Computed tomography–based determination of a safe trajectory for placement of transarticular facet screws in the subaxial cervical spine

Clinical article

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Object

Placement of transarticular facet screws is one option for stabilization of the subaxial cervical spine. Small clinical series and biomechanical data support their role as a substitute for other posterior stabilization techniques; however, the application of transarticular facet screws in the subaxial cervical spine has not been widely adopted, possibly because of surgeon unfamiliarity with the trajectory. In this study, the authors' objective is to define insertion points and angles of safe trajectory for transarticular facet screw placement in the subaxial cervical spine.

Methods

Thirty fine-cut CT scans of cervical spines were reconstructed in the multiplanar mode and evaluated for safe transarticular screw placement in the subaxial cervical spine (C2–3, C3–4, C4–5, C5–6, C6–7). As in placement of lateral mass screws, the vertebral artery and exiting nerve root were bypassed posterolaterally. The entry point was set 1 mm medial and 1 mm caudal to the center of the lateral mass. From this entry point, the sagittal angulation was set to traverse the facet joint plane approximately perpendicularly. For the axial angulation, the exit point was set posterolaterally to the transverse process. After ideal insertion angles and screw lengths were identified, the trajectory was simulated on CT scans of 20 different cervical spines to confirm safe screw placement.

Results

The mean optimal mediolateral insertion angles (± SD) were as follows: 23° ± 5° at C2–3; 24° ± 4° at C3–4; 25° ± 5° at C4–5; 25° ± 4° at C5–6; and 33° ± 6° at C6–7. The mean sagittal insertion angles measured to the sagittal projection of the facet joint space were as follows: 77° ± 10° at C2–3; 77° ± 10° at C3–4; 80° ± 11° at C4–5; 81°± 8° at C5–6; and 100° ± 11° at C6–7. The mean trajectory lengths were 15 ± 2 mm at C2–3; 14 ± 1 mm at C3–4; 15 ± 1 mm at C4–5; 16 ± 2 mm at C5–6; and 23 ± 4 mm at C6–7. Simulation of these insertion angles on 20 different cervical spine CTs yielded a safe trajectory in 85%–95% of spines for C2–3, C3–4, C4–5, C5–6, and C6–7.

Conclusions

The calculated optimal insertion angles and lengths for each level may guide the safe placement of subaxial cervical transfacet screws.

Abbreviations used in this paperAP = anteroposterior; DICOM = Digital Imaging and Communications in Medicine.

Article Information

Address correspondence to: Meic H. Schmidt, M.D., Department of Neurosurgery, University of Utah, 175 North Medical Drive East, Salt Lake City, Utah 84132. email: neuropub@hsc.utah.edu.

Please include this information when citing this paper: published online January 27, 2012; DOI: 10.3171/2011.12.SPINE11141.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Sagittal reconstruction and axial CT scan showing angles for transfacet trajectories in the subaxial cervical spine. A: The angle α for the sagittal trajectory was referenced to the sagittal projection of the facet joint space. B: The angle β for the mediolateral trajectory was referenced to a straight AP line, parallel to the vertebral body.

  • View in gallery

    Three-dimensional CT reconstructions showing the trajectories for transfacet screws in the subaxial spine (C2–3, C3–4, C4–5, C5–6, C6–7). A: The entry point is 1 mm medial and 1 mm caudal to the midpoint of the superior lateral mass at C-3, C-4, C-5, C-6 and in the medial upper quadrant of the C2 isthmus. B: The trajectory traverses the facet joint plane perpendicularly and exits posterolaterally to the transverse process of C-3, C-4, C-5, and at the outer upper quadrant of C-6. C: The exit point at C-7 is at the tip or lateral ventral surface of the transverse process.

  • View in gallery

    Photographs of cervical spine specimen in oblique and lateral views showing the relative size of the lateral masses and transverse processes of C-3 to C-7. The lateral masses of C-3, C-4, and C-5 (+) are relatively large and their corresponding transverse processes (arrowheads) small. C-7 has a transitional morphology: the lateral mass (*) is more shallow whereas the transverse process (arrows) is larger and extends more laterally than at C3–6. The anatomy of C-6 (#) falls in between that of C3–5 and C-7.

  • View in gallery

    Computed tomography images showing the transarticular anatomy at C5–6. A and B: Sagittal view (A) with corresponding axial plane (B). The groove (*) between the lateral mass of C-6 and its transverse process may be very small, thereby precluding a trajectory that exits on the C-6 lateral mass posterolateral to this transverse process. C and D: Sagittal view (C) with corresponding axial plane (D). A less steep and less lateral trajectory exits in the safe upper outer quadrant of C-6.

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