Benedito Ferreira Caetano , Rodrigo Guerra Sabongi Benedito Caetano Edie , Luiz Angelo Vieira , João José Sabongi Neto ,Maurício implications Anterior interosseous nerve: anatomical study andclinical Original Article

SOCIEDADE BRASILEIRA DE ORTOPEDIA E TRAUMATOLOGIA

w w w . r b o . o r g . b r

Original Article

Anterior interosseous nerve: anatomical study and clinical implications ^夽

Edie Benedito Caetano

, Luiz Angelo Vieira

, João José Sabongi Neto

, Maurício Benedito Ferreira Caetano

, Rodrigo Guerra Sabongi

aPontifíciaUniversidadeCatólicadeSãoPaulo(PUC-SP),FaculdadedeCiênciasMédicasedaSaúde(FCMS),Sorocaba,SP,Brazil

bConjuntoHospitalardeSorocaba(CHS),Servic¸odeCirurgiadaMão,Sorocaba,SP,Brazil

cUniversidadeFederaldeSãoPaulo(Unifesp),EscolaPaulistadeMedicina,SãoPaulo,SP,Brazil

a r t i c l e i n f o

Articlehistory:

Received1June2017 Accepted6June2017

Availableonline2August2018

Keywords:

Skeletalmuscle/innervation Mediannerve

Nervecompressionsyndrome Pronation

a bs t r a c t

Objective:Thegoalofthisstudywastodescribeanatomicalvariationsandclinicalimpli- cationsofanteriorinterosseousnerve.Incompleteanteriorinterosseousnervepalsy,the patientisunabletoflexthedistalphalanxofthethumbandindexfinger;inincomplete anteriorinterosseousnervepalsy,thereislessaxonaldamage,andeitherthethumborthe indexfingerareaffected.

Methods:Thisstudywasbasedonthedissectionof50limbsof25cadavers,22weremale andthree,female.Agerangedfrom28to77years,14werewhiteand11werenon-white;

18werepreparedbyintra-arterialinjectionofasolutionof10%glycerolandformaldehyde, andsevenwerefreshlydissectedcadavers.

Results:Theanteriorinterosseousnervearosefromthemediannerve,anaverageof5.2cm distaltotheintercondylarline.In29limbs,itoriginatedfromthenervefasciclesofthe posteriorregionofthemediannerveandin21limbs,oftheposterolateralfascicles.In 41limbs,theanteriorinterosseousnervepositionedbetweenthehumeralandulnarhead ofthepronatorteresmuscle.Intwolimbs,anteriorinterosseousnerveduplicationwas observed.Inallmembers,itwasobservedthattheanteriorinterosseousnervearosefrom themediannerveproximaltothearchoftheflexordigitorumsuperficialismuscle.In24 limbs,thebranchesoftheanteriorinterosseousnerveoccurredproximaltothearchandin 26,distaltoit.

Conclusion: Thefibrousarchesformedbythehumeralandulnarheadsofthepronatorteres muscle,thefibrousarchoftheflexordigitorumsuperficialismuscle,andtheGantzermuscle (whenhypertrophiedandpositionedanteriortotheanteriorinterosseousnerve),cancom- pressthenerveagainstdeepstructures,alteringitsnormalcourse,bynarrowingitsspace, causingalterationslongusandflexordigitorumprofundusmuscles.

©2017SociedadeBrasileiradeOrtopediaeTraumatologia.PublishedbyElsevierEditora Ltda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://

creativecommons.org/licenses/by-nc-nd/4.0/).

夽StudyconductedatFaculdadedeCiênciasMédicasedaSaúde,PontifíciaUniversidadeCatólicadeSãoPaulo(PUC-SP),Sorocaba,SP, Brazil.

∗ Correspondingauthor.

E-mail:ediecaetano@uol.com.br(E.B.Caetano).

https://doi.org/10.1016/j.rboe.2018.07.010

2255-4971/©2017SociedadeBrasileiradeOrtopediaeTraumatologia.PublishedbyElsevierEditoraLtda.Thisisanopenaccessarticle undertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Nervointerósseoanterior:estudoanatômicoeimplicac¸õesclínicas

Palavras-chave:

Músculoesquelético/inervac¸ão Nervomediano

Síndromecompressiva Pronac¸ão

r e s u m o

Objetivo:Analisarasrelac¸õesanatômicaseasvariac¸õesdonervointerósseoanterioresuas implicac¸õesclínicas.Aparalisiacompletadonervointerósseoanteriorresultanaincapaci- dadedefletirasfalangesdistaldopolegareindicador;naincompleta,ocorremenordano axonaleapenasopolegarouoindicadorsãoafetados.

Método: Esteestudobaseou-senadissecc¸ãode50membrosde25cadáveres,22eramdo sexomasculinoetrêsdofeminino.Aidadevariouentre28e77anos,14daetniabrancae 11nãobranca;18forampreparadosporinjec¸ãointra-arterialdeumasoluc¸ãodeglicerinae formola10%eseteforamdissecadosafresco.

Resultados: Onervointerósseoanteriororiginou-sedonervomedianoemmédiade5,2cm distalàlinhaintercondilar.Em29membros,originou-sedosfascículosnervososdaregião posteriordonervomedianoeem21membros,dosfascículosposterolaterais.Em41mem- bros,onervointerósseoanteriorposicionava-seentreascabec¸asumeraleulnardomúsculo pronadorredondo.Emdoismembros,observou-seaduplicac¸ãodonervointerósseoante- rior.Emtodososmembros,registramosqueonervointerósseoanteriorsedesprendiado nervomedianoproximalmenteàarcadadomúsculoflexorsuperficialdosdedos.Em24 antebrac¸osaramificac¸ãodonervointerósseoanteriorocorreuproximalmenteàarcadado músculoflexorsuperficialdosdedosem26,distalmente.

Conclusão: Asbandasfibrosasformadaspelascabec¸asumeraleulnardomúsculopronador redondo,aarcadafibrosadomúsculoflexorsuperficialdosdedoseomúsculodeGantzer, quandohipertrofiadoeposicionadoanteriormenteaonervointerósseoanterior,podem comprimironervocontraestruturasprofundas,alterarseucursonormal,porestreitaro espac¸odesuapassagem,causaralterac¸õesnomúsculoflexorlongodopolegarenoflexor profundodosdedosdamão.

©2017SociedadeBrasileiradeOrtopediaeTraumatologia.PublicadoporElsevier EditoraLtda.Este ´eumartigoOpenAccesssobumalicenc¸aCCBY-NC-ND(http://

creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

Theanteriorinterosseousnerve(AIN)emergesontheposte- riorsurfaceofthemediannerveindifferentlocations.Atits origin,itisinitiallypositionedparalleltothemediannerve;

moredistally,itliesintheintervalbetweentheflexorpolli- cislongus(FPL)laterally,andtheflexordigitorumprofundus (FDP),medially,sendingbranchestothesetwomuscles.Ithas aconstantbranchtotheflexorindicisprofundusandpartially suppliestheflexordigitorumprofundusofthemiddlefinger.

Theflexordigitorumprofundusoftheother fingersissup- pliedbytheulnarnerve.TheAIN,afterbranchingtotheFDP andFPL,followsalongtheanteriorinterosseousartery,rest- ingontheanteriorfaceoftheinterosseousmembraneand distallyinnervatingthepronatorquadratus(PQ) muscle.Its thinnerterminalbranchpassesthroughthedorsalaspectof thePQmuscle,sendingsensorybranchestothecarpaljoints.¹ However,thereisconsiderablevariationintheproportionin whichthemedianandulnarnervessupplytheflexordigito- rumprofundus.^1–3

AstheAINisdeeplylocated,itisprotectedbyseveralstruc- tures, which makelesions to it rare;however, while these structuresprotecttheAIN,theycanbecausesofitscompres- sion.TheAINcanbecompressedbytheStruthersligament;

bicipitalaponeurosis;fibrousarchesbetweenthehumeraland ulnar heads of the pronator teres muscle (PT); the fibrous

archformed bytheorigins ofthe flexordigitorum superfi- cialis(FDS)muscle;presenceofanomalousmusclessuchas theGantzermuscle;vascularchanges,suchasthrombosisor vesselhypertrophy thatcrossthe nerve;tumorformations;

cysts;hematomas;abscesses;iatrogenesisinfracturereduc- tion ordrugs injectedinthe forearm;and trauma,suchas supracondylarfractureofthehumerusandtheproximalthird oftheforearm.^3,4

AINcompressivesyndromeisararecompressiveneuropa- thyoftheupperlimb.ItwasfirstdescribedbyParsonageand Turner⁵ in1948,andlaterbyKilohandNevin²in1952.Itis characterizedbytheinabilitytoflexthedistalinterphalangeal joints ofthe thumb and index finger,causing an inability to make a pulp pinch, hyperextension of the distal inter- phalangealjoint,andflexionoftheproximalinterphalangeal joint; inthe thumb, there isflexion of the metacarpopha- langealjointandhyperextensionoftheinterphalangealjoint, whichresultsinacontactareaofthethumbpulpwiththeindi- catormuchmoreproximalthannormal.⁶InincompleteAIN, less axonaldamageisobservedandonlytheflexionofthe distalphalanxofthethumborindexfingeriscompromised.

PQmuscleimpairmentcanbedemonstratedbyresistedactive pronationoftheforearmwithafullyflexedelbowtoneutral- izePTmuscleaction.Nosensorydeficitsareobservedinthe clinicalevaluationofthehandandforearm.^3,4

Itisdifficulttodeterminethe etiologyofAINsyndrome preciselybecausetherearenosignsorclinicalteststhatcan

indicatewhetheritisacompressiveneuropathyorabrachial plexusneuritis. Kiloh and Nevin² have proposed that it is causedbyAIN neuritis;incontrast,FearnandGoodfellow⁷ have suggested that it is a compressive neuropathy, and bothhypothesesremainwidelyaccepted.AINsyndromehas beenincreasinglyunderstoodasaneuritisandoftenresolves spontaneously afterprolonged observation.^4,8,9 In contrast, thosewhoconsideredittobeanervecompressionadvocate treatmentoftheconditionbysurgicalexplorationanddecom- pressionofthenerve.¹⁰

Thisstudywasaimedatanalyzingtheanatomicalrelation- shipandthevariationsoftheAIN,anditsclinicalimplications, suchascompressiveneuropathy.

Material and methods

Thisstudywasbasedonthedissectionof50limbsfrom25 cadavers,22malesandthreefemales.Agerangedfrom28to 77years;14ofthecadaverswerewhiteand11werenon-white.

Ofthetotal,18werepreparedwithanintra-arterialinjection ofglycerin and 10% formaldehyde, whileseven specimens were freshly dissected cadavers. Cadaverswhose forearms weredeformedbytraumas,malformations,and scarswere excluded.Nocasesofmuscularatrophyintheforearmswere observed.Eachforearmwasdissectedwiththeelbowinexten- sion,wristinneutralposition,andforearminsupination.The skin,subcutaneousmuscle,andfasciaoftheflexorsurfaceof thedistalthirdofthearm,forearm,andwristwereremoved, thus exposingall the musculature. Themedian nervewas identifiedinthe arm anddissected inaproximal todistal direction.Thebicipitalaponeurosiswassectioned.Theprona- tor’ssuperficialheadwasdistallydisinsertedandretracted.

Thetendonsoftheflexorcarpiradialisandpalmarislongus musclesweresectionedintheirdistalthird,afteridentifica- tionoftheirnervebranches.TheAINanditsnervebranches totheFPL,FDP,andPQmusclesweredissectedafterthelon- gitudinaldivisionoftheFDSmuscleanditsfibrousarch.The vascularstructureswerenotpreservedtofacilitatenervedis- section.Theoriginofeachmusclebranchofthemediannerve wasmeasuredfromtheintercondylarline.TheratiosofAIN withthehumeralandulnarheadsofthePTmuscleandwith thearcadeoftheFDSmuscleweremeasured,aswellasthe branchesoftheAIN. Thesedissectionsalsoaimedatiden- tifyingtheanatomicalsitesthatmayberesponsibleforthe compressionofthemedianandanteriorinterosseousnerves, suchastheStruthersligament,bicipitalaponeurosis,andthe sitebetweenthehumeralandulnarheadsofPTmuscleand thefibrousarchoftheFDSmuscle.ThepresenceoftheGantzer muscleandoftheMartinGruberanastomosiswererecorded.

Incertainphasesofthedissection,a2.5×magnifyingglass wasused.Theanatomicalvariationswereregisteredandpho- tographed.ThisstudywasapprovedbytheEthicsCommittee underopinionNo.1,611,295.

Results

Inthepresentstudy,theAINdetachedfromthemediannerve atadistanceof1.5to7.5cm,atameanof5.2cmdistaltothe intercondylarline.In29limbs,it originatedfromthenerve

B A

(a)

(a) (b)

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(b) (c)

Fig.1–(A)In29limbs,theAIN(a)originatedfromthe nervefasciclesoftheposteriorregionofthemediannerve (b).PTmuscle(c);(B)in21limbs,theAINfascicles(a) originatedfromtheposteromedialfasciclesthemedian nerve(b).FDSmusclearch(c).

B A

(a)

(a) (b) (c)

(c)

(b)

Fig.2–(A)AIN(a)branchedoffthemediannerve(b) proximallytothePTmuscle(c)in12limbs;(B)AIN(a) branchedoffthemediannerve(b)distallytothePTmuscle (c)insixlimbs.

fasciclesoftheposterioraspectofthemediannerve(Fig.1A), andin21limbs,fromtheposterolateralfascicles(Fig.1B).In 12limbs,theAINdetachedfromthemediannerveproximally tothePTmuscle(Fig.2A),andinsix,distallytoit(Fig.2B).In

B A

(a)

(a) (b)

(c)

(c)

(c)

(b)

Fig.3–(A)In32limbs,theAIN(a)branchedoffthemedian nerve(b)underthemusclemassofthePTmuscle(c);(B)in thevastmajorityofcases,theAIN(a)waspositioned betweenthehumeral(b)andulnar(c)headsofthePT muscle.

B A

(a)

(a) (b)

(d)

(c)

(c)

(b)

Fig.4–(A)Inthreelimbs,theAIN(a)andmediannerve(b) werepositionedposteriorlytothehumeral(c)andulnar(d) headsofthePTmuscle,whichweredisinsertedand retractedlaterally;(B)AIN(a)andmediannerve(b)crossthe humeralhead(c)ofthePTmuscle.

32limbs,itwasdetachedfromthemediannerveunderthePT musclemass(Fig.3A).Inmostcases(41limbs),theAINwas positionedbetweenthehumeralandulnarheadsofthePT muscle(Fig.3B).Intwolimbs,theAINwaspositionedposte- riorlytothetwoheadsofthePTmuscle(Fig.4A).Intheseven

(a2)

(a1)

FPD FLP

FLP (e)

(a)

A

B

(b)

(b)

(d)

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(c) (c)

(e)

FPD

Fig.5–(A)DuplicationoftheAIN,theproximalbranch(a1), andthedistalbranch(a2).Mediannerve(b)FPL(c),FDP(d).

BranchtothePQ(e);(B)mediannerve(a).AIN(b).Abranch totheFPL(c)andonetotheFDP(d).BranchtothePQ(e).

limbsinwhichtheulnarheadwasnotpresent,itwasobserved thattheAINwaspositionedposteriorlytothehumeralhead infivelimbs(Fig.3A)andpassedthroughitintwo(Fig.4B).

Inonelimb,itwasobservedthattheAINwasdoubled.The proximalbranchdetachedfromthemediannerve4cmdis- tallytotheepicondylarline,supplyingtheFDP;inturn,the distalbranchemerged1.0cmbelowit,supplyingpartofthe FDPandFPL,andwasdirectedtowardthePQmuscleandthe carpaljoint(Fig.5A).Inanotherlimb,thebranchestotheFDP and FPLoriginatedseparately fromthemediannerve,with thebranchingtotheFDSmusclebetweenthem.Inallthedis- sectedlimbs,itwasobservedthattheAINbranchesoffthe mediannerveproximaltothearchformedbetweentheinser- tionsoftheflexorsuperficialismuscle(Fig.1B).In24forearms, theAINramificationoccurredproximallytothearch(Fig.3B), andin26,distallytoit(Fig.1B).Thefibrousarchwasidentified in32forearms(Fig.1).Inthreeforearms,thearchwascon- sideredtobeirregular,astherewasadiscontinuitybetween thefibersthatformedthearch.Thestudyalsoassessedthe number oframificationsfrom thebranches tothe FPL and FDPmusclesthatpenetratedatdifferentpointsofthemus- cles.Thepresenceofonebranchtoeachofthemuscleswas recordedinsixlimbs(Fig.5B);oftwobranchestotheFDPand onetotheFPL,in14limbs(Fig.6);oftwobranchestoeach muscle,in14limbs(Fig.3A);ofthreebranchestotheFDPand onetotheFPL,insevenlimbs(Fig.6B);ofthreebranchesto theFDPandtwototheFDS,infivelimbs(Fig.7A);ofthree totheFDPandthreetotheFPL,inthreelimbs(Fig.7B);and another sixbranches toFDPand twotoFPL, (Fig.5A).The longestbranchwasalwaystotheFPL(Figs.5Aand6B).The AIN left themediannerve proximallytothe mediannerve

(c)

(c) (d)

(e)

(a)

(a) (a)

A FPD

FLP

A

B

(b) (f)

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Fig.6–(A)Mediannerve(a),AIN(b),twobranchestothe FDPandonetotheFPL.Twosuperfluousbranchesofthe mediannervetothePTmuscle(c),branchtothepalmaris longus(d),branchtotheflexorcarpiradialis(e),FDS(f).

TheycanbeneurotizedtotheAIN(d);(B)mediannerve(a).

AIN(b).Ulnarnerve(c).ThreebranchestotheFDP,andone totheFPL.

branchtotheFDSmusclein41limbs(Fig.8A);infour,atthe samelevelasthebranchtotheFDS;inthree,distallytoit;in one,itoriginatedbetweentheAINbranchestotheFDPand FPLmuscles.Intwolimbs,itoriginatedfromacommontrunk withthemedianbranchestotheFDSandflexorcarpiradi- alismuscles.Infourlimbs,theFDSmusclereceivedabranch from theAIN, as well asthe innervationfrom the median nerve(Fig.8B).Inonelimb,thepresentationwasabsolutely atypical;theAINbranchedoffthemediannerve,proximally totheintercondylarlineand proximally tothe innervation ofallthe musclesoftheforearm, including thePTmuscle (Fig.9A).FourlimbspresentedanexpansionoftheAINthat providedramificationtothe flexorprofundusofthefourth finger(Fig.5B).TheGantzermusclewasobservedin34fore- arms,andinninecasesthismusclewaspositionedanteriorly totheAIN(Fig.9B).Inalltheforearms,theGantzermuscle receivedexclusiveinnervationfromtheAIN.MartinGruber’s anastomosiswasidentifiedin13forearms,andineightthe nervecommunicationoccurredbetweentheulnarnerveand theAIN(Fig.7A).

Discussion

Thereis somecontroversy astowhere theAIN leaves the median nerve. Sunderland¹ reported that the AIN left the mediannerve between2.3 and 8cm distally tothe medial

(b)

(d) FPD

FLP (a)

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FPD

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Fig.7–(A)Mediannerve(a).Ulnarnerve(b).AIN(c).Martin Gruber’sanastomosis(d).ThreebranchesoftheAINtothe FDPandtwobranchestotheFPL;(B)mediannerve(a).

Ulnarnerve(b),AIN(c)ThreebranchesoftheAINtothe FDP(c)andthreebranchestotheFPL,onedirectedtothe Gantzermuscle(d).

(b)

(a)

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A

B

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(b) FSD

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Fig.8–(A)TheAIN(a)branchedoffthemediannerve(b) proximallytothebranchtotheFDS(c)into41limbs.PT muscle(d).Palmarislongusmuscle(e).Flexorcarpiradialis muscle(f);(B)infourlimbs,theFDSmusclereceiveda branchfromtheAIN(a)inadditiontotheinnervationofthe mediannerve(b).

(c) (a)

(b)

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(b) (a)

A

B

Fig.9–(A)Atypicalcase,AIN(a)branchedoffthemedian nerve(b),proximallytotheintercondylarlineand

proximallytotheinnervationofallmusclesoftheforearm, includingthePTmuscle(c);(B)Gantzermuscle(a)is positionedanteriorlytotheAIN(b)andposteriorlytothe mediannerve(c).

humeralepicondyle.Tubbsetal.¹¹observedthattheAINorig- inatedfromthemediannerveatameanof5.4cmdistallyto thejointline.AccordingtoFreiberg,¹²theAINleftthemedian nerve8cmdistallytothemedialepicondyle.Vinceletetal.¹³ dissected35cadavericlimbsandobservedthattheAINorig- inatedonaverage45mmbelowtheintercondylarline.Inthe presentstudy,theAINleftthemediannerveatadistanceof 1.5–7.5cm,atameanof5.2cmdistallytotheintercondylar line.

Sunderland¹ and Tubbs et al.¹¹ reported that the fasci- clesthatformedtheAINwereposteriorlyorposterolaterally locatedinthemediannerveand thatagroupofnervefas- ciclesseparatedfromthemediannervehadalreadyformed approximately 2.5cmbefore the emerging site. Dellon and Mackinnon⁹observedthattheAINoriginatedfromthepos- terioraspectofthemediannervein12of31limbsandfrom theposterolateralaspect,in19.Inthepresentstudy,thenerve originated inthe posterior fasciclesin 29,and in the pos- terolateralfasciclesin21.Thepresentauthorshadthesame impressionasDellonandMackinnon⁹ thattheAINismore susceptibletocompressionwhenitoriginatesfromthelat- eralaspect,andthatitismoreprotectedfromfibrousarches whenit islocatedposteriorly.Botte¹⁴ reportedthattheAIN originates5–8cmdistaltothemedialepicondyle,justbelow thearchoftheFDSmuscle;thepresentresultsarenotinagree- mentwiththisstatement,sincetheAINwasnotobservedto branchofffromthemedialnervedistallytotheFDSmuscle

archinanyofthestudiedlimbs.Tubbsetal.¹¹statedthatAIN canbecompressedbythefibrousarchbetweentheinsertions of the FDS muscle. In the present study,in 32 of 50 fore- arms, thefibrousarchwassupportedonthemediannerve andtheAIN;thiscouldbethecauseofnervecompression.

Parketal.,¹⁰inastudywith11patientswithspontaneousAIN syndrometreatedbysurgicalexploration,observedthatthe mostcommon structureresponsible fornervecompression wasafibrousarchoftheFDSmuscle.DellonandMackinnon⁸ identifiedthat,in16of31limbs,theAINwasrelatedtofibrous arches.Inthree,thefibrousarcheswereformedonlybythe archoftheFDSmuscle;intwo,bythefibrousarchesofthe ulnarheadofthePTmuscle;inthree,bythehumeralhead ofthePTmuscle;andineight,bythecombinationoffibrous arches formed by the arch of the FDS muscle and by the humeralandulnarheadsofthePTmuscle.GuoandWang¹⁵ reportedthattheAINbrancheddistallytothearchoftheFDS musclein74%oftheforearms,butthoseauthorsdidnotmen- tionthefrequencywithwhichtheAINleftthemediannerve distallytothearcadeoftheFDSmuscle.Intheirstudy,the AIN originatedfrom the ulnarside ofthe mediannervein only8%ofthecases;thoseauthorsrecommendthatthesur- gicaldecompressionofthearchshouldbedoneontheulnar sideofthemediannerve,thuspreservingthebranchesofthe AIN. Incontrast,Tubbs etal.¹¹ reportedthat theAIN origi- natedproximallytothearchoftheFDSmuscleinallthe60 limbsassessed.ThepresentfindingsconfirmthosebyTubbs etal.,¹¹astheAINoriginatedproximallytothearchinallfore- arms,oftenveryclosetothearchbutnotdistallytoit.TheAIN ramificationwasproximaltothearchin26limbsanddistalto itin24.

Therearecontroversiesintheliteratureregardingtherela- tionshipsbetweenAINandPTmuscle;somereportthatthe AINoriginatesdistallyfromthehumeralandulnarheadsof thePTmuscle,whileothershavedescribeditasoriginating betweenthetwoheadsofthatmuscle.Tubbsetal.¹¹reported thattheyalwaysobservedtheAINpositionedposteriorlyto thehumeralheadofthePTmuscle;iftheulnarheadispresent, theAINliesbetweenthem.ChidgeyandSzabo¹⁶reportedthat theAINmaybepositionedposteriorlytothetwoheadsofthe PTmuscle.Johnsonetal.¹⁷dissected40preservedcadavers;

in90%ofthedissections,theAINbranchedoffthemedian nerve distallytothe PTmuscle; inonly10% ofthe dissec- tions,didtheAINbranchoffthemediannerveunderthePT muscle,andnoseparationbetweentheAINandthemedian nerveproximaltothePTmusclewasobserved.In41limbs assessedinthepresentstudy,thehumeralandulnarheadsof thePRmusclewerewellpartitionedastwodistinctstructures.

In 39 limbs, the AIN was positioned betweenthe humeral andulnarheadofthePTmuscle;intwolimbs,itwasposi- tionedposteriorlytothetwoheadsofthePTmuscle.Inseven limbs,the ulnarhead wasabsent;infiveofthese,the AIN waspositionedposteriorlytothehumeralhead,andintwo itpassedthroughit.TheAINoriginatedproximallytothePT musclein12limbs,distallytoitinsixlimbs,andunderitin 32limbs.

Regarding the anatomical variations of the typical AIN innervation,ChidgeyandSzabo¹⁶reportedthattheextension oftheinnervationtotheflexordigitorumprofundusofthe middlefingerwasobservedinapproximately50%ofthecases.

KilohandNevin²considerthatthereisaconsiderablevaria- tionintheproportionthatthemedianandulnarnervessupply theflexordigitorumprofundusofthefingers.Theseauthors describedtwoclinicalcasesofAINinvolvement;inthefirst, therewasparalysisoftheFPLandFDP,andinthesecondcase, onlytheFPLwasparalyzed.Ulrichetal.³reportedthatseven oftheir14patientspresentedisolatedtotalparalysisofthe FPLfunction;infive,theFPLandthedeepflexoroftheindica- torwereparalyzed,andintwopatients,onlytheindexflexor wasimpaired. Noneoftheirpatientshad totalparalysis of theflexordigitorumprofundus ofthe middlefinger,but in fourcasestherewassomelossofflexionforceofthefinger.

Twopatientspresentedadecreaseinpronationforce.Botte¹⁴ describedthatinallofthemedialhand,theAINalsosupplies theflexorsofthefourthandfifthfingers.Inthepresentstudy, infivelimbsitwaspossiblethattheAINcouldsupplythethird andfourthfingers,butduetotheirsyncytialnature,itisdif- ficulttomakethisaffirmationbasedonlyonthedissections.

Sunderland¹identifiedabranchoftheAINtotheFDSmuscle in30%ofcases.Inthepresentstudy,theFDSwasinnervated bytheAINandbythemediannerve,infourlimbs.Outofthe 13limbswhereMartin-Gruber’sanastomosiswasidentified, ineightacommunicationwasobservedbetweentheAINand theulnarnerve;inthesecases,itispossiblethatmotornerve fibersfromtheAINinnervatetheintrinsicmusclesofthehand usuallyinnervatedbytheulnarnerve.^6,18

Dellon and Mackinnon⁹ reported that a hypertrophied GantzermusclemaycompresstheAIN.Theyreportthat,in themostdistalpartofitscourse,thenervecancrossposteri- orlytothetendonoftheGantzermuscleandthuscompress thebranchoftheAINtothePQmuscle, causingweakness at forearm pronation. Those authors also reported that a hypertrophiedGantzermuscle,evenpassinganteriorlytothe AIN,may compressthe nervebetweenthe Gantzermuscle andthemusculoaponeuroticstructuresofthePTmuscleand FDS.In the present study,the Gantzer muscle was identi- fiedin34limbs;theauthorsspeculatethatahypertrophied Gantzer muscle cancause AIN syndrome,but this was an infrequentfinding,asthishypertrophywasonlyobservedin threelimbs.Tabitetal.¹⁹reportedtheclinicalcaseofapatient withincompleteAINandparalysisonlyoftheFPL,whichwas interoperativelyproventobecausedbytheGantzermuscle.

Thepresentfindingsindicatedthatinnineofthelimbs,the AINcrossedonlythebranchestotheFPLorFDP;inthesecases, itcouldonlycauseincompleteAINsyndrome,whichcanbe misdiagnosedasatendoninjury.²⁰Digitalpressurealongthe courseofthemusculotendinouscomponentproducesaslight flexionoftheinterphalangealjointandconfirmstheintegrity ofthetendon.

Conclusion

Thefibrousarchesformedbythehumeralandulnarheads ofthePTmuscle,thefibrousarchoftheFDSmuscle,andthe Gantzermusclewhenhypertrophiedandpositionedanterior totheAINcancompressthenerveagainstdeepstructures, changingitsnormalcoursebynarrowingthespaceofitspas- sage,andcausealterationsintheFPLandFDPmuscles.

Conflicts of interest

Theauthorsdeclarenoconflictsofinterest.

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