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Peer-Review Reports

Biomechanical Stability of a Posterior-Alone Fixation Technique After CraniovertebralJunction RealignmentRoy Thomas Daniel1, Aditya Muzumdar2, Aditya Ingalhalikar2, Mark Moldavsky2, Saif Khalil2

INTRODUCTION

Craniovertebral junction anomalies (non-traumatic) are primarily of two types: reduc-ible atlantoaxial dislocation and basilar in-vagination. Basilar invagination syndrome ischaracterized by upward migration of theodontoid process of the second cervical verte-bra. Basilar invagination is usually seen in pa-tients with bone diseases such as rheumatoidarthritis, hyperparathyroidism, Paget disease,osteogenesis imperfecta, and rickets. It maylead to narrowing and stenosis of the fora-men magnum and result in fatal compres-sion of the brain stem, manifesting as sud-den death (2).

The standard and most accepted form oftreatment of patients with basilar invagina-tion is a transoral decompression (3, 5, 22).Because of the instability of the region result-ing from this procedure, many authors haverecommended simultaneous posterior occip-itoaxial or atlantoaxial fixation surgery (3, 5,6, 12, 23, 26, 28). An alternate technique, anoccipitocervical fixation after cervical tractionto reduce basilar invagination, has been pro-posed (12). Nevertheless, in some of the pa-

tients, transoral decompression was requiredbecause the implants could not sustain thereduction of basilar invagination.

Recently, a new technique to treat basilarinvagination by distraction and realign-ment through a posterior approach hasbeen developed and is similar to the tech-nique popularized by Goel et al (8-10, 16).The initial clinical results with this proce-dure have been favorable (16). Long-termclinical and radiological assessments arenow in progress, and the early data ob-tained attest to the stability of the fixation(unpublished data). The biomechanics ofthis surgical procedure would further quan-tify the stability provided by it. The objectiveof this study was to biomechanically evalu-

ate the surgical constructs performed dur-ing this procedure.

MATERIALS AND METHODS

Specimen PreparationSeven fresh human cadaver occipitocervicalspines (occiput-C4) were used. The speci-mens were obtained from the MedCure(Portland, Oregon, USA) tissue bank. Thespecimens were harvested from two femaleand five male cadavers (mean age of death,53 � 4 years) and stored at �20°C beforethawing. The spines had been radio-graphed previously in the anteroposteriorand lateral planes to ensure the absence of

� OBJECTIVE: The aim of the current study was to investigate the biomechanicalstability and fixation strength provided by a posterior approach reconstructiontechnique to realign the craniovertebral junction.

� METHODS: We tested seven human cadaver occipito-cervical spines (oc-ciput-C4) by applying pure moments of �1.5 Nm on a spine tester. Each specimenwas tested in the following modes: 1) intact; 2) injured; 3) spacers alone at C1-C2articulation (S); 4) spacers plus C1-C2 Posterior Instrumentation (S�PI); and 5)spacers plus C1-C2 posterior instrumentation plus midline wiring (S�PI�MLW).C1-C2 range of motion for each construct was obtained in flexion-extension,lateral bending, and axial rotation.

� RESULTS: In all the loading modes, S, S�PI, and S�PI�MLW constructssignificantly reduced range of motion compared with the intact and injuredcondition (P < 0.05). There was no statistical difference between any of the threeinstrumentation constructs (P > 0.05).

� CONCLUSIONS: This study investigated the biomechanics of the posteriorapproach technique for realignment of the craniovertebral junction and alsomade comparisons with additional posterior fixations. The stand-alone spacerswere stable in all three loading modes. Posterior instrumentation increased thestability as compared to stand-alone spacers. The third point of fixation, carriedout by using midline wiring, increased the stability further. However, there wasnot much difference in the stability imparted with the midline wiring versuswithout. The present study highlights the biomechanics of this novel concept andreaffirms the view that distraction of the C1-C2 articular facets and directarticular joint atlantoaxial fixation would be an ideal method of management ofbasilar invagination.

Key words� Basilar invagination� Biomechanics� Craniovertebral junction realignment

Abbreviations and AcronymsMLW: Midline wiringPI: Posterior instrumentationROM: Range of motionS: Spacers

From the 1Centre HospitalierUniversitaire Vaudois, Lausanne,

Switzerland; and 2Globus Medical, Inc., Audubon,Pennsylvania, USA

To whom correspondence should be addressed:Aditya Muzumdar, M.S.[E-mail: amuzumdar@globusmedical.com]

Citation: World Neurosurg. (2011) xx, x:xxx.DOI: 10.1016/j.wneu.2011.06.039

Journal homepage: www.WORLDNEUROSURGERY.org

Available online: www.sciencedirect.com

1878-8750/$ - see front matter © 2011 Elsevier Inc.All rights reserved.

WORLD NEUROSURGERY xx [x]: xxx, MONTH 2011 www.WORLDNEUROSURGERY.org 1

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fractures, deformities, and any metastaticdisease. The spines were carefully denudedof paravertebral musculature while preserv-ing the spinal ligaments, joints, and diskspaces. Each spine was potted proximally atthe occiput and distally at C4 in a 3:1 mix-ture of Bondo auto body filler (Bondo Mar-Hyde Corp, Atlanta, Georgia, USA) and fi-berglass resin (Bondo Home Solutions AllPurpose Putty; Bondo MarHyde Corp).

Plexiglas markers, each having three infra-red light-emitting diodes, were secured rig-idly to the anterior aspect of the occiput, C1,C2, C3, and C4 vertebral bodies by the use ofbone screws to track its motion with OptotrakCertus (NDI, Inc. Waterloo, Canada) motion

analysis system. The location of the markers(denoting a rigid body) was approximatelyaligned sagittally along the curvature of thespine. The Optotrak Certus software was ableto superimpose the coordinate systems of twoadjacent vertebral bodies to inferentially de-termine the relative Eularian rotations in eachof the three planes.

Flexibility TestingThe specimen was fixed to the load frame ofa six degree of freedom spine simulator anda pure moment was applied to the occiputthrough servomotors (7, 24). The specimenwas kept moist throughout the test by

spraying it with 0.9% saline. All tests wereperformed at a room temperature of 25°C.Each of the test constructs were subjected tothree load– unload cycles in each of thephysiologic planes generating flexion-ex-tension, right-left lateral bending, andright-left axial rotation load-displacementcurves. This was achieved by programmingthe motors to apply continuous moments ineach physiologic plane. A typical load-un-load cycle in the sagittal plane was com-posed of neutral � full flexion � full exten-sion � neutral (three times). Data from thethird cycle were considered for analysis.The design of the load frame enables un-constrained motion of the spine in re-sponse to an applied load. There was nocompressive preload applied on the speci-men. A load control protocol was used toapply a maximum moment of � 1.5 Nm at arate of 1°/sec (7, 17, 24, 30).

The three-dimensional intervertebral ro-tation was obtained from the Optotrak Cer-tus data files in the form of Euler angles(degrees) about the X, Y, and Z axes: �Rx/�Rx, �Ry/�Ry, and �Rz / �Rz denotingflexion-extension, right-left axial rotation,and right-left lateral bending range of mo-tion (ROM), respectively. The Euler se-quence used in this study was xzy.

Study DesignEach specimen (occiput-C4) was tested byapplying pure moments of �1.5 Nm. ROMat C1-C2 was obtained in flexion-extension,lateral bending, and axial rotation. Eachspecimen was tested in the followingmodes: 1) intact; 2) injured; 3) spacers aloneat C1-C2 articulation (S); 4) spacers plusC1-C2 posterior instrumentation (S�PI);and 5) spacers plus C1-C2 posterior instru-mentation plus midline wiring construct(S�PI�MLW; Figure 1). After the intacttesting, injury was simulated by disruptingthe articulation between C1 and C2 bilater-ally. Spacers (COLONIAL 5-mm small 0°;Globus Medical Inc, Audubon, Pennsylva-nia, USA) typically used for cervical inter-body fusion were inserted between C1 andC2 articulations bilaterally. Posterior in-strumentation was carried out at C1-C2 bythe use of C1 lateral mass screws (4 � 32mm) and C2 pedicle screws (4 � 22 mm)with the use of PROTEX CT (Globus Medi-cal Inc) polyaxial screws. A poly-ether-ether-ketone spacer was wedged betweenthe posterior arch of atlas and the C2 spi-

Figure 1. Surgical constructs. (A) Spacers alone at C1-C2 articulation (S). (B) Spacers plus C1-C2posterior instrumentation (S�PI). (C) Spacers plus C1-C2 posterior instrumentation plus midlinewiring construct (S�PI�MLW)

PEER-REVIEW REPORTS

ROY THOMAS DANIEL ET AL. POSTERIOR-ALONE FIXATION TECHNIQUE

2 www.SCIENCEDIRECT.com WORLD NEUROSURGERY, DOI:10.1016/j.wneu.2011.06.039

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nous process, and then a midline C1-C2modified Gallie’s fixation was performedwith the use of 1.1-mm stainless-steel wire.Subsequent to intact testing, the instru-mented constructs were subjected to thesame load control protocol for flexibilitytesting as described previously.

Data AnalysisROM data were normalized to intact (100%).Statistical analysis was performed on raw

data. Comparison of data was performed bythe use of repeated-measures analysis of vari-ance for independent samples followed byTukey’s post-hoc analysis for multiple com-parison procedures. Significance was ac-cepted at P � 0.05.

RESULTS

The means � SD for ROM (%) in all loadingmodes are presented in Table 1 and Figure 2.

Flexion-ExtensionThe injured ROM increased to 131% � 37%(P � 0.05) as compared with intact. The S(18% � 9%), S�PI (13% � 9%), andS�PI�MLW (10% � 7%) constructs signif-icantly reduced ROM when compared withintact (P � 0.05). When compared with theinjured state, all three constructs signifi-cantly reduced ROM (P � 0.05).

Lateral BendingThe injured ROM increased to 106% � 51%(P � 0.05) as compared with intact. The S(8% � 6%), S�PI (4% � 3%), andS�PI�MLW (4% � 3%) constructs signif-icantly reduced the ROM when comparedwith intact (P � 0.05). When comparedwith the injured state, all three constructssignificantly reduced ROM (P � 0.05).

Axial RotationThe injured ROM increased to 140% � 76%(P � 0.05) as compared with intact. The S(2% � 1%), S�PI (1% � 0.5%), andS�PI�MLW (1% � 1%) constructs signifi-

Figure 2. Comparison of C1-C2 ROM (% values) in flexion-extension, lateral bending, and axial rotation. The intact ROM is 100%.Figure shows comparison between 1) spacers alone at C1-C2 articulation (S); 2) spacers plus C1-C2 posterior instrumentation(S�PI); and 3) spacers plus C1-C2 posterior instrumentation plus midline wiring construct (S�PI�MLW). �Representssignificantly different than intact (P � 0.05). #Represents significantly different than injured (P � 0.05). Intact and Injuredconditions have not been presented in the graph to appreciate the difference between the instrumentation constructs.

Table 1. C1-C2 ROM (% values) During Different Loading Modes (Mean � SD) forIntact; Injured; S; S�PI; and S�PI�MLW

Loading Modes Intact Injured S S � PI S � PI � MLW

Flexion-extension 100 � 0 131 � 37 18 � 9*† 13 � 9*† 10 � 7*†

Lateral bending 100 � 0 106 � 51 8 � 6*† 4 � 3*† 4 � 3*†

Axial rotation 100 � 0 140 � 76 2 � 1*† 1 � 0.5*† 1 � 1*†

S, spacers alone at C1-C2 articulation; S�PI, spacers � C1-C2 posterior instrumentation; S�PI�MLW, spacers � C1-C2posterior instrumentation � midline wiring construct.

*Represents significantly different than intact condition (P � 0.05).†Represents significantly different than injured condition (P � 0.05).

PEER-REVIEW REPORTS

ROY THOMAS DANIEL ET AL. POSTERIOR-ALONE FIXATION TECHNIQUE

WORLD NEUROSURGERY xx [x]: xxx, MONTH 2011 www.WORLDNEUROSURGERY.org 3

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cantly reduced the ROM when compared tointact (P � 0.05). When compared with theinjured state, all three constructs signifi-cantly reduced ROM (P � 0.05). There wasno statistical significance between the ROMof the instrumented constructs in any of theloading modes (Table 1 and Figure 2).

DISCUSSION

Basilar invagination may be possibly re-duced by distracting the articular surfacesbetween atlas and axis. In this technique,the patient is placed prone with the headend of the table elevated to approximately35°. Cervical traction is given, and theweights are progressively increased to amaximum of one-fifth of the body weight toattain optimum realignment of the cranio-vertebral junction. The distracted positionis maintained by insertion of spacers intothe lateral atlantoaxial joints. Stand-alonespacers may be used in select cases (such as

in children, in whom the posterior pediclescrew fixation may be difficult because ofthe size of the lateral masses and pedicle orbecause of anatomical variations). Addi-tional fixation could be provided by poste-rior instrumentation and midline wiringconstructs to further stabilize the joint.

Several authors have shown that the useof screw/rod systems with placement of C1lateral mass screws and C2 pedicle screws isa safe and effective system for achievingC1-C2 fusion. It has a good safety profile, isassociated with a low complication rate,and may also be applicable to most patients(1, 11, 13-15, 18-20). For a patient with moreposterior instability, standard cable con-structs provide adequate stability in con-junction with additional posterior instru-mentation (25). In basilar invagination,either congenital or of acquired causes suchas rheumatoid arthritis or tuberculosis, theligamentous structures supporting theC1-C2 lateral facetal joints are diseased orincompetent. This technique focuses on the

realignment at these joints and thereforehas the advantage of effectively immobiliz-ing the joint with preparation of the articu-lar surfaces and implantation of adequatelysized spacer filled with bone chips. Thisjoint treatment not only reduces the disloca-tion, but also favors the development of astrong bony lateral pillar fusion that wouldimpart added stability to the craniovertebraljunction.

Our clinical results with this procedurehave been favorable (Figures 3 and 4). Aprospective study of 27 patients with basilarinvagination who underwent this proce-dure is currently in progress. The estima-tion of the distraction achieved was as-sessed by well-established craniometricmeasurements. The median distractionachieved at the midline by the use of Cham-berlain (4), McRae (21), and Thiebaut et al.(29) lines were 8.7 mm, 8.8 mm, and 12.4mm, respectively.

The present study highlights the biome-chanical stability provided by stand-alonespacers combined with the posterior instru-mentation and midline wiring constructs.For this evaluation, human cadaveric cervi-cal spines were used. Human spines areideal for performing in vitro spinal recon-struction and biomechanical testing. How-ever, in vitro biomechanical evaluationshave inherent limitations, such as exclusionof the effect of muscles, weight of the torsoabove the instrumented level, and complexmovements occurring in vivo. The spinesimulator currently doesn’t have the capa-bility of applying follower load (preload)(27) to the specimen.

With clinical basilar invagination, thereis pathological vertical settling, presumablyas the result of deformation of the bonesand laxity of the ligaments. However, theinjury model simulated in our study doesnot closely approximate this condition. Theinured model did not have any ligamentouslaxity, which were then jacked up to a newheight far exceeding the physiologicalheight. Ligaments were likely stretched ex-tremely tight in this model, which might bedifferent than the case in which height isrestored to normal in a patient with pathol-ogy. This difference would account for thestability in the condition of spacers only.

This study investigated the biomechanicsof the posterior-alone fixation technique,compared with additional posterior fixa-tions. The stand-alone spacers were stable

Figure 3. Sagittal computed tomography scans through the lateral C1-C2joints (patient with basilar invagination attributable to rheumatoid arthritis)show the reduction of the listhesis and maintenance of the distractionwith the spacers.

Figure 4. Midsagittal computed tomography scans showing the distractionachieved.

PEER-REVIEW REPORTS

ROY THOMAS DANIEL ET AL. POSTERIOR-ALONE FIXATION TECHNIQUE

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in all three loading modes, and additionalposterior instrumentation further increasedstability. The third point of fixation per-formed with the use of midline wiring in-creased the stability further. However, therewas not much difference in the stability im-parted with the midline wiring versus with-out. Hence, spacers with posterior instru-mentation may provide adequate stability toreduce basilar invagination. Considering thatthis is a biomechanical cadaveric study andnot a model that would replicate dynamics ofa long-standing bone fusion after realign-ment, it would not be possible to concludeelimination of the third point of fixation fromthe procedure in clinical practice. It shouldalso be noted that in this study we evaluatedthe initial stability offered by the constructsand do not account for the effect of fatiguetesting (sequential loading) on the con-structs. Long-term clinical results with thissystem of realignment and fixation, therefore,need to be studied to assess stability at thecraniovertebral junction after bony fusion hasoccurred.

This technique of craniovertebral junc-tion realignment provides neural decom-pression and stabilization through a solelyposterior approach and has demonstratedfavorable short-term clinical results (16).The present study effectively highlights thebiomechanics of this novel concept. It reaf-firms the view that distraction of the C1-C2articular facets and direct articular joint at-lantoaxial fixation in the distracted positionwould be an ideal method of managementof basilar invagination.

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Conflict of interest statement: Dr. Roy Daniel, surgeondesign team, Globus Medical Inc; Aditya Muzumdar,employee, Globus Medical Inc; Aditya Ingalhalikar,employee, Globus Medical Inc; Mark Moldavsky, employee,Globus Medical Inc; and Saif Khalil, employee, GlobusMedical Inc.

received 25 February 2011; accepted 23 June 2011

Citation: World Neurosurg. (2011) xx, x:xxx.DOI: 10.1016/j.wneu.2011.06.039

Journal homepage: www.WORLDNEUROSURGERY.org

Available online: www.sciencedirect.com

1878-8750/$ - see front matter © 2011 Elsevier Inc.All rights reserved.

PEER-REVIEW REPORTS

ROY THOMAS DANIEL ET AL. POSTERIOR-ALONE FIXATION TECHNIQUE

WORLD NEUROSURGERY xx [x]: xxx, MONTH 2011 www.WORLDNEUROSURGERY.org 5

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tapraid3/wne-wneu/wne-wneu/wne99910/wne0803p10z

KnepperBP S�3 8/22/11 14:21 Art: 797