A Novel Parsimonious Method as Accurate as Extravagant Counterparts: Streolithographically-Modeled Patient-Specific Drill Guide Template for Subaxial Cervical Pedicle Screw Insertion

This study was conducted to develop a modified, parsimonious, faster to produce, easier to implement, and patient-specific drill guide template and also to examine if such a modification might affect the accuracy.

On two cadaveric spines, using reverse engineering, the orientation of pedicles and safe corridors for pedicle screw were determined. A drill template was designed with a surface that was the inverse of the posterior vertebral surface. The drill template was manufactured using rapid prototyping technique. To decrease the costs, the cervical spine corresponding prototypes were not manufactured. In contrary to previous studies, to preserve the stability from posterior element, the templates were designed in such a way that removing interspinous and supraspinous ligaments was not necessary. The accuracy was evaluated by computed tomography (CT) images and classified into three grades of 0: correct placement, 1: malposition by less than a half screw diameter, and 2: malposition by more than a half screw diameter.

Of 20 positions available, we inserted 19 screws, because the trajectory of one of the patient-specific drill guide templates was misdirected. The overall accuracy rate for cervical pedicle screw (CPS) placement was 84.2% (16 of 19). Safely inserted screws, combining the grades 0 and 1 categories, were as high as 100%. We observed no “unsafe screw placement”.

The total cost and the latency period before the operation was reduced and the interspinous and supraspinous ligaments were preserved. A good applicability and high accuracy was obtained for subaxial CPS (SCPS) insertion.