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
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 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/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 / periosteal thickening - variable, and may appear aggressive including formation of a Codman's triangle 6
- focal bony lysis
- 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 / involucrum. Appearance are otherwise similar to plain films.
MRI is most sensitive and specific and is able to identify soft-tissue/joint complications 5.
- 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
- bone marrow oedema
- central high signal (fluid)
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.
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
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)
Gallium67 scintigraphy -
- 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 may 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
FDG - CT/PET
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
- sinus track formation with occasional superimposed squamous cell carcinoma (Marjolin's ulcer)
- secondary sarcoma (e.g. osteosarcoma) : rare
- pathological fracture
- secondary amyloidosis
General imaging differential considerations include
- 1- Kumar V, Abbas AK, Fausto N et-al. Robbins and Cotran pathologic basis of disease. W B Saunders Co. (2005) ISBN:0721601871. Read it at Google Books - Find it at Amazon
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- 13.Wu JS, Gorbachova T, Morrison WB et-al. Imaging-guided bone biopsy for osteomyelitis: are there factors associated with positive or negative cultures?. AJR Am J Roentgenol. 2007;188 (6): 1529-34. doi:10.2214/AJR.06.1286 - Pubmed citation
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