20 Bone and Soft Tissue Infection A. Mark Davies

20 Bone and Soft Tissue Infection

A. Mark Davies and Richard William Whitehouse

A. M. Davies, MD

Consultant Radiologist, The MRI Centre, Royal Orthopaedic Hospital, Birmingham, B31 2AP, UK

R. W. Whitehouse, MD

Department of Clinical Radiology, Manchester Royal Infi r- mary, Oxford Road, Manchester, M13 9WL, UK

sacroiliac joints, hip joints and proximal femur.

The first two are covered in Chaps. 10 and 19. The presentation and imaging features of osteomyelitis of the proximal femur are similar to that of any long bone and will not be discussed in this chapter nor will infections associated with sepsis from the intrapelvic organs. This chapter will concentrate on infections involving the pelvic bones and surround- ing soft tissues.

20.2

Acute and Subacute Osteomyelitis

20.2.1

Incidence, Age and Sex

Osteomyelitis of the pelvic bones is rare with an estimated incidence of 1%–11% of all cases of hae- matogenous osteomyelitis (Beaupre and Carroll 1979; Edward et al. 1978; Evans et al. 1985; Morrey et al. 1978; Weld 1960; Young 1934). As with all osteomyelitis this tends to be a disease of children with an average age between 8 and 14 years (High- land and LaMont 1983; Mah et al. 1994; Rand et al. 1993; Sucato and Gillespie 1997; Davidson et al. 2003), although it is well recognised in infants and throughout adult life (Choma et al. 1994). There is an unexplained male preponderance of approxi- mately two-thirds of cases (Highland and LaMont 1983; Mah et al. 1994; Rand et al. 1993; Sucato and Gillespie 1997; Davidson et al. 2003).

20.2.2 Detection

Pelvic osteomyelitis can have very variable present- ing features mimicking spinal, abdominal, sciatic and hip conditions (Morgan and Yates 1966; Ham- mond and Macnicol 2001; Macnicol 2001; Oud- jhane and Azouz 2001; Davidson et al. 2003). Only 20.1

Introduction

Osteomyelitis is an infection of bone and bone marrow caused by blood-borne organisms, (hae- matogenous) spread from a contiguous source of infection or by direct inoculation from a penetrating injury or surgical wound. The term osteomyelitis also encompasses granulomatous infections such as tuberculosis (TB) or fungal infections. The mor- bidity associated with osteomyelitis has long been recognised. Each year, acute osteomyelitis affects approximately 10 of every 100000 people in the developed world (Lidgren and Lindberg 1972). Its incidence is related to socioeconomic factors and increases in hot and humid climates. The risk of developing chronic osteomyelitis is substantially reduced with modern antibiotic therapy. The com- monest sites of bone infection in the pelvis are the

CONTENTS

20.1 Introduction 323

20.2 Acute and Subacute Osteomyelitis 323 20.2.1 Incidence, Age and Sex 323

20.2.2 Detection 323 20.2.3 Site 325 20.2.4 Diagnosis 325

20.3 Chronic Osteomyelitis and Chronic Recurrent Multifocal Osteomyelitis 327

20.4 Tuberculosis 328 20.5 Soft Tissue Infection 328

20.6 Immunodeficiency Associated Infections 329 20.7 Parasitic Infections 330

20.8 Conclusion 330 References 331

Fig. 20.1a–c. Acute osteomyelitis of the right pubis. a AP radiograph of the pubis shows a little rarefaction of the right pubis but is otherwise normal. b Anterior image from bone scintigraphy showing increased activity over the right pubis partially obscured by the bladder activity. c Axial T1-weighted fat suppressed contrast enhanced MR image showing a small area of destruction in the right pubis with fl orid surrounding marrow and soft tissue oedema. The small signal void within the lesion is likely to represent an early sequestrum

a

b c

imaging to first detect the focus of infection and then to suggest the correct diagnosis. The radio- graph is routinely the first imaging investigation and, as with acute osteomyelitis at more common sites, is usually normal in the first 1 to 2 weeks (Sammak et al. 1999). Indeed, there is arguably a greater problem with pelvic osteomyelitis as David- son and co-workers (2003) reported normal initial radiographs in 84% cases (Fig. 20.1a). The subtle, early lesions in the pelvis are easily overlooked due to the curvature of the bones and may also be obscured by overlying soft tissues, bowel gas, faecal material and, in older patients, vascular calcifica- tions. One of the cardinal signs of osteomyelitis, periosteal new bone formation, may be missed on the conventional anteroposterior (AP) radiograph of the pelvis unless it involves the inner or outer margins of the iliac bones or the superior or inferior margins of the pubic rami (Fig. 20.2a).

If there is a clinical suspicion of osteomyelitis, in the presence of normal radiographs, then fur- ther imaging is indicated. Ultrasound is virtually essential in the patient with suspected septic arthri- tis of the hip (see Chap. 10) but is of doubtful value

arthritis, which can have a similar presentation and tends to be a more rapidly progressive condi- tion with potentially more serious sequelae. Bone scintigraphy using the most commonly available radioisotope, technetium 99m (Tc 99m) will usu- ally demonstrate increased activity on all phases of a triple-phase bone scan within hours of the onset of infection (Sammak et al. 1999). Bone scintigraphy can be normal in up to 10% cases of pelvic osteo- myelitis (Highland and LaMont 1983; Sucato and Gillespie 1997; Davidson et al. 2003). Radio- isotope activity in the bladder may obscure pubic or distal sacral lesions on the routine frontal and posterior projections (Fig. 20.1b). The conspicuity of pubic lesions can be improved by performing a

“brim view”. The sensitivity in the pelvis is likely to

be increased with the use of single photon emission

computed tomography (SPECT). Alternative agents

include gallium-67, indium-111-labelled leukocytes

and Tc-99m-labelled leukocytes. Although these

may have some merits they require more laborious

preparation, involve a greater radiation burden to

the patients, can give false positive diagnoses and

persistent red marrow activity can cause problems

with interpretation (Wukich et al. 1978; Sammak et al. 1999). The ability of bone scintigraphy to image the entire skeleton is useful in suspected multifocal infection and in neonates and young children where there may be limited localising signs.

Computed tomography (CT) has in the past been reported to be useful in the detection of pelvic osteo- myelitis and in defining the extent of the inflamma- tory process (Fig. 20.2b) (Rand et al. 1993; Viani et al. 1999; Davidson et al. 2003). This role, particu- larly in the early case, has been largely superseded by magnetic resonance (MR) imaging. CT can still be helpful in chronic osteomyelitis in confirming/

excluding the presence of sequestra. Now that it is becoming increasingly available MR imaging is the investigation of choice in demonstrating acute and subacute osteomyelitis (Fig. 20.1c). It is more sensi- tive to marrow abnormalities than the other rou- tinely available imaging techniques.

20.2.3 Site

Haematogenous spread of osteomyelitis typically begins in the metaphyseal equivalent areas which are the margins of the iliac blade, particularly adja- cent to the sacroiliac joint, around the acetabulum (Fig. 20.3), and adjacent to the pubic symphysis (Nixon 1978; Kricun 1993). The percentage inci- dence at different sites varies between studies, though this is undoubtedly influenced by many

reports including only a handful of cases. Compil- ing their data with previously reported cases in chil- dren, Davidson and co-workers (2003) found the most commonly involved site was the ilium (35%) followed by the acetabulum (24%), the pubis (18%) and the ischium (17%).

20.2.4 Diagnosis

Establishing the diagnosis of osteomyelitis is based on a combination of appropriate clinical presen- tation, serological results, imaging findings and, where possible, microbiological proof. Many pub- lished reports detail the efficacy of MR imaging in identifying osteomyelitis (Morrison et al. 1993;

Jaramillo et al. 1995). While the sensitivity of MR imaging in detecting marrow change is not in doubt the remarkably high sensitivity in the diagnosis of osteomyelitis is questionable. Simple detection of an abnormality is relatively straightforward. It depends on the acumen of the reporting radiolo- gist as to whether osteomyelitis is diagnosed cor- rectly or, as is often the case in reality, is only one of a relatively large differential diagnosis. It has long been recognised that osteomyelitis, notably subacute, may be misdiagnosed as a bone tumour, particularly in the age group where both infection and tumour are at their most common (Gledhill 1973; Roberts et al. 1982; Baxter Willis and Rozencwaig 1996; Rasool 2001a; Oudjhane and

Fig. 20.2a,b. Acute osteomyelitis of the ilium initially thought to be a Ewing’s sarcoma. a AP radiograph showing a lytic lesion within the blade of the ilium and a lamellar periosteal reaction along inner rim. Biopsy defect noted laterally. b The CT more clearly reveals the periosteal new bone formation anteriorly, bone destruction posteriorly and overlying soft tissue mass

a

b

granuloma) and more aggressive appearing lesions with a malignant round cell tumour such as Ewing’s sarcoma (Fig. 20.2a). Obviously, the converse may occur with potentially more serious consequences.

In the adolescent on MR imaging, early osteomyeli- tis can mimic avulsion stress to the ischium as well as osteoid osteoma in a juxtaarticular origin and visa versa. Infection of the ischiopubic synchondro- sis in children may mimic an osteochondritis (Van Neck phenomenon) (Iqbal et al. 2004).

Conventional radiographs of pelvic osteomyeli- tis can be relatively non-specific. In many cases MR imaging can help distinguish tumour from infection.

Radiographic features of an aggressive lesion with cortical breaching and a solid soft tissue component on MR imaging favours a sarcoma. Bone abscesses typically exhibit a target-like appearance with four layers on MR imaging (Martí-Bonmati et al. 1993).

A useful indicator of infection rather than tumour is the “penumbra sign” which refers to the relatively hyperintense granulation tissue lining on T1-weighted

abscess (Fig. 20.3b) (Grey et al. 1998; Marui et al.

2002). The conspicuity of this granulation layer on MR imaging can be increased by the administration of a gadolinium chelate (Munk et al. 1993; Tehran- zadeh et al. 1992). It should be noted that any cyst lining, regardless of the nature of the lesion, would also tend to show contrast enhancement. In the pelvis the penumbra sign is most frequently seen in cases arising around the acetabulum (Fig. 20.3b).

Biopsy and culture are fundamental to establish- ing a diagnosis. These are necessary to ensure that the diagnosis of infection is correct and that the appropriate antibiotic treatment instituted, which is important in countries where numerous different infective agents, not just staphylococcus aureus, are endemic. Image-guided biopsy is advised utilising an approach which would not prejudice subsequent surgery if a tumour were identified. CT-guided biopsy is probably preferable to fluoroscopy to ensure accurate needle positioning in deep-seated pelvic lesions (Fig. 20.4). Most units are not set up

Fig. 20.3a–c. Subacute osteomyelitis. a Coro- nal CT reconstruction showing a multilocu- lated lytic lesion containing fl ecks of calcifi - cation arising in the roof of the acetabulum. b Coronal T1-weighted MR image showing low signal intensity marrow oedema. The rela- tively high intensity thin granulation tissue around the abscess cavity indicates a positive

“penumbra sign”. c Coronal short T1-inver- sion recovery (STIR) image allowing distinc- tion of the central loculated abscess from the surrounding oedema. There is a reactive hip joint effusion

b c

a

to use MR for image-guided procedures. The pre- biopsy MR scan is helpful in showing the most suit- able site for the biopsy.

20.3

Chronic Osteomyelitis and Chronic Recurrent Multifocal Osteomyelitis

Chronic osteomyelitis of the pelvic bones exhibit- ing the classic features of sequestrum, involucrum formation etc. is uncommon in the pelvis. The ilium is, however, a well recognised site for the scleros- ing form of chronic osteomyelitis (Steinbach 1966; Jurik et al. 1988). The radiographic features are those of dense sclerosis involving the blade of the ilium (Fig. 20.5a). The differential diagnosis

in the younger patient includes osteosarcoma and in the elderly Paget’s disease. Bone scintigraphy is useful as sclerosing osteomyelitis of the ilium is most frequently found in chronic recurrent mul- tifocal osteomyelitis (CRMO) (Fig. 20.5b). CRMO is an inflammatory disease of unknown aetiology, which predominantly affects children and adoles- cents. It belongs to a variety of aseptic osteomyelitis- like conditions of bone, many of which are associ- ated with dermatological lesions. These have been lumped together under the acronym SAPHO syn- drome (synovitis, acne, pustulosis, hyperostosis and osteitis). Whether CRMO is a distinct clinical entity or part of the spectrum of diseases including SAPHO syndrome continues to be debated (Earwaker and Cotten 2003). The clavicle and long bones are more frequently involved than the pelvis. In the authors experience identification of bone-forming lesions arising concomitantly in the medial two thirds of the clavicle and the ilium almost invariable indi- cates CRMO (Fig. 20.5) Biopsy is advised to exclude other conditions but in CRMO typically just shows non-specific inflammatory changes and culture is usually negative. Monitoring disease activity in the presence of so much sclerosis can be difficult using radiographs. MR imaging will show increasing water content in active exacerbations. It has been sug-

Fig. 20.4. CT-guided needle biopsy of a left acetabular lesion which was proven to be osteomyelitis on histology

Fig. 20.5a,b. Chronic recurrent multifocal osteomyelitis. a AP radiograph of the pelvis showing sclerosing osteomyelitis of the right ilium. b Anterior bone scintigraphy image showing involvement of the right ilium, left proximal femur, sternum, right clavicle and acromion

a b

20.4

Tuberculosis

Tuberculosis (TB) remains a major cause of skeletal infection in many developing countries (Rasool 2001b). The resurgence in some countries has been largely attributed to the acquired immunodeficiency syndrome (AIDS) (see Sect. 20.6), but is also due in part to inefficient immunisation programmes,

cate a predominantly solitary presentation (Rasool 2001b). In the experience of the authors, multifocal osseous TB is re-emerging, particularly in patients of Asian ethnicity (Fig. 20.6). The two most impor- tant aspects of the disease are that concomitant pul- monary disease occurs in only 12%–50% of patients and that the radiographic appearances are non-spe- cific (Fig. 20.7) (Watts and Lifeso 1996; Abd El Bagi et al. 1999). It can frequently be mistaken for other infective, traumatic and neoplastic conditions (Yao and Sartoris 1995; Rasool 2001b). Late pre- sentation with a history of symptoms extending back months or even years is typical. In the pelvis, TB characteristically involves the sacroiliac and hips joints with subsequent spread to the adjacent bones.

Uncommonly the pubic bones and ischia may be involved (Moore and Rafii 2001). One of the com- moner manifestations of TB in the pelvis is extension of a spinal abscess down the iliopsoas muscles.

20.5

Soft Tissue Infection

Soft tissue infections of the pelvis may be primary or secondary to a joint infection. Again, they are commoner in immunocompromised patients (see Sect. 20.6). The advent of cross-sectional imaging

Fig. 20.6. Multifocal osseous lytic lesions with central seques- tra, in the sacrum and left ilium. Left SI joint involvement is also evident, other lesions were present in the spine. Biopsy confi rmed TB

Fig. 20.7a,b. Tuberculosis of the ilium. a AP radiograph just shows non-specifi c periosteal new bone formation along the left ilium and superior pubic ramus. b The oblique radiograph reveals some ill defi ned lysis medial to the acetabulum a

b

has dramatically improved our ability to detect and diagnose these infections (Struk et al. 2001). Ultra- sound will readily detect the predominantly cystic soft tissue abscesses. The degree of echogenicity can be very variable dependent on the quantity of debris and semisolid contents (Struk et al. 2001).

Colour or power Doppler will show absent blood flow within the abscess but hyperaemia in the wall and immediate surrounding soft tissues (Bureau et al. 1999). Both CT and MR imaging will demon- strate the abscess to good effect (Fig. 20.8), as well as confirming/excluding the presence of adjacent bone or joint involvement. In the immobile or chroni- cally debilitated patient pressure sores may result in decubitus ulcers over the ischia and greater trochan- ters with secondary infection of the bone. This can be identified on radiographs by the resorption of the cortex with varying degrees of bone destruction as well as gas within the soft tissues (Fig. 20.9).

The more diffuse form of muscle infection, pyo- myositis, may be a particular diagnostic problem if affecting the intrapelvic muscles (Orlicek et al.

2001; Peckett et al. 2001). Pyomyositis is uncom- mon in temperate climates unless, as stated, the patient is immunocompromised. Pyomyositis of the iliacus and obturator internus muscles in chil- dren tend to have non-specific presenting features.

MR imaging will show diffuse swelling and loss of definition of the muscle with or without discrete abscess formation (Yuh et al. 1988; Orlicek et al.

2001; Peckett et al. 2001).

20.6

Immunodeficiency Associated Infections

The worldwide epidemic of acquired immunodefi- ciency syndrome (AIDS) due to infection with the human immunodeficiency virus (HIV) is now the commonest cause of severe musculoskeletal infec- tions associated with immunocompromised states.

Other conditions causing immunodeficiency, such as diabetes mellitus, may be commoner and also associated with an increased risk of infection, but is rarely life threatening. Septic arthritis of the knee and hip is the most prevalent form of musculoskel- etal infection in HIV-positive patients (Tehranza- deh et al. 1996; Barzilai et al. 1999). Osteomyelitis is the second commonest infection in these patients with the femoral head the commonest site involv- ing the pelvis. Unlike conventional osteomyelitis, HIV related osteomyelitis can be associated with a

mortality of up to 20% (Tehranzadeh et al. 2004).

Staphylococcus aureus is the commonest causative pathogen but many other more exotic organisms have been reported including mycobacterial species.

A disease apparently unique to immunocompro- mised patients, particularly the HIV-infected popu- lation, is bacillary angiomatoid osteomyelitis due to gram negative Rickettsia-like bacilli (Bureau and Cardinal 2001; Tehranzadeh et al. 2004). This is a multisystem infectious disease which involves bone in approximately a third of cases producing aggressive lytic bone lesions with surrounding soft tissue inflammation (Baron et al. 1990).

The AIDS epidemic has been largely responsible for the resurgence of mycobacterium tuberculo- sis infections in the developed world over the past two decades. The commonest sites of involvement in descending order are the spine, the hip and knee joints, the metaphyses of the long bones of the lower limb and the hands and feet (Tehranzadeh et al.

2004). The commonest site in the pelvis is the sac- roiliac joint (see Chap. 19). A point worthy of note is that AIDS-related skeletal TB may have a multicen- tric distribution in about 30% cases.

Fig. 20.8. Axial fat suppressed post-contrast T1 weighted image through the pelvis demonstrates a tuberculous abscess within piriformis muscle

Fig. 20.9. AP radiograph in a paraplegic showing decubitus ulcers eroding both ischia and an old untreated left femoral neck fracture

be fatal. MR imaging is excellent in demonstrating the epicentre of infection, necessary when deter- mining treatment by aspiration or surgical drainage (Major and Tehranzadeh 1997; Steinbach et al.

1993).

20.7

Parasitic Infections

Parasitic infestations that may calcify or cause cal- cification and thereby be visible on radiographs of the pelvis include cysticercosis and guinea worm in the muscles and schistosomiasis in the urinary tract. The calcified dead cysts of cysticercosis are typically oval with a lucent centre, up to 1 cm in length, and oriented in the direction of the muscle fibres (Fig. 20.10a). The dead guinea worm is seen as long coiled or curled calcifications that in time can break up due to the action of the surrounding muscles (Fig. 20.10b).

The only parasitic infection to involve the bones of the pelvis is hydatid disease most commonly due

epiphyses of long bones, the ilium, skull and poste- rior ribs (Beggs 1985; Merkle et al. 1997; Abd El Bagi et al. 1999). The radiographic appearances are those of a multiloculated “soap bubble” lesion with marginal sclerosis usually arising in the ilium (Abd El Bagi et al. 1999).

20.8 Conclusion

Musculoskeletal infections of the pelvis can present in a large variety of ways. Early diagnosis depends on a high index of clinical suspicion and early use of cross sectional imaging, particularly MR imag- ing (Hammond and Macnicol 2001; Macnicol 2001). In problem cases where the imaging features cannot clearly differentiate infection from tumour the old adage “Biopsy the infection and culture the tumour”, cannot be overemphasized. Culture and sensitivity is important as many different organ- isms can give rise to similar imaging features in the pelvis.

Fig. 20.10. a AP radiograph of the hip showing the oval calcifi cations of cysticerco- sis, aligned with the muscle fi bres. b AP radiograph of the hip showing the linear and coiled calcifi cations of dead guinea worms

a b

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