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CaseReport

Correspondence to: Jiraporn Sriprapaporn E-mail: sijmh@mahidol.ac.thReceived 7 December 2011Revised 12 March 2012Accepted 13 March 2012

INTRODUCTION

tressfracturesaredividedintotwotypes.Afatigue fractureoccursinanormalbonewhenthebone is overused, while an insufficiency fractureoccurswhenanormalstressisappliedoveranabnormalstructuralbonesuchasanosteoporoticbone.1 Fatiguefracturesthatareusuallyfoundinmilitaryandathleticpopulationsarethemostimportantstressfracturesfoundinclinicalpractice.Diagnosisofsuchconditionsissomewhatunderestimatedandsometimesmaynotbesimple. Theauthorreportsayoungmanwithahistoryofheavyrunningabout10km/dayforafewmonths,pre-sentingwith left shinpain.MRIfindingssuggestedaninfectiveorneoplasticprocess,whichrequiredbonebiopsy,butthree-phaseradionuclideboneimagingimpliedastressfracture,whichavoidedanunnecessarybiopsyprocedure.

CASE REPORT

A19-year-oldpolicecadetwithahistoryofheavyrunningabout10km/dayforafewmonthspresentedwithleftshinpainespeciallyduringexerciseforafewweeks.Hehadnootherhistoryoftraumaticinjurybefore.Onphysicalexamination,hehadnofever,nobruiseovertheleftleg,buthehadapointoftendernessattheposteromedialaspectofhisleftlegwithoutsignsofinflammation. Hisfirstplainradiographsataprivateclinicwereclearlynormal.Hehadobligatorilycontinuedthesametrainingprogramandthepaintendedtobeaggravated.

Tibial Stress Fracture: A Diagnostic PitfallJiraporn Sriprapaporn, M.D.Department of Radiology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand.

ABSTRACT

Theauthorreportsayoungpolicecadetwithahistoryofheavyrunningabout10kmperdayforafewmonths,presentingwithleftshinpainespeciallyduringexercise.HisfirstplainradiographwasnothelpfulandhisMRIfindingsfavoredaninfectiveorneoplasticprocess.However,three-phasebonescintigraphysuggestedastressfractureresultinginavoidanceofadispensablebiopsyprocedure.

Keywords:Stressfracture,bonescan,bonescintigraphy,pitfall

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Furtherlaboratoryinvestigationsfoundnormalcompleteblood counts, normal erythrocyte sedimentation rate,suggestingsosignsofinfection.Hismagneticresonanceimaging (MRI) fromanother imagingcenterperformedonemonthafteronsetof the symptomdemonstratedapatchylesioninthemedullaofthemidlefttibiawithiso-to-lowsignalonT1-weightedimagesandhighsignalonT2-weightedandSTIR(shorttauinversionrecovery)imageswithmoderategadoliniumenhancement.Therewasalsosofttissueedemaattheposteromedialaspectofthelesion,whichsuggestedaninfectiveorneoplasticprocess.(Fig1)Hethenwasreferredforbonescintigraphyforwhole-bodyevaluation.Athree-phasebonescintigraphofbothlegswasobtainedusinganintravenousinjectionof20mCiTc-99mmethylenediphosphonate(Tc-99mMDP).The technique included 120-second immediate vasculardynamicimaging,5-minutestaticblood-poolimages,anddelayed 3-hour bone images. The study revealedmildhyperemiatotheleftleg,withfocalincreaseduptakeatthemedialaspectofhisleftcalf,andfusiformincreaseduptakeinthecortico-medullaryregionattheposteromedialaspectatmidshaftofhislefttibia(Fig2).Theskeletonelsewhere appeared unremarkable. These findingswerecompatiblewithgradeIIItibialstressfractureaccordingtoZwas’ classification.2Theserialplainradiographafewdaysfollowinghisbonescandemonstratedasolidperios-tealreactionathisposteriorcortexofthelefttibia(Fig3).Nevertheless,thepatientwasconcernedaboutabonetumor,thusabonebiopsywasthenscheduled. Whilewaitingforabonebiopsyappointment,hehadbeenadvisedtodiscontinuerunningactivity.Afterthat,hispainwasgraduallyrelieved.Serialplainradiographsoftheleftlegonemonthlaterdemonstratedprogressiveperiostealthickening,whichfavoredahealingprocessofthestressfracture.Thus,bonebiopsywascanceledandafollow-up3-phasebonescintigraphy(Fig4)wasobtainedatthethree-monthinterval.Althoughhehadbeenback

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foranintermediatetrainingprogramrecentlypriortothesecondbonescanning,thestudyconfi rmedmuchimprove-mentwithcorrespondingsclerosisalongtheposteromedialaspectofhislefttibiaseenonSPECT/CTimages(Fig5).

DISCUSSION

Stressfracturesarecommonoveruseinjuriesamongathletesandmilitaryrecruits.Thetrueincidenceofstressfracturesishardtodetermineandtheytendtobeunder-estimatedsincemanyofthemmightbeleftunrecognized.However,itwasestimatedthatstressfracturesaccountfor0.7%-20%ofallsportsmedicine injuries.3Femalesaremoresusceptiblethanmalestodevelopstressinjuriesoftheboneespeciallyinthemilitarypopulation.4-5 Approximately75%ofstressfracturesoccurrbeforetheageof40years.6Inaddition,thetrendofsuchcondi-tioninyoungadultshasbeenincreasingduetoincreasedparticipationinsportingactivities.7Onestudyalsofoundthat9%offracturesoccurredinchildrenagedlessthan15yearsand32%inthosebetween16-19yearsofage.8The site of fractures depends on the type of activityinvolved.9-10Runninginjuriesplayanimportantrolefordevelopingstressfracturesinbothathletesandmilitarypopulations.Most stress fractures occur in the lower

extremities,especiallyinlong-distancerunners.Runningmorethan20milesaweekisatincreasedrisktodeveloplower-extremityinjuries.11Thisinjurytypicallyoccurs6to8weeksafterachangeintrainingdurationorintensity,but can occurwith repetitive stress alone.12 Themost

Fig 1.CoronalMRIimagesoflefttibiashowedpatchylesioninthemedullaofmidlefttibiawithiso-to-lowsignalonT1-weightedimages(A)andhighsignalonSTIRimages(B)withmoderate gadoliniumenhancement (C).Associated soft tissueedemaatposteromedialaspectofthelesionwasalsonoted.

Fig 2.Three-phasebonescintigraphyrevealedmildhyperremiatotheleftleg(A)withfocalincreasedblood-poolactivityatthemedialaspectofleftcalf(B).Thedelayed3-hourimagesintheanteriorandlateralprojectionsshowedfocalfusiformincreaseduptakeinthecortico-medullaryregionatposteromedialaspectatmidshaftoflefttibia.(C)Thesefi ndingsarecompatiblewithgradeIIItibialstressfracture.

Fig 3.Serialplainradiographs(AP-Lateral)oflefttibiarevealedperiostealreactionatposteriortibialcortex.

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commonsiteofstressfracturesinmostreportsisinthetibia,whichmaybeashighas72%.2,13-14 Diagnosisofstressfracturesprimarilydependsonahigh indexofsuspicionsince itmaybeproblematicbecauseseveralmusculoskeletaldisorderscancauseexer-cise-inducedlegpainsuchassofttissueinjuries,medialtibialstresssyndrome(MTSS)orshinsplints,arteryornerveentrapment,compartmentsyndrome,bonetumors,andboneinfection.15-16Theclassicclinicalfeatureofastressfractureisinsidiousonsetofactivity-relatedlocalpainwithweightbearing.Thepain is relievedbyrestandbecomesworsewhentheactivityiscontinued.Localtendernessandswellingareoftenfoundat thefracturesite.16Nevertheless,stressfracturesshouldbeconsideredinanypatientwhopresentswithlocalpainafterarecentincreaseinactivityorrepeatedactivitywithlimitedrest.9,17 Earlydefi nitediagnosisisessentialforpreventingprogressionoflesions,avoidingcomplicationsandguidingpropermanagement.Sinceclinicaldiagnosismaybein-conclusive,variousimagingtestsshouldbeperformedto

confi rmstressfractures.TheseimagingmodalitieshavetheirownadvantagesanddisadvantagesaslistedinTable1. Plainradiographyisusuallythefi rstimagingmodalityobtainedbecauseofitsavailabilityandlowcost.19Plainradiographisusuallynegativeinitially,butismorelikelytobecomepositiveovertime.21Theinitialplainradiographhasquitelowsensitivityforstressfracturesrangingbetween10%-30%,asinthiscase.2,14,18Onfollow-upexaminations,thesensitivityofradiographsishigherwithreportedvaluesrangingbetween40%to54%.2,19 Radiographicmanifestationsoftibialstressinjuriesincludedecreasedcorticaldensity,socalled“graycortex” sign,periostealreaction,endostealthickening,andacorti-calfractureline.16,20 Iftheinitialradiographisnegative,moreadvancedimagingmodalitiessuchasthree-phasebonescintigraphyandMRImay be helpful for further evaluation. Bothmodalitieshavesimilarlyhighsensitivityfordetectionofstressfractures,butMRIhasgreaterspecifi city.16Radio-nuclideboneimaginghadtraditionallybeenthestandardtooltoconfi rmstressfracturesinmoststudiespriortothedevelopmentofMRI,becauseofitshighsensitivity.14,21-22

Fig 4.Thefollow-up3-phasebonescanofthelegintheanteriorviewshowedsymmetricalbloodfl owtobothlegs(A),almostnormalblood-poolimage(B),andlessintenseuptakeatlefttibialshaft,suggestingimprovementofstressfracture.

Fig 5.SPECT/CTimagesoftheleg(acquiredwith15second/stepx60 steps, 360degreeof rotation forSPECTand130kV,50mAsforCT)demonstratedincreaseduptakealongtheposteromedialaspectoflefttibialshaftwithassociatedsclerosisobservedonthecorrespondingCTscan.

Imaging Advantages Disadvantages -PoorinitialsensitivityPlainradiography -Lowestcost -Someradiationexposure -Widelyavailable -Limiteddifferentialdetails -Limitedavailability -LimiteddifferentialdetailsBonescinitigraphy -Highercost -Someradiationexposure -Highsensitivity -Canbefalselypositiveinbonyinfectionortumor -Bestdifferentialdetails -Noradiationexposure -HighestcostMagneticresonanceimaging(MRI) -Highestdifferentialdetails -Limitedavailability -Highestspecifi city -Sometimesmaybefalselypositiveinbonyinfection -Equalorslightlybetter ortumor sensitivitythanscintigraphy

TABLE 1.Advantagesanddisadvantagesofimagingmodalitiesforstressfractures(modifi edfromPatel,etal.)16

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However, nowadays,MRI is currently considered themethodofchoiceforthispurposeduetoitshighimageresolutionofbone,bonemarrow,andsurroundingsofttissue,resultinginbetteranatomicalevaluationandhigherspecificityascomparedtobonescintigraphy.23-25 Bone scintigraphy is really sensitive andmaybepositiveasearlyas48-72hoursafterinjuries.26Inacutestressfractures,allthreephasesofthebonescanareposi-tive.Zwas,etal.,developedascintigraphicclassificationofstress fractures,whichwasdivided into fourgradesaccordingtolesiondimension,boneextension,andtraceraccumulationinthelesions.2 Three-phase bone scintigraphy is also helpful inthedifferentiationbetweenstressfracturesandMTSS.InMTSS,onlythedelayedthirdphaseispositiveandtendsto be longitudinally-oriented along the posterior cortexof the tibia. Incontrast, the increaseduptake in stressfracturestendstobemorefocal(withfusiformshapeinadvancedstages).27 Apart from planar bone imaging, SPECT (singlephoton emission tomography) acquisition, which is a3-dimentionalimagingtechnique,whichprovidesbetterdelineationofbonylesions,canbeused.CombinedSPECTandcomputedtomography(CT)imagingtogetherinthesame instrument, so called SPECT/CT scanning, addsevenmore anatomical details over the SPECT imagesalone.Todate,moreandmoreSPECTandSPECT/CTimaginghavebeenusedinthefieldofSportsMedicinetoevaluateseveralpartsofthebody,suchasthespine,hips,knees,andfeet.28-29 AlthoughMRI, isnow thegold standard for thediagnosisofstressfractures,itmightbeoflimitedavail-abilityduetoalongwaitinglist,especiallyinmostbusytertiarycenters.Furthermore,theMRIabnormalitiessuchasperiostealreactionandbonemarrowedema,ifthereisnodemonstrationofafractureline,arenonspecific.30Thus,anunawareradiologistmaybeconfusedwithotherconditionssuchasosteomyelitisorinfiltrativeneoplasms.Asinthereportedcase,MRIresultswereindicativefortumorpathology,sotheattendingclinicianhadscheduledbonebiopsyforpathologicalevidence. Prior to performing a biopsy, one should alwayskeepinmindthatthisprocedureshouldbestrenuouslyavoidedinacasewithpotentialstressfracturesincethisprocedurecandisturbbonehealingandthebiopsyspeci-menmaycontainimmaturecellsandosteoid,relatedtothehealingprocess,whichcouldbemistakenasmalignancies.Follow-up imagingstudies in thenext fewweeksmayrevealcharacteristicfindingsofstressfracturesandthusavoidanunnecessaryinvasiveprocedure.9,31

CONCLUSION

AlthoughMRIiscurrentlyacceptedastheimagingof choice for evaluationof stress fractures, it still hassomepitfalls.Three-phasebonescintigraphyontheotherhand,isgenerallylessspecific,butcarefulinterpretationalongwithpreciseclinicalcorrelationcanovercomethislimitationresultinginaccuratediagnosisforstressfractures.Therefore,three-phasebonescanningcouldbeusedsolelyorasanadjunctwhentheMRIfindingsarenon-diagnosticincasespresentingwithhighclinicalsuspicionofstressfractures.

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