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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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Fig. 9.2a–d A patient presenting with PJK who was treated with extension of the posterior fusion and posterior
osteotomiesonly.(a)Lateralradiograph.(b)Hyperextensionview,clearly showingaflexibledeformity.(c)Computed
tomography(CT)scan.(d)Preoperative(left)andpostoperative(right)standingradiographs.
Fig.9.3a–cApatientwitharigiddeformity.(a)Preoperativeimagesofthe proximalkyphoticdeformity.(b)CTscan
andrecumbentfilmshowasolidfusionextendinguptotheupperinstrumentedvertebra(UIV)witharigiddeformity.
Thispatientrequiredathree-columnosteotomytocorrectthePJK.(c)Pre-andpostrevisionradiographs.
Pearls
The appropriate magnitude of correction and global sagittal alignment are critical to achieving successful
outcomes and avoiding the development of PJK.An SVA of 4 cm is a reasonable goal, especially for those
patientsover60yearsofage.
Proximaljunctionalkyphosisisanearlypostoperativephenomenon,andmostcasescantypicallybeobserved
within2to6months postoperatively.Similarly,casesofjunctional failure typicallyareevidentwithinthefirst
year.
WhenrevisingcasesofPJK,itiscrucialtoconsidertherigidityofthedeformity,asflexiblePJKmaybetreated
withinstrumentationandfusiononly,withoutanosteotomyorposteriorcolumnosteotomiesonly.
Pitfalls
Carefulattention mustbepaidtoglobal sagittalalignment;attemptingtoaggressivelycorrectallpatientsto an
SVAof0cmhasbeenshowntopredisposepatientstoPJK.
Although no consensus exists on preventing PJK, surgeons must pay close attention to the integrity of the
posteriorsoft tissues and therigidityofthe construct, and mustselectanappropriateUIV.Failureto consider
thesefactorsmayleadtothedevelopmentofPJK.
Although mostcases of PJK are asymptomatic,these patients may have increasedpain and worsefunctional
scores.Theymustbefollowedregularlytoensurestablekyphosisandacceptableoutcomes.
References
FiveMust-ReadReferences
1.GlattesRC,BridwellKH,LenkeLG,KimYJ,RinellaA,EdwardsCII.Proximaljunctionalkyphosisinadultspinal
deformityfollowinglonginstrumentedposterior spinal fusion: incidence, outcomes, andriskfactoranalysis.Spine
2005;30:1643–1649PubMed
2.KimHJ, Lenke LG, Shaffrey CI, Van Alstyne EM, Skelly AC. Proximaljunctionalkyphosisasadistinctformof
adjacent segment pathology after spinal deformity surgery: a systematic review.Spine 2012;37(22, Suppl):S144–
S164PubMed
3. Lee JH, Kim JU, Jang JS, Lee SH. Analysis of the incidence and risk factors for the progression of proximal
junctionalkyphosisfollowingsurgicaltreatmentforlumbardegenerativekyphosis:minimum2-yearfollow-up.BrJ
Neurosurg2014;28:252–258PubMed
4.YagiM,RahmM,GainesR,etal;ComplexSpineStudyGroup.Characterizationandsurgicaloutcomesofproximal
junctionalfailureinsurgicallytreatedpatientswithadultspinaldeformity.Spine2014;39:E607–E614PubMed
5. YagiM, King AB, Boachie-Adjei O. Incidence, risk factors, andnatural courseof proximaljunctional kyphosis:
surgicaloutcomes review ofadult idiopathic scoliosis.Minimum 5 yearsof follow-up. Spine2012;37:1479–1489
PubMed
6.CammarataM,AubinCE,WangX,Mac-ThiongJM.Biomechanicalriskfactorsforproximaljunctionalkyphosis:a
detailednumericalanalysisofsurgicalinstrumentationvariables.Spine2014;39:E500–E507PubMed
7.PanjabiMM,WhiteAAIII.Basicbiomechanicsofthespine.Neurosurgery1980;7:76–93PubMed
8. Anderson AL, McIff TE, Asher MA, Burton DC, Glattes RC. The effect of posterior thoracic spine anatomical
structuresonmotionsegmentflexionstiffness.Spine2009;34:441–446PubMed
9. Denis F, Sun EC, Winter RB. Incidence and risk factors for proximal and distal junctional kyphosis following
surgicaltreatmentforScheuermannkyphosis:minimumfive-yearfollow-up.Spine2009;34:E729–E734PubMed
vertebraprovidemoregradualmotiontransitionthanpediclescrews.Spine2014;39:E826–E832PubMed
level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long
posteriorspinalfusion.SpineDeformity2013;1:299–305
deformity after segmental posterior spinal instrumentation and fusion: minimum five-year follow-up. Spine
2008;33:2179–2184PubMed
patientswithidiopathicscoliosis.Injury2010;41:415–420PubMed
important risk factor for developing proximal junctional kyphosis in idiopathic scoliosis. Clin Orthop Relat Res
2012;470:1633–1639PubMed
of20degreesasacriticalangle.Neurosurgery2013;72:899–906PubMed
withfusionfromthethoracicspinetothesacrum:acomparisonofproximalanddistalupperinstrumentedvertebrae.
JNeurosurgSpine2013;19:360–369PubMed
adultspinaldeformity.Spine2013;38:E1469–E1476PubMed
adolescent idiopathic scoliosis following pedicle screw, hook, or hybrid instrumentation. Spine 2010;35:177–181
PubMed
higherpostoperativelumbarlordosisandlargersagittalbalancecorrections.Spine2014;39:E576–E580PubMed
resultsfromincompleterestorationofthelumbarlordosisrelativetothemagnitudeofthethoracickyphosis.Iowa
OrthopJ2011;31:199–206PubMed
adolescentsandadults.Spine1998;23:211–215PubMed
asymptomaticmiddleandolderagedvolunteers.Spine1995;20:1351–1358PubMed
idiopathicscoliosis.Spine1999;24:795–799PubMed
deformitypatients.Spine2013;38:896–901PubMed
over age 65: surgicalconsiderations andtreatment options in patients with poor bone quality. Spine 2006;31(19,
Suppl):S144–S151PubMed
adults after spinal deformity surgery using pedicle screw constructs: analysis of morphological features. Spine
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radiographic characteristics, and management of proximal junctional kyphosis following adult thoracolumbar
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10
Biomechanics and Material Science for
DeformityCorrection
ManabuIto,YuichiroAbe,andRemelAlingalanSalmingo
Introduction
Thefirstapplicationofametallicimplanttothehumanspinewasreported byHadra
1
in
1891.Silverwireswereplacedinthethoracicspinefortreatmentofaspinalfracture.The
mostimportant historicaleventin spinalreconstructionsurgerywas theinventionof the
Harringtoninstrumentationinthemiddleofthe20thcentury.
2
PaulHarringtondeveloped
thesespinalimplants,consistingofhooksandrodsmadeofstainlesssteel,fortreatmentof
severespinaldeformityandfracture-dislocationsofthespine.Sincetheintroductionofhis
device,surgeriestocorrectandstabilizethespinewithmetallicimplantshaveundergone
dramaticdevelopment, and surgeriesusingmetallic implants tocorrect andstabilize the
damaged spine became known as spinal instrumentation surgery. The Harrington
instrumentation surgery was modified by his successors, who added sublaminar wires,
tapes,andpediclescrews.
3
Since2000,pediclescrewsandrodshavebeenwidelyusedfor
spinaldeformitysurgeryduetotheirbiomechanicalsuperiority.Ofthebiomaterialsused
forspinaldeformity surgery, titaniumalloysarethe mostpopularmaterialat thepresent
timedue totheir improvedbiocompatibility andbecause theyentail fewermetal-related
artifacts in magnetic resonanceimaging (MRI).
4
Historically, stainless steeland cobalt-
chromium(Co-Cr)werediscoveredmuchearlierthantitaniumalloys(Table10.1).Spinal
implantsmadeofstainlesssteelorCo-Crarecurrentlyusedforpatientswithrigidspinal
curvesduetotheirsuperiormechanicalpropertiestotitaniumalloys.
There is no consensus on how to select metallic materials for spinal deformity
correction today, and spine surgeons depend on their personal experience through their
medicalcareer.This chapterdiscusses metallic spinalimplants andthebiomechanics of
thedeformity correction ofthe spine.It is imperativethat practitionersbe familiarwith
thesetopics,astheyareindispensableinprovidingpatientswithsafeandeffectivespinal
deformitycorrection.
MechanicalPropertiesofMetals
Metalshaveacommonpatternofstress–straincurveconsistingoftheelasticdeformation
zoneand the plasticdeformation zone.In theelastic deformationzone, thestress–strain
relationshipislinearandthemetaldeformsinproportiontotheappliedforce(Fig.10.1).
After the yieldpoint,the stress–strain curve of metals becomes nonlinear.Ifincreasing
force is applied to a metallic implant, the implant will reach the ultimate strength and
finally rupture or break. Although all metals follow the same pattern of stress–strain
relationship, there are differences in their yield strength and ultimate strength and the
slope ofthe stress–strain curve. Within the elasticdeformation zone, metals are ableto
returntotheoriginalshapeaftertheforceisremoved.Oncemetalswereover-benttotheir
plastic deformation zone, the metal is not able to return to the original shape and
permanent deformation of metallic implants occurs. If plastic deformation of the rods
occursafterscoliosiscorrection,significantlossofcorrectionmayresult,andtheoriginal
purpose of correcting the spinal deformity would not be fulfilled. For this reason, it is
importantforspinesurgeonstoknowhowmuchforceisputonthespinalimplantsduring
correction procedures and mechanical responses of the metallic implants to the forces
createdbycorrectionprocedures.
Table10.1HistoryofMetallicImplantsUsedforSpineSurgery
Year Event
1890s
Application of sliver wires for spinal
fractures
1910s Developmentofstainlesssteel
1920s
Development of cobalt-chromium alloys
(Vitallium
®
in1932)
1940s
Development of SUS316, 317 stainless
steel(Harringtoninstrumentation)
1960s
Developmentoftitanium alloys(Grade1
—4: commercially pure titanium, other
titanium alloys: Ti-6Al-4V, Ti-6Al-7Nb,
Ti-6Al-2.5Fe,Ti-13Zr-13Ta,etc.)
Abbreviation:SUS,steelusestainless.
The metals currently used for spinal deformity surgery include stainless steel, pure
titanium,titaniumalloys,and Co-Cralloys.Themechanicalpropertiesof eachmetalare
shown in Fig. 10.2. Commercial pure titanium (cpTi) has four grades based on its
mechanicalproperties.Grade1hasthehighestvalueofelongationatbreak,butithasthe
lowest tensile strength. As the grade increases, the tensile strength increases, and the
capabilityof elongationdecreases. Thetensile strengthof Ti alloysis muchhigher than
thatofpuretitanium,buttheelongationcapacityofTialloysisthelowestamongallthe
types of titanium. Although titanium rods are bent by surgeons during surgery to the
desired contour, the mechanical stiffness of titanium decreases significantly around the
bendingpoints. Co-Crshowsthe highesttensile strengthandrelatively highbreak point
forelongation.ThisstiffermechanicalpropertyofCo-Crisfavoredbyspinesurgeonsfor
treatmentofrigidspinaldeformities.Stainlesssteel(grade,SUS316L)isalittleweakerin
itstensilestrengthcompared withCo-Cr,butit showsmuchbetterelongationdurability.
Newstainlesssteelmaterialswithhighertensilestrengthhavebeeninventedrecentlyand
willbeavailableforsurgeryverysoon.
Fig.10.1Thestress–straincurveofatypicalstructuralmetal.LineAshowstheapparentstressandlineBshowsthetrue
stress.Point1showstheultimatestrength,andpoint2showstheyieldstrength.Amaterialdemonstratesruptureatpoint
3.Theareaof linear relationshipbetweenthestressandstrainindicates the elasticdeformationregionbeforepoint2.
Afterpoint2,thestressincreasesuptotheultimatetensilestrength(point1)inregion4.Beyondpoint1,aneckforms
wherethelocalcross-sectionalareasignificantlydecreasesandthematerialbecomesweakerinregion5.
Fig.10.2Therelationshipbetweentensilestrengthandelongationatthebreakofeachmetal.Therearefourgradesfor
commercialpuretitanium(cpTi),withsmalldifferencesinmechanicalproperties.Thoughtitaniumalloysshowgreater
tensilestrengththancpTidoes,thebreakpointsoftitaniumalloysunderelongationaremuchlowerthanthoseofcpTi.
StainlesssteelshowsthehighestcapabilityforelongationandCo-Crshowsthehighesttensilestrengthamongall.
ChangesinMechanicalPropertiesofRodsAfterManual
Bending(Table10.2)
Becausethe metallicrods suppliedby medicaldevice companiesfor useduring surgery
are straight,the surgeonneeds to bendthemby hand tothe desired contourjust before
performingthe correctionprocedure.Withregardto therods’mechanical properties,the
yieldstrength oftitaniumalloy rodswitha 5.5-mmdiameterdecreases from803.9N to
324.0N(40.3%)afterthree-pointbendingby20degrees.
5
Theyieldstrengthoftitanium
alloyrodswitha6.0-mmdiameterisreduced54.1%after20-degreethree-pointbending.
Co-Cr alloy rods with a 6.0-mm diameter showed the same tendency as the 6.0-mm
titanium alloy rods, with their yield significantly decreasing to 56.4% after 20-degree
three-point bending. If multiple rod bending operations were performed, even the
mechanically stiffestCo-Cr rods can exhibit a significant reduction in their mechanical
properties.Spine surgeonsshould beaware ofthe reductionin mechanicalproperties of
eachmetaltoavoidmechanicalfailuresofrodsduringsurgeryandpostoperativefollow-
upperiodsbeforesolidbonyfusionisobtained.
Themechanicalforcesonspinalimplantsdecreaseovertimeaftersurgery,asbiological
bonyfusionmaturesandbecomessolid.Themaximumforceonthespinalimplantsmay
occur during the correction procedure.Precise in-vivo forces on spinal implants during
correction procedures are still unknown. Previous biomechanical studies have tried to
measurein-vivoforcesonrodsbyvariousengineeringmethods.
6,7
Itisdifficulttoobtain
in-vivo dataduring operative procedures dueto ethical restrictions,andreliable in-vivo
datainspinaldeformitycorrectionarelacking.Medicaldevicessuchasmetallicrodsand
screws for spinal deformity should be designed and manufactured based on the
biomechanicalandbiologicalenvironmentinwhichtheywillbeusedinthehumanbody.
Reliablein-vivobiomechanicalinformationregardingtheforceonspinalimplantsduring
spinaldeformitycorrectionmayadvancethesafetyandeffectivenessofdeformitysurgery
inthefuture.
ViscoelasticityoftheSpine
Thespinalcolumn,consistingofbone,ligaments,andintervertebraldisks,isacomposite
material with significant viscoelasticity. Viscoelasticmaterials show two biomechanical
characteristics:creepphenomenonandstressrelaxation.Thecreepphenomenonstatesthat
when stress is held constant, the strain on the material increases with time. Stress
relaxationstatesthatwhenthestrainisheldconstant,thestressdecreaseswithtime.
Consideringtheviscoelasticpropertyofthespine,rapidcorrectionproceduressuchas
quickrod rotationmaneuversmay makethespine muchstifferandmay hinderefficient
spinal correction, resulting in a lower correction rate than the surgeon anticipated
preoperatively. Thus, destabilization procedures, such as bilateral facetectomies,
diskectomies, and release of costotransverse ligaments, are very important in obtaining
bettercorrection.Besidesthesetechnicalissues,thebiomechanicalprinciplesindicatethat
slowrodrotationandtranslationprocedurescanobtainbetterfinalcorrectionrates.After
the correction procedure is completed,the rod within an elastic deformation range will
tendtospringbacktotheoriginalshapeduetothestressrelaxationeffectaftercorrection
procedures (Fig. 10.3). Fast rod rotation may cause significant increase of mechanical
loadsontheimplantsandresultindramaticchangesintheshapeoftherods.Iftheforces
ontherodswerewithintheelasticdeformationzoneofmetal,therodswouldstillhavea
potential to return to the original shape. Spine surgeons should be familiar with the
mechanicalcharacteristicsofmetalsandthespinalcolumntoobtainbettercorrectionrates
andprovidepatientswithsafesurgery.
CorrectionProceduresforAdolescentIdiopathicScoliosis
There are many surgical procedures to correct adolescent idiopathic scoliosis (AIS)
reported after the introduction of the Harrington instrumentation. The Cobb angle
correctiononanteroposterior(AP)radiographswas40%withHarringtonrods,55%with
thedual-rodmultihooksystem(CD,Cotrl-Duboussetinstrument),and65%withthedual-
rod multiplepediclescrew constructs in the coronalplane. Recent studies reported that
pediclescrewconstructsinthesagittalplaneincreasedthelordosisofthethoracicspine.
8
While the surgeon is performing the direct vertebral rotation technique to decrease
rotationaldeformity aroundthe apexof thethoracic curve,the majorforce onthe spine
pushesthethoracicribhumpdowntolessentherotationaldeformityofthespine,which
eventuallycausesdekyphosisinthethoracicspine.
9
There have been several attempts to create thoracic kyphosis by posterior spinal
instrumentationsurgery.Becauseatitanium rodis mechanicallyweaker thana stainless
steel or Co-Cr rod, some surgeons utilized stainless steel or Co-Cr rods rather than
titaniumrodssoasnottoyieldtotheforceontherods.Anothermethodwastouseanin-
siturodbendingtechniqueafter asinglerod-rotationtocreatethoracickyphosis. Oneof
the problems ofan in-situ rod-bending procedure is thatamuch greater load would be
applied to the pedicle screws around the area of rod bending, which may increase the
possibility of vertebral fractures or screw loosening due to a higher concentration of
mechanical force on the screws. Also, multiple rod-bending procedures will reduce the
originalmechanicalstrengthofthemetallicimplant.
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