The Accuracy of 3D Printing Assistance in the Spinal Deformity Surgery

Abstract

Background: The pedicle screw is one of the main tools used in spinal deformity correction surgery. Robotic and navigated surgeries are usually used, and they provide superior accuracy in pedicle screw placement than free-hand and fluoroscopy-guided techniques. However, their high cost and space limitation are problematic. We provide a new solution using 3D printing technology to facilitate spinal deformity surgery.

Methods: A workflow was developed to assist spinal deformity surgery using 3D printing technology. The trajectory and profile of pedicle screws were determined on the image system by the surgical team. The engineering team designed drill templates based on the bony surface anatomy and the trajectory of pedicle screws. Their effectiveness and safety were evaluated during a preoperative simulation surgery. The surgery consisted in making a pilot hole through the drill template on a computed tomography- (CT-) based, full-scale 3D spine model for every planned segment. Somatosensory evoke potential (SSEP) and motor evoke potential (MEP) were used for intraoperative neurophysiological monitoring. Postoperative CT was obtained 6 months after the correction surgery to confirm the screw accuracy.

Results: From July 2015 to November 2016, we performed 10 spinal deformity surgeries with 3D printing technology assistance. In total, 173 pedicle screws were implanted using drill templates. No notable change in SSEP and MEP or neurologic deficit was noted. Based on postoperative CT scans, the acceptable rate was 97.1% (168/173). We recorded twelve pedicle screws with medial breach, six with lateral breach, and five with inferior breach. Medial breach (12/23) was the main type of penetration. Lateral breach occurred mostly in the concave side (5/6). Most penetrations occurred above the T8 level (69.6%, 16/23).

Conclusion: 3D printing technology provides an effective alternative for spinal deformity surgery when expensive medical equipment, such as intraoperative navigation and robotic systems, is unavailable.

Figure 1

(a) The trajectory, diameter, and length of the pedicle screws are planned on the Avizo software. (b) The drill template is designed on Geomagic Design X software based on the profiles of pedicle screw and the anatomic traits of a certain level. (c) The drill template is composed of three parts, namely, the foot template (arrowhead), drilling cannula (asterisk), and connecting bar (arrow). (d) Finished products of 3D printing technology in this study, including the drill template and the 3D spine model. (e and f) The surgeon uses a power drill (arrow) to create a pilot hole on the full-scale spine model during the simulation surgery. The trajectory is guided by the drilling cannula (asterisk) of the drill template, and the route can be directly visible (arrowhead). (g) The drill template is mounted on the patient's spine.

Figure 2

(a) Anteroposterior and lateral scanography of spine and photograph before surgery (Case 1). (b) Anteroposterior and lateral scanography of spine and photograph after surgery (case 1).

Figure 3

(a) Medial breach (arrowhead) is defined as violation of medial pedicle wall. (Case 10, T5). (b) Lateral breach (arrowhead) is defined as violation of lateral pedicle wall. (Case 10, T12). (c) Inferior breach (arrowhead) is defined as violation of inferior pedicle wall. (Case 2, T7). (d) Anterior breach (arrowhead) is defined as penetration of vertebral body without pedicle wall violation (not in this case series).

Figure 4

(a) The drill template is the female die (dotted line) on the target zone of the patient's spine. In practice, the target zone should be carefully selected to avoid inapplicability (box). (b) The sharply curved template (dotted line) generates high friction between the template and the target zone (arrowhead), resulting in a gap that causes inaccuracy. (c) The smoothly curved template (dotted line) creates a close fit between the template and the target zone (arrowhead), insuring the planned trajectory.