protrusio acetabulipresentationgrafix.com/_dev/cake/files/archive/pdfs/645.pdf · 2015-02-22 ·...

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Intrapelvicprotrusionoftheacetabulumwasfirstrecog-nizedbyOtto1in1824onadesiccatedspecimenfromtheNaturalHistoryMuseumofBreslauinPoland(Fig.

1).Ottooriginallydescribedtheabnormalityasbeinganimportantconsiderationinobstetricdeliveryandascribedhisfindingsto“anabnormalgout.”Todate,hehasremainedtheonlyauthortomakethisassociation.WhitereportedthefirstcaseintheBritishliteraturein1883.2Protrusioacetabulireceivedscantattentionuntiltheturnof19thcentury.Sincethen, this conditionhasbeen the subjectofmuchdebateregardingdiagnosis,etiology,andmanagement.Thisinwardprotrusionof theacetabulum is commonly referred to asOtto pelvis, protrusio acetabuli, or arthrokatadysis (fromancientGreek:literallymeaningsubsidenceofajoint).ThetermOtto-ChrobakpelviswasusedforanumberofyearsafterEppingeraddedthenameofaProfessorChrobakasabirthdaytribute.3However,Chrobak’snamewasdroppedashehadnoconnectionwiththecondition.

Historical ConsiderationsOtto described the macroscopic appearance of theacetabulum and head of femur as being “smooth and

polishedwithalossofthecartilagecover.”Eppinger,in1903,reportingonfoursimilarpelvicspecimens,sug-gestedthatthedeformityresultedfromadisturbanceofgrowthaffectingthedelayedossificationofthetriradiatecartilage.Thecontinuedpressurefromthefemoralheadwasthensufficienttocausetheprotrusion. Over the following 30 years, a number of authorspublishedreportswhichsuggestedavarietyofalternativepathologies.Kuliga(citedinPomeranz4)andSchertlin5favoredan“osteo-arthritisdeformans,”whileHertzler,6Pomeranz,4andLewin(citedinPomeranz4)describedan“osteoarthriticprotrusionoftheacetabulum.”Infectionwasalsoidentifiedasapossiblecause,withtuberculo-sis,gonorrhea,andsyphilisbeing themostcommonlyreportedconditionsresponsible.Streptococcus,staphylo-coccus,andechinococcuswerealsoidentifiedindiseasedhips.4,7

In 1929, Doub8 found 50 cases of protrusio with awide variance of opinion as to the exact etiology. Hefailedtoidentifyanyoneetiologicalfactorandsuggestedthatthemostlikelyexplanationwasthatsomegeneraldiseaseinearlylifecausingsofteningofthebonesledtothedeformity,withosteoarthritisalaterstageintheprocess. Pomeranz,4 in 1932, identified 79 cases, acceptedthatitwasincorrecttoseekonecommonetiology,anddescribed twomaingroupsofpatients.Onegroupde-velopedprotrusiosecondarytoinfection.Inthesecondgroup, the protrusio was associated with generalizedosteomalacia. Schaap9andGolding,10in1934,agreedwithPomer-anz’shypothesisandincludedafurthergroupconform-ing with Eppinger’s opinion that, in some, a growthdisturbance primarily initiated the deformity. Schaapalsocomparedpatientsinthisgroup,whoweremostlyfemalesandinwhomtheconditiontendedtobebilateral,

Protrusio Acetabuli

Colin C.R. Dunlop, M.R.C.S., Charles Wynn Jones, F.R.C.S., and Nicola Maffulli, M.D., M.S., Ph.D., F.R.C.S.(Orth.)

ColinC.R.Dunlop,M.R.C.S.,isaSpecialistRegistrarinTraumaandOrthopaedicSurgery,DepartmentofOrthopaedicsandTrauma,Tayside University Hospitals NHSTrust, Ninewells Hospital,Dundee,Scotland.CharlesWynnJones,F.R.C.S.,isaConsultantOrthopaedic Surgeon, Department ofTrauma and OrthopaedicSurgery,KeeleUniversitySchoolofMedicine,StokeonTrent,England. Nicola Maffulli, M.D., M.S., Ph.D., F.R.C.S.(Orth.),isaProfessorofTraumaandOrthopaedicSurgery,DepartmentofTraumaandOrthopaedicSurgery,KeeleUniversitySchoolofMedicine,StokeonTrent,England.Correspondence: Nicola Maffulli, Department ofTrauma andOrthopaedicSurgery,KeeleUniversitySchoolofMedicine,NorthStaffordshire Hospital,Thornburrow Drive, Hartshill, Stoke onTrent,Staffordshire,ST47QB,England.

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withpatientswithdevelopmentaldysplasiaofthehip.9 The suggestion of more than one etiology for theconditionbecamewidelyaccepted,with theexceptionofRechtman11who,in1936,suggestedthatall casesofprotrusio had an initially “too deep” acetabulum as acongenitaldeformity. Overgaard,12 in 1935, first distinguished betweenprimary and secondary protrusio. He subdivided theprimarygroupintothosewithosteo-asthenic protrusionandthosewithosteo-arthriticprotrusion.Thesecondarygroup showed evidence of a destructive inflammatoryprocessinthejoint. Gilmour,13 in 1939, further simplified this classifi-cation so that theprimarygroupcontainedonly thosepatients in whom no other underlying pathology wasdemonstrable, while subjects in the secondary groupdevelopedprotrusioasaresultofanyunderlyingpathol-ogy.Thisistheclassificationusedtoday.

EtiologySecondary ProtrusioThecausesofsecondaryprotusioacetabuliarenowwelldocumented(Table1).Inmostsubjectswithsecondaryprotrusion,theprogressionofthedeformitycanbeex-plainedintermsoftheunderlyingcondition. Wherethereisadestructiveprocess,suchasinfection,thefemoralheadwillmigrateaxially,closetothelineof

thejointreactionforceasthebonysubstructureisweak-ened.Similarly,ininflammatoryconditions,migrationwilloccurduetoincreasedboneturnoverunderload.Thejointreactionforceactsat69°fromthehorizontal14andmigrationof the femoralhead inprotrusiooccurs justmedialtothisat65°.15

Conditions which weaken the bone matrix, eitherlocally,suchasradiationinducedosteonecrosis,orglo-bally,suchasPaget’sdisease,willalsoallowthefemoralhead to migrate superomedially under normal loadingconditions. However,thesesimpleexplanationsdonotexplaintheprogressionoftheconditioninalltherecognizedcausesofsecondaryprotrusio.OneexampleistheoccurrenceofthedeformityinMarfan’ssyndrome.Marfan’ssyndromeiscausedbyadefectinthefibrillin1gene.Thisdefectgivesrisetocardiovascular,skeletal,ocular,pulmonary,integument,andduralabnormalities.Theskeletalmani-festationsofMarfan’ssyndromearebestexplainedbyovergrowthofthelongbonesasopposedtoaweaknessintheirstructure.Alackofnormalfibrillinintheperios-teummightgivelessrestrictiontolongitudinalgrowth,and subsequent overgrowth occurs due to this lack ofnegative feedback.16 To simply attribute the protrusiointhesepatientstoaweaknessintheacetabularfloorisunfounded.Itwouldbeequallylikelytoresultfromthegrowthdisturbancewhichgivesrisetotheotherfeatures.Thishypothesiswouldsupportapossibledevelopmental

Table 1 EtiologyofSecondaryProtrusioAcetabuli

Infection Gonococcus Echinococcus Tuberculosis Syphilis Streptococcus Staphylococcus Inflammatory RheumatoidArthritis Spondyloarthritides IdiopathicChondrolysis Metabolic Paget’sDisease Osteomalacia Hyperparathyroidism Genetic OsteogenesisImperfecta Acrodysostosis MarfanSyndrome Ehler-DanlosSyndrome Trisomy18 SticklerSyndrome Neurofibromatosis SickleCellDisease TrichorhinophalangealSyndrome Homocystinuria Neoplastic PrimaryNeoplasm(e.g.,Hemangioma) MetastaticDisease RadiationInducedOsteonecrosis Trauma AcetabularFracture

Figure 1LithographofpelvicspecimenoriginallydescribedbyAWOttoin1824.Hisdescriptionisasfollows:“Thispelviswithaglobularextendedacetabulumofanadultwomanhasanormalsizeandshapeingeneral.Thespecialfeatureisthatbothheadsoffemurreachquitedeepintotheacetabulum,thefloorsofwhichextendintothepelviccavity,andtheyshowanimperfectankylosis…Duetotheextensionoftheacetabulumintothepelvis…notonlytheheadofthefemurbutalsotheneckofthefemurareprojectedintotheacetabulumandthegreatertrochanterisarticulatingwiththeupperpartoftheedgeoftheacetabulum.”(Reproducedwithkindpermissionfrom:SteelHH:Protrusioacetabuli:itsoccurrenceinthecompletelyexpressedMarfanSyndromeanditsmusculoskeletalcomponentandaproceduretoarrestthecourseofprotrusioninthegrowingpelvis.JPediatrOrthop1996;16:705)

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etiologyinsomeprotrusionpatients.

Primary ProtrusioPrimaryprotrusioacetabuliremainsadiagnosisofexclu-sion,andassuchmanyofthecasesreportedinthepastmayhaveinfactbeensecondarytoundiagnosedcondi-tions. In thesearchforanetiologyresponsiblefor theprimaryprotrusiogroup,mostinvestigatorshavereferredtothemechanismsresponsibleforsecondaryprotrusio.Thus,thepossibleetiologieshavebeenconsideredunderthefollowingthreeheadings:

1. Aninflammatoryordestructiveconditionofthehipjoints;

2. Aqualitativedeficiencyofacetabularbone;and

3. Adevelopmentalabnormalityorgrowthdisturbance.

Clinical FeaturesTheconditionpresentsinthreemainagegroups17:pa-tientspresentingintheirteens,thosepresentingbetween35and50years,andthosepresentingbetween51and85years.Thisdivisionwasbasedonagraphicalrepresenta-tionoftheageatpresentationin59patients.Whilethereisadefinitepeakintheyoungerpatients,18-21divisionofthe lateronsetpatients into twodistinctgroups is lesswell-defined. Inyoungerpatients,thediagnosisofprotrusioisoftenoverlooked,asitisunusualforanyfeaturesofdegenera-tivechangetobepresentatthisstage.Theconditioninthisagegroup,althoughproducingtheassociatedsignsandsymptomsofprotrusio,oftenlacksthestrikingradio-graphicalappearanceseeninlaterlife.18Belowtheageof25,osteoarthriticstigmataareminimal.Abovethisage,thedegreeofosteoarthriticchangescorrelatespositivelywiththeageofthepatient.22Theyoungerpatientspresentwhensymptomsarisefromtheanatomicalabnormality.Theolderpatients, instead,presentwhendegenerativechange occurs secondary to this abnormality. This isin agreement with Gilmour’s13 belief that there was aprimaryphaseinwhichthedeformityexistedinanun-complicatedstate,butthatitwasmoreoftendiscoveredwhenosteoarthritisorotherchangeshadset in. In theelderlypatientsstudiedbyHooperandJones,protrusiowas invariably accompanied by osteoarthritis,17whichinitselfisacauseofsecondaryprotrusio.Bythetimethatthegrosschangesofosteoarthritisaremanifest,itisimpossibletodistinguishthoseinwhomtheprotrusioresultsfromosteoarthritisfromthoseinwhomthepro-trusiopre-datedthedegenerativechanges. There is a marked female to male preponderance.Scandalisandcolleagues23reportedtheincidenceaccord-ingtosexof85%female(35outof41patients).Gilm-our13foundasimilarratioof30femalesto7males. The condition is characteristically bilateral.22,24 In

Overgaard’s12reviewin1935,35of44patientshadbilat-eralprotrusio.Oftheninepatientsreportedasunilateral,sixhadanabnormallydeepenedcontralateralacetabu-lum. Details of the contralateral hip of the remainingthreewereunavailable. Rechtman11firstsuggestedahereditarypatterninthreeofthefivepatientshedescribed.Thiswassuggestedfromhistoryalone,withtheprobandsreportingasimilarfunc-tionaldisabilityinmembersoftheirimmediatefamilies.Rechtmanwasunabletoaccesstheotherfamilymemberstoconfirmthisradiographically.Sincethen,thisfamiliallinkhasbeenconfirmedradiographicallybyanumberof investigators25-29 andhas alsobeendemonstrated inidenticaltwins.30Thesuggestedpatternsofinheritancehave been in accordance with an autosomal dominantgenewithincomplete27orcomplete29penetrance. There is also a racial influence on the condition,31withagreatlyincreasedincidenceintheBantuwomeninNatal.PelvicradiographsofpregnantBantu,Indian,andEuropeanwomenreferredforpelvimetrywerecom-pared.The incidenceofprimaryprotrusioacetabuli inthesegroupswas25.7%(58outof226),5.7%(6outof105),and2.9%(3outof105),respectively. In1939,Gilmour13reportedanabnormal“rhythmofadolescentdevelopment” inpatientswithprotrusio. Inanumberofhispatients,thesignsandsymptomsofthedeformityhadprecededmenarche,andthesegirlsshowedacceleratedepiphysealgrowthandfusionpriortothis.Friedenberg,32 in a report of two patients, commentedthat one of them experienced menarche at 9 years ofage and that both had experienced early fusion of theepiphyses. With theexceptionof twopatientswhohadrepeat-edlyelevatedcalciumlevelsintheseriesbyHooperandWyn Jones,17 all the patients reported showed normalbiochemistry.Anabnormalitymay indeedexist,but itmaybe too subtle tobedetectedby routine screeningmethods. Shoreandassociates21describedthehistologicalap-pearanceofbiopsyspecimensfromjuvenilepatientsasshowingasignificantbutnon-diagnosticinflammatoryreaction. Specimens from arthroscopic examinationshoweddegenerativechangeswithfibrocartilagereplace-ment.WroblewskiandHillman33performedhistologicalexaminationonspecimensoftheacetabularfloorfrompatientswiththefeaturesofidiopathicprotrusiounder-going total hip arthroplasty. The marrow spaces werereplacedwithactivevascularfibrousgranulationtissuewithdisruptionofthetrabecularpatterninsomeareas. Typically, patientswithprimaryprotrusio acetabulipresent with increasing stiffness rather than pain. Of-ten,stiffnesshasbeennoticedinadolescence.Historiesfrom the various series include one young lady whonoticedthatshecouldnotsitcrossleggedonthefloorand another who found that she lacked hip flexibility

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while attempting gymnastics at school. Of those whopresentlaterwithsymptomsofsecondarydegenerativechange,closequestioningoftenrevealssimilarhistoriesofprecedinglimitationsofmovement.Deepeningoftheacetabulumleads topainful limitationofabductionasthe femoral neck impinges on the superior acetabularmargin.Thepainispresumedtoarisefromsynovialstruc-turesinthisregion.34Furtherprogressionleadstoadductorspasm,andfixedflexiondeformitiesdevelop.Thereisoftenhyperlordosisofthelumbarspinewhichcompensatesforthefixedflexiondeformitiesatthehips.Untreated,thepatientultimatelydevelopsankylosisoftheaffectedhip.

Radiological FeaturesTheterm“ProtrusioAcetabuli”simplyreferstothepro-trusionoftheacetabulumintothelesserpelvis,andthisoccurstovaryingdegrees.Itiscommonlyreportedasachronicprogressivedeformity.Brailsford,18followingtheprogressofthedeformityinyoungpatients,concludedthat theonsetof theconditionwasininfancyandthatthedeformityfollowedaslowlyprogressivecourse.Inthe same year, however, Friedenberg19 suggested that,althoughtheprotrusionoccurredatayoungage,itdidnotalwayscontinuetoincrease.Heprovidedevidenceforthisstatementwithoneofhisownpatientsashavingnoprogressionofdeformitybetweenradiographstakenat31yearsand39years.HealsopointedoutthatSchaap9andGolding10hadbothfollowedupfemalesof45and40yearsofage,respectively,forfiveyearswithnopro-gression of acetabular protrusion. Hubbard,35 in 1969,describinghisfindingson27patientswithprotrusion,confirmedthatnoteverycaseofprotrusiowasprogres-sive.Inhisseries,20hipsin15patientsprogressedbyanaverageof6mminameanof8.4years,10hipsinsixpatientshadnoprogressioninatimerangingfrom1to10years,and10hipsinsixpatientsactuallyshowed

adecreaseinprotrusion. Thenon-progressivecasesofFriedenberg,19Schaap,9and Golding10 showed preservation of the articularcartilage in the presence of marked protrusio. Figure2demonstrates the typical radiographicappearanceofthe condition in a patient with minimally progressivedeformity. Sherlock,36in1995,suggestedthatprimaryprotrusioacetabuli and acute idiopathic chondrolysis may havebeen the same condition. In the absence of Marfan’ssyndrome,infection,trauma,orrheumatoidarthritis,adiagnosisofprimaryprotrusioacetabuliwasmadeifanadolescentpresentedwithpainandstiffnessofthehipandacetabularprotrusioandlossofjointspacewereap-parentonplainradiography.Thesamepicture,butintheabsenceofprotrusio,wouldbegthediagnosisofacuteidiopathicchondrolysis.Offiveadolescentfemaleswiththeformerclinicalpicture,threehadbeendiagnosedashavingprimaryprotrusioacetabuli,and twoashavingacute idiopathic chondrolysis. The similarities in theclinicalfeaturesandCTfindingsledSherlocktobelievethattheywerelikelytobeonecondition.Heproposedthatthosepatientswithpainasthepredominantfeaturehadtendedtobelabeledasacuteidiopathicchondrolysis,whereasthosewithamoredefiniteprotrusiowerelabeledprimaryprotrusioacetabuli.Theassociationofprotrusiowithacuteidiopathicchondrolysishasbeencommentedonbyotherinvestigators.37-41

Chondrolysisofthehiphasbeendescribedsecond-arytoslippedupperfemoralepiphysessince1930.42In1971,Jones43firstdescribedidiopathicchondrolysisinthe absence of any other pathology. This uncommonconditionwasinitiallyreportedasbeingmorecommoninblack,adolescentfemales.40,43Morerecentliteraturehas recorded its occurrence in Caucasian, Indian andHispanic males and females.37,38,41 Bilateral cases are

Figure 2A,Radiographshowingthetypicalappearancesofprimaryprotrusioacetabuliina19-year-oldCaucasianfemale.Notethemarkedprotrusioandthepreservationofthejointspace.B,Radiographofthesamepatientat29yearsofagedemonstratingthenon-progressivenatureofthecondition.

BA

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exceptional.Theradiographicfindingsinacuteidiopathicchondrolysisshowagloballossofarticularcartilageonthefemoralheadandacetabulum. In the juvenile groupof patientswithprimarypro-trusioacetabuliofHooperandWynJones,17malesandfemaleswereequallyaffected.Insome,thesymptomsprogressedrapidly,becomingincapacitating.Therefore,intheyoungeragegroup,acuteidiopathicchondrolysisandprimaryprotrusioacetabulimaywellbetwodistinctpathologies thataredifficult toseparateat the timeofpresentation.Inacuteidiopathicchondrolysis,thechon-drolyticprocesscausesgloballossofarticularcartilageandsecondaryprotrusiodevelops.Thisgroupcontainsamoreevenmaletofemaleratioandtheprocessisoftenunilateral.Inprimaryprotrusioacetabuli,theprotrusiodevelops with sparing of the articular cartilage. Thisgrouphasthecharacteristicfemalepreponderanceandbilateraldeformity.Otherinvestigators7,44haveincludedacuteidiopathicchondrolysisintheirlistsofcausesofsecondaryprotrusioacetabuli. Several investigators have disagreed with Otto’sdescriptionofhisoriginal specimenashaving“anor-mal size and shape ingeneral.”Overgaard12describeda typical “clover leaf” appearanceon radiographs andseveralotherinvestigatorsfoundawidenedintercristaldistance on anteroposterior pelvic radiographs.13,22,30Alexanderalsodemonstratedthattheratiobetweentheintertuberousdistanceandtheintercristalmeasurementwas lower inpatientswithprimaryprotrusioacetabuliandhigherinpatientswithdevelopmentaldysplasiaofthehip.22Littlefurtherworkhasbeenpublishedinthisarea.Thisfindingcouldbeexplainedbyforwardtiltingofthepelvisduetofixedflexiondeformityatthetimeofradiography,but,toourknowledge,todatethiseffecthasnotbeeninvestigated. Coxa vara and decreased femoral anteversion havebeenlinkedwithprimaryprotrusioacetabuli.22,45Hooper17evaluatedthefemoralneck-shaftangleintheirpatients:allmeasurementswerewithinthenormalrange,averag-ingbetween123°and127°.Tonnis,45inacomprehensivereviewoffemoralandacetabularanteversion,foundthatprimary protrusio acetabuli was associated more withincreasedfemoralandacetabularanteversionratherthanretroversion,butheincludedtoofewpatientstodrawanydefinite conclusions. It would certainly seem unlikelythat there isanymarkedrelationshipbetweenprimaryprotrusio acetabuli and either coxa vara or decreasedfemoralanteversion. With these clinical and radiographical features, theetiologyofprimaryprotrusioacetabuliwouldmostlikelybedevelopmental.Abilateralconditionaffectingonlytheacetabula,withpreservationofthearticularcartilageandnormal inflammatorymarkers, isunlikely tohavean inflammatoryordestructiveetiology.Likewise, thenon-progressivenatureoftheacetabulardeformity,the

normalbiochemicalprofile,andthelackofothersitesaffected would make a qualitative deficiency of boneunlikely. Currently, two hypotheses exist regarding adevelopmentaletiology:

1. Adisturbanceofgrowthmayleadtoadelayinos-sification of the triradiate cartilage.3 Subsequentstressesontheunfusedepiphyseswouldproducethedeformity.

2. The converse of this might be the case: abnormalaccelerationofepiphysealossification in thepelvisreplacing the triradiatecartilagewithnew,vascularandplasticbone that allowsmoldingunderweightbearing.13

Alexander22showedthatadegreeofprotrusiowasnormalinchildren.Hedescribeda“beaking”ofthetriradiatecarti-lagewhichisatamaximumateightyearsofageandwhichsubsequentlyremodels.Healsosuggestedthatacceleratedepiphysealclosuremayleadtopreservationofthisphysi-ologicalprotrusiothatthenfailedtoremodel. Experiments on rabbits have shown, however, thatpremature surgical fusion of the triradiate cartilageleadstoathickmedialacetabularwallwithprogressiontosubluxationofthehip.46,47Thisfindinghasalsobeendemonstrated in children following acetabular frac-tures.47-49Prematureclosuresuspectedtobetheresultofsepticarthritisofthehiphasalsobeenreported50withsubsequentsubluxation.Surgicalclosureofthetriradiatecartilagehasalsobeendescribedasaneffectivetreatmentinarresting thedevelopmentofprotrusio in skeletallyimmatureMarfan’spatients.51,52SteelrecommendsthisprocedureinchildrenwithMarfan’swhoarebetweentheagesof8and10yearsandwhohavedocumentedpro-gressionofacetabulardeepening.Thisevidencewouldimplythatearlyepiphysealclosureleadstoashallow,dysplastic acetabulum rather than protrusio.Any linkbetweenlateepiphysealclosureandabnormalacetabulardevelopmenthasyettobedocumented.

Radiographic DiagnosisTheterm“protrusioacetabuli”simplyreferstothepro-trusionoftheacetabulumintothelesserpelvis.ThiswasfirstdemonstratedradiographicallybySchertlinin1910.5Grossexamplesoftheconditionareobviousonstandardantero-posteriorradiographsofthepelvis,withtheout-lineoftheacetabulumextendingbeyondtheiliopectinealline.Sotelo-GarzaandCharnley53gradedthedeformityin these patients.The distance between the projectionoftheuppermarginofthesuperiorpubicramusandtheoutlineoftheacetabulumprotrudingintothepelvis wasestimated,givingthreegradesofprotrusio.

GradeI(mildprotrusio)measured1to5mm; GradeII(moderateprotrusio),6to15mm;and GradeIII(severeprotrusio),morethan15mm.

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Atearlierstagesintheprogressionoftheconditionthediagnosisisnotalwayssoapparent.Asaresult,variousradiographiccriteriahavebeenusedtotrytoidentifythemostconsistentmethodofdiagnosis.Beforethesemeth-odsaredescribed,ithastobestressedthatinaconditionsuchasprotrusio,wheretherangeofdeformityextendsalongacontinuumfromnearnormaltogrosslyabnormal,anyvaluechosenisnecessarilyarbitrary. Thethreemostwidelyusedcriteriaarethecenter-edgeangleofWiberg,54thedistancebetweenthemedialwalloftheacetabulumandKohler’s55ilioischialline,andtheconfigurationoftheteardrop. Thecenter-edgeangle,originallydescribedtodiag-nosedevelopmentaldysplasiaofthehip,54 isshowninFigure3A.Ananglelessthan20°wastakenasdiagnos-ticofdevelopmentaldysplasiaofthehip,andananglegreaterthan46°asdiagnosticofprotrusio.“Greyareas”probablyliebetween20°and25°,and40°and46°,withanormalrangebetween25°and40°.Someinvestigatorshavesupportedtheuseofthisangle,19,27butothershavefounditunreliableinthediagnosisofprimaryprotrusioacetabuli.17,22,56,57Freidenburg19andMacDonald27foundcenter-edgeanglesgreaterthan50°andashighas90°inradiographsofobviousprotrusion.Armbuster,57inananatomicalstudyoftheadulthip,foundnocorrelationbetween the center-edge angle and either the teardropconfigurationorthedistancebetweentheacetabularlineandtheilioischial line.Healsofoundawiderangeofvalues,ashighas59°,inthenormalpopulation. Sharp57observedseverallimitationsofthecenter-edgeangle.Thecenterpointofadeformedfemoralheadisdifficulttolocate,andsubluxationorsimplelossofjointspace of either hip leads to inaccurate measurements.These patients also develop superolateral osteophytesthat make identification of the superolateral edge dif-

ficult. Therelationshipoftheacetabularline,representingthemedialwalloftheacetabulum,toKohler’silioischialline,hasalsobeenusedbyseveralinvestigators(Fig.3B).Alexander,22asoneofhisdiagnosticcriteria, requiredthefemoralheadtoreachKohler’sline.Headmittedthathiscriteriaweretoorigidandleadtounder-diagnosis.Hubbard35requiredtheacetabularlinetocrosstheiliois-chiallineaspartofhisdiagnosticcriteria.InaseriesbyArmbusterandcoworkers,56however,inwomentheac-etabularlinecrossedtheilioischiallinebyanaverageof1mminthenormalpopulation,andinmen2mmlateralto it.They recommended thatadiagnosisofprotrusiobemadeiftheacetabularlinecrossedtheilioischiallineby3mminmenandby6mminwomen.Intheirhands,thismeasurementwasreliableevenwithminordegreesofrotation,astheyconsideredboththesestructurestobecentrallyplaced.Goodmanandcolleagues,58however,radiographeddriedpelvicspecimenswhileseriallysec-tioningandreamingthespecimens.Theydemonstratedthat,whiletheteardropwasaconsistentrepresentationofthemedialandlateralwallsoftheflooroftheacetabulumjustabovetheobturatorforamen,theilioischiallinewasaprojectionofaportionofthequadrilateralsurfacepos-teriortotheacetabulum.Thus,theseparationofthetwolineswouldbesensitivetorotationofthepelvisatthetimeofradiography.Theysuggestedthat,astheteardropwasaconsistentfinding,aCartesiancoordinatesystembasedonthisstructureshouldbeusedfordiagnosisandchartingprogression.ThissuggestionhasbeenfurtherreinforcedbyGates and associaties,59 and is useful tochartprogressionofthecondition. TheappearanceofKohler’s“teardrop”figurehaslongbeencommentedonwithregard toacetabulardeepen-ing. In 1935, Overgaard12 noted that inversion of the

Figure 3A,Center-edgeangleofWiberg.Alinepassingthroughthecentersofbothfemoralheadsisconstructedasabaseline.Twofurtherlinesaredrawnfromthecenterofthefemoralhead,oneperpendiculartothebaselineandonetangentialtothesuperolateralmarginoftheacetabulum.Thecenter-edgeangleismeasuredbetweenthesetwolines.B,DistancebetweentheacetabularlineandKohler’silioischialline(arrows).

A B

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“pear-figure”(teardrop)representedadeepeningoftheacetabulum. The various appearances of the teardropareshowninFigure4.ThedefinitionsshownarethoseemployedbyArmbusterandcolleagues.56Alexander,22HooperandJones,17andHubbard35allusedthecross-ingoftheteardropasoneoftheirdiagnosticcriteriaforprotrusio. However,Armbuster56 showed that this ap-pearanceisalsoverysensitivetopelvicrotation.Kohlerhimselfdemonstratedthisvariabilityonpoorlycenteredfilms.55

In the absence of a proven “standard” diagnosticcriterion,itisimpossibletoseparatenormalhipsfromthosewithanabnormalprotrusion.Onemustthereforebecarefulinwhatconclusionsaredrawnfromcompari-sons between different proposed diagnostic measures.Theevidencewouldsuggestthatpatientswithprotrusiodemonstratehighcenter-edgeanglesabovethereportednormalrange.However,thismeasurementisunhelpfulindiagnosisduetoitslackofspecificity.Toovercomethese inaccuracies57 the inter-teardrop line should beusedasthebaselineratherthanthecenter-centerlineinthismeasurement.Boththeappearanceoftheteardropandtheseparationoftheacetabularandilioischiallinesarehelpful indiagnosis,providedthatcare is takentoensureaccuratecenteringof films.Theonlymeasure-mentmethodthatwouldeliminatetheeffectofrotationwouldbetouseCartesiancoordinateswiththeoriginatthetipoftheteardrop. Severalotherindicesofacetabularmorphologyhavebeen described in the investigation of developmentaldysplasiaofthehip.Theseincludetheacetabularindexor sourcil angle, the acetabular inclination angle, andthe acetabular depth-to-width index. Murphy and co-workers60assessedtheusefulnessof thesevariablesasprognosticindicatorsindevelopmentaldysplasiaofthe

hip.Thesourcil tends tobeconcaveinferomedially inprotrusion,12 givinganegative acetabular index.Apartfromthisobservation,noworkhasbeencarriedoutusingthesemeasurementsinagroupofprotrusiopatients.

ManagementThemanagementoptionsforprimaryprotrusioacetabulihavebeenrecentlyreviewed7(Table2).Themostappro-priatetreatmentisbasedontheageandskeletalmaturityofthepatient,thedegreeofprotrusio,andtheextentofdegenerativechangesinthejoint. Inskeletallyimmaturepatients,surgicalfusionofthetriradiate cartilage has been proposed.7 Steel52 recom-mendsthisprocedureinMarfan’sSyndromefollowinghis resultsonaseriesof22patients,as thesecondaryprotrusio thatdevelops isprogressive.Atpresent, it isnotpossibletoanticipatewhichcasesofjuvenileprimaryprotrusioacetabuliwillprogressandwhichwillfollowamoreindolentcourse.Therefore,theuseofthispro-cedureinprimaryprotrusioacetabulicurrentlyremainsunclear. Inthetimepredatingtotalhiparthroplasty,surgicalmanagementoftheconditionincludedresectionarthro-plasty,arthrodesis,andacetabuloplasty.34Acetabuloplastywithresectionoftheanteriorwalloftheacetabulumwasperformedforthefirsttimeona55-year-oldfemalewithbilateralprotrusiotoremovetheareaofimpingementthat

Figure 4Variationinappearanceoftheteardrop.

Table 2 OperativeProceduresforProtrusioAcetabuli

Arthrodesis ExcisionArthroplasty TotalHipArthroplasty Acetabuloplasty ValgusIntertrochantericOsteotomy SurgicalClosureofTriradiateCartilage

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gaverisetoherpainfulsymptoms.Thepatientwasabletoreturntoworkasahousekeeperfourmonthslater.Atthetimeofpublicationtheprocedurehadbeenusedon11patients.Eightofthesewereperformedfor“malumcoxaesenilis,”twoforoldslippedcapitalfemoralepiphy-sis, and theoriginalprocedurewasperformed foronepatientwithprimaryprotrusioacetabuli.Nolong-termfollow-upstudyhasbeenpublishedonthisprocedure,although a similar acetabuloplasty or “rimectomy” isstillbeingperformedinsomecenters(C.Wynn-Jones,personalcommunication). Thetwomostwidelyusedoperativeproceduresforprimaryprotrusioacetabuliarevalgusintertrochantericosteotomy and total hip arthroplasty. Pauwels61 firstdescribedvalgusintertrochantericosteotomyforuseinprotrusio hips, and reasoned that the procedure led tomorecranialresultantforcesatthehipjoint,thusreduc-ingthepressureontheflooroftheacetabulum.Theotherbeneficialeffectoftheprocedureisthatitreducestheimpingement at the superior acetabular margin. Sincethen,severalinvestigatorshavereportedexcellentresultsin selected patients following valgus intertrochantericosteotomy,withthebestresultsinyoungerpatientswithminimalarthriticchangeinthehipjoint.62Inastudywitheightoperatedhipsinsixpatientsandamaximumfollowupof5years,theresultsweregoodexceptfor1patient,whowas49yearsoldatthetimeofoperationandhadmoderate arthritic changes visible radiographically.63McBrideandassociates7 reportedon19hipsoperatedonin12patientswithanaveragefollowupof7.2years.Goodtoexcellentresultsweredocumentedin12hipsinpatientsagedbetween21and33years.Poorresultswerenotedinfivehipsinthefourpatientsaged42andover.McBride and associates concluded that this procedureshould not be performed on patients over 40 years ofageor inwhomsignificantdegenerativechangeswereevidentonplainradiographs.7Ofthesevenpatientsinthepersonalseriesoftheauthorsofthisarticle,threeover40yearsofagewereoperatedonandalloftheseachievedagoodorexcellentresultatanaveragefollowupof5.3years.Noneofthesepatientshadsignificantdegenerativechangesevidentintheirpreoperativeradiographs. Inolderpatientsorinpatientswithsignificantdegen-erativechanges,thebestoperativeprocedureforprimaryprotrusioacetabuliistotalhiparthroplasty.Sotelo-Garzaand Charnley53 demonstrated no significant differenceinoutcomein totalhiparthroplastyperformedon253protrusiohipscomparedwiththoseperformedonnon-protrusiohips.Theimportanceofreplicatingtheoriginalanatomyofthehipjointhasbeenstressed.Ranawatandcoworkers64 demonstrated cement-bone demarcationaroundtheacetabularcupinallthreezones,asdescribedby DeLee and Charnley,65 in 16 of 17 hips positioned1cmormorefromtheanatomicalposition.Failureofacetabularprosthesesmalpositioned to thisdegreehas

been further confirmed by Gates and colleagues59 andBayleyandassociates.66Crowninshieldandcoworkers67performed finite element analysis on various types ofprosthesis when positioned anatomically or with me-dialdisplacement.Highermedialstressesresultedfrommedialplacementoftheacetabularcomponent.Lateralplacementwithmetalbackingofthecomponentortheuse of a protrusio cup lessened these medial stresses.Prosthetic reinforcement of the medial wall had littlebenefit other than that of containment of any cementused.Theuseofamorselizedorfragmentedbonegraftisaneffectivemeansoflateralizationofthecup.15,68Aswellasreturningthehipjointtoitsanatomicalposition,thegraftisincorporatedintothemedialwallimprovingbonestockinthisregion.7RanawatandZahn69reportedon27arthroplastiesperformedforprotrusio.Whenthedegreeofprotrusiowaslessthan5mm,theydidnotusebonegraft.Whenthedegreeofprotrusiowasgreaterthan5mmbut themedialwall remained intact,autologousbonegraftwithoutartificialfixationdeviceswasrecom-mended.Finally,inpatientswithagrosslydeficientme-dialwall,reconstructionwithbonegraftandadditionalfixationisrequired.Hirstandassociates70reportedtheirfindingson61hipsoperatedonin51patientsandfol-lowedupforanaverageof4yearsand3months.Inthesepatients,thedegreeofprotrusiowasdeterminedbythegradingusedbySotelo-GarzaandCharnley53ratherthanameasurementdefiningmigrationfromtheanatomicalposition.All patients were operated on with the sametechnique.Themedialwallwasreinforcedwithwafersof autologous bone graft cut from the femoral head,sufficienttolateralizethecup.Cementwaspressurizedoverthislayerandintoperipheralkeyholes.Aflangedcupwasthenpushedfirmlyintoposition.Therewasnorelapseofprotrusioinanyofthesecases.

ConclusionsPrimaryprotrusioacetabuliremainsadiagnosisofex-clusion inpatientswithabnormalmedializationof theacetabulumandinwhomthesecondarycauseslistedinTable1havebeenruledout. While the exact etiology remains obscure, primaryprotrusio acetabuli is a developmental condition withhereditaryandracialinfluences.Itisprogressiveinado-lescence,priortofusionofthetriradiatecartilage,afterwhichthedeformityremainsstaticuntilsuperimposedwithsecondarydegenerativechanges.It ismost likelythatdelayedossificationratherthanearlyfusionofthetriradiatecartilageisresponsible.Alternatively,thede-formitymaydevelopduringanacceleratedgrowthspurt.Furtherworkisrequiredtoidentifytheexactdevelop-mentalmechanismandanypossiblegeneticetiologicalfactors. The current methods employed in radiographicaldiagnosisofprotrusioacetabulialluseanarbitrarycut-

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offpointbetweennormalandabnormalanatomy.Whenlookingataconditionwithaspectrumofdeformity,thisinevitablyleadstoover-orunder-diagnosis,dependingon the cut-off point chosen. Certainly, patients withprotrusio acetabuli tend to have a center-edge anglegreaterthanthereportednormalupperlimitof46°;theteardrop configuration tends to be closed, crossed, orreversed;andtheacetabularlinetendstocrossKohler’slinebymorethan3mminmenandmorethan6mminwomen.However,noneofthesemeasurementshavethesensitivityorspecificitytoprovideareliablediagnosticsystem.Gradingsystems,suchasthoseusedbySotelo-GarzaandCharnley53orRanawatandZahn69aremoreusefulastheyallowcategorizationofpatients. The management of protusio acetabuli depends onage and degree of degenerative change. In the young,skeletallyimmaturepatientwithprogressivesecondaryprotrusioaceabuli,earlysurgicalfusionofthetriradiatecartilagewithorwithoutvalgusintertrochantericoste-otomyisappropriate.Asitisnotpossibletoanticipatewhichpatientswithprimaryprotrusioacetabuli in theyoungeragegroupwillprogresstoaseveredeformity,fusionofthetriradiatecartilagecannotberecommendedforprimaryprotrusioacetabuli.Valgusintertrochantericosteotomyisrecommendedinskeletallymaturepatientswithnodegenerativechangeunder40yearsofage.Over40years,thisprocedurecanstillbecarriedoutinpatientswithnodegenerativechangesintheirhipjointandwhoarecapableofundertakingtheassociatedrehabilitation.In older patients, total hip arthroplasty with medialbonegraftingandmeticulousattentiontoreturningthehip joint to its anatomical center is the procedure ofchoice.

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