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4
Indications and Techniques for Sacral-Pelvic
FixationinAdultSpinalDeformity
KristenE.Jones,RobertA.Morgan,andDavidW.Polly,Jr.
Introduction
Fusion attempts across L5/S1 in adult spinal deformity are plagued by a high rate of
pseudarthrosis and implant breakage/failure due to the unique anatomic and
biomechanicalcharacteristicsofthelumbosacraljunction.
1,2
Inasingle-institutionreview
ofadultdeformitypatientswithconstructsgreaterthanfourlevels,constructsendingatS1
hadasignificantlyhigherrateofpseudarthrosiscomparedwithconstructsendingatL5or
more cephalad levels.
1
Other pseudarthrosis risk factors were age older than 55 years,
more than 12 levels included intheconstruct, and T10-L2 kyphosis of greater than 20
degrees. The addition of sacral-pelvic fixation increases the strength and stability of
constructs spanning the lumbosacral junction and is a formidable tool in the spinal
deformitysurgeon’sarmamentariumforcorrectingspinalimbalance.
AnatomicandBiomechanicalConsiderations
As a transition zone from the mobile lumbar spine to the stiff pelvis, the lumbosacral
junction experiences significant forces challenging arthrodesis attempts across the
segment. Despite bearing axial loads of more than double body weight, the osseous
anatomyofthesacrumprovidesrelativelylittlestrengthforfixation.
3
Thesacrumconsists
ofathin rimofcorticalbone surroundingacancellouscore, withlargepediclediameter
precluding the engagement of both medial and lateral cortical walls via pedicle screw
instrumentation.
Thelumbosacraljunctionisabiomechanicallydistinctlocationthatissubjectedtothe
highestleveloftranslationalshearforceandthemostlimitedrangeofmotionwithinthe
spine, with the L5/S1 disk bearing the largest summation of load vectors.
2,4–6
These
unique stresses, combined with the relatively small amount of sacral cortical bone
available for fixation, resultin increased pseudarthrosis and implant breakage/failure in
longinstrumentationconstructsendingatS1.
2,7
McCordetal
6
introducedtheconceptofthelumbosacralpivotpointatthejunctionof
the L5-S1 disk and the middle osteoligamentous column to describe the considerable
flexion moments and cantilever forces acting at the lumbosacral junction. Extending
fixationanterior tothispivot pointincreases constructstrength.Screw insertioninto the
iliumprovidesthelongestfixationlengthanteriortothispivotpoint,andwasfoundtobe
theonlyinstrumentation typeatthe lumbosacraljunctionthatsignificantly increasedthe
maximumflexionmomentatfailure.Comparedwiththeweakcancellouscompositionof
thesacrum,theposterioriliumoffersabundantcorticalboneforanchoringinstrumentation
and enablesincreased screwlength and diameter, makingsacral-pelvic fixationa useful
techniqueforincreasingconstructstrength.
IndicationsforSacral-PelvicFixation
Therigidfixationprovidedbysacral-pelvicinstrumentationisausefuladjuncttotreating
a widearrayof pathological entities. Sacral-pelvicfixation is indicated for lumbosacral
arthrodesis extending cephalad to the L2 vertebra, augmentation for poor quality or
osteoporoticbone,sacrectomyfortumororinfection,unstablesacralfractures,correction
of flat-back syndrome via lumbar osteotomy, correction of pelvic obliquity, and high-
grade spondylolisthesis.
8
The addition of iliac fixation in these conditions significantly
reducesthestressplacedonS1instrumentationandincreasesconstructstrength.
As with all spinal surgery,selection of the appropriate approach for each individual
patientand meticulousattention tosurgicaltechnique isthe keytosuccessfultreatment.
Althoughsacral-pelvicfixationisnotrequiredformanypatientsundergoinglumbosacral
arthrodesis,theforcevectorsrequiredforthecreationandmaintenanceofpropersagittal
alignmentrelativetothepatient’sindividualbonequalitymustbeconsidered.
Sacral-PelvicInstrumentationSelectionandTechniques
SacralFixation
Screws atS1 can beplaced through thepedicles in amedially convergentmanner with
bicorticalend-plateortricorticalpurchase,orthroughthealainadivergentmanner(Fig.
4.1a).Sublaminarhooks andwiresandS2pedicle screwscanbeusedto supplementS1
pedicle/alarscrewsbutshouldnotbereliedonforanchoringalongconstruct.
S1PedicleScrews
The sizable medially convergent sacral pedicles accommodate large screw length and
diameter while simultaneously preventing “filling” the pedicle to achieve bicortical
purchaseofthemedialandlateralpediclewallwithasinglescrew.Thelargelycancellous
sacral pedicles provide relatively little pullout strength in unicortical fixation, and
unicortical pedicle screws should be avoided.
3
Bicortical fixation anchored into the
anterior sacral cortex provides increased pullout strength compared with unicortical S1
pedicle screws; however, additional trajectories can be employed to further enhance
pullout strength. Luk et al
9
compared bicortical S1 pedicle screw insertion torque and
pulloutstrengthtothatofS1pediclescrewsadvancedthroughtheS1superiorendplate.
S1pediclescrewstraversingtheendplatehadsignificantlyhigherinsertionaltorqueand
pulloutstrengthcomparedwithbicorticalS1screws.
Tricorticalfixation,definedasascrewtrajectorytowardthemedialsacralpromontory,
captures purchase in the dorsal, anterior, and superior end-plate cortex (Fig. 4.1b).
Lehman et al
10
found that this tricortical trajectory doubles the insertional torque
compared with bicortical S1 pedicle screws parallel to the S1 end plate. Tricortical S1
screwstrengthhas notbeendirectly comparedwithtrans–end-platescrew strength;both
provideenhancedstrengthcomparedwithtypicalbicorticalpurchaseparalleltotheS1end
plate. Triangulationof the pedicle screw trajectory increases pullout strength compared
withstraight-aheadtrajectoryandshouldbeuniversallyemployed.
SacralAlarScrews
Alarscrewinsertion utilizesa lateral trajectoryinto low-densitycancellous sacral bone.
Bicortical alar fixation is technically possible but fraught with risk of injuring the L5
nerveroots orcommon iliacvessels drapedanteriorlyacrosstheala.
3
Thus,alar screws
are utilized mainly as unicortical supplements to bicortical/tricortical S1 pedicle screw
constructs.AChopin orColoradoplateoraTacoma blockcanbeutilizedtoconnectS1
andalarscrews.Disadvantagesofthistechniqueincludeconstrainedscrewstartingpoint
andpotentialimpairmentoftheidealtrajectory.
Fig.4.1a,bSacralscrewtrajectories.(a)TrajectoriesforS1pedicle(A)andalar(B)screwplacement.(b) Intraoperative
view demonstrating probe insertion in tricortical S1 screw trajectory. Tricortical purchase utilizing dense sacral
promontorycorticalboneshouldbeemployedtomaximizescrewpulloutstrength.
SacralSublaminarWiresandHooks
Althoughsublaminar wiresand hookslack sufficientbiomechanicalstrength toserve as
anchorstolongconstructs,theycanbeusedassupplementsforshort-segmentfusions.
3,11
Hooks areoptimally placed in thedorsal sacral neuroforaminawhereimproved cortical
purchase can be achieved. The Harrington instrumentation system initially employed
sacral hooks as anchors to long constructs, but the rate of pseudarthrosis and hook
dislodgmentatL5/S1 wasunacceptablyhigh.Sacralsublaminarwiresand hooksshould
notbeusedasanchorstolongconstructs.
S2Screws
S2 pedicle screw strength is typically limited by a short pedicle length and a location
dorsal to the lumbosacral pivot point described by McCord et al
6
and Kebaish.
11
S2
pediclescrewsandS2screwsdirectedlaterallyintothealacanbeusedasadjunctsupport
toshort-segmentfusionsbutlackthebiomechanicalstrengthtoanchorlongconstructs.
12
JacksonIntrasacralRodTechnique
JacksonintrasacralrodsareinsertedverticallythroughthealafromS1tothelevelofS2
and then can be connected to a construct including S1 pedicle screws. Insertion is
technicallydifficultandcanbeprecludedbyalaranatomicvariations.Thistechniquehas
been shown tobe biomechanically inferior to a lumbosacral pediclescrews–iliac screw
constructandismentionedforhistoricalcontextonly.
5
TranssacralFixation
Kellogg Speed first described transvertebral strut grafting at L5/S1 from an anterior
approach for patients with high-grade spondylolisthesis. The Speed technique involves
drivingafibularstrutgraftthroughtheL5vertebralbodyandintothesacrumviaanterior
exposure, and is a useful technique in lieu of interbody cage placement, which has an
increasedriskofanteriorsubsidenceforpatientswithhigh-gradespondylolisthesis.
13
Duetotheriskoftheanteriorexposuretothelumbosacraljunction,includinginjuryto
the great vessels during mobilization or sympathetic plexus dysfunction causing
retrograde ejaculation in males, H.H. Bohlman popularized the posterior approach for
transvertebralfibularstrutgraftingatL5-S1forpatientswithhigh-gradespondylolisthesis.
AnteriorfixationthroughL5/S1canalsobeperformedviaaparacoccygealapproachina
minimally invasive fashion utilizing synthetic implants. As with all constructs, the
additionofposteriorcolumnsupportincreasesstability.
IliacFixation
Anchoring a construct with iliac fixation creates a longer lever arm to resist cantilever
forces across the lumbosacral junction via extension anterior to the lumbosacral pivot
point,increasingbiomechanicalstrengthofsacral-pelvicconstructs.
6
Incorporationofthe
iliuminto aconstruct offloadsstressfrom sacralscrews anddecreasesthe rateof sacral
instrumentationfailureandpseudarthrosisacrossthelumbosacraljunction.
4,14
TransiliacFixation
HarringtonThreadedSacralRod
Developed forpelvic fixation inadjunct with Harrington distractionrods, this deviceis
mentionedforhistoricalpurposes.Twoseparateposterioriliacincisionsareusedtoinsert
the threaded rod through the posterior iliac wings with compression applied.
Pseudarthrosis rates have been reported above 40%m and the distraction forces of
Harringtonrodscanresultinsagittalplaneimbalance.
2,8
KostuikTransiliacBar
Inserted fromthe midline, theKostuik transiliac baris placed 1to 2 cmanterior to the
posteriorsuperioriliacspineandthenattachedviacustomconnectorstoS1pedicleoralar
screws. The bar is smooth and has a contoured shape that accommodates the midline
sacraldorsalprominence,andhasbeenreportedtohaveahighfusionrateofupto97%.
8
IliacFixation
LuqueL-fixation
Thefirsttodevelopsegmentalinstrumentation,Luqueextendedlumbosacralconstructsto
the pelvisusing L-shaped rodswhose ends were insertedinto the posteriorilium at the
posteriorsuperior iliacspine. Thiscircumvented thedistractionproblem associatedwith
theHarringtontechniqueandimprovedfusionrates,butpistoningoccurringbetweenthe
rodsdecreasedthestabilityoftheLuqueL-fixationintorsionandflexion.
2,8
GalvestonTechnique
Ben L. Allen and Ron L. Ferguson developed the Galveston technique in the 1980s.
Smoothrodsinsertedfromtheposteriorsuperioriliacspineintotheiliumwereconnected
tosegmentallumbosacral instrumentation.Rodcontouringrequiredsignificant expertise.
Toobviatetheneedforthis,rodswithpre-bentsagittalcontourandbilateraliliacfixation
werecreatedin asinglepiece.
3
This constructstillrequired substantialrod management
skills. Because the smooth rods had inferior pullout strength compared with threaded
screws,iliacscrewsquicklybecameamorepopularmethodoffixation.
IliacScrews
Iliacfixationwithsingleorstackedunilateralorbilateralscrewsisperformedwithfullyor
partiallythreadedscrews.Thepulloutstrengthand rotationalstabilityissuperiortonon-
threaded rod techniques. The starting point for screw insertion is at the level of the
posteriorsuperioriliac spinewitha trajectorytargetingeitherthesupra-acetabular notch
or the anterior superior iliac spine (Fig. 4.2). The screw trajectory is planned via
visualizationofthe“iliacteardrop”oneitherfluoroscopyorcomputedtomography (CT)
imageguidance.Theiliacteardropisaregionabovetheacetabulumborderingthemedial
iliac wall,the lateral iliac wall,and the zenith of thesciatic notch. The screwachieves
greatestpurchaseinthelateralmarginoftheteardrop, directedthrough thecorticalbone
justabovethe sciaticnotch.Santosetal
15
analyzedvariousscrew lengthsanddiameters
andfoundasignificant increaseininsertionaltorqueforiliacscrewsoflength≥80 mm
and diameter ≥ 9.5 mm. Nodifference in insertional torque existed between the supra-
acetabular notch and the anteroinferior iliac spine-directed trajectory. Given the risk of
acetabularjointviolationwiththesupra-acetabular trajectory,theauthors concludedthat
optimaliliacscrews areinsertedin theanteroinferioriliacspine trajectorywithlength ≥
80mmanddiameter≥9.5mm.
Iliac screws can be used in combination with sacral screws to provide increased
construct strength and are biomechanically superior to other pelvic fixation options.
Comparing the Galveston technique to iliac screws in a series of 20 neuromuscular
scoliosispatients,iliacscrewsenabledbettercorrectionofpelvicobliquityanddecreased
implantbreakage.
16
In a biomechanicalcomparison between the modified Galveston techniquewith iliac
fixationbutnoS1fixationversusS1pediclescrewsplusiliacscrews,orS1andS2screws
without iliac fixation, Tis et al
17
found that constructs with iliac screws conferred
significant strength via decreased range of motion in multidirectional flexibility testing
andincreasedloadtofailure.
Bridwell’s group
4
performed a laboratory investigation comparing multidirectional
flexibilityand flexuralload tofailure amongthe followingconstruct types:lumbosacral
pediclescrews,interbodycage,andiliacscrews.Iliacscreworinterbodycageplacement
significantly reduced multidirectional flexibility at the lumbosacral junction compared
withpediclescrewsalone,butiliacscrewfixationwassuperiorinprotectingsacralscrews
frompulloutorplowthrough.
4
Fig.4.2a–dIliacscrewplacementusingintraoperativecomputedtomography(CT)-basedframelessnavigation.(a)Iliac
screw insertion pointisat the posteriorsuperior iliac spine,directed toward theanterior inferior iliacspine.(b) The
teardropview is utilizedtooptimize screw purchaseinthe lateral corticalwall,just above thesciaticnotch. (c) The
sagittalviewis usedtodemonstratescrewangulation towardtheanteriorinferioriliac spine,avoidingviolationofthe
acetabulum.(d)Stacked iliac screw construct in patient with progressivedeformityfromhigh-gradespondylolisthesis
previouslyfusedinsitu,requiringsacralosteotomyfordeformitycorrection.
Lebwohl et al
5
performed a laboratory biomechanical analysis comparing construct
stiffness, S1 screw strain, and ultimate failure load among several techniques of
supplementary sacral fixation, using S1 pedicle screws with and without S2 screws, as
wellasanintrasacralrodandiliacscrews.AlltechniquesdecreasedtheS1screwstrainin
flexion-extension, but only iliac screws decreased S1 screw strain in axial loading. In
destructivetestingwithflexionloading,onlyiliacscrewssignificantlyincreasedthe load
tofailure.Theauthorsconcludedthattheadditionofiliacfixationsignificantlyincreases
thebiomechanicalstrengthofsacralconstructs.
Asignificantdisadvantagetoiliacscrewsisthepotentialprominenceofthescrewhead
attheposteriorsuperioriliacspine(PSIS).Althoughthestartingpointcanbemodifiedor
the PSIS notched to allow burying of the screw head, implant prominence cannot be
completelyeliminated.Anotherdisadvantageistherequirementofanoffsetconnectorfor
segmentallumbosacralinstrumentation.
TranssacralIliacFixation
S2-Alar-IliacScrews
Developed to address the problematic implant prominence and offset location of iliac
screws, S2-alar-iliac (“S2-iliac”) screws are associated with a lower complication rate
relatedto implant prominence-relatedpain, necessitatingscrew removal, comparedwith
traditional iliacfixation.
11
Implantprominence is minimized, withthe mean distance of
the insertion point to the skin 15 mm deeper for the S2-iliac technique compared with
traditionaliliac screwinsertionat thePSIS.
18
The S2-iliacscrewstarting pointis 2to4
mmlateraland 4to8mm caudaltothedorsal S1foramen,witha trajectorytowardthe
anterior inferioriliac spine (Fig.4.3a) atanangle of ~40 degrees lateraland20 to 30
degreescaudal.
11,18
Theiliactear-dropisagainemployedfortrajectoryalignment.Unlike
traditional iliac screws, S2-iliac screws do not typically require an offset connector for
joiningrodstolumbosacralpediclescrewconstructs(Fig.4.3b).Biomechanicaltestingof
S2-iliacscrewshasshownequivalentstabilitytoconventionaliliacscrews.
19
TheS2-iliac
screwtrajectory resultsin crossingthesacroiliac joint,which hasnot beenshown tobe
problematicinthefirst5yearsoffollow-upofthistechnique,butwhichrequiresongoing
surveillance(Fig.4.3c,d).Althoughhaloingaroundiliacscrewsmaybeobservedinover
25%ofpatients,iliacscrewpulloutorbreakageisveryrare.
1,11,14
AdjunctiveAnteriorInterbodySupport
Polly and colleagues
20
found that load-bearing interbody structural grafts increase
construct stiffnessand therefore candecrease the strain onposterior instrumentation, in
additiontoincreasingthesurfaceareaavailable forarthrodesis.Theyalsofoundthatthe
locationof theinterbody graftin thesagittalplane hasbiomechanical significance,with
anteriorly placed grafts having increased stiffness compared with central or posteriorly
placed interbody grafts. Compared with stand-alone pedicle screw and combination
pedicle-iliac screw constructs, pedicle-iliac screw constructs combined with interbody
cages significantly reduce segmental movement across the lumbosacral junction in
laboratoryanalysis.
4
PatientPositioning
Patients undergoing sacral-pelvic instrumentation via open or minimally invasive
technique shouldbe positioned prone on anoperating table that enablesthecreation or
maintenanceofanatomiclumbarlordosis.Fixationofthelumbosacraljunctioninaflator
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