BurrHolesandBoneFlapsLastUpdated:September27,2018

Figure1:HarveyCushingillustratedanosteoplasticcraniotomyandcorticalmappinginapatientwhosufferedfromagunshotwound(illustrationcourtesyoftheCushingBrainTumorRegistryatYaleUniversity).

TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.

Thecreationofburrholesandboneflapsisamongthemostfundamentalproceduresincranialsurgeryandtheirtimelycompletionimprovesoperativeefficacy.Althoughsimpleandstraightforward,theseproceduresarenotwithoutrisks.Mostimportantly,theoperatorshouldbeintimatelyfamiliarwithcomplicationavoidancestrategiestominimizemorbidityfromtheserelativelybasicstepsoftheoperation.

Figure2:Theskullbonecontainstheouterandinnercorticalbonelayers(tables)separatedbycancellousbone.Thisanatomicrelationshipisimportantduringcreationoftheburrholebecausetheoperatorcansafelydrillexpeditiouslythroughtheoutertableandcancellousbone,butshouldgentlyshellouttheinnercorticallayertoavoiddirecttransmissionofthedrillvibrationstotheduraandcortex.

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

hematomaformation,especiallywithyoungpatients.

Figure3:Placementofaholealongtherootofthezygomaisshownhere.Aftertheoutertableandcancellousbone(yellowarrow)havebeendrilledunderampleirrigation,theinnercorticallayer(bluearrow,rightupperimage)iscarefullythinnedoutandasmallareaoftheduraexposed(leftmiddleimage).Next,aKerrisonrongeurorcuretteremovesadditionalpiecesofthe

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.

Figure4:Theburrholeforastandardpterionalcraniotomyisplacedatthejunctionofthefrontotemporallineandthefrontalprocessofthezygomaticbone.Becauseofthedifferenceinthethicknessoftheskullatthefrontalandtemporalbones(thefrontalboneisthicker),thetipoftheacorn-shapedbitcanreachthedurainsideaportionoftheburrholebeforetheentirebaseoftheburrholeisremoved.Thisphenomenoncanleadtocorticalinjury,especiallyinpatientswitha“tight”brain.Carefuluseofthedrillwillminimizetheriskofthiscomplication.ThisisoneofthereasonswhyIprefertouseasingleburrholejustinferiortothesuperiortemporallineforpterionalcraniotomies.Anotherreasonistheneedtominimizeboneresectionincosmeticallyvisibleregions(suchastheareainfrontofthehair

line,suchasthekeyhole,thatisnotcoveredbythetemporalismuscle).

Figure5:Iuseonlyoneburrholeforcreationofabifrontalcraniotomytominimizecosmeticdeformityaslongastheduraiseasilyseparable.AB1drillbitwithoutafootplatemaybeusedtocreatethemostinferiorosteotomyparalleltotheskullbase.Thismaneuverwillallowcontrolledtransectionoftheanteriorandposteriorfrontalsinuswalls.Theuseofthefootplateinthisregioncanbeproblematicbecausetheskullisverythickneartheairsinuses,andforcefulmaneuveringofthedrillcanleadtoduralandcorticalinjuries.

Figure6:Thefearofaduralvenousinjuryshouldnotdetertheoperatorfromgenerouslyuncoveringthesestructurestoallowexpansionofthesupracerebellaroperativecorridorthroughmobilizationoftheduralsinusessuperiorly.Placementofburrholesoverthetransversesinusesisrelativelysafe,andcorrectuseofthefootplatetippursuingthecontoursoftheinnertableleadstoasingle-pieceboneflapexposingtheduraandtheroofofthevenoussinuses.

Figure7:Theunevenskullthickness(forexample,withinthekeyholeoraroundthevenoussinuses)canbeasignificantcauseofcortical,duralandvenoussinusinjurybecausethecuttingburrsalongtheequatoroftheacornbitcanreachtheduracoveredbythethinnertemporalsectionoftheskullorthewallofthesinuswhilethetipofthedrillisstillworkingontheadjacent

thickerfrontaloroccipitalbonewithintheburrhole,respectively.

Thiseventisespeciallyproblematicwhenthesurgeonismakingburrholesovertheduralsinusesforparasagittalandretromastoidcraniotomies.Becausethewallsoftheduralsinusesembedthemselveswithintheinnertable,thecuttingburrsonthesideofthedrillbitcaninjurethesinuswallwhilethedrilltipworksonthethickerbonecoveringthesidewallsofthesinus.

Idonotroutinelyplacetwoburrholes,oneoneachsideoftheduralvenoussinuses.Infact,asingleburrholeandeffectiveseparationofthewallofthesinusisoftenadequatetosafelycompletethecraniotomy.Boneworknearthesigmoidsinusisanexceptionbecausethissinusisnotonlyveryadherent,butalsoisalmostalwaysembeddedinthemastoidbone.

Figure8:Increasedintracranialpressureisarealriskforsubduralhematoma,subarachnoidhemorrhage,andcorticalinjuryduringdrillingandconsequenttransmissionofvibrationstothepartofthebrainpushedagainsttheskull.Thedrillshouldbeusedcarefullyandthefootplatemaneuveredsmoothlytominimizedirecttransmissionoftheunnecessaryexcessivevibrationstothebrain.TheuseoflumbarCSFdrainagecandramaticallydecreasetherisk.

CraniotomyandElevationoftheBoneFlap

Oncetheduraisseparated,theburrholescanbeconnectedusingaB1bitwiththefootplateattachment(craniotome).Theoperatorpracticingthetechniqueofboneflapcreationshouldconsiderthefollowingdetails:

1. Thefootplatecannotmakesharpturns,ratheritisdesignedtomakewideturns;wideturnswillminimizetheriskofaduraltear.

2. Thesurgeonmustconsidertheanatomyofthecalvariuminrelationtotheskullbase,includingthelateralsphenoidwing,orbitalroof,frontalsinuses,theobliquefloorofthemiddlefossa,andtheforamenmagnum.

3. Thedrillshouldeffortlesslycuttheboneandfollowtheinnercontoursofthecalvarium.Ifthisisnotthecase,thefootplateismostlikelynottangentialtotheinnersurfaceoftheskullortheduraisveryadherent.Theoperatorshouldpause,assessforthesetwopossibilities,andresolvetheunderlyingcauses.Theoperatorshouldnotcontinuetoforcefullyhandlethedrill.

Figure9:Forthefootplatetoremaintangentialtothecontoursoftheskull,thedrillhandleshouldbeperpendiculartotheskullsurface(topimage).Ifthehandleisnotperpendicular(lowerimage),thedrillwillnotadvancesmoothlyandtheoperator’sstruggletopushthedrillforwardwillleadtoduraltearandrisk

transmissionofvibrationstothebrain.

Iftheduraisundulyadherenttotheskull,theB1bitwithoutafootplateisusedtocreateatroughdowntothethinned-outinnertablethatcanberemovedusingfineKerrisonrongeursorgentlyfracturedtocompletetheosteotomy.

Bonewaxwillstopbleedingfromtheexposedcancellousbonealongthecraniotomyedgesandthrombin-soakedgelfoampowdercanstopepiduralvenousbleeding.Theduraistackeduptothecraniotomyedges.Tensebluishduramostlikelyrepresentsthepresenceofasubduralhematomacausedduringthebonework.Ifthisoccurs,thedurashouldbeimmediatelyopenedandthehematomaevacuated.

Figure10:Thefootplateshouldstaytangentialtotheinnersurfaceoftheskulltoavoidinadvertentduraltears,especiallyaroundtheduralvenoussinuses(insetimage).Thisnuanceisoftenignoredbyjuniorresidents.Theduralsinusescanembedthemselveswithintheinnertableoftheskull.

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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