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Anteropost