BONE INFECTION MIMICKING BONE TUMORS:BONE INFECTION MIMICKING BONE TUMORS:HOW TO MAKE THE DISTINCTION? HOW TO MAKE THE DISTINCTION?

M. ATTIA, M. CHELLI BOUAZIZ, A. KAMMOUN , M F. LADEB

MUSCULOSKELETAL : MK 25  

INTRODUCTION INTRODUCTION

• Bone infection often mimics bone tumors on imaging and

clinical information is not always relevant for the diagnosis.

An erroneous diagnosis may be responsible for delayed,

ineffective or excessively mutilating treatments having severe

consequences for the patient.

• The purpose of this presentation is to review the key

elements for establishing a correct diagnosis of bone

infection.

PATHOPHYSIOLOGY OF BONE INFECTIONPATHOPHYSIOLOGY OF BONE INFECTION

• Bone infection may result from hematogenous spread, direct extension

from a contiguous source or direct implantation and post surgery

• Osteomyelitis is the prototype of bone infection. Infective organisms

present in the blood stream embolize in the sinusoidal vessels of the

metaphysis. Septic venous thrombosis provokes interruption of endosteal

and periosteal blood supply and secondarily bone necrosis

• If infection is not eradicated during the acute phase, subacute or chronic

osteomyelitis can occur. This can be related either to inadequate therapy or

to a specific host resistance to the infection.

Chronology of radiological changesChronology of radiological changes

Normal

Soft tissue swelling

Demineralization

Osteolysis

Periosteal reaction

MRI

Radiographs/CT

Abnormal bone marrow signal

D7 D15 D21

RADIOGRAPHS AND CT: OSTEOLYSISRADIOGRAPHS AND CT: OSTEOLYSIS

• Results from inflammatory reaction and septic necrosis of

bone trabeculae

• Most common in long bone metaphyses and diaphyses

• Presents as an ill defined radiolucent medullary area

(Lodwick IC) (fig. 1) secondarily extending to the cortex in a

moth eaten (Lodwick II) or permeative (Lodwick III) pattern

• CT provides a better analysis of trabecular and cortical

bone osteolysis and may show non specific increased bone

marrow density due to inflammatory infiltration. Presence of

gas (fig. 2) or fat-liquid level in the medullary cavity is very

specific of osteomyelitis in a non traumatic context.

Ram PC et al. CT detection of intraosseous gas: a new sign of osteomyelitis. AJR 1981;137:721-723Rafii M et al,. Hematogenous osteomyelitis with fat-fluid level shown by CT. Radiology 1984;153:493-494Naidoo P. Extramedullary fat fluid level on MRI as a specific sign for osteomyelitis. Australas Radiol 2003;47:443-446

Fig 1: Acute osteomyelitis of the humerus: ill defined osteolysis

Fig 2: Activation of chronic osteomyelitis Transverse CT view of the tibia showing gas in the medullary cavity

RX AND CT MANIFESTATIONSRX AND CT MANIFESTATIONS: PERIOSTEAL : PERIOSTEAL REACTIONREACTION

• A periosteal reaction is observed in half of acute,

subacute or chronic osteomyelitis cases.

• It usually occurs early and is better diagnosed in

children (fig. 3)

• It is detected earlier by CT rather than by

radiographs

• Periosteal reaction may be unilamellar, plurilamellar

or compact. However, it is not specific of

osteomyelitis

Fig 3: Subacute osteomyelitis of the tibia.

AP radiograph shows a well circumscribed lytic lesion with compact periosteal reaction

RADIOGRAPHIC AND CT MANIFESTATIONS:RADIOGRAPHIC AND CT MANIFESTATIONS: SEQUESTRUMSEQUESTRUM

• The sequestrum is a devitalized bone fragment

surrounded by granulation tissue. It is present in more

than 50% of chronic osteomyelitis

• It usually appears on radiographs as a condensed bone

fragment with irregular margins and radiolucent rim (fig.

4)

• The sequestrum is better detected by CT than by

radiographs

• The sequestum is characteristic of bone infection

• However, the sequestrum is not absolutely specific of

infection because it may be observed in histiocytosis,

fibrosarcoma and may mimick a calcified osteoid

osteoma nidus

Fig 4: intracortical sequestrum in a patient with subacute osteomyelitis of the tibia. Note also the bone reaction surrounding the infection (involcrum)

RADIOGRAPHIC AND CT MANIFESTATIONS: RADIOGRAPHIC AND CT MANIFESTATIONS: SOFT TISSUE SOFT TISSUE ABNORMALITIESABNORMALITIES

• Radiographs and CT are relatively insensitive to soft tissue abnormalities

• Ultrasound is the first method to diagnose subperiosteal or soft tissue

collection

• MRI allows an exhaustive study of bone and soft tissue

• Soft tissue calcifications or ossifications are rare in musculoskeletal

pyogenic infections and usually observed in tumors (osteosarcoma) and

pseudotumors (myositis ossificans circumscripta). However, they are

characteristic of musculoskeletal tuberculosis and may also be observed in

hydatidosis or mycoses.

MRI FINDINGS: BONE MARROW MRI FINDINGS: BONE MARROW EDEMAEDEMA

• MRI is very useful for the early diagnosis of

musculoskeletal infection

• In acute osteomyelitis, MRI typically shows low to

intermediate T1 ( fig 5) and high T2 signal

intensity areas involving the medullary canal the

cortex and the soft tissues, corresponding to

bone marrow edema

Fig 5: Acute osteomyelitis of the distal tibia. Bone marrow edema of low T1 signal

MRI FINDINGS: BONE MARROW EDEMAMRI FINDINGS: BONE MARROW EDEMA

A

B

• In subacute osteomyelitis, signal abnormalities are

better circumscribed: intraosseous low T1 and

high T2 signal intensity collections, sometimes

containing a sequestrum of low T1 and T2 signal

intensity (fig.6)

• In chronic osteomyelitis, the bone shows a

heterogenous signal associating low T1 and high

T2 intensity areas (fluid) and low T1 and T2 areas

(sclerosis).

Fig 6: Bone sequestrum with periosteal reaction. CT(A) and T2 weighted MR (B)

appearance

MRI FINDINGS: BONE MARROW EDEMAMRI FINDINGS: BONE MARROW EDEMA

• Intravenous contrast administration enhances the periphery of bone

collections and makes their detection easier (fig. 7)

• Bone marrow edema is a sensitive but not specific sign of osteomyelitis.

• Optional signs are more specific of this diagnosis

– Fat globules

– Penumbra

– Double line

MRI FINDINGS: FAT GLOBULESMRI FINDINGS: FAT GLOBULES

• Acute osteomyelitis causes septic necrosis of bone marrow

• Fat sediments with pus may resulting in intra or extra-osseous fat-

liquid levels

• The presence of fat in acute osteomyelitis may result from either the

persistence of normal bone marrow surrounded by edema or the

presence of linear or globular foci of necrotic bone marrow (fig. 7)

• The presence of fat in soft tissues is an indirect sign of cortical

disruption (fig. 8)

A B

Figure 7 : Acute osteomyelitis of the humerus (same patient as fig 1). Note the presence of fat into the osteomyelitis focus on T1 weighted sequence (A). Bone and soft tissue collection are well depicted by IV Gadolinium administration (B).

Figure 8 : Subacute osteomyelitis of the distal femur: the presence of fat in the soft tissue (B) and the cortical sequestration (A) are very specific of bone infection.

A B

MRI FINDINGS: PENUMBRA SIGNMRI FINDINGS: PENUMBRA SIGNFour concentric layers are observed on MRI

(Fig 9):

– the center (pus) has a low T1 and high

T2 signal intensity

– the internal ring (abcess wall) has an

intermediate T1 and high T2 signal

intensity

– the external ring (reactional sclerosis)

has low T1 and T2 signal intensity

– the peripheral area (bone edema) has

low T1 and high T2 signal intensity Figure 9 : Penumbra sign in a Brodie’s abcess of the proximal tibia. The abcess wall internal aspect shows a high signal on T1 weighted sequence (A) and after fat suppression and Gadolinium administration (B).

MRI FINDINGS: PENUMBRA SIGNMRI FINDINGS: PENUMBRA SIGN

• The penumbra sign (fig. 9) described on T1 weighted

sequences corresponds to the relative hyperintensity of

the internal ring compared to the other three layers

• Histologically, it corresponds to granulation tissue at the

abcess wall (in bone, soft tissue or any other organ)

• This sign is specific of abcess (99%) but its sensitivity

ranges from 27 to 75%

MRI FINDINGS: DOUBLE LINE SIGNMRI FINDINGS: DOUBLE LINE SIGN

• The double line sign, described

on T2 weighted sequences is

less interesting . It corresponds

to internal (hyperintense) and

external (hypointense) rings (Fig

10).

Its sensitivity is 22%Fig 10: Brodie’s abcess of the humerus. Transverse T2 weighted MRI shows a « double line » sign

TUBERCULOUS OSTEITISTUBERCULOUS OSTEITIS

•Rare localization of musculoskeletal tuberculosis

•Axial and appendicular skeleton

•Pseudotumoral appearance

•May be multifocal and mimick bone metastases

Fig 10:Tuberculous osteitis of the ischium

Fig 11: Tuberculous osteitis of the ilium. CT appearance

Fig 12: tuberculous osteitis of the distal femur

Fig 13: Tuberculous spondylitis. CT appearance

BONE HYDATIDOSISBONE HYDATIDOSIS

•Parasitic infection caused by Echinococcus

granulosus

•Bone is rarely involved

•It mainly occurs in thoracic spine, long bone

localizations are rare

•Radiographs and CT typically show a multiloculated

lytic lesion with sometimes pseudotumoral pattern

•Transarticular extension is possible

•US and MRI: multicystic appearance

CASE ILLUSTRATIONS

BRODIE ABSCESSBRODIE ABSCESS

• Brodie abscess is a form of subacute osteomyelitis

manifesting as a focal bone lucency that may contain a

sequestrum and be surrounded by bone sclerosis. It

usually involves long bone metaphyses, typically the

distal femur or proximal tibia. Periosteal reaction may be

absent

• Differential diagnosis is the nidus of an osteoid osteoma

that is a benign bone tumor occurring in children and

young adults and presenting as a focal osteolysis

(nidus), that may contain a central calcification, and

peripheral bone sclerosis.

Chronic osteomyelitis of the femur

Osteoid osteoma of the glenoid process

• Subacute osteomyelitis rarely presents as an isolated cortical hyperostosis.

• In this case, differential diagnosis includes osteoid osteoma and stress fracture

Osteoid osteoma of the femoral neckStress fracture of the tibia

Subacute osteomyelitis of the tibia

SUBACUTE OSTEOMYELITIS

CHRONIC OSTEOMYELITIS

• Chronic osteomyelitis occurs when acute

osteomyelitis is insufficiently or

inadequately treated. It usually manifests

as a mixed lytic and sclerotic lesion. Moth

eaten and permeative bone osteolysis,

periosteal reaction and pathological

fracture can also be observed

• Diiferential diagnosis of these aggressive

bone osteolysis patterns include

osteosarcoma and Ewing tumor, which

are malignant primitive bone neoplasms.Chronic osteomyelitis of the humerus

Ewing Tumor of the fibula

• Brodie’s abcess usually presents on

radiographs as a well circumscribed

metaphyseal or metaphyso-epiphyseal lytic

lesion surrounded by a sclerotic rim. Bone

sequestrum and periosteal reaction may be

absent

• This radiographic pattern may be observed

in giant cell tumor, and other benign or

malignant bone tumors. In this example,

telangiectasic osteosarcoma of the

proximal tibia.

Brodie’s abcess of the femur

Telangiectasic osteosarcoma of the proximal tibia

BRODIE’S ABCESS

• mixed lytic and sclerotic bone

appearance associated with

periostal reaction may correspond

either to chronic osteomyelitis or

osteosarcoma.

• In this case, MRI clearly depicted

intra-osseous and soft tissue

abcesses which allowed the

diagnosis of chronic osteomyelitis

Chronic osteomyelitis of the femur

CHRONIC OSTEOMYELITIS

BONE LYMPHOMABONE LYMPHOMA

• mixed lytic and sclerotic bone

appearance associated with

periostal reaction.

• doubt about the presence of a

bone sequestrum on CT images.

• MRI clearly show intra-osseous

and soft tissue involvement.

• Bone biopsy: bone lymphoma

TUBERCULOUS SPONDYLITISTUBERCULOUS SPONDYLITIS

• Lateral radiograph and Sagittal

CT image showing sclerotic

collapsed thoracic vertebra.

• The presence of intrasomatic

gas and péri-vertebral

 calcification are against tumor

• MRI show disc preservation

and bilobed epidural abcess

• vertebral biopsy : tuberculous

spondylitis

• Osteosarcoma is a primitive malignant

bone tumor occuring mainly in children

and young adults. Radiographs may

show osteolytic, sclerotic or mixed lytic

and sclerotic pattern.

• The differential diagnosis is chronic

osteomyelitis.

• In this case, the patient reported a two-

year history of knee pain but

inflammatory biological tests were

negative.

• Bone biopsy concluded at a conventional

osteosarcomaConventional osteosarcoma of the femur

OSTEOSARCOMA

• Another example of chronic osteomyelitis mimicking primary bone tumor (Ewing Tumor).

• In both cases, radiographs and CT show lytic and sclerotic bone appearance with soft tissue swelling.

Chronic osteomyelitis of the ilium

Ewing tumor of the ilium

CHRONIC OSTEOMYELITIS

CONCLUSIONCONCLUSION

• Bone infection often mimics bone tumors on imaging

•Clinical and biological information may be helpful but it is not

always relevant for the diagnosis.

•An accurate analysis of radiographic, CT and MRI signs may

be very useful to establish a correct diagnosis because some

signs are very specific and allow to differentiate both entities.

•However, in some cases, positive diagnosis remains very

difficult and bone biopsy is required.