Cartilage cap thickness is used as a primary criterion on MRI. The majority of benign osteochondromas have a cartilage cap less than 1 cm thick and the majority ...viewallclinicsClinicalHistory:A19year-oldmalepresentswithdistalfemurpainfromarunninginjury1weekago.Whatarethefindings?Whatisyourdiagnosis?1a1b1cFigure1:APandlateralradiographsoftheknee(1A),T1-weightedcoronalimagethroughbothknees(1B),andSTIRcoronal(left)andfat-suppressedprotondensity-weightedaxial(right)images(1C). Findings:2a2b2cFigure2:(2a)TheAPandlateralradiographsofthekneedemonstratewideningofthemetaphyseswithmultipleosteochondromas(arrowheads).(2b)TheT1-weightedcoronalimagesdemonstratebilateralosteochondromas(arrowheads).(2c)TheSTIRcoronal(left)andprotondensity-weightedfat-suppressedaxial(right)imagesrevealafracture(arrows)atthebaseofapedunculatedosteochondromaofthefemurwithadjacentsofttissueedema.Thefractureisnotvisibleontheplainfilms. DiagnosisHereditarymultipleexostoses(HME)withfracture. IntroductionHereditarymultipleexostoses(HME)isanautosomaldominantdisorderinwhichmultipleexostoses,orosteochondromas,form.Thelesionsarecausedbyseparationofacartilagefragmentfromtheepiphysealgrowthplatewhichherniatesthroughtheperiosteumsurroundingthegrowthplate.Thefragmentgrowsandundergoesenchondralossificationresultinginasubperiostealbonyprotuberancewithacartilagecap.1 Thedisorderwasfirstdescribedin1786andafamilialoccurrencewasfirstrecordedin1814.1,2 Around60to70%ofpatientshaveapositivefamilyhistory.2 HMEhasalsobeenseeninotheranimals,includinghorses,cats,dogsandlizards.2Mostpatientsarediagnosedbetweentheagesof5and12andsomecanbediagnosedatbirth.1 Osteochondromascontinuetogrowuntilthechildreachesskeletalmaturityatwhichpointnonewlesionswilloccur.2 Spontaneousregressionhasbeenreportedinchildhoodandpuberty,particularlyfollowingafracture.1,2,3 ComplicationsandImagingOsteochondromas,whethersingleormultiple,aretypicallyasymptomaticunlesscomplicationsoccur.MostcomplicationsarecommontobothHMEandsolitaryexostosesbutaremorecomm