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Bone and Soft Tissue Infection
Acute Osteomyelitis
Acute Osteomyelitis
Definition
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Acute infection of the bone diagnosed within 2 weeks of symptoms (subacute if diagnosed > 2 weeks) ▸ causes:
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Staphylococcus aureus : commonest organism in any age group (80%)
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Gram-negative organisms ( Pseudomonas/Enterobacter ): remaining 20%
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Acute infections prosthetic implants: S. aureus
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Spontaneous MSK infections in adults are less common than in children and are usually die to trauma, previous surgery or underlying immunodeficiency ▸ in adults haematogenous infection is mostly responsible for vertebral osteomyelitis (cf. common in childhood osteomyelitis)
Clinical presentation
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Local pain ▸ soft tissue redness and swelling (± discharging abscess) ▸ reduced function and mobility ▸ pyrexia and systemic ill health
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The presence of any infected dead bone or debris makes treatment difficult
Radiological features
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NB: The appearance of acute (and chronic) osteomyelitis can simulate almost the entire spectrum of bone tumour appearances
Early
XR / CT
This is often normal (soft tissue gas is an ominous sign)
Skeletal scintigraphy
This is sensitive but non specific ▸ there is early increased uptake ▸ it can be problematic in children as the growth plates are often adjacent to any involved areas
US
A subperiosteal fluid collection
MRI
May be positive as early as 3–5 days ▸ STIR/T2WI (fat-suppressed): high SI within bone marrow (oedema) ▸ it initially extends beyond the limits of the true bone infection ▸ T1WI + Gad: enhancement
Intermediate (after several days)
XR
New periosteal reaction with a thin, thick or laminated (‘onion peel’) appearance (it can also have a ‘fluffy’ margin) ▸ a Codman's triangle may also be present ▸ lytic lesions with a narrow zone of transition usually in the metaphyseal region
US
Increasing soft tissue oedema and subperiosteal fluid ▸ thickened synovium ▸ thickened bursae with increased Doppler flow and fluid
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Pyomyositis: abnormal echogenicity in the early stages, abscess formation in the later stages
MRI
T1WI: ‘penumbra’ sign (representing granulation tissue) ▸ T2WI/STIR: a double line observed at the lesion margin
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Differentiating between acute osteomyelitis and acute medullary bone infarction:
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Osteomyelitis: thick, irregular peripheral enhancement around a non-enhancing centre
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Infarction: thin, linear rim enhancement or a long segment of serpiginous central medullary enhancement
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Pearls
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For management purposes the diagnosis is a clinical one – it should not be delayed by imaging
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Causative organisms:
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Haematogenous : Staphylococcus aureus (the most important) ▸ Haemophilus influenzae (in immunocompromised patients) ▸ Streptococcus pneumoniae ▸ beta-haemolytic streptococci ▸ aerobic Gram-negative rods
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Foreign body or implant : coagulase-negative staphylococci (skin commensals of low virulence)
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Most infections reflect contamination at the time of operation
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Open fracture : aerobic Gram negative rods (e.g. Pseudomonas) and anaerobic Gram-positive rods (eg. Clostridium spp.)
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Early sepsis reflects direct implantation with environmental organisms
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General patterns
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Bacterial infection: this is rapid and destructive
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Fungal infection : this occurs in immunocompromised patients ▸ there is a slow, chronic, infiltrative pattern that may mimic malignancy ▸ it is hard to eradicate
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TB : an aggressive, indolent or reparative pattern
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Location:
Differential diagnosis
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Tumour : this tends to demonstrate a more homogenous appearance than infection ▸ infection is more likely to produce soft tissue fluid filled cavities ▸ diagnosis is often only resolved with biopsy
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Langerhan's histiocytosis : with disseminated disease multiple lesions of the same age are less likely to be infective
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Aggressive degenerative disease : Milwaukee shoulder (rapidly progressive osteoarthritis) may mimic a septic arthritis
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Irradiation : subsequent bone necrosis with osteopaenia traversing a joint can mimic infection
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SAPHO syndrome
Haematogenous osteomyelitis of tubular bones †
| Features | Infant | Child | Adult |
|---|---|---|---|
| Localization | Metaphyseal with epiphyseal extension | Metaphyseal | Epiphyseal |
| Involucrum | Common | Common | Not common |
| Sequestration | Common | Common | Not common |
| Joint involvement | Common | Not common | Common |
| Soft tissue abscess | Common | Common | Not common |
| Pathological fracture | Not common | Not common | Common * |
| Fistulas | Not common | Variable | Common |
Imaging findings in osteomyelitis
| Plain radiograph | CT | MRI | NM | |
|---|---|---|---|---|
| Acute | Minimal findings Soft-tissue swelling may be seen |
Not useful | Bone marrow oedema can occur as early as 24–48 h, seen as low T1, and high T2 signal | May show increased uptake, but takes a few days |
| Subacute | Lucent or sclerotic lesion, periosteal reaction, soft-tissue swelling | Cortical and marrow abnormalities, including abscess, periosteal reaction, soft-tissue oedema and abscess | Bone marrow changes, cortical abnormalities seen as thickening, bone abscess, periosteal reaction, increased T2 signal in soft tissues, abscess formation. Post-gadolinium T1W sequences outline abscess cavities clearly | Three-phase bone scintigram, 111 indium WBC scan and combined studies are useful, especially to assess multifocal involvement. PET-CT generally not used in this context, but may be useful in exceptional circumstances |
| Chronic | Bone sclerosis, cortical thickening, sequestrum and cloaca, bone destruction, resorption and deformities | Much better than plain radiographs to demonstrate cloaca and sequestrum, periosteal new bone formation and abscess | Better soft-tissue and bone marrow resolution to demonstrate medullary and cortical changes, sequestra and cloaca well demonstrated, useful to outline soft-tissue abscess and sinus tracts | Generally useful if there is a problem with diagnosis. Combined WBC and bone marrow scintigram is useful. May highlight multiple sites of involvement |
Chronic Osteomyelitis
Chronic Osteomyelitis
Definition
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Bone (marrow) infection > 6 weeks in duration
Clinical presentation
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Asymptomatic or intermittent flare ups:
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Pain ▸ swelling ▸ general debility ▸ weight loss ▸ a discharging sinus ▸ anaemia
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There is rarely renal dysfunction secondary to amyloid deposition
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Radiological features
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If an abscess cavity forms within bone pus can interfere with the local blood supply and lead to necrotic bone that is surrounded by granulation tissue:
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Sequestrum : a mechanically separate avascular bone fragment ▸ it appears dense (due to surrounding hyperaemia) ▸ it is a foci for recurrence
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Involucrum : a shell of thickened sclerotic living bone surrounding dead bone ▸ it is formed beneath vital periosteum that is elevated by pus
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Cloaca: a defect within the involucrum that can allow pus to escape (sometime to the skin via a sinus)
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US
Increasing soft tissue oedema and subperiosteal fluid ▸ thickened synovium ▸ thickened bursae with increased Doppler flow and fluid
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Tenosynovitis: thickening of the tendon sheath with fluid surrounding the tendon itself ▸ non-compressible thickening of the tendon sheath with increased Doppler flow
XR/CT
Local osteopaenia is present with a superimposed mixed pattern:
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Aggressive or rapidly changing features : lysis ▸ cortical breach ▸ fracture
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Slower, more indolent and reparative reactions: sclerosis ▸ heterotopic new bone ▸ periosteal reaction of increasing maturity
CT
Periosteal reaction ▸ subtle bone erosion ▸ cortical destruction ▸ abscess formation ▸ soft tissue swelling ▸ trabecular thickening and medullary abnormalities
99m Tc-MDP
This has a high sensitivity but a low specificity ▸ it allows differentiation from cellulitis (which will have increased uptake during the ‘blood pool’ image but no bone uptake on delayed imaging)
111 In-labelled white cell studies and 99m Tc-sulphur colloids
>90% accuracy in diagnosing prosthetic infection
MRI
T2WI / STIR: high SI ▸ necrotic areas: loss of SI with no enhancement
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Periostitis: thin linear pattern of oedema with enhancement
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Chronic periostitis and periosteal reaction: thickening of low SI of cortical bone (T1WI and T2WI)
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Sequestrum: cortical bone: low SI (higher if derived from cancellous bone) ▸ any exudate which may surround the sequestrum shows low T1WI and high T2WI SI ± enhancement
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Involucrum: SI of normal living bone (commonly thickened and sclerotic and may show oedema)
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Cloaca: high SI defect within cortical bone
Pearls
Brodie's abscess
A walled off intraosseous abscess ▸ it is seen in paediatric acute on chronic or chronic osteomyelitis ▸ it is usually located within a metaphysis (± epiphyseal extension)
XR
An oval lytic lesion with a well-defined reactive sclerotic border
MRI
Central fluid (T1WI: low SI, T2WI: high SI) surrounded by a sclerotic rim (T1WI + T2WI: low SI)
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Differential: an osteoid osteoma
Sickle cell disease
A homozygous disease associated with dactylitis and bone infarction ▸ there is a higher incidence of Salmonella osteomyelitis (sickling in the gut vasculature leads to impaired gut defence and organism entry)
Infected prosthesis
It usually mimics chronic aseptic loosening ▸ there is peri-implant osteopaenia with periosteal reaction and progressive bone destruction
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Early (within 3 months): due to contamination during surgery of the early postoperative period ( S. aureus )
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Subacute (3–24 months): virulent coagulase-negative staphylococci or S. epidermis
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Chronic (>24 months): haematogenous spread from other sources
USS
Diagnoses fluid collections and the prosthesis
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