Abstract Placement of Pedicle Screws in the Cervical Spine: Comparative Accuracy of Cervical Pedical Screw Placement Using Three Techniques

Introduction
Successful placement of a pedicle screw in the cervical spine requires sufficient three-dimensional understanding of pedicle morphology to allow accurate identification of the ideal screw axis. This investigation was conducted in two parts. First, a morphometric analysis of the subaxial cervical pedicles was performed in a human cadaver model. From this analysis, a method of cervical pedicle screw placement based on topographic anatomy was developed. The second part of the investigation employed a human cadaver model to assess the accuracy of pedicle screw placement in the cervical spine using three surgical techniques.

Methods
First, K-wires were coaxially placed in 140 pedicles from C3 through C7 in 14 human cadaver cervical spines prepared to allow circumferential visualization of each pedicle. Precise measurements were then made of pedicle dimensions, angulation, and offset relative to the lateral mass boundaries. A statistical analysis provided an assessment of variability between cervical levels, sides, sexes, and observers. Based on this analysis, guidelines for pedicle screw placement relative to posterior cervical topography could be derived. In the second part of the study, 12 human cadaver cervical spines were instrumented with 3.5 mm screws placed in the pedicles from C3 through C7 according to one of three techniques. In Group I, screws were placed using the topographic guidelines derived in the first part of this study. In Group II, the same guidelines were employed after laminoforaminotomies were performed to provide supplemental visual and tactile cues regarding the orientation of the pedicle. In Group III, screws were placed using a computer-assisted image-guided surgical system (Stealth Station, Sofamor Danek) which applies stereotactic principles to preoperative CT data allowing transformation of real-time data from the operative site into the "virtual world" data of the CT image. Cortical integrity was then assessed by obtaining postoperative CT scans (1 mm cuts) of each specimen. A cortical breach was considered "critical" if the screw encroached upon any vital structure. These findings were then confirmed by dissecting circumferentially around each pedicle. A statistical analysis comparing the accuracy of each technique was then performed.

Results
In the morphometric component of the study, linear measurements of pedicle dimensions had a wide range of values with only "fair" correlation between observers. Angular measurements revealed similar angulation in the transverse plane (-40 degrees) at each level. With respect to the sagittal plane, both C3 and C4 pedicles are oriented superiorly relative to the axis of the lateral mass while the C6 and C7 pedicles at oriented inferiorly. The dorsal entry point of the pedicle on the lateral mass defined by transverse and sagittal offset had similar mean values with wide ranges although there was often "excellent" correlation between observers. There were no significant interlevel, left/right, or male/female differences noted with respect to offset. Based on the statistical analysis, guidelines for screw placement by level were derived and appear in Table 1.

In the second part of the study, 120 pedicles were instrumented using one of three techniques. In Group I (using morphometric data), 12.5% were placed entirely within the pedicle, 21.9% had a noncritical breach, and 65.5% had a critical breach. In Group II (laminoforaminotomy and palpation), 45% were within the pedicle, 15.4% had a noncritical breach, and 39.6% had a critical breach. In Group III (stereotactic), 76.% were entirely within the pedicle, 13.4% had a noncritical breach, and 16.6% had a critical breach. When a critical breach was encountered, the vertebral artery was likely to be injured in 73.9%, while the exiting nerve root was likely to be injured in 41.5%.

Table 1: Topographic Guidelines For Cervical Pedicle Screw Placement

Discussion
Although a statistical analysis of morphometric data obtained from the cervical spine could provide guidelines for transpedicular screw placement based on topographic landmarks, sufficient variation exists to preclude safe instrumentation without additional anatomical data. Furthermore, insufficient correlation between different surgeon's assessment of surface landmarks attests to the inadequacy of screw insertion techniques in the cervical spine based on such specific guidelines. While laminoforaminotomy/palpation does improve visual and tactile access to the cervical pedicle, this technique did have a significant likelihood of injuring vital structures above the C7 vertebral body. Frameless stereotactic systems enhance accuracy and further improve the safety of transpedicular screw placement, most notably at C6 and C7.

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