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Osteomyelitis

Dr Maulik S Patel and Dr Frank Gaillard et al.

Osteomyelitis refers to bony inflammation that is almost always due to infection, typically bacterial. This article primarily deals with pyogenic osteomyelitis. 

Other pathogens are discussed separately :

Demographics and clinical presentation

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

Pathology and microbiology

In most instances, osteomyelitis results from haematogeneous spread, although direct extension from trauma and / or ulcers is also relatively common.

In the initial stages of infection, bacteria multiply setting up a localised inflammatory reaction and resulting in localised 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 agent.  Different organisms are more common in specific clinical scenarios 1,4:

  • Staphylococcus aureus : 80 - 90% of all infections
  • Escherichia coli  : IVDU (intravenous drug users) 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
Location

The location of osteomyelitis within a bone varies with age, on account of changing blood supply 1,4 :

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

Radiographic features

In some instances, radiographic features are specific to a region or particular type of infection, for example:

Below are general features of osteomyelitis.

Plain film

The earliest changes are seen in adjacent soft tissues with swelling and loss of normal fat planes. An effusion may be seen in an adjacent joint. The bone itself remains normal in appearance for 10 - 14 days. After this time a number of changes may be noted :

In chronic or untreated cases eventual formation of a sequestrum, involucrum or cloaca may be seen.

CT

CT is superior to both MRI and X-rays in depicting the bony margins and identifying a sequestrum / involucrum. Appearance are otherwise similar to plain films.

MRI

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

  • T1 :
    • intermediate to low signal central component (fluid)
    • surrounding bone marrow lower signal than normal due to oedema
    • enhancement both bone marrow, abscess margins periosteum and adjacent soft tissues
  • T2 :
    • bone marrow oedema
    • central high signal (fluid)
Ultrasound

Although ultrasound excels as a fast and cheap examination of the soft tissues, and allows soft tissue collections to be drained it has little direct role in the assessment of osteomyelitis, as it is unable to visualise within bone. 

It does however have a role to play in assessment of soft tissues and joints adjacent to infected bone, able to visualise soft tissue abscesses, cellulitis, sub periosteal collections and joint effusion. 

Ultrasound also is useful in assessing the extra-osseous components of orthopaedic instrumentation as it is not affected by metal artefact 3.

Nuclear medicine

A number of techniques may be employed to detect foci of osteomyelitis. These include 2:

Bone scintigraphy (Tc99m)

Increased osteoblastic activity results in increased levels of radiotracer uptake in the surrounding bone usually both on blood pool and delayed views. It is highly sensitive but not particularly specific.

In111-labelled WBC and Galium67 scintigraphy

Particularly useful in :

  • diabetic osteomyelitis, especially combined with Tc99m-phosphonate imaging 2,7
  • orthopaedic implants
  • vertebral osteomyelitis (Ga67 is best) 2
  • ulcers in bed ridden patients with potential underlying osteomyelitis (In111 with Tc99m-phosphonate)

Treatment and prognosis

Treatment is typically with intravenous antibiotics, usually for extended periods. If a collection, sequestrum or involucrum is present then drainage and/or surgical debridement is often necessary.

Complications include1:

Differential diagnosis

General imaging differential considerations include

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