2 Three-Dimensional Computer-Assisted Navigation Platforms
Abstract:
Intraoperative navigation technology for spine utilizes existing neurosurgical core concepts utilized with brain surgery. Over the past 50 years, advancements in imaging and computing have facilitated integration of real-time three-dimensional imaging to core neurosurgical principles of stereotaxy which utilizes external frames to provide references for difficult to visualize anatomy and pathology. With the recent advances of intraoperative fluoroscopic and CT scanning techniques along with intraoperative navigation platforms allowing instrument referencing and tracking, current spine navigated techniques for spinal surgery allow safe, effective, and accurate identification of complex spinal anatomy and assists with accurate spinal instrumentation placement. The core principles of intraoperative navigation involve successful performance and integration of imaging, tracking, registration, and image synthesis/visualization. Acquisition of intraoperative CT scans or fluoroscopy and subsequent integration with navigation platforms allows for three-dimensional recreation of the patients anatomy. While a variety of tracking platforms are available, optical systems using a camera emitting infrared light and passive optical arrays in the surgical field are the most commonly utilized techniques. In order to register and associate the patients imaging to the navigation platform, a rigid registration array is secured to the patient and mobile arrays are fixed to the navigated instruments. Last, image synthesis and visualization is performed, where the navigation platform integrates the patients anatomy to the reference arrays within the surgical field, providing real-time visualization and integration of surgical instruments with patient anatomy. Intraoperative spine navigation has been shown to have accurate identification of intraoperative anatomy and improved accuracy with pedicle screw placement over conventional open and fluoroscopic techniques.
2.1 Introduction
As surgical techniques have improved so have intraoperative technologies. Computer-assisted navigation platforms, a newer class of assistive modalities, have been developed with these techniques in mind, aiming to complement and enable the performance of increasingly complex cases. Prior to intraoperative navigation, surgeons would have to synthesize a great deal of information in their minds’ eye; the preoperative advanced imaging would be correlated with intraoperative topography and anatomy to allow successful performance of a given procedure. With the advent of intraoperative navigation, real-time three-dimensional (3D) localization and orientation of instrumentation and pathology has been enabled. This has afforded an improved understanding of the pathology and has potentially provided risk mitigation for more high-risk surgical procedures and possibly for even more routine techniques such as pedicle screw placement.
Recent publications suggest that intraoperative navigation can improve the accuracy and safety of instrumentation placement, can be time efficient, can be low risk, and can be cost-effective especially in complex spinal procedures. Despite the potential benefits of intraoperative navigation, widespread adoption has been lacking. Choo and colleagues reported that 63.4% of members in the Spine Arthroplasty Society and Society for Minimally Invasive Spine Surgery have minimal experience with navigated techniques. 1 Despite the available evidence, Härtl et al reported that only approximately 11% of north American and European surgeons routinely use navigation. 2 Adopters of the technology cite the increased accuracy, facilitation of complex surgery, and reduction in radiation as advantages, while nonusers note a lack of equipment or training and high overhead costs as obstacles to adoption of navigation techniques. 2
2.2 Evolution of Intraoperative Navigation
Navigated spine surgical techniques were conceptually designed using neurosurgical principles of stereotaxy. Due to the complex cerebral anatomy encountered, Drs. Horsley and Clarke proposed the initial use of static frames to hold the skull motionless, providing a fixed external reference for the surgeon while accessing known landmarks to improve accessing obscure areas of the brain. 3 Spiegel and colleagues built off the concepts of Drs. Horsley and Clark to create the first frame-based intraoperative referencing for use in humans, noting the advantages of limited access minimize the morbidity of the craniotomy. 3 , 4
In the 1970s and 1980s, advancements in imaging and computing allowed for the development of individualized intraoperative referencing. 5 Improved imaging techniques allowing 3D re-creations of anatomy, such as CT and MRI, and the integration of referencing computer guidance systems have facilitated development of frameless navigation techniques. In 1979, the Brown-Roberts-Wells stereotactic system allowed pairing of stereotactic instruments with CT data. 6 Roberts and colleagues in 1986 published a frameless stereotaxy technique utilizing emitted acoustic clicks from an operating microscope and detected by an array of microphones within the operating room (OR), allowing triangulation and reference of the discrete point in space. These intraoperative referenced points could be correlated with the preoperative CT scan and assist with positioning of the microscope. 7
Barnett et al described an armless and frameless stereotactic instrument in 1993 consisting of a mobile ultrasonic probe localized by microphone arrays placed in the OR, allowing for less cumbersome intraoperative triangulation of neurosurgical anatomy. 8 A subsequent technique for intraoperative navigation utilized an articulated arm, whose arm lengths and joint positions could be computed and visualized on a monitor. The position of the articulated arm could then be coregistered by referencing arm locations to known points on preoperative imaging prior to the operation. 9 , 10
Current intraoperative systems utilize optical camera arrays to detect either light-emitting diodes or reflective spheres placed statically on the patient. These “anchor” referencing points can be compared in real time to the position of reflective spheres or LEDs on surgical instruments and correlated to preoperative imaging. The superimposed position of the surgical instruments and preoperative imaging is then displayed on a monitor. Although these systems are reliant on continuous “line-of-sight” monitoring between the reference probes and tracked surgical instrument and the camera array, they continue to be a widely utilized technique for intraoperative navigation. Unlike cranial surgery, spine surgery has the obstacle of mobile anatomy due to the many motion segments found throughout the spine and changes in positioning from preoperative imaging (supine vs. prone and changes in anatomy with the progression of the surgery itself [i.e., after osteotomy, screw placement, etc.]).
2.3 Overview of Intraoperative Navigation
Successful application of intraoperative navigation technology requires the execution of four key components: imaging, tracking, registration, and image synthesis/visualization.
2.3.1 Imaging
While MRI and CT imaging techniques have been described for navigation, spine surgical applications require fine detail of bony landmarks and therefore CT is the commonly preferred imaging system. Furthermore, the imaging can be obtained real time, using an integrated CT scanner within the OR allowing for multiple intraoperative scans, or preprocedural scans taken either prior to the OR or obtained using mobile CT scanner technology. The Brainlab Airo is an example of an integrated mobile CT scanner and OR table system, facilitating multiple episodes of intraoperative CT imaging when necessary and minimizing disruptions in workflow.
Due to high radiation doses from multiple CT scans, most surgeons utilize a single preprocedural CT scan. To eliminate confounding variables to imaging fidelity such as prone positioning, mobile CT systems can create intraoperative tomographic imaging that can be utilized for navigation systems. One example is the Medtronic (Minneapolis, MN) O-arm system, a mobile cone-based CT system that can be transported into the OR for intraoperative CT scans. Comparing the use of preoperative and intraoperative CT scans for pedicle screw placement, Costa and colleagues demonstrated no significant difference in accuracy (91.8 vs. 93.5%) but noted decreased registration time (6.5 ± 2 minutes vs. 1.15 ± 0.35 minutes, respectively) with intraoperative CT by avoiding paired-point registration. 11
Fluoroscopy-based navigation operates similarly to CT-based techniques. Intraoperative fluoroscopy images are obtained in multiple planes and are uploaded to the workstation. Limitations to this technique are based on the quality of the fluoroscopy images; obesity, osteopenia, user error, and rotational deformity can reduce the quality of imaging. Using fluoroscopy-based navigation, Quiñones-Hinojosa reported less than 3 mm accuracy at up to three levels from the registration array and less than 3 mm accuracy up to 120 minutes after registration between 83 and 100%. 12 Newer 3D fluoroscopy machines can re-create CT axial cuts with active rotation around the patient. Siemens and Ziehm are examples of companies that offer fluoroscopy machines that provide 3D imaging.
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