Placement of pedicle screws in the human cadaveric cervical spine - Comparative accuracy of three techniques

Study Design. This investigation was conducted in two parts. In the first p art, a morphometric analysis of critical cervical pedicle dimensions were m easured to create guidelines for cervical pedicle screw fixation based on p osterior cervical topography, In the second part of the I study, a human ca daver model was used to assess the accuracy and safety of transpedicular sc rew placement in the subaxial spine using three different surgical techniqu es: 1) using surface landmarks established in the first part of the study, 2) using supplemental Visual and tactile cues provided by performing lamino foraminotomies, and 3) using a computer-assisted surgical guidance system. Objective. To assess the accuracy of transpedicular screw placement in the cervical spine using three surgical techniques. Summary of Background Data. A three-column fixation device implanted to sec ure an unstable cervical spine can be a valuable tool with a biomechanical advantage in the spine surgeon's armamentarium. Despite this advantage, con cerns over surgical neurovascular complications have surfaced. Cadaver-base d morphometric measurements used to guide the surgeon in the placement of a pedicle screw show significant variability, raising legitimate concerns as to whether transpedicular fixation can be applied safely. Methods. Precise measurements of 14 human cadaveric,cervical spines were ma de by two independent examiners of pedicle dimensions, angulation, and offs et relative to the lateral mass boundaries. On the basis of this analysis, guidelines for pedicle screw placement relative to posterior cervical topog raphy were derived. In the second part of the study, 12 human cadaveric cer vical spines;,were instrumented with 3.5-mm screws placed in the pedicles C 3-C7 according to one of three techniques. Cortical integrity and neurovasc ular injury were then assessed by obtaining postoperative computed tomograp hy scans (l-mm cuts) of each specimen. Cortical breaches were classified in to critical or noncritical breaches, Results, Linear measurements of pedicle dimensions had a wide range of valu es with only fair interobserver-correlation. Angular measurements showed si milar angulation in the transverse plane (40 degrees) at each level. With r espect to the sagittal plane, both C3 and C4 pedicles were Oriented superio rly relative to the axis of the lateral mass, whereas the C6 and C7 pedicle s were oriented inferiorly. The dorsal entry point of the;pedicle on the la teral mass defined by transverse and sagittal offset had similar mean value s with wide ranges, although: there often was excellent correlation between observers. There were no significant interlevel, right/left, or male/femal e differences noted with respect to offset. Using,one of three,techniques, 120 pedicles were instrumented.-ln group 1 (morphometric data): 12.5% of th e screws were placed entirely within the pedicle; 21.9% had a noncritical b reach; and 65.5% had a critical breach. In, group 2 (laminoforaminotomy), 4 5% of the screws were within the pedicle; 15.4% had a:noncritical breach; a nd 39.6% had a critical breach. In group 3 (computer-assisted surgical guid ance system), 76% of the screws:were entirely within the pedicle; 13.4% had a noncritical breach; and 10.6% had a critical breach. Regardless of the t echnique used, the vertebral artery was the structure most likely to be inj ured. Conclusions. On the basis of the morphometric data, guidelines for cervical spine pedicle Screw placement at each subaxial level were derived. Althoug h a statistical analysis of cadaveric morphometric data obtained from the c ervical spine could provide guidelines for transpedicular screw placement b ased on topographic landmarks, sufficient variation exists: to preclude saf e instrumentation without additional anatomic data; insufficient correlatio n between different surgeons' assessments of surface landmarks attests to t he inadequacy of screw insertion techniques in the cervical: spine based on such,specific topographic guidelines. Laminoforaminotomy does improve visu al and tactile access to the cervical pedicle. However, this technique did have a significant likelihood of injuring vital structures above the C7 ver tebral body. The emergence of a computer-assisted surgical system based on the principles of stereotaxis enhances accuracy and further improves the sa fety of transpedicular screw placement, most notably at C6 and C7.