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Bone and Soft Tissue Infection


Acute Osteomyelitis

Acute Osteomyelitis

Definition

  • Acute infection of the bone diagnosed within 2 weeks of symptoms (subacute if diagnosed > 2 weeks) ▸ causes:

    • ▪

      Staphylococcus aureus : commonest organism in any age group (80%)

    • ▪

      Gram-negative organisms ( Pseudomonas/Enterobacter ): remaining 20%

    • ▪

      Acute infections prosthetic implants: S. aureus

  • 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

  • Local pain ▸ soft tissue redness and swelling (± discharging abscess) ▸ reduced function and mobility ▸ pyrexia and systemic ill health

  • The presence of any infected dead bone or debris makes treatment difficult

Radiological features

  • 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

  • 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

  • Differentiating between acute osteomyelitis and acute medullary bone infarction:

    • ▪

      Osteomyelitis: thick, irregular peripheral enhancement around a non-enhancing centre

    • ▪

      Infarction: thin, linear rim enhancement or a long segment of serpiginous central medullary enhancement

Pearls

  • For management purposes the diagnosis is a clinical one – it should not be delayed by imaging

  • Causative organisms:

    • ▪

      Haematogenous : Staphylococcus aureus (the most important) ▸ Haemophilus influenzae (in immunocompromised patients) ▸ Streptococcus pneumoniae ▸ beta-haemolytic streptococci ▸ aerobic Gram-negative rods

    • ▪

      Foreign body or implant : coagulase-negative staphylococci (skin commensals of low virulence)

      • –

        Most infections reflect contamination at the time of operation

    • ▪

      Open fracture : aerobic Gram negative rods (e.g. Pseudomonas) and anaerobic Gram-positive rods (eg. Clostridium spp.)

      • –

        Early sepsis reflects direct implantation with environmental organisms

General patterns

  • Bacterial infection: this is rapid and destructive

  • Fungal infection : this occurs in immunocompromised patients ▸ there is a slow, chronic, infiltrative pattern that may mimic malignancy ▸ it is hard to eradicate

  • TB : an aggressive, indolent or reparative pattern

  • Location:

Differential diagnosis

  • 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

  • Langerhan's histiocytosis : with disseminated disease multiple lesions of the same age are less likely to be infective

  • Aggressive degenerative disease : Milwaukee shoulder (rapidly progressive osteoarthritis) may mimic a septic arthritis

  • Irradiation : subsequent bone necrosis with osteopaenia traversing a joint can mimic infection

  • 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

* Neglected areas

Acute osteomyelitis. (A) Plain radiograph of a child showing ill-defined lucency adjacent to the growth plate of the distal fibula with surrounding sclerosis. (B) Coronal CT reconstruction demonstrating the lucent areas adjacent to the growth plate with prominent surrounding sclerosis. Axial T1WI (C) and T1WI (FS), gadolinium-enhanced (D) MR images. The abnormal infected marrow is of low signal intensity on the T1-weighted pulse sequence. After administration of the gadolinium chelate, osteomyelitis is demonstrated by the high signal-enhancing area adjacent to the growth plate in the distal fibula associated with periosteal new bone formation. ©35

Penumbra sign. Sagittal (A) and coronal (B) MR images of the calcaneus show a circumscribed area of bone destruction with a halo or penumbra of granulation tissue and oedema. *

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

  • Bone (marrow) infection > 6 weeks in duration

Clinical presentation

  • Asymptomatic or intermittent flare ups:

    • ▪

      Pain ▸ swelling ▸ general debility ▸ weight loss ▸ a discharging sinus ▸ anaemia

    • ▪

      There is rarely renal dysfunction secondary to amyloid deposition

Radiological features

  • 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:

    • ▪

      Sequestrum : a mechanically separate avascular bone fragment ▸ it appears dense (due to surrounding hyperaemia) ▸ it is a foci for recurrence

    • ▪

      Involucrum : a shell of thickened sclerotic living bone surrounding dead bone ▸ it is formed beneath vital periosteum that is elevated by pus

    • ▪

      Cloaca: a defect within the involucrum that can allow pus to escape (sometime to the skin via a sinus)

US

Increasing soft tissue oedema and subperiosteal fluid ▸ thickened synovium ▸ thickened bursae with increased Doppler flow and fluid

  • 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:

  • Aggressive or rapidly changing features : lysis ▸ cortical breach ▸ fracture

  • 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

  • Periostitis: thin linear pattern of oedema with enhancement

  • Chronic periostitis and periosteal reaction: thickening of low SI of cortical bone (T1WI and T2WI)

  • 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

  • Involucrum: SI of normal living bone (commonly thickened and sclerotic and may show oedema)

  • 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)

  • 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

  • Early (within 3 months): due to contamination during surgery of the early postoperative period ( S. aureus )

  • Subacute (3–24 months): virulent coagulase-negative staphylococci or S. epidermis

  • Chronic (>24 months): haematogenous spread from other sources

USS

Diagnoses fluid collections and the prosthesis

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