Patient-Specific Drill Guide Template for Pedicle Screw Insertion into the Atlantoaxial Cervical Spine Using Stereolithographic Modeling: An In Vitro Study

Abstract

Study design: Cadaveric study.

Purpose: This study aimed to assess the accuracy and feasibility of cervical pedicle screw (CPS) insertion into the atlantoaxial cervical spine using a patient-specific drill guide template constructed from a stereolithographic model.

Overview of literature: CPS fixation is a widely accepted procedure for posterior cervical fixation because of its biomechanical advantages, particularly in the subaxial cervical region. The extremely narrow corridors of the atlantoaxial spine make CPS insertion more difficult, requiring the development of new tools to ensure accurate placement.

Methods: Fifteen atlantoaxial cervical vertebra specimens from 15 cadavers were scanned into thin slices using computed tomography. Images of the cadaver spine were digitally processed and rendered stl files so that they could be printed to scale as threedimensional (3D) plastic models. Manually molded dental acrylic drill guide templates with pins inserted in the pedicles of the plastic cervical models were placed over the 3D printed models. The drill guide templates were used for precise placement of the drill holes in the pedicles of cadaveric specimens for pedicle screw fixation. The accuracy of screw placement was evaluated by an independent evaluator.

Results: A total of 60 pedicles (combined C1 and C2) from 15 cadaveric axial cervical vertebrae were evaluated. The total acceptable accuracy for pedicle screw insertion in the atlantoaxial cervical vertebrae is 95%. An accuracy rate of 100% was achieved for C1 while an acceptable accuracy rate of 90% was achieved for C2.

Conclusions: The use of a patient-specific drill guide constructed using stereolithography improved the accuracy of CPS placement in a cadaveric model.

Keywords: Axial cervical spine; Cervical pedicle screw fixation; Patient-specific surgical template; Stereolithography; Three-dimensional printing.

Fig. 1

Summary of study methodology. CT, computed tomography; 3D, three-dimensional; stl, steriolithographic format; PLA, polylactic acid; K-wire, kirshner wire.

Fig. 2

(A) Identification and scoring of entry point prior to drilling. (B) Three-dimensional printing of drill sleeves. (C) Insertion of K-wire in the drilled hole and placement of drill sleeves. (D) Molding of drill guide template using acrylic cement.

Fig. 3

Drill guide templates with their corresponding cervical vertebrae three-dimensional printed models. (A) C1 models with drill guide templates. (B) C2 models with drill guide templates.

Fig. 4

Overview of the steps taken in screw insertion. (A) The awl is used to make a hole following the trajectory of the drill guide template. (B) Manual drilling. (C) Tapping. (D) Screw insertion.

Fig. 5

(A, B) Actual post-screw insertion computed tomography scan images of the C1 (left) and C2 (right) vertebrae.

Fig. 6

Anatomic variations as seen in the three-dimensional printed cervical models. (A) and (B) show the thickness difference of the C1 arch in 2 different cervical models seen in sagittal view. (C) and (D) show the variations in the morphology of the posterior arches of C1 vertebrae. (E) and (F) show the variations in the directions of the pedicles.

Fig. 7

Differences in trajectory between right and left pedicles of the same cervical vertebra. (A) positioning of K-wire patterned after the contralateral side. (B) Breach on the left foramen transversarium. (C) Repositioning of K-wire to correct trajectory.