or epiphysis
• children: metaphysis
• adults: epiphyses and subchondral regions

CT

CT is superior to both MRI and plain film in depicting the bony margins and identifying a sequestrum or involucrum. The CT features are otherwise similar to plain films. The overall sensitivity and specificity of CT is low, even in the setting of chronic osteomyelitis, and according to one study are 67% and 50%, respectively 17.

Some limitations CT include 20:

inability to confidently detect marrow edema; therefore, a normal CT does not exclude early osteomyelitis.

image degradation by streak artifact when metallic implants are present

MRI

MRI is the most sensitive and specific and is able to identify soft-tissue/joint complications 5,14.

Bone marrow edema is the earliest feature of acute osteomyelitis seen on MRI and can be detected as early as 1 to 2 days after the onset of infection 20.

T1

intermediate to low signal central component (fluid)

surrounding bone marrow of lower signal than normal due to edema

cortical bone destruction

T2

bone marrow edema

central high signal (fluid)

T1 C+

post contrast enhancement of bone marrow, abscess margins, periosteum and adjacent soft tissue collections