Osteomyelitis 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.
Other non-pyogenic causes of osteomyelitis are discussed separately:
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.
In most instances, osteomyelitis results from haematogenous spread, although direct extension from trauma and/or ulcers is also relatively common (especially in the feet of diabetic patients).
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
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
In some instances, radiographic features are specific to a region or a particular type of infection, for example:
Below are general features of osteomyelitis.
The earliest changes are seen in adjacent soft tissues +/- muscle outlines with swelling and loss or blurring of normal fat planes. An effusion may be seen in an adjacent joint.
In general, osteomyelitis must extend at least 1 cm and compromise 30 to 50% of bone mineral content to produce noticeable changes in plain radiographs. Early findings may be subtle, and changes may not be obvious until 5 to 7 days in children and 10 to 14 days in adults. After this time a number of changes may be noted:
- regional osteopaenia
- periosteal reaction/thickening (periostitis): variable, and may appear aggressive including formation of a Codman's triangle 6
- focal bony lysis or cortical loss
- endosteal scalloping 8
- loss of bony trabecular architecture
- new bone apposition
- eventual peripheral sclerosis
CT is superior to both MRI and plain film in depicting the bony margins and identifying a sequestrum or involucrum. Appearance is otherwise similar to plain films.The overall sensitivity and specificity of CT even in the setting of chronic osteomyelitis is low and according to one study was 67% and 50%17.
MRI is most sensitive and specific and is able to identify soft-tissue/joint complications 5,14.
- intermediate to low signal central component (fluid)
- surrounding bone marrow of lower signal than normal due to oedema
- cortical bone destruction
- bone marrow oedema
- central high signal (fluid)
- post contrast enhancement of bone marrow, abscess margins, periosteum and adjacent soft tissue collections
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 the assessment of soft tissues and joints adjacent to infected bone, able to visualise soft tissue abscesses, cellulitis, subperiosteal 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.
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 Gallium67 scintigraphy
May be useful in:
- diabetic osteomyelitis, especially combined with Tc99m-phosphonate imaging. 2,7 However MRI is now generally used in conjunction with plain films 14,15
- orthopaedic implants
- vertebral osteomyelitis (Ga67 is best) 2
- ulcers in bedridden patients with potential underlying osteomyelitis (In111 with Tc99m-phosphonate)
- radiogallium attaches to transferrin, which leaks from the bloodstream into areas of inflammation showing increased isotope uptake in infection, sterile inflammatory conditions, and malignancy.
- imaging is usually performed 18 to 72 hours after injection and is often performed in conjunction with radionuclide bone imaging.
- one difficulty with gallium is that it does not show bone detail particularly well and may not distinguish well between bone and nearby soft tissue inflammation.
- Gallium scans may reveal abnormal accumulation in patients who have active osteomyelitis when technetium scans reveal decreased activity (“cold” lesions) or perhaps normal activity.
- Gallium accumulation may correlate more closely with activity in cases of osteomyelitis than does technetium uptake
PET-CT systems are relatively novel techniques that are being applied. FDG-PET may have the highest diagnostic accuracy for confirming or excluding chronic osteomyelitis in comparison with bone scintigraphy, MRI, or leukocyte scintigraphy. It is also considered superior to leukocyte scintigraphy in detecting chronic osteomyelitis in the axial skeleton 9.
Treatment and prognosis
Complications include 1:
- sinus track formation with occasional superimposed squamous cell carcinoma (Marjolin ulcer)
- secondary sarcoma (e.g. osteosarcoma): rare
- pathological fracture
- secondary amyloidosis
General imaging differential considerations include:
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