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BurrHolesandBoneFlapsLastUpdated:September27,2018
Figure1:HarveyCushingillustratedanosteoplasticcraniotomyandcorticalmappinginapatientwhosufferedfromagunshotwound(illustrationcourtesyoftheCushingBrainTumorRegistryatYaleUniversity).
TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.
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Thecreationofburrholesandboneflapsisamongthemostfundamentalproceduresincranialsurgeryandtheirtimelycompletionimprovesoperativeefficacy.Althoughsimpleandstraightforward,theseproceduresarenotwithoutrisks.Mostimportantly,theoperatorshouldbeintimatelyfamiliarwithcomplicationavoidancestrategiestominimizemorbidityfromtheserelativelybasicstepsoftheoperation.
Figure2:Theskullbonecontainstheouterandinnercorticalbonelayers(tables)separatedbycancellousbone.Thisanatomicrelationshipisimportantduringcreationoftheburrholebecausetheoperatorcansafelydrillexpeditiouslythroughtheoutertableandcancellousbone,butshouldgentlyshellouttheinnercorticallayertoavoiddirecttransmissionofthedrillvibrationstotheduraandcortex.
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BurrHoles
Creationoftheburrholesdeservesspecialprecautionsthatinvolvethefollowing:
1. protectionoftheduralvenoussinuses,2. preservationoftheduralintegrity,3. avoidanceoftheairsinuses,4. carefulhandlingofthelesionsthatinfiltratetheskulland
obscureepidurallandmarks,and5. smooth,gentle,andseemingly“effortless”useofthedrillto
avoidthetransmissionofvibrationstothebrain(thesevibrationscancausecontusionsorsubarachnoidhemorrhage).
Forcefuluseofthedrillburrtopushagainsttheinnercortexorplungeintotheduraorbraincanleadtocorticalinjurythatwouldhavebeenavoidable.
Thenumberandthesizeofburrholesneededtosafelycompletethecraniotomydependsonmultiplefactors,including:
1. thesizeandlocationofthecraniotomy(overtheduralsinusesorcranialsutures),
2. thenatureofthelesion(itsinvolvementwiththebone),3. thepatient’sage,and4. theoveralladherenceoftheduratotheinnertableoftheskull
bone.
Iprefertoplacetheburrhole(s)behindthehairlinetominimizecosmeticdeformitycausedbyboneloss.Generousburrholesalloweffectiveseparationoftheduralbeyondthemarginsoftheburrhole.Thedurashouldbemanipulatedunderneaththeplannedbordersofthecraniotomyandnotseparatedbeyondtheedgesofthecraniotomybecausethisseparationcanleadtooccultepidural
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hematomaformation,especiallywithyoungpatients.
Figure3:Placementofaholealongtherootofthezygomaisshownhere.Aftertheoutertableandcancellousbone(yellowarrow)havebeendrilledunderampleirrigation,theinnercorticallayer(bluearrow,rightupperimage)iscarefullythinnedoutandasmallareaoftheduraexposed(leftmiddleimage).Next,aKerrisonrongeurorcuretteremovesadditionalpiecesofthe
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thinshellofinnercorticalbone(rightmiddleimage).Thistacticexpandstheinnerrimoftheburrholebeyondtheedgesoftheouterrim(undercuttingtheinnertable)toallowthetipoftheduraldissectortoachieveacuteworkinganglesandstaytangentialtotheinnertable,avoidinganinadvertentduraltear(leftlowerimage).Thesemaneuversallowtheduraldissectortotravelwellbeyondtheburrholetoeffectivelyseparatetheentiresectionoftheduraundergoingacraniotomy(rightlowerimage).Thesemethodologiesexpandtheworkingzoneofthedissectorandmostoftenfacilitateelevationoflargeboneflapsusingonlyoneortwoburrholes.
Forcefulinsertionanduncontrolledmaneuveringoftheduraldissector(Penfield#3dissector)shouldbeavoidedbecausethiscanleadtoduraltearsandpotentiallycorticalinjury.Iftheduraisnotreadilyseparatingfromtheinnersurfaceoftheskull,additionalburrholesshouldbecreated.Theduracanbeespeciallyadherentalongtheskullsutures,andthedrillfootplatehasthepotentialtoviolatetheduralintegritywhenitchangesdirectionorturnstoapproachtheskullbase.Therefore,thedurashouldbeeffectivelyseparatedintheseareas.Todoso,duringthecraniotomy,Igentlytogglethedrillfootplatetofurtherseparatethedurawiththetipofthefootplate.
Iftheduraistorninitiallyduringplacementoftheburrhole,theholeshouldbeexpandedtoexposeintactdurathatwillbeusedasalandmarktoseparatethesectionsoftheduraunderlyingtheplannedboneflap.
ThelocationoftheburrholesforavarietyofcraniotomiesisdiscussedintheircorrespondingchaptersintheCranialApproachesvolume.Inthischapter,Ireviewsomebasicconcepts.
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Figure4:Theburrholeforastandardpterionalcraniotomyisplacedatthejunctionofthefrontotemporallineandthefrontalprocessofthezygomaticbone.Becauseofthedifferenceinthethicknessoftheskullatthefrontalandtemporalbones(thefrontalboneisthicker),thetipoftheacorn-shapedbitcanreachthedurainsideaportionoftheburrholebeforetheentirebaseoftheburrholeisremoved.Thisphenomenoncanleadtocorticalinjury,especiallyinpatientswitha“tight”brain.Carefuluseofthedrillwillminimizetheriskofthiscomplication.ThisisoneofthereasonswhyIprefertouseasingleburrholejustinferiortothesuperiortemporallineforpterionalcraniotomies.Anotherreasonistheneedtominimizeboneresectionincosmeticallyvisibleregions(suchastheareainfrontofthehair
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line,suchasthekeyhole,thatisnotcoveredbythetemporalismuscle).
Figure5:Iuseonlyoneburrholeforcreationofabifrontalcraniotomytominimizecosmeticdeformityaslongastheduraiseasilyseparable.AB1drillbitwithoutafootplatemaybeusedtocreatethemostinferiorosteotomyparalleltotheskullbase.Thismaneuverwillallowcontrolledtransectionoftheanteriorandposteriorfrontalsinuswalls.Theuseofthefootplateinthisregioncanbeproblematicbecausetheskullisverythickneartheairsinuses,andforcefulmaneuveringofthedrillcanleadtoduralandcorticalinjuries.
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Figure6:Thefearofaduralvenousinjuryshouldnotdetertheoperatorfromgenerouslyuncoveringthesestructurestoallowexpansionofthesupracerebellaroperativecorridorthroughmobilizationoftheduralsinusessuperiorly.Placementofburrholesoverthetransversesinusesisrelativelysafe,andcorrectuseofthefootplatetippursuingthecontoursoftheinnertableleadstoasingle-pieceboneflapexposingtheduraandtheroofofthevenoussinuses.
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Figure7:Theunevenskullthickness(forexample,withinthekeyholeoraroundthevenoussinuses)canbeasignificantcauseofcortical,duralandvenoussinusinjurybecausethecuttingburrsalongtheequatoroftheacornbitcanreachtheduracoveredbythethinnertemporalsectionoftheskullorthewallofthesinuswhilethetipofthedrillisstillworkingontheadjacent
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thickerfrontaloroccipitalbonewithintheburrhole,respectively.
Thiseventisespeciallyproblematicwhenthesurgeonismakingburrholesovertheduralsinusesforparasagittalandretromastoidcraniotomies.Becausethewallsoftheduralsinusesembedthemselveswithintheinnertable,thecuttingburrsonthesideofthedrillbitcaninjurethesinuswallwhilethedrilltipworksonthethickerbonecoveringthesidewallsofthesinus.
Idonotroutinelyplacetwoburrholes,oneoneachsideoftheduralvenoussinuses.Infact,asingleburrholeandeffectiveseparationofthewallofthesinusisoftenadequatetosafelycompletethecraniotomy.Boneworknearthesigmoidsinusisanexceptionbecausethissinusisnotonlyveryadherent,butalsoisalmostalwaysembeddedinthemastoidbone.
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Figure8:Increasedintracranialpressureisarealriskforsubduralhematoma,subarachnoidhemorrhage,andcorticalinjuryduringdrillingandconsequenttransmissionofvibrationstothepartofthebrainpushedagainsttheskull.Thedrillshouldbeusedcarefullyandthefootplatemaneuveredsmoothlytominimizedirecttransmissionoftheunnecessaryexcessivevibrationstothebrain.TheuseoflumbarCSFdrainagecandramaticallydecreasetherisk.
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CraniotomyandElevationoftheBoneFlap
Oncetheduraisseparated,theburrholescanbeconnectedusingaB1bitwiththefootplateattachment(craniotome).Theoperatorpracticingthetechniqueofboneflapcreationshouldconsiderthefollowingdetails:
1. Thefootplatecannotmakesharpturns,ratheritisdesignedtomakewideturns;wideturnswillminimizetheriskofaduraltear.
2. Thesurgeonmustconsidertheanatomyofthecalvariuminrelationtotheskullbase,includingthelateralsphenoidwing,orbitalroof,frontalsinuses,theobliquefloorofthemiddlefossa,andtheforamenmagnum.
3. Thedrillshouldeffortlesslycuttheboneandfollowtheinnercontoursofthecalvarium.Ifthisisnotthecase,thefootplateismostlikelynottangentialtotheinnersurfaceoftheskullortheduraisveryadherent.Theoperatorshouldpause,assessforthesetwopossibilities,andresolvetheunderlyingcauses.Theoperatorshouldnotcontinuetoforcefullyhandlethedrill.
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Figure9:Forthefootplatetoremaintangentialtothecontoursoftheskull,thedrillhandleshouldbeperpendiculartotheskullsurface(topimage).Ifthehandleisnotperpendicular(lowerimage),thedrillwillnotadvancesmoothlyandtheoperator’sstruggletopushthedrillforwardwillleadtoduraltearandrisk
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transmissionofvibrationstothebrain.
Iftheduraisundulyadherenttotheskull,theB1bitwithoutafootplateisusedtocreateatroughdowntothethinned-outinnertablethatcanberemovedusingfineKerrisonrongeursorgentlyfracturedtocompletetheosteotomy.
Bonewaxwillstopbleedingfromtheexposedcancellousbonealongthecraniotomyedgesandthrombin-soakedgelfoampowdercanstopepiduralvenousbleeding.Theduraistackeduptothecraniotomyedges.Tensebluishduramostlikelyrepresentsthepresenceofasubduralhematomacausedduringthebonework.Ifthisoccurs,thedurashouldbeimmediatelyopenedandthehematomaevacuated.
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Figure10:Thefootplateshouldstaytangentialtotheinnersurfaceoftheskulltoavoidinadvertentduraltears,especiallyaroundtheduralvenoussinuses(insetimage).Thisnuanceisoftenignoredbyjuniorresidents.Theduralsinusescanembedthemselveswithintheinnertableoftheskull.
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Figure11:Forparasagittalcraniotomies,thelastbonycutisperformedovertheduralsinussothattheboneflapcanbequicklyelevatedifsinusinjuryoccurs.
PearlsandPitfalls
Whenplanningthecreationoftheburrholes,theoperatorshouldconsidertheanatomyoftheskulltopreventduralandcorticalinjuries.Theregionsoftheskullwithuneventhicknesscanplacetheduraatriskduringtheburrholeplacementandcraniotomy.Thedrillshouldbeusedsmoothly.Forcefulhandlingofthedrillwillleadtocorticalinjuryfromtransmissionofdrillvibrations.
Contributor:JonathanM.Parish,MD
DOI:https://doi.org/10.18791/nsatlas.v2.11
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