How to Distinguish Bone Infection from Tumour? 353

Diagnostic and Interventional Imaging (2012) 93, 351—359

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ICONOGRAPHIC REVIEW

Pitfalls in osteoarticular imaging: How to distinguish

bone infection from tumour?

a,∗ b c

T. Moser , M. Ehlinger , M. Chelli Bouaziz ,

c d d

M. Fethi Ladeb , J. Durckel , J.-C. Dosch

Department of Radiology, Notre-Dame Hospital, Montreal University Hospitals, 1560,

Sherbrooke Est, Montreal, Quebec H2T1H1, Canada

Department of Orthopaedic Surgery and Traumatology, Hautepierre Hospital, Strasbourg

University Hospitals, avenue Molière, 67098 Strasbourg cedex, France

Department of Radiology, Kassab Orthopaedic Institute, Ksar Said 2010, Tunisia

Department of Radiology 2, Hautepierre Hospital, Strasbourg University Hospitals, avenue

Molière, 67098 Strasbourg cedex, France

KEYWORDS Abstract In this article examining pitfalls in osteoarticular imaging we examine the diffe-

Bone infection; rential diagnosis of osteomyelitis from bone tumours. We describe the different features which

Osteomyelitis; differentiate these two types of disease in radiology and CT and MRI scanning.

MRI

The appearances of bone infection and bone tumour are often similar on imaging. The

differential diagnosis of osteoarticular infection includes above all malignant tumours but

also some benign, inﬂammatory tumours (osteoid osteoma osteoblastoma) and pseudo-

tumours (eosinophilic granuloma). The clinical and laboratory context does not always

discriminate between these and the diagnosis is occasionally only obtained on histological

examination. This confusion can lead to delays in treatment and inadequate management.

We describe here the imaging appearances of osteomyelitis, speciﬁcally those of sub-

acute osteomyelitis. It is estimated that 50% of cases in children are initially confused with

tumour [1].

∗

Corresponding author.

E-mail address: [email protected] (T. Moser).

352 T. Moser et al.

Deﬁnitions Radiological signs

Three pathways of infection of bone by pathogenic orga- The radiological signs of osteomyelitis involve a variable

nisms can be distinguished: haematogenous, direct (after combination of trabecular and then cortical osteolysis with

trauma or surgery) or contiguous infection (joint or soft tis- sequestrum, a periosteal reaction and soft tissue abnorma-

sue infection). The term osteomyelitis is usually reserved lities (collections, ﬁstulae).

for bone infection due to haematogenous spread (centrifu-

gal extension). Osteitis is a generic term combining bone Osteolysis

infections with a breach in the bone cortex from the outset

Osteomyelitis preferentially affects the metaphyses and epi-

(centripetal extension). Septic osteoarthritis is a combina-

physes of lower limb, long bones. Osteolysis is due to the

tion of septic arthritis with co-existent bone infection [2].

inﬂammatory reaction and septic necrosis in the bone layers.

Acute, sub-acute and chronic osteomyelitis can be con-

It is characterised initially by a zone of bone demineralisa-

sidered separately. The criteria for length of infection

tion with blurred edges (Fig. 1), and then extends to the

(acute = less than one month, sub-acute = one to three

cortex, taking on a moth-eaten or porous appearance. These

months, chronic = more than three months) are arbitrary and

are types IC, II and III of the Lodwick classiﬁcation [10]. Tra-

vary according to the author. Histologically, acute infection

becular and cortical osteolysis can be examined in far more

is suppurative and not contained; sub-acute infection is sup-

detail by CT scanning. This can show a non-speciﬁc increase

purative and contained (abscess) whereas chronic infection

in bone marrow density due to the inﬂammatory inﬁltrate

is variably suppurative and associated with healing bone

[7]. Finally, the occasional presence of gas (Fig. 2) or fat-

remodelling [2].

ﬂuid levels in the bone marrow cavity is very speciﬁc for

osteomyelitis outside of a context of trauma [11,12].

Clinical and laboratory features

Periosteal reaction

A periosteal reaction is present in more than 50% of cases of

Acute osteomyelitis is found predominantly in children and

acute, sub-acute or chronic osteomyelitis. It usually occurs

classically presents as pain, features of infection, local

earlier and is more clearly visible in children who have an

swelling and loss of function. It is usual to ﬁnd an acute

active periosteum, which is poorly adherent to bone. It

phase reaction with leukocytosis and raised C-reactive pro-

needs to be ossiﬁed in order to be visible (Fig. 3), although

tein. Blood cultures are positive in almost half of cases. The

can be seen more early on CT scanning than by radiographs.

organism most often isolated is Staphylococcus aureus (in

The periosteal reaction can be unilamellar, plurilamel-

90% of cases).

lar (‘‘onion skin’’) or compact. It is relatively non-speciﬁc

These signs are not speciﬁc as they can be present in some

and has many causes: physiological (premature infants),

pseudotumours and inﬂammatory tumours such as Ewing’s

fracture, osteosarcoma or eosinophilic granuloma for a unil-

sarcoma [3]. They are often absent or less pronounced

amellar periosteal reaction, Ewing’s sarcoma or osteosar-

in sub-acute or chronic osteomyelitis and in infants. The

coma for a plurilamellar reaction and fracture or osteoid

diagnosis is particularly difﬁcult in a non-febrile child in a

osteoma for a compact periosteal reaction. A spiculated

context of injury [4].

Radiographic and CT scan appearances

Radiographs are always essential in the acute phase [5],

although offer poor sensitivity, in the region of 37% with

a speciﬁcity of 73% in the study by Tumeh et al. [6]. CT

scans are particularly useful in the sub-acute or chronic

phase [7].

Chronology of radiological changes

Initially, radiographs are normal. They may then show suc-

cessive swelling of the soft tissues (early), bone demine-

ralisation (after seven days), a periosteal reaction (after

15 days), and trabecular and cortical osteolysis (after three

weeks) [8].

The duration of an infection is an important factor where

this can be assessed reliably from repeated ﬁlms. Rather

schematically according to Greenspan [9], Ewing’s sarcoma

requires approximately four to six months, osteomyeli-

tis four to six weeks and eosinophilic granuloma only

around ten days to produce the same amount of bone Figure 1. Heterogeneous demineralisation in acute humeral

destruction. metaphyseal osteomyelitis.

Pitfalls in osteoarticular imaging: How to distinguish bone infection from tumour? 353

Figure 2. Gas present in the bone marrow cavity indicating reac- Figure 4. Sequestrum appearance in sub-acute tibial osteomyeli-

tivation of chronic osteomyelitis. tis. Note also the bone reaction which surrounds the focus of

infection (‘‘involucrum’’).

zone (Fig. 4). It is important to diagnose as it is ﬁrstly very

suggestive of infection and secondly represents a reservoir

of organisms, which are not readily accessible to antibiotics,

and often requires surgical excision. The sensitivity of this

sign to diagnose infection is poor with radiographs but better

for CT scans [6,7]. It is not entirely speciﬁc as it is also seen

in eosinophilic granuloma, ﬁbrosarcoma and can occasio-

nally be confused with a calciﬁed nidus of osteoid osteoma

[14].

Soft tissue abnormalities

The fatty planes, which are visible on radiography are clas-

sically abolished by infection and displaced by tumours.

Collections and ﬁstulae are invisible on radiography unless

ﬁstulography is performed. The ﬁrst line investigation to

examine a subperiosteal or juxta-osseous collection is ultra-

sound. Bone and soft tissues can be studied in great detail

by MRI. Failing this, a CT scan with contrast injection and

multi-planar reconstructions can be used for a rapid urgent

patient assessments [15].

Soft tissues calciﬁcations or ossiﬁcations are rare in

osteoarticular infection due to the classic pyogenic orga-

nisms and are more suggestive of a neoplastic (osteosar-

coma) or pseudo-neoplastic (circumscribed myositis ossiﬁ-

Figure 3. Compact periosteal reaction associated with sub-acute

cans) process. On the other hand it is a characteristic ﬁnding

tibial osteomyelitis.

in tuberculosis and is also seen in hydatid disease and fungal

infections.

periosteal reaction with a Codman spur suggests osteosar-

coma or Ewing’s sarcoma [13]. Clinical-radiological features and differential diagnosis Sequestrum

Acute osteomyelitis

Sequestrum is a fragment of dead bone surrounded by gra-

nulation tissue and is seen in more than ﬁfty per cent of The diagnosis is suggested from clinical evidence (infection,

cases of chronic osteomyelitis. It appears on radiographs as metaphyseal pain) and laboratory ﬁndings (acute inﬂamma-

a dense, often irregular fragment with a surrounding clear tory reaction, positive blood cultures). Initial radiography

354 T. Moser et al.

is normal. Ultrasound can be used to investigate for sub- cortical bone. The most classical form of this is the Brodie

periosteal abscesses and guide aspiration puncture [18]. The abscess (Fig. 5) [16]. This is usually found in the metaphy-

initial changes in bone marrow signal can only be seen on MRI sis and may extend to the epiphysis along the epiphyseal

(see below). cartilage. It is usually 1 to 5 cm in diameter and is ovoid

or funicular along the long axis of the bone. A periosteal

reaction or appearances of sequestrum are typically absent.

Sub-acute osteomyelitis

Diaphyseal forms of disease have been described to have

This is the most difﬁcult situation, which raises the problem

far more variable appearances which may include cortical

of the differential diagnosis from tumour. We propose that

thickening or a periosteal reaction and contains sequestrum

the Gledhill and Rombouts classiﬁcation [16,19] be used as

[17]. The main differential diagnoses are eosinophilic granu-

the basis for description.Type 1 refers to the appearances

loma and osteoid osteoma (Fig. 6).Type 2 refers to a zone of

of a single osteolytic lesion, which may or may not be sur-

metaphyseal osteolysis with cortical destruction. The main

rounded by a reactive bony reconsolidation line which spares

Figure 5. Brodie’s abscess of the radial metaphysic (type 1 sub-acute osteomyelitis). Note the epiphyseal extension through the growth

cartilage, which is far easier seen on CT.

Figure 6. Sub-acute tibial osteomyelitis (type 3 sub-acute osteomyelitis). Osteoid osteoma and stress fractures can be excluded by a CT scan.

Pitfalls in osteoarticular imaging: How to distinguish bone infection from tumour? 355

Figure 7. Brodie’s abscess of the femoral head (type 1 sub-acute

osteomyelitis). The presence of bony sequestrum is atypical and

raises the possibility of osteoid osteoma.

differential diagnoses are osteosarcoma and an aneurysmal

cyst.Type 3 involves thickening of the diaphyseal cortex

caused by a compact periosteal and an endosteal reac- Figure 8. Plurilamellar femoral periosteal reaction (type 4 sub-

tion. The differential diagnosis includes osteoid osteoma acute osteomyelitis). The differential diagnosis is Ewing’s sarcoma.

(Fig. 7) and fatigue fractures or stress reactions. CT scans

are very useful to look for abnormalities which are not

Chronic suppuration may be present, the classical compli-

visible on radiographs.Type 4 refers to a plurilamellar dia-

cation of which, which is seen in 1% of cases, is squamous

physeal periosteal reaction. The main differential diagnosis

cell carcinoma developing from epithelial metaplasia in the

is Ewing’s sarcoma (Fig. 8).

sinus tract [22].

Other forms exist including epiphyseal disease imita-

ting chondroblastoma or a giant cell tumour (Fig. 9) [20]

and ‘‘metaphyseal equivalent’’ forms which affects the ﬂat

MRI appearances

bones (ilium) or short bones (calcaneum, vertebra) immedi-

ately next to the epiphyseal cartilage [21].

MRI is extremely useful for early diagnosis of acute

osteomyelitis with sensitivities and speciﬁcities of between

Chronic osteomyelitis

60 and 100% depending on the study. The lowest values

Healing bone changes predominate in the chronic stage of were obtained from studies performed without injection

osteomyelitis and episodes of reactivation are common. of contrast medium whereas values of over 90% have

Figure 9. Sub-acute osteomyelitis of the femoral head. The differential diagnosis is giant cell tumour.

356 T. Moser et al.

been achieved from the most recent work. MRI was and bone healing. The trabecular bone signal abnormalities

previously performed second line after bone scintigra- in acute osteomyelitis are homogeneous, poorly delineated,

phy and is now performed directly after radiographs and low or normal intensity on T1 and increased intensity on T2

ultrasound. (oedema), simultaneously involving the bone marrow cav-

ity, cortical bone and the adjacent soft tissues. The signal

abnormalities are better delineated, greatly reduced inten-

MRI signs sity on T1 and increased intensity on T2 (collection) in

sub-acute osteomyelitis, occasionally with reduced intensity

sequestrum on T1 and T2 (Fig. 10). Signal abnormalities in

Abnormal bone marrow signal

chronic osteomyelitis are heterogeneous and involve a com-

Abnormalities in bone marrow signal are initially due to the

bination of reduced intensity T1 and increased intensity T2

oedema, exsudate, the presence of inﬂammatory cells and

areas and reduced intensity T1 and T2 areas (sclerosis) [23].

bone ischemia and then reﬂect the formation of collections

Figure 10. a: weighted T2 MRI appearances of sequestrum and b: periosteal reaction correlated with CT scan ﬁndings.

Figure 11. Acute osteomyelitis of the humeral metaphysic (same patient as in Fig. 1). The necrosis and early intra-osseous collection are

clearly seen after injection of contrast medium (a). On the T1-weighted sequence (b), fat can be seen in the focus of osteomyelitis.

Pitfalls in osteoarticular imaging: How to distinguish bone infection from tumour? 357

The periphery of the bone collections can be highlighted by or any other organ) and appears to be due to the pres-

injecting contrast medium, making them easier to detect ence of paramagnetic free radicals produced by activated

(Fig. 11) [24]. macrophages. The penumbra sign is more apparent when

The signal abnormalities seen with oedema are a sensi- the granulation tissue is young and represents direct annu-

tive but relatively non-speciﬁc sign of osteomyelitis. Some lar uptake of contrast. It appears to be very speciﬁc (99%)

signs which are variably present are more speciﬁc for the for abscess although its sensitivity varies from 27% to 75%

diagnosis. depending on the study [28,29].

The double line sign described on T2-weighted sequences

Fat globules (acute osteomyelitis) is less useful. It represents the internal ring (increased

intensity) and external ring (reduced intensity). Its visibility

Acute osteomyelitis causes septic necrosis of the bone mar-

depends on the window used and offers a sensitivity of only

row, releasing fat particles which sediment with pus to form

22% [28].

fat-ﬂuid levels inside or outside of the bone [12]. The pres-

ence of fat in soft tissues (Fig. 12) is believed to be due

to the fat passing through the Haversian canals and is an

Periosteal reaction and sequestra

indirect sign of a breach in cortical bone [25].

The periosteal reaction is often missed on MRI although it

Davies et al. [26] have studied the presence of fat

can be seen before ossiﬁcation and therefore earlier than

in a focus of osteomyelitis. This sign is found in acute

on a CT scan. The target appearance on a T2-weighted

osteomyelitis (Fig. 11) and is due either to persistent normal

sequence (Fig. 14) represents the periosteal lamellae sepa-

bone marrow within the oedema or to linear or to globular

rated by granulation tissue [30]. The sequestra appear as

clumps of necrotic bone marrow.

reduced intensities on all sequences.

Penumbra sign (Brodie’s abscess)

Soft tissue abnormalities

The target appearance of the Brodie’s abscess on MRI was

initially described by Marti-Bonmati et al. [27]. Four con- Sequences with contrast medium injection and abolition of

centric layers are seen: the centre (pus) which is reduced in the fat signal or useful to delineate collections and ﬁstula

intensity on T1 and increased intensity on T2, the internal tracts within inﬂammatory changes [31].

ring (abscess wall) which is of normal intensity on T1 and

increased intensity on T2, the external ring (reactive scle-

Texture sign (soft tissue mass)

rosis) which is reduced in intensity on T1 and T2 and the

periphery (bone oedema) which is reduced in intensity on It is not always easy to distinguish between inﬂammatory or

T1 and increased on T2. neoplastic changes in a soft tissue mass. Classically, inﬂam-

The penumbra sign (Fig. 13) described on T1-weighted mation is diffuse and crosses the fascias whereas tumour

sequences represents the internal ring which has a rela- forms a well deﬁned mass which preserves the fascia. The

tively high density signal compared to the other layers of texture sign proposed for use in following up treated sar-

the target. Histologically, this represents the granulation tis- comas can also be used. A visible muscular framework in a

sue surrounding all abscess cavities (in bone, soft tissues T1-weighted sequence supports inﬂammatory changes when

Figure 12. Fat present in soft tissues conﬁrming the diagnosis of sub-acute osteomyelitis in a bone lesion with soft tissue mass. The

presence of sequestrum (b) is also suggestive.

358 T. Moser et al.

Figure 13. Penumbra sign in a Brodie’s abscess centred on the tibial physis. On the T1-weightedsequences (a), an internal reduced

intensity ring is silhouetted by the contents of the abscess on one side by the external ring of sclerosis and on the other by bony oedema.

After injection of contrast medium and abolishing the fat signal (b), the internal ring representing the granulation tissue picks up contrast

medium intensely.

attention to the aggressive presentation of benign inﬂam-

matory bone tumours (osteoid osteoma, osteoblastoma and

chondroblastoma), eosinophilic granuloma and stress frac-

tures which cause bone and soft tissue oedema that is often

disproportionate to the size of the lesion [33]. In these situa-

tions, careful examination of radiographs is essential and a

scan targeted on the abnormal area can occasionally reveal

subtle abnormalities (nidus, fracture line, stress reaction).

The differential diagnosis between osteomyelitis and

bone oedema as a reaction to neighbouring septic arthri-

tis or cellulitis is also difﬁcult. Bone would appear to take

up contrast medium less intensely than soft tissue in cel-

lulitis with reactive bony oedema, whereas contrast uptake

appears to be equivalent in osteomyelitis [24]. Bone oedema

as a reaction to septic arthritis appears to be less intense on

T1 sequences than in osteomyelitis [34].

An additional approach may be to use a contrast

media containing supermagnetic iron particles (SPIO) which

shorten the T1 and particularly T2 relaxation time. After

intravenous injection these particles are taken up by the

reticulo-endothelial cells and accumulate in macrophages

and ﬁbroblasts in the inﬂamed area. These contrast media

Figure 14. Appearances of a periosteal reaction on a T2-weighted

could help to delineate inﬂammatory reactions and distin-

MRI sequence. The unilamellar periosteal reaction and reduced

guish them from neoplastic disease [35,36].

intensity bone cortex are separated by increased intensity gra-

nulation tissue.

signal abnormalities are present on a T1-weighted sequence Disclosure of interest

[32].

TM is a consultant for Arthrovision.

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