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ccipitoatlantal and Atlantoaxial Dislocation

. Fernando Gonzalez, MD, Nicholas Theodore, MD, Curtis A. Dickman, MD, and

olker K. H. Sonntag, MD

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ertical distractive forces at the craniovertebral junction canffect the occipitoatlantal joint or the atlantoaxial joint. These

esions are part of the same spectrum of injuries. They share theame mechanism of injury and high mortality rate. They usuallyepresent a pure ligamentous injury that causes severe instabilitynd requires early fixation.opyright 2004, Elsevier Inc. All rights reserved.

istractive injuries at the craniovertebral junction can causethe occipitoatlantal or atlantoaxial joints or both to widen.

hese unstable lesions must be reduced and fixated quickly tovoid a devastating outcome. We present different alternativesor fixating these unstable injuries, emphasizing the use ofransarticular screws to preserve motion at levels not initiallynvolved in the injury.

Anatomy of the Craniovertebral Junction

he craniovertebral junction (CVJ) is composed of the occipitalone, the two first cervical vertebrae (the atlas and axis, respec-ively), and a complex system of ligaments and synovial joints.he special anatomic configuration of the CVJ allows it func-

ional mobility whereas the neural contents are protectedithin a wide range of motion in different directions.The occipital condyles articulate with the lateral mass of C1

n a cup-shaped joint that allows motion primarily for flexionnd extension. C1 rests on top of the shoulders of C2, which areelatively flat and slope laterally and inferiorly. The odontoidrocess is an essential part of this complex. It consists of aertical fulcrum that allows atlantoaxial rotation around thexis. This mechanism is responsible for a significant portion ofhe lateral rotation of the head. The configuration of C1 on topf C2 predicts and facilitates this axial rotation while minimiz-ng lateral bending and flexion-extension of the head.

The functional alignment of this complicated anatomic con-guration also is maintained by a complex ligamentous array.he cruciate ligament is composed of a vertical and a horizontalegment. Its fibers are interwoven in a cross. The vertical seg-ent extends from the posterior aspect of the body of C2 and

ttaches rostrally on the anterior aspect of the foramen mag-

From the Division of Neurological Surgery, Barrow Neurological Insti-ute, St. Joseph’s Hospital and Medical Center, Phoenix, AZ, USA.

Address reprint requests to Volker K.H. Sonntag, MD, Neuroscienceublications, Barrow Neurological Institute, 350 W. Thomas Road, Phoe-ix, AZ 85013.Copyright 2004, Elsevier Inc. All rights reserved.1092-440X/04/0701-0004$30.00/0

odoi:10.1053/j.otns.2004.04.004

Operative6

um. Its main function is to keep the atlas under compressionetween the axis and the occipital bone. The horizontal bandorresponds to the transverse ligament. It has two lateral inser-ions on a bony tubercle on each side of the medial aspect of theateral mass of C1 and another insertion with its own synovialoint behind the odontoid process. The transverse ligamenterves as a “seat belt” to prevent the odontoid process fromosterior translation that would compress the medulla.The paired alar ligaments run from the lateral and superior

spects of the odontoid process and fan out to the inferiorspect of each occipital condyle. Their main function is toontract simultaneously when the head rotates to either side,hereby preventing excessive axial rotation at the occipito-1–C2 complex.1 The apical ligament extends from the tip of

he odontoid process and fans out to the anterior rim of theoramen magnum.

The function of the tectorial membrane, which is the mostostral extension of the posterior longitudinal ligament, is lesslear. It inserts on the anterior rim of the foramen magnum. Itsain function is to serve as a restraint during flexion of theead. The cranial extension of the anterior longitudinal liga-ent is known as the anterior atlantooccipital membrane. Itsain function is to restrain hyperextension of the head.The stability of the CVJ primarily reflects the integrity of the

urrounding ligaments and synovial joints. Distractive injuriest the CVJ, typically caused by high-speed accidents, usuallyause immediate death or severe neurological deficits.2

Occipitoatlantoaxial dislocation (OAD) constitutes a devas-ating injury that few survive. OAD is a well-characterized en-ity with established diagnostic criteria based on multiple land-arks. Power’s ratio3 is the ratio between the distance between

he basion and the most anterior part of the arch of C1 and theistance between the opisthion and the anterior arch of C1. Ifhis ratio is greater than 0.7, OAD is present. This ratio consti-utes one of the most sensitive and objective measurements ofhis type of injury.2

Few reports have described vertical subluxation of the1–C2 articulation.4-10 No radiological landmarks have beenefined to characterize this type of injury. On the coronal planef computed tomography (CT) reconstruction, the distanceetween the lateral mass of C1 and C2, known as the lateralass interval, was measured in 93 normal subjects (Gonzalez

003, unpublished data). A threshold of 2.6 mm was two stan-ard deviations above the mean in these 93 normal subjects andonsidered abnormal.

Hyperintensity within the joint on magnetic resonance im-ging (MRI) during fast spin-echo inversion recovery and shortau inversion recovery sequences are considered corroborative

f blood and fluid within the synovial joint. This finding is

Techniques in Neurosurgery, Vol 7, No 1 (March), 2004: pp 16-21

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specially important in the pediatric population where liga-entous laxity can result in excessive false-positives if only CT

s used for evaluation.The basion-dens interval (BDI) has been described as a usefuleasurement for the identification of OAD.11,12 We hypothe-

ized that vertical distraction of C1–C2 would also widen theDI. During C1–C2 vertical distraction, the BDI widens. There-ore, a normal BDI does not exclude vertical dislocation of1–C2. An increased BDI, however, represents an ancillaryatum that indicates a CVJ injury. It should direct attention tooth the atlantooccipital and atlantoaxial articulations. In pa-ients with a grossly increased BDI and an intact atlantooccipi-al articulation, a C1–C2 vertical distraction injury must beighly suspected.Lee and co-workers13 reported the BDI based on plain radio-

raphs without correcting for magnification. Such measure-ents are likely less accurate than those obtained from CT

cans. BDI values greater than 9.0 mm (derived from sagittal CTeconstruction) indicate the likelihood of traumatic disruptionf the CVJ.A vertical distraction injury of C1–C2 can occur when the

ead strikes an immobile object with a rostrally oriented forceector. The result is severe distraction associated with posteriorcceleration-deceleration.6 Experimentally, OAD can be repro-uced by sectioning the tectorial membrane (the alar and apical

igaments). Despite severe associated ligamentous injuries, theransverse ligament often remains intact.4,6,7,14 We corrobo-ated this finding with CT and MRI (Gonzalez 2004, unpub-ished data). Vertical distraction injuries at C1–C2 are inti-

ately related to OAD. Both are caused by a significantistractive force and the transverse ligament remains intact.Based on the position and function of the cruciate ligament,

amage to its vertical portion is unavoidable in a vertical dis-raction injury. Hence, a vertical distraction injury would bessociated with a tear of at least one of the two vertical attach-ents of the cruciate ligament. If the vertical portion is dis-

upted above the transverse portion, the remaining fibers of theruciate ligament should act to keep C1 together with C2,esulting in OAD. If the vertical portion is disrupted below theransverse portion, the intact fibers of the cruciate ligamenthould act to keep C1 together with the skull base, resulting inC1–C2 vertical distraction injury.No real guidelines have been established for the treatment of

his rare injury. Only a few cases have been reported, and theyay underrepresent the actual incidence of distraction injuries.he incidence, however, may be found to increase if three-imensional images and reconstruction become a routine partf the initial assessment of trauma patients.An OAD implies severe ligamentous disruption that will not

eal by itself15 or that will heal with shortening (retraction) ofhe ligament that makes it nonfunctional. Consequently, imme-iate reduction and internal fixation are necessary.

Surgical Techniques

AD and vertical C1–C2 subluxation share similar features andhould be approached as part of the same spectrum of distrac-ive injuries. If vertical instability is present, measures thateproduce the mechanism of trauma, such as cephalic tractionnd collars with extension, should be avoided to prevent neu-

ological worsening or the appearance of new deficits. The a

CCIPITOATLANTAL AND ATLANTOAXIAL DISLOCATION

ogical treatment for a vertical distraction injury is compressionnd reduction of the distracted space. The head should be keptligned with sandbags to prevent motion. In most cases, earlylacement of a halo brace with axial loading or head compres-ion during placement reduces distraction. Real-time fluoros-opy during reduction is recommended.

Treatment can be classified into two general groups: tech-iques that use wires and techniques that involve plates andcrews. Wire-and-screw techniques imply anchorage to theuboccipital surface connected to a piece of hardware, usually aod, affixed to the spine with sublaminar wiring. Plate-screwechniques require fixation to the occipital bone with screwsnserted into the occipital bone. Epidural “washers” can also beonnected to a piece of hardware anchored to the subaxial spineith transarticular (C1–C2) or lateral mass screws.The ideal instrumentation should provide enough support to

nclude only the involved segments (occipitoatlantal joint) or1–C2 joint. It should provide immediate fixation, and noardware should be inserted into the spinal canal that can bebnormally compressed.16

Occipitoatlantal Dislocation

ith these premises in mind, we developed a technique thatxates the CVJ with bilateral transarticular screws beneath the

ateral mass of C1 into the occipital condyle (C0). This tech-ique provides immediate fixation at the occipitoatlantal com-lex. Transarticular screws are inserted from the lateral mass ofhe atlas into the occipital condyles. This technique provideshe only stand-alone method for fixating the CVJ without com-romising axial rotation and limiting the range of motion of theeck during flexion and extension.Occipitoatlantal transarticular screws are indicated for oc-

ipitoatlantal instability caused by trauma. Occipitoatlantalransarticular screws provide immediate, direct, rigid screwxation of the occipitoatlantal joints bilaterally. This technique

s based on a principle similar to that underlying the Magerlechnique for C1–C2 transarticular screw fixation.17 The use ofhe occipital condyle as an anchorage point was first describedy Grob in 2001.16 The anatomy and biomechanics of occipi-oatlantal fixation with transarticular screws were investigatedn terms of the feasibility and safety of inserting screws in bonytructures in relation to the surrounding vascular structuresvertebral arteries), hypoglossal nerves, and spinal cord.18 Thistudy found that occipitoatlantal transarticular screws providetrong and rigid fixation of the occipitoatlantal joint compara-le to that provided by other systems designed for the subaxialpine.

Transarticular C1–C0 screws effectively prevent unseating ofhe articular surfaces of the occipital condyle and the articularurface of C1, creating a lag effect that apposes the distractedragments. Pinning the occipitocervical joint maintains its nor-

al ball-and-socket configuration, preventing it from furtherwiveling and flexion-extension movements. Our techniquerevented lateral bending and axial rotation as effectively asrevious techniques, but it was less effective in restrainingotion during flexion and extension. A posterior buttress isecessary to promote definitive fusion through a solid bridge ofone between the stabilized structures. A posterior bone graft

ccomplishes both goals by providing an adequate source for

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usion and simultaneously by limiting flexion-extension move-ents of the head.

echnique

he widening at the occipitoatlantal joint must be reducedefore surgery. Appropriate reduction can be obtained with theatient in a halo brace and progressive compression of the headnder direct fluoroscopic visualization.Direct screw fixation with a cannulated screw system has

een used to reduce OAD and to stabilize the occipitoatlantaloint.18 The posterior arches of C1 are exposed laterally usingubperiosteal dissection to expose the inferior edge of the C1rch where the ring joins the lateral masses of C1. The entryoint for the pilot hole is made with a pneumatic bur at theaudal junction of the C1 arch and C1 lateral mass while the C2erve root is retracted caudally. The pilot hole prevents slidinguring drilling. Veins are cauterized carefully with a bipolaroagulation device, and bleeding is controlled with Nu-Knit®Johnson & Johnson, Arlington, TX).

Under fluoroscopic guidance, an end-threaded, 1.5-mm di-meter, Kirschner-wire (K-wire) is drilled through the center ofhe lateral masses of C1 across the occipitoatlantal joints intohe occipital condyles. The tip of the K-wire is directed to beositioned in the condyle. On lateral fluoroscopy the target is 1m above the anterior arch of C1 (the anterior tubercle is therimary landmark on the lateral radiograph for C1–C2 transar-icular screws). The occipital condyles can be visualized on ailted anteroposterior fluoroscopic view. This positioningvoids the hypoglossal canals, which are in the anterolateralnd rostral portions of the occipital condyles. The cranial tra-ectory of the K-wire is about 45° from the horizontal plane andto 10° relative to the sagittal plane.Pilot holes are drilled over the K-wires and tapped using a

annulated system (Universal cannulated screw system; Sofa-or-Danek Memphis, TN). Next, a 4.0-mm diameter, 30-mm

ig 1. Illustration showing the trajectory of a transarticular srajectory is about (A) 45° from the horizontal plane and (B)o the sulcus anteriosus of C1.

ong screw is placed and the K-wire is removed. As soon as the o

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crew pierces the joint, the lag effect causes the fragments toppose each other. The screw length is determined preopera-ively using sagittal CT reconstruction of the occipitoatlantaloint. Screws are placed bilaterally (Fig 1). C1–C0 transarticu-ar screws are a potential threat to neurovascular structures.he vertebral artery is especially susceptible as it travels

hrough the sulcus arteriosus or if it projects as a caudal loophat obscures insertion of the screw at the midaspect of theateral mass of C1. The length of the screw is critical becauseong screws can violate the dura and injure the medulla. Ifcrews are too short, they will not obtain enough purchase onhe occipital condyle and fixation will be limited. Based onreliminary measurements from normal patients, we found that28- to 32-mm long screw inserted along the established tra-

ectory provides adequate anchorage and good mechanical re-ults.

Preoperative planning, based on CT scans of the CVJ, ismportant to define particular bony relations. CT angiographys key for defining the relationship between the vertebral arterynd the lateral mass of C1.19

Autologous corticocancellous iliac crest bone strut is fixedith occipitoatlantal sublaminar wires (braided titanium ca-les) and compressed between the occiput and C1.Frameless stereotactic navigation and intraoperative CT can

lso be used. When severe instability is present or involves1–C2, theoretically C1–C2 transarticular screws could be in-luded in the construct (Fig 2). The screws do not share theame trajectory at any point. Therefore, both screws can be placedimultaneously because their trajectory is almost parallel.

C1 Lateral Mass and C2 Pedicle Screw Fixation

his technique is indicated for C1–C2 instability when patientsave an anomalous vertebral artery that precludes placement ofC1–C2 transarticular screw (Magerl technique). The course

from the lateral mass of C1 into the occipital condyle. The10° relative to the sagittal plane. The entry point is inferior

crew0° to

f the vertebral artery is anomalous in about 20% of the popu-

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ation.19,20 This screw-fixation technique provides rigid inter-al fixation of the atlantoaxial joint and is biomechanicallyquivalent to transarticular screws (JS Hott, 2003, unpublishedata).

echnique

he dorsal aspect of the C1 posterior arch is decorticated wheret joins the surface of the lateral mass. (The entry point on C1 ishe same as for C1–C0 transarticular screws and C1 lateral masscrews. The trajectory is quite different.) Screws are insertedFig 3) into the center of the lateral mass in a unicorticalashion, and they are oriented parallel to the plane of C1 with a

ig 2. Combined construct using transarticular screws at theA) Sagittal and (B) posterior views of entry points and traject1–C2 transarticular fixation.

onverges on the axial plane as determined by the patient’s ana

CCIPITOATLANTAL AND ATLANTOAXIAL DISLOCATION

light convergent trajectory in the anteroposterior plane. Usinguoroscopic imaging, frameless stereotatic guidance can beseful. Screws are inserted into the medial and superior quad-ant of the C2 pars interarticularis. The screw is directed 20° to0° in a convergent direction and with a cephalad orientationhat is determined by the superior and medial aspect of C2.21

he screw-fixation procedure can be performed unilaterally orilaterally. If the anatomy of the contralateral joint is suitableor transarticular screws, a transarticular screw can be placedn the contralateral side (Fig 4). Autologous graft is wiredetween the posterior arches of C1 and C2 to provide long-termtability to the construct. Both screws are linked with a titanium

iput-C1 and at C1–C2. Note that the trajectories are parallel., which are the same for isolated occipital-C1 fixation as for

occories

ig 3. Illustrations showing that the entry point of the C1-lateral mass screw is the same point on C1 as that used for C1–C0ransarticular screws. (A) The C1 lateral mass screw is parallel to the plane of the C1 articular surface with an unicorticalurchase. The C2 screw is almost parallel to the C1 screw following the pars articularis of C2. (B) Both screws are linkedogether with an adjustable rod that helps reduce the distraction between C1 and C2. The trajectory of both C1 and C2 screws

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od. For C1–C2 vertical distraction, the rod re-approximateshe widening between the lateral masses.

If the vascular anatomy precludes the use of transarticularcrews and the C2 pars interarticularis is small, use of C2 in theonstruct is contraindicated. In this case the fixation shouldnclude C3 with lateral mass screws (Fig 5). C2 can be con-ected with wires to the rod that extends from C1 to C3.Definitive treatment should be individualized. We recom-end C1–C2 fusion for patients with isolated C1–C2 subluxa-

ion (with no additional fractures, OAD, or combined injuries).

C1–C2 Transarticular Screw Fixation

midline suboccipital approach is performed to expose theosterior arch of C1 and C2. (The technique is detailed else-here.17) It is very important to expose the C2–C3 joint. A-wire is inserted through the isthmus of C2, piercing the

ateral mass of C1. The trajectory is followed with real-timeuoroscopy oriented cephalad with the anterior tubercle of C1s the target. A cannulated drill is used to tap across the C1–C2oint. Following the K-wire, the drill is removed and a cannu-ated 4-mm diameter screw is inserted. Short-threaded screwsroduce a lag effect to appose C1 to C2.CT angiography can be used to determine the suitability of

lacing screws at C1 and C2. An anomalous vertebral artery canreclude the use of screws on one side.19,20

Associated injuries (eg, odontoid fractures, C1–C2 antero-osterior dislocation, OAD) increase instability and should beddressed. Occipitocervical fixation should be used to treatnisolated C1–C2 distraction injuries.

Conclusions

1–C2 vertical distraction and OAD are produced by the sameechanisms. These injuries affect the same ligamentous com-

lex at different levels. Preoperative vascular evaluation is key

ig 4. When the size of the C2 pars interarticularis or theatient’s vascular anatomy precludes the placement ofransarticular screws, this combined technique is an alter-ative (transarticular screw in one side and C1 lateral

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o identifying the course of the vertebral artery and to defininghe feasibility of using C1–C2 transarticular screws or C1 lat-ral mass and C2 pedicle screws. Frameless image-guided nav-gation with intraoperative CT is a useful adjunct but is notequired.

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CCIPITOATLANTAL AND ATLANTOAXIAL DISLOCATION

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