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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6051_Библиотеки_им_академика_М_И_Перельмана
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kyphotic angulation must be absolutelyavoided due to the resultant sagittal imbalance.
Allowingtheabdomen tohangfreelywithoutventral compressionhelpsminimizeintra-
abdominalpressureandvenousbleeding.
OperativeTechniques
SacralFixation
Pedicle screw pullout strength is increased by medialization of insertion trajectory
comparedwith “straight-ahead”insertion withoutmedial angulationofthe screwtip.
3,12
An obese body habitus, an iliac crest overhang, or triangulated vertebral bodies may
presentobstaclestoadequatemedializationofpediclescrewtrajectory(Fig.4.4a).Ifthis
problem is encounteredwhileusing a midline open skin incision, thebailout technique
requires splitting the fascia in a paramedian fashion for a transmuscular approach to
enable a more lateral starting point and medialization of the trajectory. Vigilance is
required to prevent an unintended straight-ahead trajectory from violating the anterior
sacral or lumbar cortex with resultant injury to neurovascular structures. The aortic
bifurcation occurs at approximately L4/L5, with the common iliac vessels traveling
laterallyfromthebifurcation.TheL4andL5nerverootstraversetheanterolateralsacral
cortexpriortojoiningthelumbosacralplexuslocatedatthelevelofthesacralala,andthe
colonisincloseoppositiontotheventralsurfaceofS2.Arelative“safezone”existsinthe
ventral midline of thesacral promontory; however, individual patientvascular anatomy
shouldbe incidentallyvisualizedand appreciatedonpreoperativespineimagingpriorto
proceedingwithinstrumentation.
Fig.4.3a–dS2-alar-iliacscrews. (a) Intraoperative CT-guided planningdemonstratingstartingpoint and trajectory of
S2-alar-iliacscrew.(b)PostoperativeCTscanshowingalignmentofS2-alar-iliacscrewheadwithlumbosacralpedicle
screws, eliminating the need for an offset connector typically required by traditional iliac screws. (c,d) X-rays
demonstratingusageofS2-alar-iliacscrewstoanchorlongconstructfordeformitycorrection.
Fig.4.4a–cComplicationsofsacral-pelvicfixation.(a)Failuretoadequatelydirectpediclescrewsinmedialtrajectory,
combined with significant screw perforation of anterior cortex, resulting in screws abutting internal iliac veins
bilaterally.(b,c)Asymptomatichaloformation(arrows;dottedline)around iliac screws doesnotnecessitate revision
unlessresultinginpainorlumbosacralpseudarthrosis.
IliacFixation
Misdirectionof iliacorS2-iliac screwsthrough thesciaticnotch cancause injurytothe
superiorglutealarteryorsciaticnerve,whichisararebutseriouscomplication.Violation
ofthe acetabulummust beavoided. Familiarizationwiththe iliacteardrop viewenables
correctscrewtrajectoryselection.
Placementofscrewsintotheiliumrequiressignificantinsertionaltorquethatcancause
breakageof the screwdriverunless careis taken tosequentially tapthe screw trajectory
completely to the desired depth. Once iliac screw insertion is initiated, one should not
pauseduringinsertion,asthemechanicalthermalenergygeneratedbyscrewinsertionin
the bone assists in temporarily lessening insertional torque, a benefit that is lost when
screwinsertionpauses.Iliacscrewheadprominenceandoffsetdistancefromlumbosacral
pediclescrewheadsisminimizedutilizingtheS2-alar-iliactrajectory.
RevisionofScrews
Asmentioned,iliacscrewsmayundergoasymptomatichaloing(Fig.4.4b,c).Unlesspain,
implantbreakage,orinstabilityoftheconstruct occurs,screwrevisionisnotrequired.If
iliac screw replacementis required, using larger diameterscrews confers more stability
thanusinglongerscrews.
ChapterSummary
Sacral-pelvic fixation is a powerful tool for increasing the strength and stability of
lumbosacral constructs. Extreme biomechanical forces at the lumbosacral junction and
relatively poor bone quality of the sacrum result in a high rate of lumbosacral
pseudarthrosis and implant failure in adult spinal deformity correction. Sacral-pelvic
fixation is indicated forlumbosacralarthrodesis extending cephalad to the L2 vertebra,
augmentationofconstructsforpoor-qualityorosteoporoticbone,sacrectomyfortumoror
infection, unstable sacral fractures, correction of flat-back syndrome via lumbar
osteotomy, correction of pelvic obliquity, high-grade spondylolisthesis, or as a salvage
mechanismduringrevisionforpseudarthrosis.
Sacral fixation is optimally performed using bi- or tricortical S1 medially directed
pediclescrews.AlarandS2screwsandhooksthroughthedorsalsacralforaminamaybe
addedforsupplementation,butlackthebiomechanicalstrengthtoanchorlongconstructs
inadultspinaldeformity.Caremustbetakentooptimizescrewsizeforcorticalpurchase
whileavoidinginjurytoneurovascularstructuresanteriortothelumbosacraljunction.
Iliacfixationis optimallyperformed usingiliac or S2-alar-iliacthreaded screws,in a
trajectorytowardthe anteroinferioriliacspine, withlength≥ 80mmanddiameter ≥9.5
mm. Knowledge of sacral-pelvic anatomy and use of the iliac teardrop view on
intraoperativeimagingiskeyforiliacscrewplacement.
Theadditionofinterbodystructuralgraftsincreasesthesurfaceareaforarthrodesisand
stiffnessoftheconstructandshouldbeperformedfor longconstructsatthelumbosacral
junction.
The spinal deformity surgeon must have excellent knowledge of indications,
instrumentation options, and techniques of sacral-pelvic fixation. Familiarization with
sacral-pelvicanatomyisnecessaryto optimizethesizeandtrajectoryofinstrumentation
andavoidcomplications.
Pearls
Sacral-pelvicfixationincreasesthestrengthandrigidityofconstructsspanningthelumbosacraljunction.
Iliac fixation decreases the rate of sacral instrumentation failure and reduces the incidence of lumbosacral
pseudarthrosis.
S2-alar-iliacscrewsprovidestrongbiomechanicalfixation,minimalimplantprominence,andfavorableimplant
alignmentwithlumbosacralpediclescrewsforeaseofrodcontouring.
Pitfalls
Sacral pedicle and alar screws have inadequate strength to anchor constructs extending cephalad to L2,
predisposingthepatienttolumbosacralpseudarthrosisunlesssacral-pelvicfixationstrategiesareemployed.
Acetabular joint impingementor sciatic notch violation with resultant neurovascularinjury can occur during
placementofiliacscrewsunlessintraoperativeimagingwithteardropviewisused.
References
FiveMust-ReadReferences
1. Kim YJ,Bridwell KH, LenkeLG, Cho KJ,Edwards CCII, Rinella AS.Pseudarthrosis in adultspinal deformity
followingmultisegmentalinstrumentationandarthrodesis.JBoneJointSurgAm2006;88:721–728PubMed
2.KostuikJP.Treatmentofscoliosisintheadultthoracolumbarspinewithspecialreferencetofusiontothesacrum.
OrthopClinNorthAm1988;19:371–381PubMed
3.SantosER,RosnerMK,PerraJH,PollyDWJr.Spinopelvicfixationindeformity:areview.NeurosurgClin NAm
2007;18:373–384PubMed
4. Cunningham BW, Lewis SJ, Long J, Dmitriev AE, Linville DA, Bridwell KH. Biomechanical evaluation of
lumbosacralreconstructiontechniquesforspondylolisthesis:an in vitro porcine model. Spine2002; 27:2321–2327
PubMed
5.LebwohlNH,CunninghamBW,DmitrievA,etal.Biomechanicalcomparisonoflumbosacralfixationtechniquesin
acalfspinemodel.Spine2002;27:2312–2320PubMed
6.McCord DH, Cunningham BW,ShonoY, MyersJJ,McAfeePC. Biomechanical analysis of lumbosacralfixation.
Spine1992;17(8,Suppl):S235–S243PubMed
7.KimYJ,BridwellKH,LenkeLG,RinellaAS,EdwardsCII.Pseudarthrosisinprimaryfusionsforadultidiopathic
scoliosis:incidence,riskfactors,andoutcomeanalysis.Spine2005;30:468–474PubMed
8. Moshirfar A, Rand FF, Sponseller PD, et al. Pelvic fixation in spine surgery. Historical overview, indications,
biomechanicalrelevance,andcurrenttechniques.JBoneJointSurgAm2005;87(Suppl2):89–106PubMed
9.LukKD,ChenL,LuWW.Astronger bicortical sacral pedicle screw fixation through the S1 endplate: an in vitro
cyclicloadingandpull-outforceevaluation.Spine2005;30:525–529PubMed
into the apexof the sacralpromontory over bicorticalfixation: a biomechanicalanalysis. Spine 2002;27:806–811
PubMed
pediclescrewscombinedwithS2-alarscrews.OperOrthopTraumatol2013;25:294–314PubMed
ClinOrthopRelatRes2002;394:73–83PubMed
sacropelvicfixation(bilateralS1andiliacscrews)forspinaldeformity.Spine2006;31:303–308PubMed
length,trajectory,anddiameter.JNeurosurgSpine2011;14:219–225PubMed
deformitycorrection:Galvestonrodversusiliacandlumbosacralscrews.Spine2006;31:2392–2398,discussion2399
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fixation.Spine2009;34:E866–E872PubMed
alar-iliacfixationversustraditionaliliacfixation.Spine2009;34:436–440PubMed
corticalpurchaseascomparedwithiliacfixation.Spine2013;38:E1250–E1255PubMed
anteriorcolumnsupportinasimulatedsingle-levelspinalfusion.JSpinalDisord2000;13:58–62PubMed
5
Instrumentation Strategies in Osteoporotic
Spine:HowtoPreventFailure?
AhmetAlanayandCaglarYilgor
Introduction
Osteoporosisisanimbalancebetweenboneformationandresorptionthatprimarilyaffects
trabecularbone.Progressiveboneminerallossandconcomitantbonyarchitecturechanges
resultinpain,deformity,increasedriskoffracture,andpossibleneuralcompression.
The spineis the mostcommon site of osteoporoticfractures. Although mostpatients
withacutevertebralcompressionfracturesimproveregardlessofthetreatmentapplied,no
patient experiences spontaneous restoration of the vertebral height and achieves a
realignedspine.Therefore,spinalinstrumentationiseventuallyrequiredforsomepatients.
With aging comes a higher incidence of comorbidities that further complicates the
managementofosteoporoticspine.Theelderlytodayhavemoreactivelifestylesthandid
theelderlyofpreviousgenerations,andtheyrefusetoacceptdisabilityanddeformityasa
part of the aging process. Modifiable conditions such as pulmonary, coronary, and
cerebrovascular diseaseand diabetes mellitus shouldbeaddressed in collaboration with
the consulting medical and anesthesiology specialists to minimize the surgical risk and
optimizetheoutcome.Patientswhosmoke,haveanutritionaldeficiency,aredepressed,or
aresubjecttootherlifestressorsshouldbecounseledpreoperativelytoreducetheimpact
ofthesefactors.
Performingadultspinalreconstructioninpatientswithosteopeniarequirescarefulpre-
operative planning, as osteopenia has impact on both idiopathic and degenerative
disorders. Similarly, careful preoperative planning is required when performing a
reconstruction on younger patients with secondary osteoporosis due to factors such as
hypercortisolism, hyperthyroidism, hyperparathyroidism, alcohol abuse, and
immobilization.
Inpatientswithlowbonemineraldensity(BMD),spinalimplantscannotbeplacedas
securely as in patients with normal BMD, and thus application of corrective forces
throughtheweakbone–implantinterfaceisdifficult.Toavoidfailureinsuchsituations,it
is important to understand thebiomechanicsof the osteoporotic spine and to recognize
thatosteoporosisisasystemicdisease. Themainsurgicalgoal shouldbeset totreatthe
symptoms.Thischapterdiscussesthepre-andpostoperativemeasuresthatcanbetakenin
treatingpatientswith osteoporosis,andthe surgicalstrategiesthatcan beusedtoreduce
theriskoffailure.
UnderstandingtheModesofFailureinOsteoporoticSpine
In the osteoporotic spine, the two most common surgical problems are failure of the
fixationor of the bone–implantinterface, and adjacent segment failure,either of which
mayresultinpseudarthrosis.
Intheearlypostoperativeperiod,pedicleandadjacentvertebralfracturesare themost
commonfailures, whereas inthe latephase, pseudarthrosis withinstrumentation failure,
adjacent disk degeneration, and late compression fractures with progressive kyphosis
occurmorefrequently.
Because the osteoporotic spine is less able to withstand force, even the stresses and
strains that areentailed in the activities ofdailyliving can cause postoperative implant
failure, which may present as a sudden pain, a neurologic problem, or implant
prominence.
FixationFailure
Becausetheelasticmodulus oftheboneissmallerthanthatoftheimplant,and because
theforcetransmissionsfollowthepathofleastresistance,thebonesurroundingthescrews
fails before the implant does. This phenomenon is called screw toggling, and, under
repetitivecycling loading,pedicle screwstypically failbycephalocaudal toggling.Then
loosening and eventually pullout occur, stripping or fracturing the pedicle. The thinner
lateralwallofthepedicleismoreoftenfracturedthanthemedialwall.Thepackingofa
stripped screw hole with corticocancellous graft does not usually augment the pullout
strengthofosteoporoticpedicles,whichisapossiblesalvagemethodinhealthybone.
1
Inacement-augmentedpedicle,thescrewcanbepulledoutalone,causingnodamage
totheboneorthecement,orthescrewandthecementcanbepulledouttogether,creating
eitheranenlargedholeinthepedicleorapediclefracture.
The dorsal lamina has a thicker cortical shell than does the ventral aspect, which
contributes to its success in theosteoporotic spine. The main failure mechanism of the
laminar hooks is lamina breakout, breaking the “ring” formed by the lamina, posterior
vertebralbody,andmedialpediclewalls.
Fracture of the upper-instrumented vertebra is another commonly seen failure in the
osteoporoticspine.
AdjacentSegmentProblems
Afterfixationof theosteoporoticspine,almost80%oftheproximaljunctional kyphosis
occursdue toadjacentvertebra fractures.
2
Instability andadjacent diskdegeneration are
otherpossiblemechanismsofadjacentsegmentfailure.
Thepreoperativestatusoftheadjacentsegmentanddiskisthegreatestpredictorofthe
developmentof postoperativeadjacent segmentfailure. Onemust avoidending afusion
adjacent to a severely degenerated disk or to a segment with fixed obliquity or
subluxation.
NonunionandPseudarthrosis
Similartoahealthybone,anosteoporoticboneisalsosubjecttopseudarthrosis,especially
infusions extendingtothe sacrum.Known riskfactorsinclude thoracolumbarkyphosis,
positivesagittalbalancegreaterthan5cm,presenceofhiposteoarthritis,andincomplete
sacropelvicfixation.
PreoperativeMeasures
QuantifyingBoneQuality
Grading scales from X-rays, dual-energy X-ray absorptiometry (DEXA), quantitative
computed tomography (QCT), and microdensitometry can be used to diagnose and
quantify osteoporosis in an adult surgical candidate. QCT provides separate BMD
estimatesoftrabecularandcorticalbone,andhasahighersensitivityduetoitsimagingin
across-sectionalplane.AlthoughQCTisusefulinpredictingthefracturerisk,thereisno
clear consensus on a correlation between the quantity of osteoporosis and the type of
strategiesthatshouldbeapplied.
TheDEXAvaluesacquired fromthefemoral neckshouldbe interpretedwithcaution
becausethe bonedensityin thespine decreasesearlierthan inother skeletalsitesin the
earlypostmenopausalyearsduetoturnoverinthishighlytrabecularbone.Bonedensityat
variousskeletalsites beginstocoincideat aboutage70.Also, DEXAacquiredfrom the
vertebrae may be falsely elevated due to degenerative changes. Therefore, the surgeon
mustbereadytodealwithaweakboneregardlessofthepreoperativeDEXAvalues.
MedicalTreatment
ItiswelldocumentedintheliteraturethatBMDcorrelateswithimplantpulloutstrength.
Therefore,preoperativemedicaltreatmentwithbisphosphonates,recombinantparathyroid
hormone(rPTH),calcitonin,selectiveestrogenreceptormodulators,calcium,orvitaminD
should be considered. It is also important to determine whether the benefit of medical
treatmentissufficientenoughtooffsetthedelayinsurgicaltreatment.
The choice, timing, and duration of postoperative pharmacological treatment for
osteoporosis also remain controversial because these drugs may interfere with bone
healing.
IntraoperativeMeasures
Thelossofthequantityaswellasthearchitectureoftheosteoporoticbonemayincrease
the risk of spinal surgery or make the surgical goals difficult to achieve. In these
situations,specificpediclescrewcharacteristicsandinsertiontechniquescanbe adopted,
and surgical strategies such as addressing the pathomorphology of the osteoporotic
vertebrae, handling soft tissue meticulously, enhancing anchor points, applying
prophylactic vertebroplasty, using interbody support, and protecting the bone–implant
interface are utilized to improve the rate of successful fixation. These techniques and
strategiesarediscussedinthefollowingsubsections.
PathomorphologyoftheOsteoporoticVertebrae
Itis wellestablished thatthe bonequality variesin differentpartsof thevertebrae. The
vertebralbodyitselfisthemostaffectedpartoftheosteoporoticvertebrae.Thelamina,on
the otherhand, whichis predominantly cortical,is relatively sparedand ispotentiallya
strongeranchor.Themorphometryofthe pediclesarevariable.Thispatternofboneloss
causesthepediclescrewfixationtobelesseffectiveintheosteoporoticbone.Thefixation
of the pedicle screws is achieved either by taking advantage of the relatively stronger
corticalbonewithinthepediclebyincreasingthescrewdiameterandavoidingtappingthe
screwpath,orbyaugmentingthepediclescrewinvariousways.Sublaminarfixationwith
wires, cables, hooks, and bands is also a good alternative because the lamina is less
affectedbyosteoporosis.
TheBMDalsovariesindifferentregionsofthesacrum.MedialsidehasahigherBMD
thanthelateralside,andthesuperiorsacralendplatehasthehighest.Thescrewsshould
thereforebedirectedmediallyinatriangularfashionandtowardthesacralpromontory.
TheT2pedicleisgenerallystrongerthanT3–T6pedicles,makingT2agoodoptionfor
screwfixationorpediclehooksasastrongupperanchorpoint.
3
PedicleScrewFactors
Noconsensushasyetbeenreachedonthe optimalscrew diameter,length,andshapefor
fixationintheosteoporoticbone.However,severalpediclescrewcharacteristics,together
withtheholepreparationandscrewinsertiontactics,areshowntoachieveabetterfixation
andpreventimplantfailure.
PedicleScrewCharacteristics
Double-threaded pedicle screws have a cancellous threaded tip followed by a cortical
thread. The wider pitch of cancellous thread provides additional grip in the cancellous
bone,andthescrewadvancesfasterwithhigherinsertiontorque.Thecorticalthreadinthe
pedicleareaprovideshighergripandlesstoggleduetodenserthreads.
4
Conical(tapered)screwsalsoincreaseinsertionaltorque,buttheycannotbereversedor
backedout,becausedoingsoeradicatesthescrew’scontactwiththebone.
Theexpandablepediclescrewusesanovelscrewdesignthatenablesthedistalpartof
thescrewtoenlargewithinthevertebralbodyasaposteriorlydirectedforceisappliedto
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