The accuracy of a novel pedicle screw insertion technique assisted by a special angular scale in the subaxial cervical spine using lateral mass as a reference marker

Abstract

Background: Posterior cervical pedicle screw (CPS) internal fixation has better biomechanical stability than other posterior cervical fixation methods. However, this technique is limited in clinical practice due to the complex anatomical structure and the adjacent relationship of the cervical pedicle, and the high risk of neurovascular injury. The purpose of this study was to describe a novel subaxial CPS insertion technique assisted by a special angular scale using lateral mass as a reference marker and to evaluate the accuracy of CPS placement and the distribution characteristics of CPS misplacement.

Methods: A total of 36 patients with subaxial cervical spine diseases who underwent posterior CPS fixation were consecutively selected. The optimal entry point on the posterior surface of the lateral mass was identified on the three-dimensional cervical model reconstructed from preoperative computed tomography (CT) images. The pedicle transverse angle (PTA) and pedicle-lateral mass angle (PLMA) were measured on the transverse and sagittal CT images respectively. The pedicle screws were inserted according to the preoperatively planned entry point and angles. We analysed the postoperative CT images for CPS misplacement rates and perforation directions following the Lee classification.

Results: Overall, 177 pedicle screws were inserted, of which 119 (67.2%) were classified as grade 0, 43 (24.3%) as grade 1, 12 (6.8%) as grade 2 and 3 (1.7%) as grade 3 by the postoperative CT images. The accuracy rate of CPS placement was 91.5%. Of the 15 misplaced pedicle screws (grades 2 and 3), 11 were lateral pedicle perforations, 3 were superior perforations and 1 was an inferior perforation. There were no neurovascular injuries related to CPS misplacement.

Conclusions: With our technique, the optimal entry point and two angles (PTA and PLMA) were identified for CPS insertion. The novel CPS insertion technique assisted by a special angular scale provides high accuracy and few complications.

Keywords: CPS; Lateral mass; Optimal entry point; PLMA; PTA; Special angular scale.

Fig. 1

The ideal pedicle trajectory was defined as a line passing through the centre of the pedicle on the transverse and sagittal CT images (left column). The three-dimensional cervical model was reconstructed using cervical spine transverse CT images. On the three-dimensional reconstructed cervical model, the ideal pedicle trajectory would penetrate the posterior surface of the lateral mass, and this point was then identified as the optimal entry point passing through the centre of the pedicle (right column)

Fig. 2

Vertical and horizontal offsets of the optimal entry point (a, b) were measured from the centre of the lateral mass

Fig. 3

The preoperative pedicle transverse angle (PTA) and pedicle-lateral mass angle (PLMA) were measured on the transverse and sagittal CT images, respectively (a, b), and other parameters, including the pedicle height (A), pedicle width (B) and pedicle axis length (C), were also measured (b, c). The PTA referred to the included angle between the pedicle trajectory and the vertical line of the posterior edge of the vertebral body on the transverse CT images, and the PLMA was defined as the included angle between the pedicle trajectory and the posterior edge of lateral mass on the sagittal CT images

Fig. 4

The diagram of the pedicle-lateral mass angle (PLMA) on a specimen

Fig. 5

The lateral masses of target segments were clearly exposed during operation

Fig. 6

The entry hole was made by using a power drill at the optimal entry point on the posterior surface of the lateral mass

Fig. 7

A special angular scale that controls the angle (a). The pedicle screw was inserted according to preoperative measurements of pedicle transverse angle (PTA) and pedicle-lateral mass angle (PLMA) (b). The detailed processes were as follows: during the operation, the reference axis was placed parallel to the spinous processes of the cervical spine, and then the adjusting spindle was regulated to achieve the preset PTA. To control PLMA as accurately as possible in clinical practice, after identifying the entry hole, the wire tapping inside the adjusting spindle was first placed perpendicular to the lateral mass and then slightly adjusted to the head or tail side according to the preset PLMA

Fig. 8

The rod with the appropriate size was bent in accordance with normal cervical curvature and the connections between the screws and rod were tightened

Fig. 9

A typical case with successful cervical pedicle screw (CPS) fixation. Postoperative CT scans and multiplanar reconstruction showed the good placement of pedicle screws at C3, C5, C6 levels (a, b, c, d, e, f)