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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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thescrewtoresistpulloutfailure.Thetipofthescrewbecomesanchoredagainsttheinner
cortex of the dorsal vertebral body, resulting in a 76% increase in holding strength in
comparison to conventional pedicle screws,
5
by taking advantage of the relatively
uncompromised cortical bone rather than depending solely on weakened osteoporotic
cancellousbone.However,inpatientswithseverelylowBMD,expandablescrewsmaybe
unable to overcome the extreme biomechanical disadvantage, resulting in failure.
Moreover,screwrevisionremainsanissueintheclinicalapplicationofthesescrews.
PedicleScrewTractAugmentation
It is possible to augment the pedicle screw tract by preparing the hole, injecting
polymethylmethacrylate (PMMA) bone cement into the hole, and inserting the screw
afterwards.Augmentation mayalsobe donewith bioactivecements,calcium phosphate,
or calcium sulfate using traditional or fenestrated pedicle screws. Coating the pedicle
screw with hydroxyapatite is a time-dependent augmentation technique that increases
osteointegration.
CementAugmentationandFenestratedScrews
AcadavericbiomechanicalanalysisofPMMA-augmentedpediclescrew fixationusing a
novelfenestratedbone tapincreasedthe pulloutstrengthby 199%and162%in primary
and revision procedures, respectively.
6
Clinical series also demonstrated good outcome
with no screw loosening, migration, or pullout detected in the follow-up X-rays, no
fractureattheaugmentedlevels,andnoimplantfailurerequiringreintervention.
7,8
AlthoughPMMAaugmentationofthepediclescrewsprovidesgoodfixationinpatients
with low BMD, it is not free of complications. Extravasation, intracanal leakage,
hypotension,increaseinpulmonaryarterypressure,pulmonarycementemboli,superficial
infections,andthermalnerveinjurieswerereported.Therefore,strategiesweredeveloped
to reduce the likelihood of cement leakage. Higher viscosity cement can be used, and
fluoroscopycanprovideadditionalassistance.Itisgenerallyrecommendedtoinject1to3
mLofcementbecauseusingalargeramountfailstodemonstrateanysignificantbenefitin
pulloutstrength.
6
AttentionwasalsopaidtothemethodofPMMAaugmentation.Injectingcementintoa
cavity prepared by an inflatable balloon followed by insertion of the pedicle screw
demonstratedalmosttwicethepulloutstrengthofscrewsaugmentedwithstandardcement
injection.
9
Fenestrated screws have been used more recently, with promising results.
10
Although clinical long-term results are yet to be seen, there is a potential theoretical
advantageofusingfenestratedscrewsoverinjectingcementfollowedbyscrewinsertion.
Injectingthecementintothepreparedholefillsthetract,andwheninsertingthepedicle
screw,thecement coatsthescrew threadsandtherebyreduces effectivescrewpurchase.
Alternatively,cementinjectionthroughafenestratedscrewenablesthecementtoinfiltrate
inthevertebralbodywithoutalteringthebone–implantinterface.
11
Althoughitiswidelyusedwithpromisingresults,PMMAistoxic,isunabletoundergo
remodeling aftermicrofracture within thecement, and isdifficult toremove in revision
surgery. Hence, osteobiologic cement is an area of interest and development for screw
augmentation.Calciumphosphateandcalciumsulfateavoidtheexothermicreactionand
reduce the risk of leakage. Moreover, they are bioresorbable and potentially
osteoconductive, and integrate in the natural process of bony remodeling. A cadaveric
study comparing osteobiologic cement and PMMA for the use of screw augmentation
foundnosignificantdifferencesinaxialpulloutstrength.
12
HydroxyapatiteCoating
The increased osteointegration of the hydroxyapatite-coated pedicle screws is time
dependent;withtime,optimumstabilityisachieved.Ithasbeenshowninanosteoporotic
animal model thathydroxyapatite-coated pedicle screws are 1.6 timesmoreresistant to
pulloutandthattheyhavesuperiorbiologicalbondingtothesurroundingbone,occurring
as early as 10 days after surgery.
13
However, they do not allow for the application of
additionalforcesduringintraoperativecorrectionmaneuvers.
InsertionTechniqueandInsertionalTorque
The startingpoint, holepreparation, tapping, theinsertion angle, andthe trajectoryof a
pediclescrew,aswellasitslength,depthofpenetration,anddiameter,affectitsinsertional
torqueandtherebyitsresistancetofailure.
LengthofScrewandDepthofScrewPenetration
The length of a pedicle screw is linearly related to its pullout strength. As the screw
penetratesfurtherintothevertebralbody,thecutoutloadtofailureincreases.Byengaging
the ventralcortex of the vertebralbody,screws can beplaced in a bicorticalfashion to
provide up toanadditional 30% of pullout strength.
14
The risk–benefit ratioshould be
considered,andcaremustbetakentoavoidinjurytoadjacentstructureswhenusingthis
technique.
Diameter
Thediameterofthepediclescrewshouldbeaswideaspossibletoenable bettercortical
bone purchase. Increasing the diameter increases the pullout strength in milder cases;
however,insevereosteoporosisthepulloutforceislowregardlessofthescrewdiameter.
Instead,usinglargerdiametersmaycausedilationorfractureofthepediclethatdecreases
itsstrength.
15
StartingPoint,InsertionAngle,andTrajectory
Whenthescrewsareplacedparallel,the volumeofcancellousbonebetweenthethreads
of the screw determines the resistance to pull-out for each screw. Triangulated screws
provide better pullout strength with a larger volume of cancellous bone available for
resistanceto pull-outbecause theconstructiscontributedby thevolume ofbone within
thetrapezoidareainthevertebralbodyformedbylongerandtriangulatedscrews.
16
ThoracicSpine
In the anatomic trajectory, the screw is in line with the pedicle axis and therefore is
directed to the inferior corner of the vertebral body in the sagittal plane. In the
straightforwardtechnique,thescrewisparalleltothevertebralendplateandtriangulation
in the transverse plane can be achieved. This technique provides at least 39% higher
maximum insertional torque and 27% greater pullout strength.
17
Pedicle-rib screws
increasetheeffectivetransversediameterwhencomparedwiththepediclealoneandcan
beusedforsaferinsertionofthescrews,althoughitmaydecreasethepulloutstrengthby
25%.
18
Thestartingpointshouldbeselectedinaccordancewiththetrajectoryused.
LumbarSpine
Placingthepediclescrewsinconvergencealsoincreasesthepulloutstrengthinthelumbar
spine.
SacrumandPelvis
Sacral fixation is a big challenge in the osteoporotic spine. Restoration of the sagittal
balanceismoreimportantthanthefixationitself.
Whenthefusionisextendedtothesacrumandalongfusionisperformed,multipleand
bicortical screwfixation should beused in addition toconsideration of anteriorcolumn
supportoriliacfixation.Thetricorticaltechnique,whichentailsdirectingthescrewsinto
thesacralpromontory,increasestheinsertionaltorque.
19
HolePreparationandTapping
Appropriate preparation of the hole improves screw purchase. High insertional torque
improves the screw pullout strength. In healthy vertebral bodies, the screws are placed
after tappingto avoid microfracturing within thedense bony matrix ofthe bone during
screwinsertion. Inosteoporotic cancellousbone, however,tapping resultsin removalof
bonewithin thepedicle track andpreventsbone compression aroundthe screw threads.
Evenscrewremovalandimmediatereinsertiondecreasesthemechanicalinsertiontorque.
Therefore, under-tapping or not tapping at all is advised in osteoporotic bone. When
compared with same-size tapping, under-tapping by 0.5 and 1.0 mm increases the
insertionaltorqueby47%and93%,respectively.
20
Under-tappingis morebeneficial in thelumbar spinethan inthethoracic spine.This
maybe duetothefactthat thethoracicpedicle screwsare probablymoredependent on
corticalpurchasewithinthepediclewalls.
EnhancingAnchorPoints
The weak link in the osteoporotic spine instrumentation is the implant–bone interface.
Fixation strategies for osteoporotic bone are targeted toward taking advantage of the
relatively stronger cortical bone. Anchor options, in addition to screws, include hooks,
wires,cables,andbands.
LoadSharingbyMultilevelFixation
Because the implant–bone interface in the osteoporotic bone is prone to failure, the
number of points of fixation must be increased to distribute the contact forces more
evenly. Longer constructs with at least three sets of fixation points at each end can be
beneficial,keepinginmindtheaddedmorbidityentailedwithusingadditionalscrews.
Aspreviouslystated,usinghooks,wires,cables,andbandsaswellascross-linkshelp
inimprovingtheperformanceofthepediclescrews.
SelectionofFusionLevels
End-instrumented vertebrae should be carefully selected. Ending the construct in a
kyphoticregionorattheapexofkyphosisshouldbeavoided.
Anotherfrequentdecision-makingdilemmaintheosteoporoticspineiswhetherornot
tofusetosacrum.CertainscenariosthatrequirelumbosacralfixationaresymptomaticL5-
S1spondylolisthesis,over15degreesofscoliosisat theL5-S1segment,andthe needto
achieve proper sagittal balance.
21
Stopping at L5 entails the risk of increased adjacent
segmentdisease,whereasfusingtothesacrumisfoundtohavemorecomplications.
21
L5
pediclesareusuallyshortandcontainmorecancellousbone.Therefore,itmayberiskyto
endalongfusionatL5inosteoporoticpatientsbecauseL5pediclescrewsmayfail.
Cross-Link
The use of a rigid or semirigid cross-link, especially when the screws are triangulated,
increases the torsional stiffness by making the construct perform effectively as a
quadrilateral frame. The use of a cross-link is especially advantageous in longer
constructs,asitpreventsrodsfromtelescoping.
Hooks,Wires,Cables,andBands
The use of sublaminar and pediculolaminar hooks, wires, cables, and bands takes
advantageofthecorticalbonecompositionofthespinallamina.
A polyesterband may be usedto increase thesurface of bony contactand to fitany
anatomy. It may be used in a sublaminar, subpars, transversal, or laminotransversal
fashion to enable translation, distraction and compression, in situ bending, and rod
derotation.
ProphylacticVertebroplasty
Inthe settingof osteoporosis,junctional failure,especially inthe craniallevels, isnot a
rare occurrence. Prevention is the best way to overcome adjacent segment failure.
Prophylacticvertebroplastyentailscementaugmentationoftheadjacentnoninstrumented
segment/segments. Although there is a paucity of clinical and biomechanical studies,
prophylacticvertebroplastyseemstobehelpfulindecreasingtherevisionarthrodesisrates
becauseofadjacentvertebraefractures.
22
Furtherstudiesareneededtoclarifytheoptimumamountofcementrequiredandhow
manylevelsshouldbeprophylacticallycemented.Efficacyatthedistaladjacentlevelalso
needstobefurtheranalyzed.
InterbodySupportinOsteoporoticSpine
Anteriorcolumnsupportisbeneficialinloadsharingbecausethegraftorcagelessensthe
stress directed toward the screw–rod construct. Anterior interbody support may further
improvesagittalspinalbalanceandratesofarthrodesis.
Interbodygraftsserveamorecriticalroleatthecaudalendoftheconstruct,particularly
atthelumbosacraljunction.Graftscanbeplacedwithabiastowardtheconcavityofthe
deformity to assist correction. Figs. 5.1, 5.2, and 5.3 demonstrate the use of the
intraoperative measures mentioned above in a 64-year-old patient with a BMD of –3.2
complainingofbackandlegpain.
Fig.5.1a,b(a)PreoperativeposteroanteriorX-ray.(b)PreoperativelateralX-ray.
Fig.5.2a,b(a)Preoperativesagittalmagneticresonanceimaging(MRI)view.(b)PreoperativetransverseMRIview.
In severeosteoporosis, there isa risk ofsubsidence of thegraft or cageinto the end
platesthatmayleadtoanteriorcolumncollapseand subsequentkyphotic deformity.The
cageshouldbeplacedtocontacttheperipheralapophysealringwherethecorticalboneis
stronger.Agraftwithanelasticmodulusthatissimilartothenativebonealsoreducesthe
risk of subsidence. Choices of interbody graft materials include bone (autograft or
allograft), metal, carbon fiber, polyaryletheretherketone (PEEK), and other synthetic
materials.Iliac crestautograft istypically thebest match,but itis associatedwith well-
establishedharvest-relatedmorbidity.
RoleofAnteriorFixationintheOsteoporoticSpine
Continuousloading ofanteriorscrew constructson alow-BMDspine canlead toscrew
cutout. Although newer implant designs demonstrate improved anchorage,
23
anterior
fixationhasalimitedroleintheosteoporoticspinebecauseitisthemostaffectedpartof
thevertebrae.
RoleofSemirigidFixationintheOsteoporoticSpine
Theloading ofthe spineinvariousaxesof motioncreates increasedstress atthebone–
implant interface. The difference of the rigidity within the instrumented and
noninstrumentedsegmentsofthespinecanaccelerateadjacentsegmentdegenerationand
potentiallycausepseudarthrosis.Semirigidfixationmayprovidesufficientstabilizationto
facilitatebony fusionwhile permittingsomedegree offlexibility tooffloadstress atthe
adjacentsegmentandthebone–implantinterface.
ProtectionoftheBone–ImplantInterface
Handlingintraoperativesoft tissuemeticulously,providing extensiverelease,performing
osteotomies to increase flexibility and thus minimize the corrective forces, maintaining
sagittalalignment,andobtainingasolidfusionareessentialfortheprotectionofthebone–
implantinterface.
MeticulousSoftTissueHandling
Care should betaken to preserve the supraspinous ligament, intraspinousligament, and
ligamentumflavumbetween thecranialfusedlevel andtheadjacentsegment,aswellas
between the cranial levels of fusion. Doing so provides a segment of higher posterior
tension, which may prevent the development of junctional deformity and instability.
Supra-andinfra-adjacentfacetsshouldbepreservedandthecranialdiskspaceshouldnot
beviolatedwithpediclescrews.
Fig.5.3a,b(a) Last follow-up posteroanterior X-rayofthesame patient demonstrating the useofcement-augmented
pediclescrews,anteriorinterbodysupport,prophylacticvertebroplasty,andcross-link.(b)Lastfollow-uplateralX-ray
ofthesamepatientdemonstratingtheuseofcement-augmentedpediclescrews,anteriorinterbodysupport,prophylactic
vertebroplasty,andcross-link.
ExtensiveRelease
The release of the diskoligamentous and bony constraints such as diskectomy,
facetectomy,orvariousosteotomies shouldbeoptimizedbeforereductionto decreasethe
stressappliedtothebone–implantinterfacewhenperformingspinalcorrectivemaneuvers.
MaintainingSagittalAlignment
Aligningtheosteoporoticspinetophysiologicalcoronalandsagittalcontoursneutralizes
thedeformingforces,reducesthejunctionalforces,anddecreasestheenergyrequiredfor
ambulation.
ObtainingFusion
Obtainingarapidandsolidfusionensureslong-termspinalstability,reducingtheloadon
instruments and on the relatively poor bone–implant interface. A thorough fusion
procedure with appropriate bone-bed preparation and appropriate use of bone grafts or
substitutesisthereforeofparticularimportanceintheosteoporoticspine.Theuseofbone
morphogenetic proteinmay facilitate an earlierand more vigorousfusion, and decrease
the risk of implant-relatedfailure. The use of bone morphogenetic protein mayalso be
associatedwithcomplicationsrelatedtosofttissueswelling,inappropriateboneformation
aroundneuralelements,andsubsequentradiculitis.
PostoperativeMeasures
Enhancingthepurchaseofinternalfixationandprotectingthebone–implantinterfaceby
handling soft tissue meticulously, providing extensive releases, maintaining sagittal
alignment,and usinganteriorinterbody supportdecreases thedemandon fixationin the
postoperativeperiod.External braceimmobilizationandrestriction ofthespine rangeof
motionaswellasphysicaltherapyandrehabilitationaremeasuresthatcanbetakeninthe
postoperative period to serve the same purpose. As stated earlier, the timing of
postoperativepharmacologicalosteoporosistreatmentremainscontroversial.
If a brace is to be used, it must be custom-molded postoperatively, after surgical
deformitycorrectionisestablished.Rehabilitationshouldfocusongaittraining,balance,
and general conditioning, together with range-of-motion and flexibilityexercisesof the
hipandknee.Thereisnoconsensusyetonthedurationofbraceapplication.
ChapterSummary
Primarilyaffectingthetrabecularbone,osteoporosiscausesprogressivebonemineralloss
and concomitant bony architecture changes that result in pain, deformity, increased
fracturerisk,andpossibleneuralcompression.Althoughmostpatientswithacute,painful
vertebral compression fractures improve regardless of the treatment applied, no patient
spontaneously restores the vertebral height and achieves a realigned spine. Spinal
instrumentation is eventually required in some osteoporotic patients. In the setting of
osteoporosis,however,the fixationof thespinalimplantsisinsecure, andapplication of
corrective forces through a weak bone–implant interface is difficult, complicating the
surgicaltreatment.Thefirststepforanadultspinalsurgicalcandidateisthediagnosisand
quantification of the osteoporosis. Understanding the biomechanics and the modes of
failureoftheosteoporoticspineisimportant.Thevertebralbodyitselfisthemostaffected
partoftheosteoporoticvertebrae.Thelamina,ontheotherhand,whichispredominantly
cortical,isrelativelysparedandispotentiallyastrongeranchor.Themorphometryofthe
pedicles is variable. Failure of the fixation or of the bone–implant interface, adjacent
segment failures, and pseudarthrosisarethe three main problems in osteoporotic spine.
Severalpre-andpostoperativemeasuresmaybetaken,aswellasapplyingseveralsurgical
strategies intraoperatively to prevent failure. The pedicle screw characteristics together
withhole preparationandscrew insertiontactics areshown toachieve abetter fixation.
Anterior column support is beneficial in load sharing, improving sagittal balance and
reducingtheratesofarthrodesis.Theresultsofcementaugmentationofthepediclescrews
andtheadjacentnoninstrumentedvertebraeseempromising,butitisnotacomplication-
freeprocedure.
Pearls
Inelectivecases,increasingtheBMDpreoperativelywithparathyroidhormonemightbeconsidered.
Under-tappingisadvisedintheosteoporoticbonetoincreasetheinsertionaltorque.
Triangulated screws provide better pullout strength, with a larger volume of cancellous bone available for
resistancetopulloutbecausetheconstructiscontributedby thevolumeofbonewithinthetrapezoidareainthe
vertebralbodyformedbythelongerandtriangulatedscrews.
Theuseofacross-linkisespeciallyadvantageousinlongerconstructs,asitpreventsrodsfromtelescoping.
Theuseofhooks,wires,cables,andbandstakeadvantageoftherelativelystrongercorticalboneforfixationof
osteoporoticbone.
Cement-augmented pediclescrews are advantageous for betterfixation and forallowing additionalcorrective
forces.
Prophylacticvertebroplastyis helpful indecreasingtherevisionarthrodesisrates because ofadjacentvertebrae
fractures.
The release of the diskoligamentous and bony constraints such as diskectomy, facetectomy, or various
osteotomiesshouldbeoptimizedbeforereductionwhenperformingspinalcorrectiveprocedurestodecreasethe
stressappliedtothebone–implantinterface.
Acustom-moldedpostoperativebracehelpsprotectthebone–implantinterface.
Pitfalls
Avoid ending a fusion adjacent to a severely degenerated disk or to a segment with fixed obliquity or
subluxation.
DEXA scans in elderly patients must be interpreted with caution because degenerative changes may falsely
elevatetheBMDvalues.
In the osteoporotic bone, tapping results in removal of bone within the pedicle track and prevents bone
compressionaroundthescrewthreads.
Avoidendingaconstructinakyphoticregionorattheapexofkyphosis.
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