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).Sagittalprotondensity(a)andaxialfat-suppressedT2-weighted(b)MRimagesofanormalknee.Notethelowsignalpatellar(PT)andquadriceps(QT)tendonsandthethick,homogeneous-appearingpatellarcartillage(redarrows).Notethelateralandmediaretinacula,passivestabilizersofthepatella.In1941,Wibergclassifiedpatellarshapeintothreedifferentmorphologies:TypeI(a)demonstratesroughlysymmetricandequal-sized,concavemedial(MF)andlateral(LF)patellarfacets.TypeII(b)showsamedialfacetthatisslightlysmallerthanthelateralfacetandaconcavelateralfacet.TypeIII(c)alsoshowsasmallerandmoreverticallyorientedmedialpatellarfacet,whichisassociatedwithmaltrackingdisorders[18].5-year-oldmalewithhereditaryosteo-onychodysplasia(nail-patellasyndrome).AP(a),later(b),andaxial(c)viewsofthekneedemonstratecompleteabsenceofthebilateralpatellarossificationcenters.Anteroposteriorandaxialradiographs(a)showbilateral,well-corticatedossifiedfragmentsinthesuperolateralaspectofthepatellas(arrows).CoronalandaxialT2-weightedfat-suppressedMRimage(b)showthewell-corticatedossifiedfragment.Notethenormalbonemarrowsignalandcartilageacrossthesynchondrisis,Thewell-corticatednatureofthefragmentandlackofabnormalmarrowsignalhelptodifferentiatethisentityfromapatellarfracture.Anteroposterior,lateral,andaxialradiographs(s)showalucent,roundlesionwithwell-definedmarginsatthesuperolateralaspectofthepatella(arrows).SagittalprotondensityandaxialT2-weightedfat-suppressedMRimages(b)showafocalsubchondralosseousdefectwithintact-appearingoverlyingcartilage;thecartilageisthickened,andfillsthedefect.Thereisnormalbonemarrowsignalandsmooth,homogeneoussignalofthearticularcartilage.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.Anteroposterior(a)andlateral(b)radiographsofa15-year-oldfemalepatientwithcingenitalright-sidedpatellabaja.Lateralradiographsofapatientoneyearfollowingtotalkneearthroplastydemonstratespatellabaja.Thepatellartendonisscarredtotheuppertibia(arrow).Patellabajamayalsobeseeninassociationwithneuromusculardiseases.Fromtal(c)andlateral(d)radiographsinthispatientwithahistoryofpolioshowmarkedpatellabaja.Alsonitethat...
