Journal of Clinical Neuroscience 19 (2012) 757–760

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Journal of Clinical Neuroscience

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Technical Note

Occipital condyle screw placement and occipitocervical instrumentationusing three-dimensional image-guided navigation

Tien V. Le, Clint Burkett, Edwin Ramos, Juan S. Uribe ⇑Department of Neurosurgery and Brain Repair, University of South Florida, 2 Tampa General Circle, 7th Floor Tampa, FL 33606, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 April 2011Accepted 21 April 2011

Keywords:Entry pointImage guidance navigationOccipital condyleOccipital condyle screwOccipital condyle screw trajectory

0967-5868/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jocn.2011.04.037

⇑ Corresponding author. Tel.: +1 813 259 0965; faxE-mail address: juribe@health.usf.edu (J.S. Uribe).

Occipital condyle (OC) screws are an alternative cephalad fixation point in occipitocervical fusion. Safeplacement of occipital, C1 lateral mass, and C2 pars screws have been described previously, but not OCscrews. The craniocervical junction is complex, and a thorough understanding of the anatomy is needed.

Three-dimensional (3D) image-guided navigation was used in six patients. There were no complica-tions related to image-guided navigation during the placement of 12 OC screws and we found that thisnavigation can serve as a useful adjunct when placing an OC screw. Technical considerations of placingOC and C1 lateral mass screws are discussed with particular reference to patient positioning and theStealthStation� S7™ image-guided navigational platform (Medtronic, Minneapolis, MN, USA). The refer-ence arc is attached to the head-clamp and faces forward. The optical camera and monitor are positionedat the head of the table for a direct, non-obstructed line-of-sight. To minimize intersegmental movement,the OC should not be drilled until all other screws have been placed. We conclude that 3D image-guidednavigation is a useful adjunct that can be safely and effectively used for placement of instrumentation ofthe upper cervical spine including the OC.

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1. Introduction

Occipitocervical fixation can be challenging due to the anatomiccomplexity of the craniocervical junction. Occipital condyle (OC)screws are a feasible alternative to traditional occipital plates asthe cephalad fixation points in occipitocervical fusions.1 Placementof these screws requires a thorough understanding of the cranio-cervical junction and condylar anatomy.

Image-guided navigation can be a valuable tool, especiallywhere the anatomy cannot be thoroughly appreciated. The use ofintraoperative image guidance is not a new concept; however, itsuse for placement of an OC screw has not previously beendescribed.

We present one illustrative case of a series of six patients anddiscuss the use of three-dimensional (3D) image-guided navigationas it pertains to OC screw placement in the setting of occipitocer-vical fusion. To our knowledge, these are the first patients reportedin the English literature.

2. Illustrative case report

A 61-year-old male with a 20 pack-year smoking history and noprior medical history presented to the emergency room complain-

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ing of progressive neck pain and decreased strength in the rightupper extremity for 6 weeks. Cervical spine plain radiographsand CT scans revealed a lytic lesion of the body and dens of C2 withsevere anterolisthesis of C1 on C2 (Fig. 1) with compromise of thelateral masses of C1 that precluded the use of C1 as a fixation point.A right Pancoast tumor was also noted. MRI of the cervical spineshowed severe spinal cord compression at the C1–C2 level and bet-ter demonstrated the metastatic lesion. The patient was placed onbed rest in cervical traction to realign C1 and C2. Adequate reduc-tion was achieved, and the patient remained neurologically stablethroughout. He was then taken to surgery on hospital day 4 forposterior occiput to C4 posterolateral fusion and C2 biopsy.

3. Surgical technique

The patient was placed in the prone position on a Jackson framewith the head fixed in a 3-pin head-clamp. Lateral fluoroscopy wasused to confirm a neutral position of the cervical spine, and thehead was slightly flexed if more space was felt to be needed forinstrument access at the craniocervical junction posteriorly. Thereference arc was then secured to the head clamp (Fig. 2). The opti-cal camera of the image guidance system was placed at the head ofthe bed to avoid line-of-sight issues related to the camera and theimage-guided instruments and to place the reference arc betweenthe surgeon and the optical camera (Fig. 3). The operative site wasshaved followed by prepping and draping in a sterile fashion.

Fig. 1. Preoperative (A) sagittal CT scan showing severe anterolisthesis of C1 on C2;(B) sagittal T1-weighted unenhanced MRI showing metastatic disease and spinalcord compression; and (C) sagittal CT scan post cervical traction showing adequateclosed reduction.

Fig. 2. Photograph of reference arc placement showing the prone patient and thehead secured in 3-pin head-clamp. The reference arc is attached directly to theclamp and faces forward.

Fig. 3. Photograph of camera and monitor placement in the operating roomshowing the optical camera and monitor at the head of the table. The camera has adirect line-of-sight with the reference arc and is unobstructed during the operation.

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The posterior craniocervical dissection was carried out in typi-cal fashion until all relevant bony anatomy was exposed, includingdissection of the occipital condyles laterally up to the condylar fos-sa. Drilling of the lateral masses was done freehand without navi-gation using the Magerl technique with a plan to place the screwslater.2

To define the craniocervical junction, intraoperative 3D cone-beam CT images (O-arm�, Medtronic, Minneapolis, MN, USA) werethen acquired and auto-registered with an image guidance plat-form (StealthStation� S7™, Medtronic).3 Accuracy was confirmedusing a registered probe to touch multiple different bonylandmarks.

The probe was used to define the entry points on the OC and C1.A high-speed drill was used to drill the initial pilot holes. C1 lateralmass trajectories were drilled under image guidance. The holeswere tapped and screws were then placed. Once completed, thedesired trajectories and projected lengths for the OC screws werethen re-established using the probe (Fig. 4). We drilled to a depthof 20 mm along the set trajectory without violating the hypoglos-sal canal cranially or the atlanto-occipital joint caudally. The initialentry point was also clear of the vertebral artery inferiorly and con-dylar fossa laterally. The tract was tapped, and a 3.5 mm by 32 mmpolyaxial shank screw was placed bicortically into the OC (Vue-Point� OCT, NuVasive LLC, San Diego, CA, USA). Approximately12 mm of the unthreaded portion of the screw remained superfi-cial to the posterior cortex of the condyle, allowing the polyaxialportion of the screw to lie above the posterior arch of C1 to avoidcompromise of the vertebral artery by the rods. This was repeatedfor the opposite condyle. Once completed, a second ‘‘spin’’ of the O-arm was taken to confirm adequate placement of all screws. Onceproper placement of screws was confirmed, the rods were placed,

and an allograft was placed in decorticated areas (Fig. 5). A subfas-cial suction drain was placed and the operative site was thenclosed in standard fashion.

Fig. 4. Monitor projections of a three-dimensional image-guided navigation of theoccipital condyle: (A) anteroposterior view with the trajectory based on probeposition; (B) lateral view with trajectory base on probe position; (C) axial view withtrajectory and projected depth; and (D) lateral view with trajectory and projecteddepth.

Fig. 5. (A) Anteroposterior and (B) lateral fluoroscopic views of completedoccipitocervical fusion aided by three-dimensional image-guided navigation con-firming instrumentation placement.

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4. Results

There were a total of six patients who underwent image-guidednavigation for OC screw placement. Four patients underwentplacement with the StealthStation� S7™ platform with intraoper-ative 3D CT imaging. One patient underwent placement withStealthStation� TREON™ using pre-operative images and manualregistration. One patient underwent placement with VectorVision�

(BrainLAB, Westchester, IL, USA) using pre-operative images andmanual registration. There were a total of 12 OC screws placedwithout any breaches.

5. Discussion

OC screws have evolved to become a feasible alternative tostandard occipital plates for occipitocervical fusion.1,4,5 Biome-chanically, they are similar, if not superior, to standard occipitalplates, and clinical outcomes have been encouraging to this point.6

OC screws have a number of advantages including a decreasedlength of the lever arm, increased screw length, a low profile toprevent protuberance under the skin, a larger available bony sur-face area along the subocciput for grafting, and the potential to stillplace an occipital plate for supplementation if needed.7 In addition,rod placement can be less challenging compared to occipital plates,and they can still be used in the setting of a previous suboccipitalcraniectomy.

The placement of an OC screw can be technically challenging gi-ven the relatively small area to negotiate and the risks of neurovas-cular injuries in the immediate surrounding areas. There have beenthree different techniques described for OC screw placement in the

absence of any image-guided navigation, each with differences inthe entry point as well as the trajectory taken in medial as wellas cranial-caudal angulation.1,4,5 Cadaveric and radiographic stud-ies have demonstrated variability in occipital condyle morphologyand have emphasized that a thorough understanding of the sur-rounding anatomy is paramount.1,4,8–11

Recently, our group has proposed an optimal trajectory forplacement of an OC screw (Le et al., unpublished data). The opti-mal entry point, or ‘‘condylar entry point,’’ was identified as apoint as close to mid-condyle as possible, but medial to thecondylar fossa and at least 2 mm caudal to the skull base. Theoptimal medial trajectory is one that is most parallel to thelongest axis, which is typically at least 20�. The cranial–caudaltrajectory should be parallel to the skull base, but minor manip-ulations not to exceed 10� in either direction may be necessarygiven a patient’s specific condylar anatomy. These parametersshould make placement of OC screws more uniform and moreeasily achieved in most cases where appropriate preoperativeplanning is conducted and where reliable intraoperative fluoros-copy is available.

Intraoperative two-dimensional fluoroscopy has its limitations,however, and technological advances in the form of intraoperativeimage-guided navigation have the potential to be a useful adjunctin placement of the OC screw. Advantages over traditional two-dimensional fluoroscopy include the ability to provide accurate,real-time 3D depictions of condylar anatomy for more preciseplacement and a reduction in radiation exposure.12

Nottmeier and Young recently described their experience usingimage-guided placement of occipitocervical instrumentation.12

They concluded that image guidance not only can be done safely,but also reduces the radiation exposure, which is beneficial tothe patient, surgeon, and ancillary OR staff.13–15 In their study, im-age guidance was helpful when placing C1 lateral mass screws, C2pars screws, as well as occipital screws. To maintain navigationalaccuracy and to minimize intersegmental movement associatedwith manipulation of the spine from tapping the bone or placingscrews, they recommended placing the reference arc on the headclamp and drilling all holes for screws before tapping the boneand inserting the screws. Borrowing from this, we recommendplacing any screws below the occiput prior to the OC screws giventhat the condyle is affixed to the skull and will be less susceptibleto intersegmental movement. Another technical point to minimizeintersegmental movement is to keep the self-retaining retractor inplace when scanning the operative field. Because of their relativelylow profile, any streak artifact introduced was found to beminimal.

We have used image guidance to place OC screws in six patientsto date without any complications. We currently use the O-arm�

coupled with the StealthStation� S7™ for navigation. Early in ourexperience, we have also used two other image guidance plat-forms, StealthStation� TREON™ and VectorVision�, without anycomplications. We have been satisfied with the resolution of thebony anatomy with our current platform and its relative ease ofuse. This is particularly important when dealing with such a smallarea as the OC.

The ability to have real-time 3D image-guided navigation is cer-tainly a sign of continued advances in medical technology. Someadvantages have been outlined above, but it should be noted thatnot every facility, and subsequently not every surgeon, has accessto these devices for a variety of reasons, with cost being one factor.In addition, it cannot be emphasized enough that there is no sub-stitute for a surgeon’s own understanding of the anatomy andexpertise in the operation performed, especially if the navigationis malfunctioning or unavailable. It is imperative that calibrationof the navigation is carried out meticulously and deliberately in or-der to maximize utility and safety.

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6. Conclusion

Understanding the optimal OC screw entry point and trajectorycan be useful in safely placing an OC screw. 3D image-guided nav-igation is safe and effective, and can be a useful adjunct for place-ment of OC screws. In addition, image-guided navigation alsoreduces the radiation exposure for all in the operating room. Sur-geons should be cautious of over-reliance of technological ad-vances, however, especially if the observed anatomy does notcoincide with what is projected. A second scan done after place-ment of screws can be useful to confirm proper screw placement.

Disclosure/disclaimer

This work was not supported by any external sources of fund-ing. There is no commercial interest in any of the devices men-tioned in the article by any of the authors. Dr. Uribe is a paidconsultant and receives research grants from NuVasive, LLC (SanDiego, CA), USA.

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