Construction and accuracy assessment of patient-specific biocompatible drill template for cervical anterior transpedicular screw (ATPS) insertion: an in vitro study

Abstract

Background: With the properties of three-column fixation and anterior-approach-only procedure, anterior transpedicular screw (ATPS) is ideal for severe multilevel traumatic cervical instabilities. However, the accurate insertion of ATPS remains challenging. Here we constructed a patient-specific biocompatible drill template and evaluated its accuracy in assisting ATPS insertion.

Methods: After ethical approval, 24 formalin-preserved cervical vertebrae (C2-C7) were CT scanned. 3D reconstruction models of cervical vertebra were obtained with 2-mm-diameter virtual pin tracts at the central pedicles. The 3D models were used for rapid prototyping (RP) printing. A 2-mm-diameter Kirschner wire was then inserted into the pin tract of the RP model before polymethylmethacrylate was used to construct the patient-specific biocompatible drill template. After removal of the anterior soft tissue, a 2-mm-diameter Kirschner wire was inserted into the cervical pedicle with the assistance of drill template. Cadaveric cervical spines with pin tracts were subsequently scanned using the same CT scanner. A 3D reconstruction was performed of the scanned spines to get 3D models of the vertebrae containing the actual pin tracts. The deviations were calculated between 3D models with virtual and actual pin tracts at the middle point of the cervical pedicle. 3D models of 3.5 mm-diameter screws were used in simulated insertion to grade the screw positions.

Findings: The patient-specific biocompatible drill template was constructed to assist ATPS insertion successfully. There were no significant differences between medial/lateral deviations (P = 0.797) or between superior/inferior deviations (P = 0.741). The absolute deviation values were 0.82±0.75 mm and 1.10±0.96 mm in axial and sagittal planes, respectively. In the simulated insertion, the screws in non-critical position were 44/48 (91.7%).

Conclusions: The patient-specific drill template is biocompatible, easy-to-apply and accurate in assisting ATPS insertion. Its clinical applications should be further researched.

Figure 1. 3D model of cervical vertebra with virtual pin tracts.

3D model of each vertebra was reconstructed in Mimics software. The 2 cylinders with 2 mm-diameters were then imported and their locations were ensured at the central cervical pedicle by visual observation. With the tool “subtraction” under “Boolean operation” in the Geomagic studio® software, the 3D model of the cervical vertebrae with bilateral pin tracts was obtained and saved in group 1 in (.stl) file format.

Figure 2. Production of biocompatible navigation template with RP model.

The 3D model of cervical vertebrae with virtual pin tracts was rapid-prototyped with Z Corporation 3D printer Spectrum Z™510. A Kirschner wire was then inserted into the pin tract of the RP model and polymethylmethacrylate (PMMA) was used to construct the drill template.

Figure 3. Actual drill with navigation template in cadaveric cervical specimens.

Anterior soft tissue was removed from the vertebrae. The drill template was put in place by hand and compressed slightly to the anterior surface of cervical vertebrae. A 2 mm-diameter Kirschner wire was then drilled into the cervical pedicle with the assistance of the drill template.

Figure 4. Accuracy evaluation with screw simulation insertion in the axial (A) and sagittal (B) planes.

The blue lines in Figure 4A are the border of cervical pedicle in axial plane. The structures inside and outside the blue lines are vertebral canal and vertebral artery, respectively. And the red lines in Figure 4B are the border of cervical pedicle in sagittal plane. The structures upper and lower the red lines are foramen intervertebrale. A pre-designed 3D screw model (3.5 mm in diameter), which aligned with the pin tract of the 3D model of group2, was imported into Mimics to simulate the screw insertion. The screw positions were graded according to the distance between the screw thread and the border of pedicle cortex.

Figure 5. Illustration of grades of ATPS positions in cervical pedicle.

Grade 1: Screw positioned at the center of the pedicle. Grade 2: Less than one-third of the screw cross-section (≤1.2 mm with a 3.5-mm diameter screw) penetrating the cortex. Grade 3: Between one-third and one-half of the screw cross-section penetrating the cortex (or deviation <2 mm). Grade 4: More than one-half of the screw cross-section penetrating the cortex (or deviation ≥2 mm). Grade 5: Deviation equal or greater than the screw diameter.

Figure 6. Absolute deviation values in the axial and sagittal planes.