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Normalradiographsofthekneewithanteroposterior(a),lateral(b),andaxial(c)biewdemonstratenormalpatellarpositionandmorphology.Theanteroposteriorprojection(a)isusefulforevalutingthefemurandproximaltibia,femoralandtibialplateaus.Thelateralprojectionisusefulforevaluatingpatellarheight,patellofemoralcompartment,suprapatellarrecess(SR),quadricepstendon(QT),patellartendon(PT).Theaxialviewofthepatellahelpsinassessmentoftheshapeofthepatella,notemedia(MF)andlateral(LF)patellarfacetsandmedianridge(MR).Alsonotenormalandroughanteriorpatellarcortex(bluearrow).Fig.1:Normalkneeradiographs图1：正常膝关节Sagittalprotondensity(a)andaxialfat-suppressedT2-weighted(b)MRimagesofanormalknee.Notethelowsignalpatellar(PT)andquadriceps(QT)tendonsandthethick,homogeneous-appearingpatellarcartillage(redarrows).Notethelateralandmediaretinacula,passivestabilizersofthepatella.Fig.2:NormalMRimages图2：正常膝关节MR图像In1941,Wibergclassifiedpatellarshapeintothreedifferentmorphologies:TypeI(a)demonstratesroughlysymmetricandequal-sized,concavemedial(MF)andlateral(LF)patellarfacets.TypeII(b)showsamedialfacetthatisslightlysmallerthanthelateralfacetandaconcavelateralfacet.TypeIII(c)alsoshowsasmallerandmoreverticallyorientedmedialpatellarfacet,whichisassociatedwithmaltrackingdisorders[18].Fig.3:Variationsinpatellarmorphology图3：髌骨形态变异5-year-oldmalewithhereditaryosteo-onychodysplasia(nail-patellasyndrome).AP(a),later(b),andaxial(c)viewsofthekneedemonstratecompleteabsenceofthebilateralpatellarossificationcenters.Fig.4:Patellaraplasia图4：髌骨发育不良5岁男孩遗传性指(趾)甲-髌骨综合征（nail-patellasyndrome）Anteroposteriorandaxialradiographs(a)showbilateral,well-corticatedossifiedfragmentsinthesuperolateralaspectofthepatellas(arrows).CoronalandaxialT2-weightedfat-suppressedMRimage(b)showthewell-corticatedossifiedfragment.Notethenormalbonemarrowsignalandcartilageacrossthesynchondrisis,Thewell-corticatednatureofthefragmentandlackofabnormalmarrowsignalhelptodifferentiatethisentityfromapatellarfracture.Fig.5:Bipartitepatella图5：二分髌骨Anteroposterior,lateral,andaxialradiographs(s)showalucent,roundlesionwithwell-definedmarginsatthesuperolateralaspectofthepatella(arrows).SagittalprotondensityandaxialT2-weightedfat-suppressedMRimages(b)showafocalsubchondralosseousdefectwithintact-appearingoverlyingcartilage;thecartilageisthickened,andfillsthedefect.Thereisnormalbonemarrowsignalandsmooth,homogeneoussignalofthearticularcartilage.Fig.6:Dorsaldefectofthepatella图6：髌骨背侧缺损（DDP）Congenitalpatellaaltaisananatomicriskfactorforpatellofemoralinstability.Theinsall-Salvatiindexistheratioofthelengthofthepatella(PL)tothepatellartendon(PT).Thenormalvalueisbetween1.0and1.2,withincreasedvaluesindicatingpatellaaltaanddecreasedvalueindicatingpatellabaja.Lateralradiograph(a)atapproximately30degreesofkneeflxionshowsanoemallyplacedpatella,withInsall-Salvatiindexof1.1.Lateralradiograph(b)ofan8-year-oldmaleshowspatellaalta,withInsall-Salvatiindexmeasuring1.8.AxialT2-weightedtubrospinechoMRimage(c)formthissamepatientshowsfindingofalateralpatellardislocation.Thereisbonemarrowedemaofthemedialaspectofthepatella(arrow)anddisruptionofthemedialpatellarretinaculum(asterisk).Thispatienthadahistoryofrecurrentdislocations,likelyduetohiscongenitalpatellaalta.Fig.7:Patellaalta图7：高位髌骨a图正常位置髌骨，髌韧带长度（PT）/髌骨长度（PL）正常比值为1.0-1.2（国内文献一般小于0.8提示低位髌骨，大于1.2提示高位髌骨）；b图PT/PL比值为1.8；c图示髌骨脱位状态，局部骨髓水肿。高位髌骨通常无症状，尽管它是膝关节不稳定的重要解剖危险因素之一。Anteroposterior(a)andlateral(b)radiographsofa15-year-oldfemalepatientwithcingenitalright-sidedpatellabaja.Lateralradiographsofapatientoneyearfollowingtotalkneearthroplastydemonstratespatellabaja.Thepatellartendonisscarredtotheuppertibia(arrow).Patellabajamayalsobeseeninassociationwithneuromusculardiseases.Fromtal(c)andlateral(d)radiographsinthispatientwithahistoryofpolioshowmarkedpatellabaja.Alsonitethatthebineareosteopenicandgracileandthatthereisapaucityofsofttissues,inkeepingwiththepatient`shistoryofpolio.Fig.8:Patellabaja图8：低位髌骨a,b图，15岁女孩右膝先天性低位髌骨。c,d图，低位髌骨也见于神经肌肉疾病；患者既往有脊髓灰质炎病史。e图，人工膝关节置换后患者一年复查，侧位片提示低位髌骨；箭头是髌韧带疤痕形成。TrochleardysplasiaisamongthemostsignificantanstomicfactorscontributingtopatellarmaltrackingLateralradiograph(a)depictsonesign,thecrossingsign,inwhichthelineofthedeepestaspectofthetrochleargroovecrossesovertheantenoraspectofthefemoralcondyles(arrow).Sagittalprotondensityimage(b)depictsanotherhndingoftrochleardysplasia.Theventraltrochlearprominence(vtp)hasbeendetinedasthedistancebetweenthelineparallelingtheventralcorticalsurfaceofthedistalfemurandthemostanteriorpointofthefemoraltrochlearfloor.Inthisimageisseenastep-likedeformityattheintertaceoftheanteriorfemoralcortexandtrochieawithavtemeasuring9mm,consistentwithtrochleardysplasia.AxialT2-weightedfat-suppressedimage(c)showsacongenitalydysplastictrochleawithamarkedlyshallowtrochieardepth(arrow),consistentwithtrochleardysolbsiaAddisanallynotedismarkedasymmetryofthemedial(MF)andlateral(LF)trochlearfacets.Alateraltomedalfemoralfacet.ranootgreaterthan1.75isgenerallyconsidereddiagrosncfortrochleardysplasia.Inthiscasetheratiomeasures23.representinganothertindingoftrochleardoplasi