Musculoskeletal infections: ultrasound appearances

PICTORIAL REVIEW

Musculoskeletal infections: ultrasoundappearances

C.L.F. Chaua,*, J.F. Griffithb

Departments of aRadiology, North District Hospital, NTEC, Fanling, and bDiagnostic Radiology and OrganImaging, Prince of Wales Hospital, NTEC, Shatin, NT, Hong Kong, Republic of China

Received 23 May 2003; received in revised form 2 February 2004; accepted 6 February 2004

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KEYWORDSSoft tissues;Ultrasound;Review;Cellulitis;Bones, infection

09-9260/$ - see front matter q 200i:10.1016/j.crad.2004.02.005

* Guarantor and correspondent: C. Ldiology, Baptist Hospital, 222 Watng, China. Tel.: C852 2683 7308; faE-mail address: [email protected]

Musculoskeletal infections are commonly encountered in clinical practice. Thisreview will discuss the ultrasound appearances of a variety of musculoskeletalinfections such as cellulitis, infective tenosynovitis, pyomyositis, soft-tissueabscesses, septic arthritis, acute and chronic osteomyelitis, and post-operativeinfection. The peculiar sonographic features of less common musculoskeletalinfections, such as necrotizing fasciitis, and rice body formation in atypicalmycobacterial tenosynovitis, and bursitis will also be presented.q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rightsreserved.

Introduction

Musculoskeletal infections are commonly encoun-tered in clinical practice. Imaging is mainly used toevaluate moderate to severe infections, both as adiagnostic and therapeutic aid. Ultrasound may beused either as the primary imaging technique or asan adjunct to radiography, computed tomography(CT), magnetic resonance imaging (MRI) andnuclear medicine studies.

Ultrasound may help to (1) differentiate acute orchronic infection from tumours or non-infectiveinflammatory conditions with a similar clinicalpresentation; (2) localize the site and extent ofinfection (e.g. subcutaneous, muscle, bursa, ten-don sheath, joint); (3) ascertain the form ofinfection (e.g. cellulitis, pre-abscess, abscess); (4)identify precipitating factors (e.g. foreign bodies,fistulation); and (5) provide guidance for diagnosticor therapeutic aspiration, drainage or biopsy.1 Thisreview focuses on the application of ultrasound invarious forms of musculoskeletal infection.

5 The Royal College of Radiolo

. F. Chau, Department oferloo Rd, Kowloon, Hongx: C852 2683 7379.(C.L.F. Chau).

Infections of superficial, deep, and specific tissues(bursa, tendons, and joints) will be discussed.

Cellulitis

Cellulitis is a spreading inflammatory reactionoccurring along subcutaneous and fascial planeswith oedema and hyperaemia.2 Cellulitis is probablythe most common form of musculoskeletal infec-tion encountered in hospital practice. It usuallyresults from a Streptococcus pyogenes or Staphy-lococcus aureus infection.1 Predisposing factorsinclude stasis, poor general health, skin lacerationor ulceration, venepuncture, eczema, and immu-nosuppression (Fig. 1).

The ultrasound appearances of cellulitis varyaccording to the site and severity of infection.Ultrasound appearances range from diffuse swellingand increased echogenicity of the skin and subcu-taneous tissues (Figs. 1 and 2(a)), to a variablecobblestone appearance depending on the amountof perifascial fluid, the degree of subcutaneousoedema and the orientation of the interlobular fatsepta. These grey-scale appearances are not, inthemselves, diagnostic of cellulitis as similarappearances occur in subcutaneous oedema

Clinical Radiology (2005) 60, 149–159

gists. Published by Elsevier Ltd. All rights reserved.

Figure 1 A 27-year-old intravenous drug abuser withleft forearm swelling and fever for 1 week. Split-screentransverse ultrasound image reveals diffuse hyperechoicthickening of the subcutaneous fat interlaced withhypoechoic strands indicative of cellulitis. A subcu-taneous vein is thrombosed containing non-compressiblehypoechoic thrombus. The muscle is normal.

C.L.F. Chau, J.F. Griffith150

resulting from non-infective conditions such asvenous insufficiency or cardiac failure. Colour orpower Doppler imaging to show concurrent hyper-aemia within the subcutaneous tissues is helpful inestablishing an inflammatory element. Hyperaemiais not a feature of non-infective forms of oedema.Cellulitis may occur in conjunction with superficialthrombophlebitis (Fig. 1) and may progress to pre-abscess (Fig. 2(b)) or abscess (Fig. 2(c)) formation.

Figure 2 A three-year-old patient with painful posteriorthigh swelling and erythema with central punctum for 3days. A series of longitudinal ultrasound examinationsrevealed (a) increased thickening and echogenicity of thesubcutaneous tissues. The muscle layer is normal. (b)Antibiotic treatment was started and 3 days later, somecoalescence and fluid accumulation towards the centre ofthe subcutaneous tissues akin to a pre-abscess state(arrows) is apparent. (c) Two days later, there is morepronounced liquefaction (arrows) of the subcutaneoustissues. Subcutaneous swelling is less pronounced than onearlier examinations. 1 ml of purulent blood-stained fluidwas aspirated.

Necrotizing fasciitis

Necrotizing fasciitis is an uncommon, severe formof cellulitis characterized by a rapid clinicaldeterioration and death approaching 50% ifuntreated.3,4 It usually affects the lower extremi-ties. Patients typically experience severe pain,disproportionate to the severity of cellulitis.Dusky cutaneous discoloration with purpuricpatches may occur.5 Group A streptococci are themost common offending organisms. Pathologically,severe inflammation and necrosis of the subcu-taneous tissues and the underlying investing fascia6

characterize necrotizing fasciitis. Necrosis may bethe result of microcirculation thrombosis andischaemia.5,7 The skin and muscle are spared inthe early stage of disease.6 Muscle necrosis occurswith involvement of the investing fascia. Theaccuracy of ultrasound in diagnosing of necrotizingfasciitis has not been fully established, a reflectionof the relative rarity of this condition. The reportedultrasound appearances are thickened distortedfascia with perifascial hypoechoic fluid collectiontogether with variable swelling of the subcutaneoustissues and muscle (Fig. 3). Early muscle necrosis

may be apparent (Figs. 3 and 4). Parenti et al.8

retrospectively reviewed the ultrasound appear-ances of 32 pathologically proven cases of necrotiz-ing fasciitis. Ultrasound revealed changes in the

Figure 3 A four-year-old patient with fever, pain andright leg swelling for a few days. Split-screen ultrasoundof right and normal left leg reveals marked subcutaneouscellulitis with thickening and increase echogenicity ofsubcutaneous fat (indicated by calipers). A hypoechoicabscess is present just deep to the disrupted investingfascia (arrows). The pre-tibial juxtacortical tissues arealso very swollen. TZtibia. FZfibula. Surgical explora-tion revealed necrotic fascia, a small fascial abscess andearly focal necrosis of adjacent muscle, consistent withnecrotizing fasciitis. Tissue culture yielded group Astreptococci. The patient responded well to debridementand antibiotic therapy.

Figure 4 An eighteen-month-old patient with fever,knee pain and left distal thigh swelling for one week.Initially treated by a herbalist. Split-screen ultrasound ofabnormal left and normal right distal thigh demonstratesa lobulated isoechoic abscess with a hypoechoic rimlocated superficial to the investing fascia. There is a focaldiscontinuity of the fascia with extension of the abscessinto the vastus lateralis muscle. Surgical explorationconfirmed necrotizing fasciitis with operative findingscorresponding to the ultrasound findings. Tissue culturegrew Staphylococcus aureus. Soon afterwards a severeosteomyelitis of the proximal tibia became apparent (notshown). This did not respond well to treatment. A residualpseudoarthrosis of the tibia remains.

Musculoskeletal infections: ultrasound appearances 151

subcutaneous fat (28 of 32), investing fascia (18 of32) and muscle (15 of 32), which correlated wellwith histological findings. However, in some casesultrasound did not reveal histologically apparentinflammation in the subcutaneous tissues (three of32) or muscle (eight of 32).8 Ultrasound-guidedaspiration of perifascial fluid can help isolate thepathogen.9 Successful treatment requires earlyrecognition, aggressive antibiotic therapy andadequate surgical debridement.

Infective bursitis

Although chronic bursitis is common, in our experi-ence it is usually non-infective.10 Bursitis usuallycomprises a sterile inflammation of the bursal wallas a result of chronic repetitive trauma or unduemechanical stress. When infection is present, S.aureus is usually the offending organism and asuperficial bursa, such as the olecranon or pre-patella bursa, the most commonly affected.1,5

Ultrasound reveals peribursal oedema, bursal wallthickening and distension by fluid or gelatinousmaterial of mixed echogenicity. Occasionallyinternal debris and calcification may be apparent.Colour Doppler imaging may reveal bursal wallhyperaemia. The ultrasound appearances of infec-tive bursitis are considered identical to those of achronic inflammatory non-infective bursitis, (whichmay be accompanied by intra-bursal bleeding),especially if a relatively non-virulent pathogensuch as tuberculosis is responsible. Rice body,comprised of aggregates of fibrin, may be a featureof typical or atypical tuberculous burstitis (Fig.5).11,12 When bursal inflammation cannot be ade-quately explained by the clinical history, bursalaspiration for culture is recommended to excludean infective element.

Infective tenosynovitis

Infective tenosynovitis is most commonly the resultof penetrating trauma and as such occurs predomi-nantly affects the hands and wrists.10 The flexortendons are more commonly affected.5 Familiariz-ation with tendon sheath anatomy and communi-cation is essential to understanding the spread oftendon sheath infection in the hand and wrist.13 Theflexor tendon sheaths of the thumb and little fingercommunicate, respectively, with the radial andulnar bursa on the volar aspect of the wrist andcarpus. In half of cases, the radial and ulnar bursacommunicate via an intermediate bursa. There are

Figure 5 A 50-year-old man with left wrist swelling for1 year. Transverse ultrasound of wrist shows low-levelinternal echoes without acoustic shadow located indeeper portion of the radio-ulnar bursa and surroundingflexor digitorum superficialis and profundus (P) tendons.Discrete rice bodies cannot be appreciated. RZradius,UZulna. (b) Axial fast spin-echo MR image (repetitiontime 3800/echo time 105) demonstrates multiple smallhypointense rice bodies (short arrows). These rice bodiesalmost completely fill the radioulnar bursa and are muchmore readily appreciable than on ultrasound (a). PZflexor digitorum profundus tendons. RZradius, UZulna.

Figure 6 A 37-year-old intravenous drug addict withhistory of recurrent lung abscess and septic arthritis ofknee with wrist pain for 1 week. Ultrasound revealedmarked tenosynovitis affecting the extensor carpi ulnaristendon with moderate tendon thickening, severe tendonsheath thickening and moderate peritendinous soft-tissueoedema. Colour Doppler (not shown) revealed diffusehyperaemia of the tendon sheath (indicating that itrepresents synovial inflammation rather than an echo-genic effusion) and peritendinous tissues.

Figure 7 A 74-year-old man with chronic renal failurewho presented with wrist swelling for 8 days. Ultrasoundrevealed a severe suppurative-type tenosynovitis of theextensor digitorum tendons from the wrist joint to themetacarpophalangeal joints. Note the echogenic fluidcollection located superficial to the thickened extensortendons (T). No tendon sheath encases the extensortendons normally at this location. The extensor carpiradialis brevis and longus tendons (not shown) were alsoinvolved. Percutaneous aspirate grew group G strepto-cocci. M, metacarpal shafts.


six discrete extensor tendon synovial sheaths thatcommunicate neither with each other or the flexortendons.14 Acute suppurative tenosynovitis isusually caused by S. aureus and S. pyogenes.

Ultrasound can demonstrate variable thickeningof the tendon and tendon sheath with hyperaemiamost commonly of the tendon sheath though alsooccasionally within the substance of the tendon.Tendon sheath thickening is usually hypoechoic andas such may resemble viscous fluid (Fig. 6). ColourDoppler is particularly helpful in this instanceallowing one to differentiate synovial sheaththickening from a synovial sheath effusion.

This distinction is important as the demon-stration of an effusion allows one to considerdiagnostic aspiration (for culture and sensitivity)and may also influence decision making as towhether urgent surgery should be undertaken.Surgery may potentially be less beneficial in thosepatients without visible tendon sheath effusion.Unequivocal tendon sheath effusions are usually nota dominant feature though small pockets of fluidmay be present with more pronounced fluidaccumulation being a feature of severe acutesuppurative tenosynovitis (Fig. 7). Occasionally

Figure 8 An 11-year-old patient with fever and leftelbow region swelling of a few days, 3 weeks aftersustaining blunt trauma to this area. (a) Transverseultrasound reveals a very swollen hyperechoic brachialismuscle compatible with acute myositis. The overlyingbiceps muscle is only mildly swollen. No biopsy or surgicalintervention was performed. (b) Follow-up ultrasound at4 months after completion of antibiotic treatment, at aslightly proximal level to 8a. The brachialis (Bra) anddistal biceps (Bi) muscles are now normal.


infection can spread from the tendon sheath intothe peritendinous tissues.

Figure 9 A 73-year-old woman with fever and rightshoulder pain. Transverse ultrasound of the right infra-spinatus muscle. There is large well-defined hetero-geneous hypoechoic abscess collection within theinfraspinatus muscle. Although the abscess appearsreasonably solid, percutaneous aspiration yielded 10 mlof purulent material. Culture revealed Enterobactercloacae.

Mycobacterial tuberculous tenosynovitis

Both typical and atypical mycobacterial tubercu-lous tenosynovitis may be associated with rice bodyformation.11,12 If rice bodies are small, ultrasound

may reveal low-level internal echoes but may fail toresolve individual rice bodies (Fig. 5(a)). In thatsituation, MRI is recommended for better delinea-tion (Fig. 5(b)).12 Whenever the possibility of aninfective tensosynovitis exists and a tendon sheatheffusion is visible, ultrasound-guided aspiration oftendon sheath fluid is helpful in differentiatinginfective from non-infectious causes of tenosyno-vitis. If moderate to severe tendon sheath thicken-ing is present, percutaneous biopsy may beundertaken.

Pyomyositis

Pyomyositis is a suppurative bacterial infection ofmuscle,1,5 most commonly affecting the largermuscles of the lower limbs.5 Pyomyositis is moreprevalent in the tropics and in immunocompro-mised patients.5 Muscle trauma and haematomamay be precipitating factors. Clinically, there isfever, myalgia and localized muscle tenderness. S.aureus is the causative organism in more than 90%of cases.1,5

Ultrasound of pyomyositis reveals diffuse muscleswelling with oedema, the latter manifested byeither diffuse muscle hyperechogenicity with orwithout localized hypoechogenicity (severe muscleoedema or early necrosis) and diffuse hyperaemia.1

At this stage, pyomyositis will usually response toantibiotic treatment (Fig. 8). If untreated, muscleabscess formation will often subsequently occurrequiring surgical or percutaneous drainage (Fig. 9).

Figure 10 A 51-year-old diabetic man with lefttuberculosis empyema, complicated (after removal ofpercutaneous chest drain) by fistula to chest andabdominal wall with abscess formation. During cleaning,a wooden probe was accidentally retained inside theabscess cavity. (a) Transverse ultrasound image of theabscess reveals the echogenic wooden probe (indicatedby calipers) with acoustic shadowing. Adjacent smallechogenic gas locules show no acoustic shadowing. Post-contrast abdominal CT (not shown) revealed a leftposterior muscle abscess cavity and a wooden probe(the latter with a similar attenuation to gas). (b) Thewooden probe, including a small broken fragment, isshown after removal with a forceps under ultrasoundguidance.

Figure 11 A 72-year-old woman with a right arm massfor 1 year that was slowly increasing in size. Obliquelongitudinal ultrasound reveals an irregular lobulatedsubcutaneous mass with internal echoes and posterioracoustic enhancement (large arrows). There was moder-ate vascularity throughout the mass, though particularlyat the periphery (not shown). There is also a small lymphnode (small arrows) present alongside the brachialneurovascular bundle. Percutaneous guided biopsy ofthe mass revealed caseating granuloma with mycobac-teria. AZbrachial artery.


Abscess formation

An abscess is a localized collection of necrotictissue, bacteria, inflammatory exudate and poly-morphs.15 Abscesses most commonly occur withineither muscle or subcutaneous tissue. Abscesses areusually round in shape though may be tubular orgeographical. Depending on location, maturity and

contents, abscess echogenicity can vary fromhypoechoic to isoechoic or hyperechoic.16 Posterioracoustic enhancement is characteristic. Internaldebris is a common feature while gas loculationsmay be seen with gas-forming organisms or if thereis open communication with bowel or skin (Fig. 10).Internal septa are a feature of more chronic, low-grade infection. Colour Doppler imaging usuallyreveals variable hyperaemia of the abscess wall andimmediate surrounding tissues. Arslan et al.17

reported that 19 out of 21 soft-tissue abscessesdisplayed wall hyperaemia apparent on by powerDoppler imaging. Breidahl et al.18 found hyperae-mia to be a feature of inflammatory rather thannon-inflammatory collections. Movement of thepart under examination or dynamic compression,if tolerated, can help identification of the fluidcomponent of the abscess. The threshold foraspiration should be low particularly as muscleabscesses may appear quite solid with no clearlydiscernible fluid yet still yield pus on aspiration(Figs. 9 and 11). The extent of the abscess,especially with respect to the adjacent bone,joint and internal structures (Figs. 12 and 13)should be carefully assessed. Therapeutic interven-tion (single or repeated aspiration, or catheterdrainage) can be readily performed under ultra-sound guidance.

Figure 12 A 36-year-old woman with systemic lupuserythematosus and mixed connective tissue disease onsteroid treatment presented with a left thigh mass of 3months duration. Split-screen ultrasound of abnormal leftand normal right thigh reveals well-defined subfascialhypoechoic collection with posterior acoustic enhance-ment. Surgical excision revealed this to be a mycobacter-ial tuberculous abscess.

Figure 13 A 50-year-old man with right posterior chestwall mass and fever for 5 days. Longitudinal ultrasound ofchest wall reveals a hypoechoic abscess in the chest wall(arrowheads), extending in the intercostal space tocommunicate with an empyema (arrows) (“empyemanecessitans”). Post-contrast thoracic CT (not shown)confirms the right pleural empyema. Percutaneousaspiration yielded a small amount of purulent fluid. Nogrowth was obtained on culture. The patient respondedto antibiotic treatment. RZribs, LZlung.

Figure 14 A 36-year-old intravenous drug abuser withright hip pain of 1 week. Longitudinal (a) and transverse(b) ultrasound of the hip reveals moderate capsular,synovial thickening and a small joint effusion. Ultrasound-guided aspiration yielded purulent fluid that grew Micro-coccus sp.


Septic arthritis

Joint infection is a particularly serious conditiongiven the propensity for long term morbidity if notrecognized and treated early. Septic arthritis mostcommonly affects the hip, knee, shoulder, elbowand ankle.19 S. aureus is the most commoncausative organism, followed by group A strepto-cocci and streptococcal pneumonia. In neonates,group B streptococci and coliform bacteria areimportant causes. In children of 3 months to 5years, Haemophilus influenzae used to be acommon causative organism, but the incidencehasdeclined significantly becauseof vaccination.20,21

Radiographic changes comprise peri-articularsoft-tissue swelling, effacement and blurring ofperi-articular fat planes, effusion, and peri-articu-lar osteopenia (the latter reflecting peri-articularhyperaemia and increased osteoclastosis). By thetime joint-space narrowing becomes apparentradiographically, articular cartilage lysis is alreadywell-established and severe morbidity with prema-ture osteoarthritis, limitation of joint movement,and limb shortening is likely to ensue.

Ultrasound more than any other imaging tech-nique has enabled septic arthritis to be identifiedearly before significant cartilage lysis occurs. The

hallmark of septic arthritis on ultrasound is thepresence of a joint effusion in a patient with clinicalsigns of joint infection. Ultrasound enables recog-nition and guided-aspiration of joint fluid at anearly stage thereby allowing early diagnosis andtreatment of septic arthritis (Fig. 14). Joint fluid inseptic arthritis may be hypoechoic and clearlydemarcated from joint synovium and capsule (Fig.14) or hyperechoic and less clearly demarcatedfrom joint synovium or capsule (Figs. 15 and 16).Particular attention has been paid to the paediatric

Figure 15 A 83-year-old woman with a right elbowmass of 2 weeks’ duration. Longitudinal ultrasound ofradiohumeral joint demonstrates a large echogenic elbowjoint effusion. (HZhumerus, RadZradius). 3 ml of puswas aspirated under ultrasound guidance. Culture yieldedtuberculous mycobacterium. Subsequently, symptomsresolved with operative drainage and anti-tuberculousmedication.

Figure 16 A 57-year-old man post renal transplant onimmunosuppressant and steriod treatment. Intermittentright hip pain for several months. (a) Transverse ultra-sound of hip joint reveals a hypoechoic joint effusioncontaining echogenic debris. The joint capsule isthickened. (b) Transverse ultrasound of hip joint alsoreveals a distended hypoechoic ilioposas bursa containingechogenic debris. Ultrasound-guided aspiration of thejoint effusion was performed. Culture was negative.Crystal analysis was not performed. The patient wastreated with non-steroidal anti-inflammatory drugs withalleviation of symptoms. No antibiotic treatment wasgiven. The final diagnosis was a non-infective inflamma-tory arthropathy. Periarticular extension does not alwaysimply the presence of a septic arthritis.


hip. The anterior joint capsule of the normalpaediatric hip consists of anterior and posteriorlayers, mainly composed of fibrous tissue with onlya thin synovial membrane.22 In children, thedistance between the cortex of the femoral neckand the outer margin of the hip capsule should notbe greater than 5 mm or more than 2 mm thickerthan the contralateral normal side.22 In adults, athickness of 9 mm or more than 2 mm thicker thanthe contralateral normal hip is considered abnor-mal.23 For most joints, a normal ultrasound exam-ination has a strong negative predictive value forseptic arthritis.24 In a study of 96 children by Zawinet al.,24 all 15 surgically proven cases of septicarthritis of the hip had hip joint effusions demon-strable by ultrasound. Increased capsular vascular-ity can be seen on colour or power Doppler imaging.In an animal study, Strouse et al.25 demonstratedincreased synovial vascularity in about 50% of septicarthritis cases by power Doppler imaging. No kneeswith aseptic arthritis demonstrated increased vas-cularity. In clinical practice, demonstrable synovialhypervascularity proved a less useful marker ofseptic arthritis being demonstrable in only one of 11patients in one recent study.26 Contrast-enhancedimaging may improve the sensitivity of ultrasoundin this respect. In most instances, ultrasoundcannot reliably differentiate infective from non-infective causes of arthritis, and, in the appropriateclinical setting, aspiration of joint fluid for analysis


is helpful (Figs. 14–16). For joints with non-distensible capsules (sacroiliac, sternoclavicular,acromoclavicular joints), the absence of a visiblejoint effusion cannot be used to exclude septicarthritis and, if suspected, MRI (or CT) examinationtogether with guided joint aspiration should beundertaken.

Figure 17 A 7-year-old patient with fever, right anklepain and swelling. (a) Transverse ultrasound of the distaltibia reveals isoechoic juxtacortical soft-tissue thicken-ing (arrows). TZtibia. (b) Longitudinal ultrasound of theankle joint reveals small sympathetic-type joint effusion(*). Medial malleolus is outlined by arrows. MZmetaphy-sis, EZepiphysis, **Zarticular cartilage. TaZtalus. Thepatient responded well to antibiotic treatment. MRIexamination was not undertaken in this case.

Osteomyelitis

Acute osteomyelitis occurs more commonly in theskeletally immature. Osteomyelitis is usuallycaused by S. aureus in young patients, and byGram-negative bacteria in the elderly. Radiogra-phically apparent osteolysis or periosteal new boneformation may not become apparent for up to 2weeks after the onset of infection. Ultrasoundexamination may show features of osteomyelitisseveral days earlier than radiographs.27 Juxtacor-tical soft-tissue swelling together with early peri-osteal thickening is the earliest sign of acuteosteomyelitis on ultrasound (Fig. 17(a)). This isfollowed by increased periosteal thickeningaccompanied by, in up to two-thirds of cases, alayer of subperiosteal exudate, and more rarely,abscess formation.28,29 Finally, cortical erosion canbecome apparent30 (Fig. 18). Sympathetic jointeffusions (Fig. 17(b)) or co-existent inflammation ofjuxtacortical tissues can occur. Ultrasound exam-ination is likely to be most sensitive in children withsuspected acute osteomyelitis of the tubular boneswhere the propensity to develop a periostealreaction is greatest. Although the sensitivity andspecificity of ultrasound examination at diagnosingosteomyelitis has not been determined, it is notlikely to be as high as MRI or nuclear medicinestudies as visualization is limited to outer corticaland juxtacortical tissues only.19,31,32 Therefore, ifthere is clinical suspicion of osteomyelitis,especially when infection of a non-tubular bone orthe intra-articular portion of a tubular bone issuspected, either MRI (or phosphonate bone scinti-graphy) should be considered on an urgent basis. Inpatients with sickle cell disease, subperiosteal fluidcollections similar to those found in acute osteo-myelitis may occur with medullary infarction,although they are tend to be less pronounced thanin acute osteomyelitis.28 Ultrasound-guided aspira-tion is helpful in confirming an infective aetiol-ogy.28 Reactivation of chronic osteomyelitis may beassociated with soft-tissue abscess, fistula or sinustract formation, all of which may be apparent onultrasound (Fig. 19).

Post-operative infection

Examination of the soft tissues adjacent to ortho-paedic hardware by CT and MRI is often limited bymetallic artefacts. Similarly, the effects of post-operative repair tissue, altered mechanical stressesin bone and the photopenic areas resulting frommetallic shielding may impair the specificity ofnuclear medicine studies. Ultrasound is less ham-pered in many of these respects and as such is oftenthe preferred investigation in this situation pro-vided transducer access is possible (Fig. 20).Infection is a complication of 0.5–2% of hip andknee replacements. Early-onset infection is usuallythe result of pre-operative wound contamination.

Figure 18 A 28-year-old woman with on and off leftside face swelling for 3 weeks with trismus. Clinically aparotid abscess was suspected. Transverse ultrasound ofangle of mandible reveals focal cortical defect containinga hypoechoic abscess (callipers). There is moderatedegree of adjacent soft-tissue cellulitis. Lateral radio-graph for the parotid (not shown) revealed a well-definedmultiloculated lytic lesion in the right angle of mandible.Incision and drainage was performed. Culture grew S.aureus.

Figure 20 A 52-year-old man with a fracture of theright tibia sustained during a road traffic accident 3months earlier and treated by ring external fixator. Hesustained a minor fall 2 weeks earlier which was followedby persistent swelling of the wound. Colour Dopplerultrasound examination reveals increased vascularity of ahypoechoic abscess collection (arrows) overlying thefracture site. Subsequent surgical debridment confirmedthe presence of an abscess cavity. Culture was negative.


Late-onset infections (i.e. more than 3 monthsafter surgery) are usually the result of haematogen-ous seeding.3,33 Ultrasound examination is able todetect small increases in joint fluid or juxtacorticalfluid collection after arthroplasty or internalfixation and guide needle aspiration. After tendonrepair, ultrasound can help to differentiate tendon

Figure 19 A 54-year-old man with acute exacerbationof chronic osteomyelitis of left femur. Transverseultrasound of femur reveals a band of hypoechoicgranulation tissue (arrows) extending from femur to thecutaneous surface. Echogenic gentamicin beads (*) arepresent.

re-rupture from other causes of impaired mobilitysuch as infection.

Foreign body detection

Ultrasound can demonstrate wood, bamboo, glassor fishbone foreign bodies in soft tissues that maynot be apparent radiographically.34 Foreign bodiesmay be a cause of persistent infection. They areoften more clearly demarcated at a later stagewhen surrounded by hypoechoic reparative granu-lation tissue and, as such, if clinical suspicionremains high or if the infection fails to settle, arepeat ultrasound in about 3 weeks or sooner shouldbe performed. Ultrasound can demonstrate woodenfragments as small as 2.5 mm with 87% sensitivityand 90% specificity.34,35 Ultrasound can also be usedto assist percutaneous removal of foreign bodies byforceps36 (Fig. 10(b)).

Conclusion

Musculoskeletal tissues are, for the most part,accessible to examination by ultrasound, allowingits deployment as a first-line investigation in a widerange of musculoskeletal infections. Ultrasoundexamination, often coupled with ultrasound-guidedaspiration, frequently provides sufficient infor-mation to enable a firm diagnosis and guidetreatment. The judicious and appropriate use of


ultrasound should allow earlier recognition andtreatment of musculoskeletal infections, therebyhelping to minimize long-term morbidity. MRI (orCT) can be reserved for situations where ultrasoundaccess is limited or whenever extensive soft-tissueinfection is present.

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Musculoskeletal infections: ultrasound appearances