SPECIAL ANNUAL ISSUE

Fusions at the craniovertebral junction

Raheel Ahmed & Vincent C. Traynelis &

Arnold H. Menezes

Received: 22 January 2008 /Published online: 4 April 2008# Springer-Verlag 2008

AbstractIntroduction The surgical management of craniovertebraljunction instability in pediatric patients has unique chal-lenges. While the indications for internal fixation inchildren are similar to those of adults, the data concerningtechniques, complications, and outcomes of spinal instru-mentation comes from experience with adult patients.Diminutive osseous and ligamentous structures andanatomical variations associated with syndromic craniover-tebral abnormalities frequently complicates the approachesand limits the use of internal fixation in children. Cervicalarthrodesis in the pediatric age group has the potential forlimiting growth potential and causing secondary deformity.Recent advances in image analysis have enabled preoperativeplanning which is critical to evaluate the size of instrumen-tation and its relation to the patient’s anatomy. Newertechniques have recently evolved and have been incorporatedin the management of pediatric patients with requirement forcraniocervical stabilization.Materials and methods Over 750 craniovertebral junctionfusions have been reviewed in children. The indications foratlantoaxial arthrodesis were: (a) absent odontoid process,dystopic os odontoideum, absent posterior arch of C1; (b)Morquio’s syndrome, Goldenhar’s syndrome, Conradi’ssyndrome, and spondyloepiphyseal dysplasia. The acquiredabnormalities of trauma, postinfectious instability, andDown’s syndrome completed the indication in children.The indications for occipitocervical fusion were: (a)anterior and posterior bifid C1 arches with instability,

absent occipital condyles; b) severe reducible basilarinvagination, unstable dystopic os odontoideum, andunilateral atlas assimilation; (c) acquired phenomenon withtraumatic occipitocervical dislocation, complex craniover-tebral junction fractures of C1 and C2, after transoralcraniovertebral junction decompression, cranial settling inDown’s syndrome and inflammatory disease such asGrisel’s syndrome. Instability was seen in children withclivus chordoma and osteoblastoma. Atlantoaxial fusionswere performed mainly with interlaminar rib graft fusionand more recently with the transarticular screw fixation inthe older patient. In the teenager, lateral mass screws at C1and rod fixation were made; C2 pars interarticular screwfixation and C2 pedicle screw fixation. A C2 translaminarscrew fixation is described. Occipitocervical fusions weremade utilizing rib grafts below the age of 6. A contouredloop fixation was made in children above the age of 7, andrecently, rod and screw fixation was also utilized.Results Abnormal cervical spine growth was not seen inchildren who underwent craniocervical stabilization belowthe age of 5. The authors have reserved rigid instrumentationfor children above the age of 10 years and dependent on theanatomy.

Keywords Craniovertebral junction instability .

Occipitocervical fusion

Introduction

Occipitocervical and atlantoaxial instability affects a num-ber of children every year. The unique developmental,anatomical, and biomechanical aspects of the immaturecraniovertebral junction (CVJ) and cervical spine areresponsible for characteristic patterns of spine injury in this

Childs Nerv Syst (2008) 24:1209–1224DOI 10.1007/s00381-008-0607-7

R. Ahmed :V. C. Traynelis (*) :A. H. MenezesDepartment of Neurosurgery,University of Iowa Hospitals and Clinics,200 Hawkins Drive, 1824 JPP,Iowa City, IA 52242, USAe-mail: vincent-traynelis@uiowa.edu

age group [17, 29]. The relatively underdeveloped neckmusculature, disproportionately large head, and the shallowoccipital–atlantal articulation all increase the risk of injuriesto the CVJ and upper cervical spine [29]. In addition, therelatively high laxity of the occipital/cervical ligaments andjoint capsules in the young predisposes this patient group toligamentous injuries, bony dislocations, and multiple-levelinjuries [17, 48]. Spinal or paraspinal infections or otherinflammatory processes may increase ligamentous laxity tosuch a point that atlantoaxial dislocation may developwithout trauma. Instability may occur in children withDown syndrome, os odontoideum, and rheumatologicdisorders like juvenile rheumatoid arthritis [41]. Finally,CVJ instability may result primarily from the growth of aneoplasm or secondarily due to surgical resection of a tumor.

The surgical management of CVJ instability in pediatricpatients is associated with certain unique challenges. Whilethe indications for internal fixation in children are similar tothose in adults, most of the data concerning techniques,complications, and outcomes of spinal instrumentationcome from experience with adult patients. Diminutiveosseous and ligamentous structures and anatomical varia-tions associated with syndromic craniovertebral anomaliesfrequently complicate operative approaches and can limitthe use of internal fixation in children. Cervical arthrodesisin the pediatric age group has the potential for limitinggrowth potential and causing secondary spinal deformity.Recent advancements in image analysis enable preoperativeplanning which is critical to evaluate the size of instrumen-tation in relation to the patient’s spinal anatomy. Thedevelopment of resorbable instrumentation may reducelong-term risks associated with spinal hardware in pediatricpatients [7].

Transodontoid screw fixation

Odontoid fractures account for up to 15% of cervical spinefractures, with an increasing incidence in the elderly agegroup (>70 years) [13]. In the younger age group, theyusually result from high energy trauma through motorvehicle accidents and are frequently complicated byconcomitant spinal injury and neurological deficits [17,29, 45].

The Anderson and D’Alonzo classification systemcategorizes odontoid fractures based on the anatomicallocation of the fracture line [4]. Type I fractures (traversingthe odontoid tip) and type III fractures (involving the bodyof the axis) generally heal well and are usually conserva-tively managed with nonoperative immobilization (traction,halo vest immobilization and rigid cervical orthosis) [36].Type II fractures, in which the fracture line extends throughthe base of the odontoid process are the most common type

and are associated with a high nonunion rate of up to 85%[30]. Risk factors that are most likely to predict nonunionwith nonsurgical treatment include the degree of fracturedisplacement (>4–6 mm) and angulation (>10°), increasingage (>40–65 years), posterior odontoid displacement,delayed diagnosis, and comminution of the dens base [5,13, 28, 30]. Given the risk of secondary myelopathy andspinal cord injury as a result of the nonunion, it hastherefore been recommended that type II fractures shouldbe treated by early surgical management with primaryinternal fixation.

Initially described by Bohler [10], direct osteosynthesiswith anterior odontoid screw fixation is now increasinglybeing advocated as the treatment of choice for primaryoperative management of type II odontoid fractures [5, 9,20, 37]. The alternative posterior approaches for surgicalstabilization of odontoid fractures are aimed at achievingstable C1–C2 fusion through posterior wiring and bonegrafting, transarticular screw fixation, or posterior instru-mentation of the C1–C2 lateral masses using plate/rodconstructs [12, 32, 35]. While achieving comparably highfusion rates (92–100%), the anterior transodontoid tech-nique affords significant advantages over the posteriorapproaches: first, it circumvents the need for atlantoaxialfusion and therefore enables preservation of axial rotation[34]; eliminates complications at the donor site [53]; andprovides immediate atlantoaxial stabilization [3, 5, 10, 20,22, 53]. The key element is preservation of the cruciateligament to do this procedure.

Patients are placed in a supine position with slightcervical hyperextension to facilitate closed reduction of theodontoid under biplanar fluoroscopic guidance. A horizon-tal incision is made at the C5–C6 level, and the anteriorcervical spine is approached by dissecting through thecervical fasciae (Fig. 1). The trajectory for the trans-

Fig. 1 Cervical incision starts at C5–C6 interspace. The trajectory forthe anterior odontoid screw is seen in the dotted line

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odontoid screw is from the anterior inferior edge of the C2to the tip of the odontoid process, along the vertical axis ofthe odontoid. The superior central portion of the C2–3annulus is removed and the anterior inferior lip of C2exposed. A 2-mm K-wire is advanced up C2 and into thefracture fragment under biplanar fluoroscopic guidance(Fig. 2). The drill bit is then advanced from the inferioredge of C2, through the C2 body and just passes the apicalcortex of the odontoid under fluoroscopic guidance (Fig. 3).The guide hole is fully tapped (Fig. 4). This is an excellentway to measure for screw length. After placement oftitanium lag screws through the drilled hole, directfluoroscopy is used to confirm realignment of the displacedodontoid fragment and to evaluate overall spinal stability inflexion–extension position [37] (Fig. 5).

The average complication rate has been estimated to be9.5% and is predominantly due to hardware failure, mostcommonly by screw pullout from the C2 body [5, 53]. Theuse of double anterior odontoid screws has not been shownto have superior biomechanical advantage over a singlescrew [5, 51, 53].

Preoperative assessment is critical to ensure successfulreduction and fixation of the odontoid fracture. Patientsneed to be evaluated for the presence of an intact transverseligament [53] and whether the fractured odontoid processcan be adequately reduced and anatomically aligned withinthe atlantoaxial complex [22]. In the setting of acuteodontoid fractures, careful patient selection and meticulouspreoperative planning enables the anterior odontoid fixationtechnique to achieve excellent fusion rates.

Atlantoaxial fusion

The atlantoaxial segment is involved in three majormotions, namely, flexion, extension, and axial rotation.

Semirigid atlantoaxial fixation approaches involving wireor cable constructs allow limited stabilization of cervicalmotion and hence need to be supplemented with rigid,external orthosis. In contrast, rigid fixation techniques usingtransarticular screws or C1–C2 screw-rod constructs requiremodest cervical orthosis if the bone quality is poor.Decisions concerning the strategy for internal fixation aredetermined by the primary surgical indication, age, andanatomical considerations and the surgeon’s expertise. Theindications are provided in Table 1.

Interlaminar fixation

Graft and wire techniques were first described by Gallie,but modifications now comprise the Brooks, modifiedBrooks, and interspinous techniques [12, 15, 19, 21, 44].Graft/cable techniques almost always require intact C1–C2posterior elements. These techniques provide effectivestabilization of sagittal plane rotation: the dorsal wiring(or cable) serves as a tension band in resisting cervical

Fig. 2 Final position of odontoid screw across fracture fragments. a Open mouth odontoid view during K-wire advancement across odontoidfracture site. b Lateral cervical radiograph showing ideal position of the advanced K-wire across the odontoid fracture site

Fig. 3 Drill over K-wire seen in a lateral view

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flexion, while the bone graft mechanically blocks exten-sion. In contrast, axial rotation is not restricted, and hencethe need for significant postoperative external immobiliza-tion [16]. Graft and cable techniques are associated withlower rates of arthrodesis [16, 27, 31, 43] than thoseenjoyed by rigid screw/rod systems probably due to

biomechanical differences. One of the major advantagesof cable techniques are greater ease of application anddecreased risk to neurovascular structures. C1C2 cable/grafttechniques are an excellent treatment option in patients withanatomical constraints that limit C1–C2 screw fixation andin young patients with immature spines.

Graft and cable constructs are implanted through amidline posterior cervical exposure. Unnecessary dissectionof the surrounding soft tissues should be avoided, and careshould be taken when exposing the posterior arch of C1 toavoid injury to the vertebral artery (ies). The classic Brooksfusion employs double set sublaminar cables bilaterally.The bone grafts, positioned between the C1–C2 laminae oneach side are encircled by the sublaminar cables. Themodified Brooks fusion utilizes only a single wire or cableon each side (Fig. 6). Sublaminar passage of the fixationcables should be carefully undertaken to avoid neuralinjury. Ideally, the dural is exposed, and the cable flowseasily through this relatively unrestricted space. The wires/cables are carefully passed using both hands to maintainadequate tension. The interspinous technique differs in thatthe sublaminar cable circles C1 in a sublaminar fashion, butC2 fixation is achieved by placing the cable around the C2spinous process. The cable is secured within notches placedbilaterally at the junction of the C2 laminae and the base ofthe spinous process. This technique eliminates the need topass a sublaminar cable at C2, and it is biomechanicallyequivalent to the classic Brooks technique, but is notrecommended by the authors [16].

Common to all of these techniques is the positioning of abone graft between the C1–C2 dorsal elements. It isimportant to ensure that the cortical bone of the inferiorsurface of the C1 lamina and the superior surface of the C2lamina are both removed, so cancellous bone contacts thegraft(s). The bone graft must be an appropriate size to fitprecisely and snugly between the lamina, while the segmentis in proper neutral alignment. Iliac crest grafts are used bymany surgeons, however, the authors prefer to use

Fig. 5 a Final screwplacement seen in open-mouthodontoid view. b Lateral viewdemonstrating the final screwplacement in ideal positionacross the fracture fragment

Fig. 4 a Tap is advanced over the K-wire and seen in the open mouthview. b Lateral view demonstrating the tap advanced into the distalodontoid fragment

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autologous rib grafts if possible. In the skeletally developedpatient, rib grafts possess greater tensile strength andcontain higher natural levels of bone morphogenetic proteinthan iliac crest autograft. Also, ribs regenerate fairly rapidlyin children, and harvesting of rib grafts is associated withlower rates of donor site morbidity compared to iliac crestgrafts [52]. The ribs are individually anchored to thelaminae of C1 and C2; spaced as far apart on the laminaeso as the restrain rotary stresses (Fig. 7). Allografts performrelatively poorly under tension and therefore should not beused if at all possible. The senior author (AHM) has usedbone morphogenetic protein in dorsal CVJ fusions inchildren with excellent preliminary results. The individualrib graft interlaminar fusion rate has been 98% [52].

Transarticular screw fixation

Rigid C1–C2 fixation using transarticular screws is associ-ated with high fusion rates and provides immediate spinal

stability in all planes at the atlantoaxial complex therebycircumventing the need for prolonged rigid external bracingpostoperatively [26, 35, 40, 44]. This approach is particu-larly useful in patients lacking intact C1–C2 posteriorelements. Careful patient selection and meticulous preop-erative imaging and surgical planning enables the trans-articular approach to be used effectively with high fusionrates (97%) and with minimal complications in pediatricpatients above the age of 9 years [11, 23].

Transarticular screw placement is technically demand-ing, and surgeons must overcome a steep learning curve.Extensive preoperative imaging studies, including plain X-rays, magnetic resonance imaging (MRI), and computedtomography (CT) scan with sagittal reformatting, areessential. Computer-based image reconstruction may helpdetermine optimal transarticular screw placement and alsoassist surgical navigation intraoperatively. Image guidanceis not necessary but an adjunct. The surgeon must becompletely knowledgeable concerning the patient’s anato-my and the surgical procedure and not rely solely on acomputer-generated trajectory. The size of the C2 parinterarticularis may limit this approach in up to 20% ofadult patients [46] and the number of children with anadequate bony channel for screw placement into C2 is evenless. Improvements in stereotactic techniques have extend-ed the application of transarticular screw fixation in thesetrying situations [49]. The risk of vertebral artery injury isthe most serious potential complication, and hence, trans-articular screw fixation is contraindicated in patients withhigh-riding vertebral artery groove at C2; seen on lateralsagittal CT [58].Fig. 6 Author’s (AHM) illustration of modified Brook’s type fusion

Table 1 Indications for atlantoaxial arthrodesis

Parameters Description

Congenital Absent odontoid processDystopic os odontoideumAbsent posterior arch of C1

Developmental Morquio’s syndrome,Goldenhar’s syndrome, Conradi’ssyndrome, spondyloepiphysealdysplasia

Acquired Down’s syndromeTraumaPostinfectiousMalignancy

Fig. 7 Author’s illustration of bilateral interlaminar atlantoaxialarthrodesis using autologous rib grafts

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The entry point for a C1–2 transarticular screw in theadult spine is approximately 3 mm superior to the C2–3facet joint line and 3 mm lateral to the lamina-lateral massjunction. This usually corresponds to the center of theinferior medial quadrant of the dorsal surface of the inferiorarticular process of the axis (Fig. 8). The medial corticalwall of the C2 pars can be palpated by carefully placing afine dissector. Utilization of a subperiosteal dissectiontechnique is important both here and when exposing theposterior surface of lateral mass of the atlas. Meticulousadherence to this plane will reward the surgeon with abloodless exposure. The medial wall of the C2 parsinterarticularis demarcates the medial boundary of thetransarticular pathway. The target point for a proper C1–2transarticular trajectory is the anterior tubercle of C1 asvisualized on a lateral fluoroscopic image. To achieve thistrajectory, one must carry the midline C1–2 incision downto the cervicothoracic junction if the entire procedure is tobe performed through a single incision. The most efficientmeans of performing this fixation is to expose only the C1–C2 complex posteriorly and then obtain the propertrajectory for screw insertion by working within a guidetrocar placed through bilateral stab incisions near thecervical thoracic junction. This technique minimizes dis-section of the paraspinous musculature, which lowersoperative time and is felt to decrease postoperativemorbidity. The 1-cm stab incisions are placed approximate-ly 2 cm lateral to the midline at the T1 level. A guide trocarwith an obturator is passed through the stab incision to thesurgical site. The C2 entry point is marked with an awl ordecorticated with a high-speed drill. The drill is passeddown the guide tube and directed toward the superioraspect of the anterior tubercle of C1. The sagittal trajectoryis either straight parasagittal or is angled 10–15° mediallydepending on assessment of the preoperative imagingstudies. The coronal trajectory is assessed fluoroscopically

by aiming for the anterior tubercle of C1. There is no needto directly visualize the C1–2 joint space. Drilling shouldbe performed using fluoroscopic guidance. The authorsstart the drill with power, and once the cancellous channelof the pars is entered, the drill is advanced carefully byhand. This technique provides excellent tactile feedbackwhich helps to “steer” the drill down the pars and decreasesthe risk of vertebral artery injury. The bony surfaces of theC2 and C1 articulation are quite dense and difficult to passwhen drilling by hand, so once the joint is encountered, thedrilling is completed using power. Drilling through and pastthe joint is fairly safe, as the vertebral artery is at risk onlywithin the pars interarticularis section. The authors prefer totap the hole fully using fluoroscopic guidance. The screwlength is determined by assessing the length of the tapneeded to reach a plane in line with the C1 anteriortubercle. It is critical to ensure proper alignment of thespine while performing this procedure. Anterior subluxationof C1 on C2 increases the risk of vertebral artery injury anddecreases the chance of obtaining adequate fixation of thelateral mass of the atlas [38]. The vertebral artery injury rateis 4.1% per patient [58], and this structure can be injured byeither the drill or the tap. If a vertebral injury has occurred,a screw should be placed to stop the bleeding. Should avertebral artery violation occur on the initial side of

Fig. 8 Author’s illustration of transarticular screw placement betweenC2 and C1 lateral mass

Fig. 9 Operative view of a dorsal atlantoaxial arthrodesis. There is aright transarticular C2 to C1 screw placed and bilateral interlaminar ribgraft fusion in this 10 year old with severe atlantoaxial dislocation

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fixation, the contralateral side should not be instrumented toavoid the possibility of bilateral vertebral artery occlusionswhich can be life-threatening. All cases of suspectedvascular injury should be promptly evaluated with angiog-raphy postoperatively. Partial occlusions of the artery andvertebral dissections should be treated with an appropriateendovascular procedure and/or anticoagulation if possible.

Placement of the screws is followed by a cable graftprocedure as described above (Figs. 9 and 10). It isparticularly important to precisely fit the graft in thisinstance. The graft helps to achieve bony fusion and is alsobiomechanically advantageous as transarticular screws donot provide optimal resistance to sagittal plane rotation.External orthosis (Philadelphia hard collar) is occasionallyused if the patient has poor bone quality or else a soft collar.

Lateral mass screws and rods

Numerous factors such as an anomalous vertebral artery,severe thoracic kyphosis, or failure to achieve a completereduction can limit the ability to safely and accuratelyinstrument the atlantoaxial complex with transarticularscrews. In these situations, bone anchors placed in the C1lateral masses can be connected to screws fixed to the C2pars interarticulari, pedicle, or laminae. Goel and Laherioriginally described atlantoaxial fixation using screws inthe lateral masses of C1. Their technique required sacrificeof the C2 nerve roots bilaterally to enable the screws to beplaced through a plate [24, 25]. Harms et al. described arigid C1–C2 fixation technique using individual fixation ofthe C1 lateral mass and the C2 pedicle with polyaxialscrews and rods [32]. Since then, several different operativeapproaches have been described for atlantoaxial lateralmass fixation each based on insertion of C1 and C2 screwsindividually. Common to all is the initial placement of a C1lateral mass screw which is then coupled to a second screwinserted either in the C2 pars interarticularis, the C2pedicle, or the C2 laminae, and this improves the safetyof the procedure.

C1 lateral mass fixation

It is critical to accurately visualize the anatomical land-marks of the C1 lateral mass before placement of a screwinto this structure. The dissection begins beneath the lateralportion of the posterior arch. It may be helpful to removeany overhanging bone in this region to improve visualiza-tion. The amount of bone removal will be small as to notdestroy the integrity of the C1 laminae, but in the propersetting, it can significantly improve the exposure. Bleedingfrom the local epidural venous plexus may be avoided bymaintaining a subperiosteal plane of dissection. The entrypoint is frequently marked by a small unnamed emissary

Fig. 10 Lateral cervical radiograph in a 12-year old with atlantoaxialdislocation and os odontoideum. Note the bilateral transarticularscrews and bilateral interlaminar rib graft arthrodesis

Fig. 11 a Sawbone illustrationof a C1 lateral mass drilling witha drill guide in place. Note thespace required for the guide. bC1 lateral mass drilling withouta drill guide

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vein and lies in the posterior midpoint of the C1 lateralmass. The C2 nerve root is depressed slightly to allowadequate visualization of the screw entry point. Greateroccipital nerve dysfunction occurs less frequently with thisprocedure than with transarticular screw placement in theauthor’s experience, despite the perception at the time ofsurgery that the nerve is undergoing a moderate degree ofmanipulation. A K-wire is used to create a pilot hole underfluoroscopic guidance. The pilot hole is expanded with a2.9 mm drill manually, without using a drill guide. Thedrilling trajectory is parallel to the plane of C1 and isdirected towards the C1 anterior arch while avoiding theforamen transversarium laterally and the thecal sac medi-ally. Using a K-wire and then a drill without a guideoptimizes the surgeon’s visualization and thereby improvesthe accuracy and safety of the procedure (Fig. 11a, b).

The hole is tapped first with a 3.5 tap and thensuperficially with a 4.0-mm tap. Taps are prone to causevenous bleeding. However, this is less problematic thanhemorrhage early in the dissection, which is prone to hinderadequate visualization for safe drilling. Though the C1lateral mass only provides 10–15 mm of bony purchase, a35–45 mm screw should be used. This ensures that thepolyaxial head extends far enough posteriorly to allow afixation rod to access the C2 pars interarticularis ortranslaminar screw (Fig. 12).

(a) C2 pars interarticularis screwThe C2 pars interarticularis is defined as the narrow

portion of the C2 vertebra connecting the superior andinferior articular facets [18]. However, there is considerable

ambiguity in the correct definition of the C2 pedicle and parinterarticularis [8, 18, 39]. This complicates the choice ofan appropriate entry point and an ideal angulation for screwplacement based on anatomical studies of the cervical spine[33, 49, 57, 60]. It is therefore recommended that the idealtrajectory for screw placement should be evaluated indi-vidually through detailed preoperative axial CT imagingand identification of appropriate anatomical landmarksduring surgery.

Placement of the C2 pars interarticularis screw is similarto that of a C2 transarticular screw: the entry point is 3 mmrostral and 3 mm lateral to the inferior medial aspect of theinferior articular surface. It is important to assess thevertebral artery position just as in preparing to place atransarticular screw. The screw trajectory parallels the C2pars at an angle of approximately 40˚ but stops short of thejoint. An appropriate screw length is determined based onpreoperative imaging (typically 12–18 mm). A longitudinalfixation rod is secured across the C1 and C2 screw heads,while ensuring adequate anatomical alignment (Fig. 13). Toensure fixation, bone fusion is essential.(b) C2 pedicle screw fixation

Insertion of C2 pedicle screws is technically difficult, butit enables rigid fixation, high fusion rates and is especiallyuseful in patients who do not have intact posterior C2elements or an inadequate pars interarticularis [2]. The C2pedicle is defined as the portion of C2 vertebra that isbeneath the superior facet and anteromedial to the trans-verse foramen [18]. The position of the vertebral artery

Fig. 12 Demonstration of screw engagement into the lateral mass ofC1. This is a 12-mm screw size that enters into the lateral mass of C1.The total screw length is 42 mm to allow access to the polyaxial head

Fig. 13 Lateral cervical radiograph in a 14-year-old male. Anatlantoaxial fusion is accomplished with C1 lateral mass screwsconnected to C2 pars interarticular screws. The screw fixation isaugmented with interspinous wiring technique and rib graft

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should be determined preoperatively with reconstructed CTscans. Many patients with encroachment of the parsinterarticularis by an ectatic vertebral artery will also havecompromise of the pedicle. The landmark for the entrypoint of C2 screw penetration is the superior-medialquadrant of the C2 isthmus [1]. The insertion angle is 30°cephalad and 15–25° medial to the midline in the axialplane. This angulation also allows the technique to be usedin patients with marked obesity or kyphosis with greaterease than the pars technique. The medial trajectory may beassociated with a decreased potential for vertebral arteryinjury in some patients. The entry point in the cortex isinitially marked with a high-speed burr drill. After the drillhole has been enlarged, a pedicle probe is inserted underfluoroscopic guidance to confirm the trajectory of the drillhole. Finally, the drill hole is tapped before placement of anappropriately sized C2 pedicle screw. Longitudinal fixationwith the C1 screw head using a rod construct is supple-

mented by a clamp/cable graft using bone graft as describedearlier.

A 15-year-old male with quadriparesis after a fall isstudied in Fig. 16a, b, c, d, e, f, g. He had an unstabledystopic os odontoideum with cervicomedullary contusionon MRI. A rigid atlantoaxial construct was achieved withC1 mass fixation.(c) C2 Translaminar Screw Fixation

Translaminar screw fixation of C2 was first described byWright [59]. This ingenious technique provides excellentrigid C2 fixation, is technically simple, does not requirefluoroscopy and eliminates the risk of vertebral arteryinjury. A high-speed burr is used to create a small corticalaperture at the junction of the C2 spinous process andlamina on one side. Using a hand drill, a hole is made downthe axis of the contralateral lamina (Figs. 14 and 15).Before tapping, the hole is probed to ensure that there areno cortical violations into the spinal canal. Preoperativeaxial CT images are used to determine the diameter of theC2 laminae and an appropriate screw diameter is chosenthat should ideally fill the majority of the cancellous center.The C2 laminae can usually accommodate a 20–26 mmscrew. Following similar screw insertion on the contralat-eral side, the bilateral C2 laminar screws are incorporatedinto an atlantoaxial fixation as described earlier (Fig. 13).

Occipitocervical fusion

The most common indications for occipitocervical fusioninclude occipitocervical instability as a result of congenitalbony/ligamentous abnormalities, trauma, neoplasm, ordegenerative bone disease (Table 2). It is also indicated inselected patients with atlantoaxial instability who are notcandidates for atlantoaxial fixation or who have failedprevious attempts at C1–C2 fusion. Similar to atlantoaxial

Fig. 14 Operative view of technique demonstrating drilling fortranslaminar screw fixation

Fig. 15 a Open mouth view todemonstrate the C1 lateral massfixation combined with C2translaminar screws. b Lateralcervical radiograph of patient in(a). There is an interspinouswiring that augments the fusion

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Fig. 16 a MidsagittalT2-weighted MRI of craniocer-vical junction in a 12-year-oldmale with spastic quadriparesispresenting after a football injury.Note the significant contusion ofthe cervicomedullary junction(CMJ). The diameter of thespinal cord here is markedlyreduced suggesting previousinjuries. b Lateral cervicalradiograph of patient in (a).Note the hypoplastic posteriorarch of C1 with a large anteriormass of the atlas. This signifiesan os odontoideum. c Midsagit-tal 2D CT reconstruction ofthe craniocervical junctiondemonstrating the dystopic osodontoideum. d 3-dimensionalCT of craniocervical junctioncut in midsagittal plane to revealthe pathology and the anatomyat C1 lateral mass. This shouldbe capable of accepting a lateralmass screw. e Author’s illustra-tion with sawbone demonstrat-ing the technique of lateral C1mass screw and a C2 pars screwwith rod fixation. f Postopera-tive lateral cervical radiographof patient with a completedatlantoaxial arthrodesis using C2par screws and lateral massscrew at C1. g Anteroposteriorview of the atlantoaxialarthrodesis

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fixation, occipitocervical fixation can be accomplishedusing either semirigid or rigid instrumentation.

An occipitocervical fusion should extend one or twolevels below the most caudal level of subaxial instability ifthat is part of the clinical scenario. The number of subaxialcervical levels to be included in an occipitocervical fusionis based on the patient’s diagnosis, clinical presentation,and radiographic findings (Fig. 16). Patients with isolatedoccipitocervical instability, intact posterior elements, and noevidence of basilar invagination or other anterior compres-sive pathology typically undergo an occiput to C2 fusion.The occipitocervical fusion may be extended to C3 or C4 inselected patients with basilar invagination or an anteriorlesion that requires decompression before stabilization,depending on the amount of vertical migration present.

The atlantoaxial fusion techniques previously describedare often incorporated into occipitocervical fusions. Thedecision to fuse the occiput to the cervical spine should becarefully evaluated, as it results in loss of 10–15° of sagittalplane rotation. The patient is positioned on a horseshoe intraction. This may be particularly effective in patients whorequire preoperative traction to reduce deformities orsettling. The pin headrest allows precise positioning andplaces no pressure on the face or eyes. The halo can be usedto apply traction and should not be attached rigidly to theoperating table with an adapter to the Mayfield device. It iscritical to position the patient in a neutral, anatomic positionto maintain sagittal balance and axial neutrality. After the

patient is positioned, plain film lateral radiography or C-arm fluoroscopy is performed to confirm satisfactoryalignment of the craniovertebral joint (CVJ).

Occipitocervical rib grafts

During operative fixation of occipitocervical instability,bone grafts are used to achieve definite bony fusion. Thetype of bone graft used is the most important factor thatdetermines the potential for successful arthrodesis [52].Autologous bone grafts offer several significant advantages.Bony arthrodesis is optimally promoted by autologousgrafts that have the advantage of being osteogenic,osteoinductive, and osteoconductive [52]. Harvesting ofautologous bone grafts circumvents graft complications dueto immunological incompatibility and risks associated withblood- and tissue-borne diseases.

Autologous grafts harvested from the iliac crest havebeen used most commonly in surgical approaches forcervical arthrodesis. The overall rate of bony fusionachieved through incorporation of iliac crest grafts isapproximately 91% [52]. However, the use of iliac crestgrafts is limited by significant donor-site morbidity includ-ing persistent donor site pain, myalgia paresthetica, andlocal wound infections [7, 54]. In pediatric patientsundergoing extensive operative fixation, the iliac crestmay yield insufficient graft material due to the relativelylarger head dimension in children. It is difficult toanatomically contour the relatively straight iliac crest graftacross the craniovertebral junction [14]. Moreover, inyounger children, the iliac may still be cartilaginous therebyrendering it unsuitable for autografting.

Autologous rib grafts offer significant advantages espe-cially for pediatric cervical stabilization procedures. Themalleable nature of ribs in pediatric patients enablesaccurate contouring across the multiplanar occipitocervicaljunction. In addition, ribs have a flatter posterior and arelatively curved anterior portion. By selecting an appro-priate rib level, the graft onlay can be optimized to matchthe patient’s anatomical configuration. Ribs regeneratefairly rapidly in children, and harvesting of rib grafts isassociated with lower rates of donor site morbiditycompared with iliac crest grafts (local complication rate of3.7% vs. 25.3% after iliac crest harvesting) [52]. Fullthickness rib grafts contain high levels of bone morpho-genic protein that promotes osseous integration at the graftsite and hence results in higher rates of bony arthrodesis(98.8% fusion rate using rib grafts vs. 94.2% using iliaccrest grafts) [52]. En bloc resection of ribs allows harvest-ing of a full thickness, corticocancellous graft that ismechanically stronger than pure cancellous bone and istherefore ideally suited to support multiaxial movementsacross the occipitocervical junction [56].

Table 2 Indications for occipitocervical fusion

Indications Description

Congenital1 Anterior and posterior bifid arches of C12 Absent occipital condylesDevelopmental1 Severe reducible basilar invagination2 Unstable dystopic os odontoideum3 Unilateral atlas assimilation with chronic rotary C0 to

C1 to C2 luxationAcquired1 Traumatic C0 to C1 dislocation (especially vertical and

posterior occipitocervical)2 Complex CVJ fractures of C1 to C23 Reducible rheumatoid cranial settling4 After transoral CVJ decompression5 Cranial settling in ankylosing spondylitis, psoriasis,

pseudogout, Down’s syndrome, inflammatory ileitis6 Inflammatory disease-chronic Grisel’s syndrome7 Primary malignancies affecting the craniovertebral

junction (CVJ, e.g. chordoma of clivus and occipitalbone, plasmacytoma, osteoblastoma chondroma,neurofibromatosis)

8 Secondary metastatic disease affecting the CVJ (e.g.breast metastasis)

Childs Nerv Syst (2008) 24:1209–1224 1219

Typically, the eighth, ninth, or tenth rib is harvested.After dissection through the overlying skin and musclelayers, the rib is exposed through careful subperiostealdissection. It is important to avoid injury to the intercostalneurovascular bundle at the inferior rib margin and to theparietal pleura beneath the rib. For partial rib grafts, the ribis excised across the most medial point of the graft usingStille rib shares. For full length rib graft, the rib istransected slightly medial to the rib angle. After inspectionof the parietal pleural, a multilayered closure is achieved.

Recently, the use of occipital calvarial bone grafts hasalso been advocated for posterior cervical arthrodesis [50,55]. Calvarial bone grafts can be harvested locally duringthe primary procedure and therefore circumvent donor sitemorbidity associated with other autologous bone harvestingprocedures. The membranous nature of the bone alsoallows better arthrodesis, enabling successful bony fusionin up to 100% of the patients [50]. The senior author hasused calvarial bone in conjunction with instrumentation andbone morphogenic protein.

Rod and wire (contoured loop) fixation

Rod and cable/wire occipitocervical fusion techniquesachieve CVJ stabilization using titanium cables passedthrough occipital burr holes rostrally and sublaminar cablesthrough the C1–C2 posterior elements caudally. A widediameter, contoured rod is used to increase the semirigidfixation afforded by the sublaminar cables and improvesagittal alignment [6]. The main advantages of thisstabilization technique are that it is technically simple, itprovides immediate, semirigid fixation of the CVJ, and hasalso been reported to provide excellent fusion results [42].However, this method of fixation construct is susceptible to

axial compression loads because of sliding of the rodsthrough the wires and hence the authors’ custom contouredoccipitocervical fixation loop has a threaded construction.This is a particularly useful technique in the very youngwho are not suitable candidates for screw fixation (Figs. 17and 18).

The occipitocervical anatomy should be carefullyassessed through detailed preoperative imaging. An appro-priate rod and loop fixation construct is critical to ensurestable occipitocervical fixation. It is also important toensure that the loop contacts the bone at each site of cablefixation. Sharp angulations in the rod should be avoided asthe rod can break across these stress points. Caudally, therod should just extend beyond the lowest fixated vertebrallevel. The cables and rod should be made of the same metalto avoid accelerated corrosion, early fatigue, and instrument

Fig. 18 Occipitocervical fusion in an 8-year old with craniocervicaldislocation and Down’s syndrome. Note the bilateral interlaminar ribgraft fusion that extends to the occiput. The rib grafts are individuallysecured to the recipient bone and not to each other

Fig. 17 Author’s illustration ofoccipitocervical fusion with C1and C2 laminectomy. Rib graftsare placed laterally. A similarfusion can be made if the lam-inectomy is not necessary. Asituation such as this occurs inchildren below the age of8–9 years of age

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failure. A single-armed titanium cable is typically used foroccipital and sublaminar fixation.

A high-speed air drill or rongeurs are used to remove theinferior posterior rim of the foramen magnum to facilitatepassage of the cranial cables. The suboccipital burr holelocations can be marked with the customized rod in place.Two burr holes are made using high-speed air drill with therostral holes placed in a relatively horizontal line, 3 cmabove foramen magnum and 1 cm from the midline. Twoadditional holes are made closer to the foramen magnum.The dura is carefully stripped from the inner table of theskull, and the upper cables from the burr holes to thecraniectomy to gain purchase of the occipital bone. Duraltears can be adequately managed with the placement of athrombin-soaked Gelfoam in the burr hole. If necessary,sutures (see below) may be used to aid the passage of thesuboccipital cables.

The posterior arches from C1 to C3 are exposed throughcomplete removal of soft tissue, interspinous ligaments, andligamentum flavum. The laminar cortex must be preservedlaterally to prevent cable pull-out. Medially, the intra-laminar space may be enlarged using Kerrison rongeurs tofacilitate passage of the sublaminar cables. The sublaminarcables are passed as medially as possible to minimize therisk of dural penetration or neural injury. Each cable ispositioned laterally for fixation with the loop that iscarefully passed around the loop anchorage points\s. Thecables are tightened sequentially using a tensioning device.It is important to ensure that the cables are adequatelytightened to achieve rigid fixation, maintain adequatesagittal alignment, and also achieve internal reduction ifnecessary.

The occiput and C1 and C2 laminae and spinousprocesses are then decorticated. A section of the autologousrib graft is split longitudinally, and each half is positionedacross the occiput, C1, and C2 and secured to the cablewith suture (Figs. 19 and 20). The remainder of the rib graftis cut into matchstick-sized pieces and used as onlay graft.Alternatively, a bicortical iliac crest autograft may beharvested through a longitudinal incision over the posteriorsuperior iliac spine. If a suboccipital craniectomy orcervical laminectomy is performed, a unicortical plate ofiliac crest bone can also be sutured to the central portion ofthe rod to facilitate fusion and to maintain decompression.A routine multilayered wound closure is performed.Postoperative immobilization using a Philadelphia hardcollar is maintained for 4–6 months until bony arthrodesisis confirmed [42].

Rod and screw fixation

Rod-screw or rod-plate implants offer rigid skeletal fixationfor occipitocervical stabilization. Rigid cervical fixation is

necessary to minimize axial rotation and is attained throughC1–C2 transarticular screw fixation or through C1 lateralmass fixation coupled with either C2 pars interarticularis,pedicle, or laminar fixation.

The first step is to achieve fixation in the upper cervicalspine using techniques described previously. The next stepis to choose the implant to be used. If a rod is chosen, itshould be used bilaterally and should also be bent to matchthe anatomical configuration of the craniovertebral junc-tion. Low-profile connectors allow the rod to be secured tothe occiput with regular bone screws. This avoids the use ofpolyaxial screws that have large heads and are therefore

Fig. 20 Operative view of dorsal occipitocervical fusion in a 12-yearold who had undergone a transoral odontoid resection for basilarinvagination. A hindbrain herniation was present also. The posteriorfossa decompression is accomplished and the dorsal occipitocervicalfusion made. Bilateral rib grafts are placed lateral to the instrumen-tation in contact with the recipient occiput and lamina-lateral mass ofC1 and C2 and held in place with cerclage ligatures

Fig. 19 Artist’s illustration of dorsal occipitocervical fusion. Acustom contoured threaded titanium loop instrumentation is securedto the recipient occiput and lamina of C1 and C2 and C3. The authorusually does not extend the fusion to C3 unless necessary

Childs Nerv Syst (2008) 24:1209–1224 1221

prone to break through skin because of inadequate softtissue in the occiput. Alternatively, rods can also be securedto an occipital keel plate that are relatively easy to use andare available in various configurations. Their larger sizemay however, hinder occipital graft placement (Fig. 21).Another option is to use a rod that converts into a smallplate at one end through which regular bone screws areplaced to obtain occipital purchase.

Bone thickness should be assessed through preoperativeaxial imaging studies. Ideal occipital fixation is achieved inthe midline because the thin, squamous portion of theocciput does not allow sufficient screw purchase and alsoincreases the risk of intracranial vascular injury. Themidline bone can accept a 10-mm or an even larger screwin many patients. Rigid fixation should be achieved throughat least four bicortical occipital screws. Six screws areoptimal, but the patient’s anatomy may not allow for thismany to be implanted. Anatomical constraints limit the useof any additional screws. Drilling is performed with a high-speed drill, and the drill holes should begin lateral to themidline and below the superior nuchal line. The drill shouldbe advanced very slowly with the help of a probe to ensurethat the drill hole is bicortical. After the drill holes havebeen made, tapping is usually necessary before placing thefixation screws. Bleeding is often encountered due toincreased local vascularity but is usually self-limited afterscrew placement.

After occipital screw fixation is achieved, the rods aresecurely fixed with the transarticular screws (and othersubaxial screws if a longer construct is used; Fig. 22). It iscritical to maintain the anatomical alignment of the CVJ as thescrews are tightened. Offset connectors are often very helpful

in aligning the rod with the polyaxial screw heads, whilecross-connectors can be used for added stability of the fixationconstruct. The rod should sit flush with the skull. Removing asmall portion of the skull outer table should be considered tominimize the rod profile. As in the rod and wire fixationtechnique, the occiput, lamina, and spinous processes of thelevels to be fused are decorticated, and an autograft is used tofacilitate fusion. A routine multilayered wound closure isperformed. External braces are used postoperatively tomaximize fixation and improve arthrodesis.

Complications

Spinal instrumentation in pediatric patients is potentiallycomplicated by issues of limitation in future growthpotential, long-term stability of fusion constructs, anddevelopment of secondary spinal deformities. However,postoperative imaging and long-term follow-up in twoindependent studies have indicated that there is nolimitation of future growth potential nor potential forpermanent spinal malalignment after cervical fusion foratlantoaxial instability [23, 47].

Potential intraoperative complications include excessivevenous hemorrhage, vertebral artery injury, and durallacerations. Most venous bleeding occurs during theexposure of the C1 lateral mass or the C2 pars interarticu-laris and can be prevented by maintaining a subperiostealplane of dissection. Hemostatic agents such as oxidizedcellulose, thrombin-soaked Gelfoam, or powdered Gelfoamcan be used as necessary. Vigorous arterial bleeding afterdrilling or tapping of the C2 pars interarticularis is

Fig. 22 Lateral radiograph of craniocervical region. The craniocer-vical junction is grossly unstable secondary to the dystopic osodontoideum. An occiput-C2 fusion is made with lateral mass screwsin C1 and pars interarticularis screws in C2. The rib graft can be seenanterior to the rods

Fig. 21 Operative view of dorsal occipitocervical fusion using plateand rod instrumentation. The occipital plate is connected with rods tothe C1 lateral mass screws and C2 translaminar screws. Note the largesurface area covered by the plate

1222 Childs Nerv Syst (2008) 24:1209–1224

indicative of vertebral artery injury. If vertebral arteryinjury occurs, the bleeding is initially controlled by placinga screw in the drill hole. The anesthesiologists should beadvised, and modestly increasing the blood pressureshould be given consideration. Prompt angiographicevaluation is mandated postoperatively. If there is evidenceof vertebral artery injury, and the injured artery is stillpartially patent, it should be occluded provided thecontralateral vessel is normal. This avoids the later devel-opment of an arterial dissection or a pseudoaneurysm orarteriovenous fistula.

The dura may be lacerated during sublaminar passage ofthe occipital or cervical cables or when occipital burr holesare made. These should be primarily repaired or a duralgraft using watertight closure may be used. CSF leaks areusually controlled by placing a thrombin-soaked Gelfoamover the burr hole.

Delayed complications include wound infections, loss ofreduction, and failure of fusion. The management of awound infection after an occipitocervical or atlantoaxialfusion is similar to that of any other postoperative spinalinfection. Subfascial fluid collections can be aspiratedpercutaneously, while deep-wound infections are managedthrough aggressive debridement. Metallic instrumentationcan usually be salvaged. Failure of arthrodesis necessitatessurgical reexploration if there is persistent bony instability.In such instances, a fresh autograft is used and consider-ation should be given to utilizing adjuvant technologiessuch as bone growth stimulators.

Summary

The evolution of rigid instrumentation has increased theability to treat complex craniovertebral disorders in a safeand effective manner. Careful preoperative planning andmeticulous attention to operative landmarks, enables thesefixation techniques to be used safely in pediatric patients.Further advances in imaging techniques and developmentof resorbable instrumentation may extend their applicationwithin the pediatric age group.

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