22.1 Introduction

The subaxial cervical spine is composed of the third through seventh vertebrae, of which the third through sixth exhibit relatively uniform anatomy. Bearing little weight, the cervical vertebral bodies are relatively small and thin compared to the size of their respective vertebral arches and vertebral foramina.

The spinal pedicle is located in the junction of vertebral body and the posterior vertebral arch. The most featured vertebral arteries transfer from the transverse foramina which perforate in the transverse processes. The pedicles get smaller caudal to C2, reaching a nadir around C3–C4. ¹ Due to the differences in individual anatomy, there is significant disparity among the cervical pedicles. The height of the cervical pedicle ranges from 5.1 to 9.5 mm and their width ranges from 3 to 7.5 mm. ^{2 ,​ 3} At C3–C4, 75% of pedicles have an average diameter less than 4 mm. ⁴

Panjabi published a study on three-dimensional anatomy of the lower cervical spine in 1991. ⁵ This study demonstrated robust variation in cervical spine anatomy with little uniformity between patients and even within individual patients regarding the height and width, the axial projection points, and the axis angle of the cervical pedicles. In addition to the diminutive nature of the pedicles, cervical pedicles are bordered medially by the spinal canal/cord, laterally by the vertebral artery, and superiorly and inferiorly by the cervical nerve roots. This challenging anatomy makes accurate placement of pedicle screws even more critical. Screw misplacement may not only lead to inadequate fixation and stability, but also may lead to neurological, vascular, or visceral injury.

22.2 Computer-Assisted Navigation and Robotic Cervical Pedicle Screw Placement

Image-based computer-assisted navigation has been utilized in spine surgery to improve cervical pedicle screw insertion accuracy, making the procedure safer and more effective. Navigation allows the patient’s anatomy to be revealed via intraoperative computer tomography (CT) registration so that the neighboring spinal cord, vertebral artery, nerve roots, and other important structures, normally not visualized, can be represented in surgery.

Although several recent studies have demonstrated improvement in pedicle screw placement in lower thoracic and lumbar spine, ^{6 ,​ 7 ,​ 8} the utility of pedicle screw placement in this setting is still debated. ⁹ Some critics claim that the navigation system distracts the surgeon’s attention during the surgery and therefore the surgeon is unable to focus on the monitor. Additionally, although the trajectory may be more easily mapped, surgeon fatigue is not necessarily mitigated, and even with practice, the surgeon’s force control, steadiness, repeatability, and durability may still be insufficient to face the challenge of complicated cervical spine surgery.

Robotics has recently been advocated as a tool to address precisely these issues. Coupled with navigation, the system allows an “eye” to reveal previously unseen structures and a “hand” to aid in steadiness and fatigue. ^{10 ,​ 11}

The SpineAssist/Renaissance and the ROSA Spine robots have demonstrated promising results in clinical applications, ^{12 ,​ 13} but neither has been studied in applications related to the cervical spine. Bertelsen et al described a new robotic system for atlantoaxial fixation in 2012, but its accuracy was not sufficient for clinical use (1.94-mm error in a cadaver trial). ¹⁰ The TIANJI robotic system (China) has been the most widely used device in China for cervical spinal surgery.

The TIANJI robot is a robot-assisted surgical navigation device based on 3D fluoroscopy, ¹⁴ and allows real-time navigation. The TIANJI robot has three main components: the robotic system, an optical tracking system, and a navigation system. The robotic arm has six degrees of freedom, and a universal tool base mounted at the end of the robot arm allows all instrumentation to be directly mounted. The optical tracking system is based on infrared reflection and consists of an infrared stereo camera and two reference frames. One reference frame is mounted on the patient’s spinous process, and the other one is mounted on the universal tool base of the robotic arm. The infrared stereo camera captures the reflection from the two frames in real time and calculates their three-dimensional vector distance to allow the robotic arm to compensate for distance. The surgical planning and navigation system is based on the intraoperative 3D fluoroscopy images, and after registration, pedicle screw trajectories can be planned on the aligned images. The surgical planning and navigation system automatically calibrates to intraoperative positioning and can calculate the distance and angle between the real and planned trajectories and subsequently guide the movement of the robot arm. In this way, the robotic system can always maintain precise position within the predefined surgical trajectory.

In 2015, results of TIANJI system use were published, one report of robot-assisted anterior odontoid screw fixation and another of posterior C1–C2 transarticular screw fixation for atlantoaxial instability. ^{14 ,​ 15} Currently, the system is in use in numerous hospitals in different provinces and cities in China serving more than 12 million people.