Customized Platform-Based Stereotactic DBS Lead Placement Technique (FHC STarFix, Medtronic Nexframe, and Robotic System Placement)

2 Customized Platform-Based Stereotactic DBS Lead Placement Technique (FHC STarFix, Medtronic Nexframe, and Robotic System Placement)

Ahmad Alhourani, Margot Samson, Joseph S. Neimat

Abstract

Traditional rigid frame-based systems have been the gold standard in stereotactic surgery for decades. Several customized platform-based stereotactic systems have recently been developed to overcome some of the limitations of traditional frames. These new systems offer comparable accuracy and precision along with added patient comfort. In this chapter, theoretical basis behind each system and its workflow for deep brain stimulation (DBS) lead placement has been described. The advantages and disadvantages between these systems and traditional frames have also been compared.

Keywords: frameless, deep brain stimulation, Nexframe, robotic, STarFix, stereotaxis

2.1 Background

The advent of stereotaxis in neurosurgery marked a huge leap by offering minimally invasive corridors to access the brain. Pioneering work by Zernov 1 in 1889 and Clarke and Horsley² in 1906 paved the way for the first routinely applied stereotactic system by Spiegel and Wycis³ in 1947. Improved frame designs, such as the Leksell frame that integrated Cartesian targeting and polar trajectory selection⁴ and the enhanced imaging of computed tomography (CT) and magnetic resonance imaging (MRI), made accurately reaching subcortical structures feasible. Stereotactic neurosurgery traditionally relied on a coordinate system contained within the frame itself and a method to relate those coordinates with those of the patient and their imaging. This relationship is calculated by acquiring patient imaging while in the frame. Although these traditional frame-based approaches remain accurate and reliable, they have several drawbacks. The main drawback is the need for the patient to be rigidly fixed in the frame throughout the procedure to maintain this relationship. This can be cumbersome for awake movement disorder patients as the weight of the frame itself requires frame and patient to be bolted to the operative table. For this reason, several stereotactic systems have been developed to allow greater patient comfort with equivalent accuracy and precision. In this chapter, we describe the three most commonly used systems, the theoretical basis for their design, and their practical workflow. We also describe the clinical results from reported experience with each system. In addition, we highlight the advantages and disadvantages to compare across systems.

2.2 Frame versus Imaging-based Coordinate Systems

The principle innovation that has enabled novel frame technologies was the advancement of imaging modalities for three-dimensional (3D) acquisition that incorporates an inherent coordinate space. Almost all CT and MRI scans currently acquired incorporate a precise parametric coordinate system such that each point on the scan has a distinct X, Y, Z designation. With this innovation, frames no longer had to provide an independent Cartesian coordinate system that was so critical when using X-ray ventriculography or 2D CT slice acquisitions ( Fig. 2.1a). New frame systems have been developed that essentially co-opt the 3D CT space as their own inherent coordinate system ( Fig. 2.1b). All platforms and trajectories within this system are simple mathematical transforms relating points of attachment and registration to targets and trajectories in the same 3D space. This is the same innovation that have made the commonly used frameless stereotactic guidance systems feasible. All the systems described below share and benefit from this simple innovation. The application of this strategy has taken different forms, each with their own unique advantages.

2.2.1 Surgical Targeting Fixture (STarFix) Platform

The STarFix system (FHC Inc., Bowdoin, ME) is an alternative method of stereotaxy that relies on custom microtargeting platforms (MTP) ( Fig. 2.2). Rather than giving the trajectory coordinates as input into the standard frame, an MTP is generated which incorporates one or more trajectories into a lightweight fixture that is directly attached to the skull. This process became feasible in the clinical setting with the emergence of rapid prototyping technology to allow the manufacture and delivery of an MTP in a relatively short time, as little as 3 days. The complete system includes planning software and bone fiducial markers. The bone fiducial markers initially used for registration become anchors for the MTP to couple to during surgery, while the planning software generates the instruction file for manufacturing of an MTP.

Fig. 2.1 Frame versus imaging-based coordinate systems. Traditional frames rely on an inherent coordinate Cartesian coordinate system contained within the frame (a). On the other hand, new frameless systems use the coordinate system inherent in the 3D image volumes (b).

Fig. 2.2 Surgical Targeting Fixture platform. A bilateral STarFix frame (FHC Inc., Bowdoin, ME) is shown with the associated guide tube.

The STarFix system retains the basic principles of traditional stereotactic frames, in that (1) the fiducial points are incorporated into the platform itself, and (2) there is a rigid relationship between registration points and the trajectory fixture. It relies on three key data points: bone fiducial anchor locations (made more accurate by recording bone fiducial anchor orientation), the target location, and the trajectory to target. Based on these data points, a transform is generated to translate the imaging space to the patient’s physical space. In addition, the orientation of the trajectory with respect to the anterior commissureposterior commissure line and midline are taken into account to allow for trajectory translation.

In contrast to traditional frames, the general workflow of the STarFix system is broken into two discrete steps over a period of 1 to 2 weeks. In Stage I (termed Step 0 at some centers), the bone fiducial markers are implanted in a separate surgical procedure. This can be performed under local or general anesthesia. At least 3 are required for unilateral cases and 4 are typically used for bilateral frames. (Implantation of 6 or more anchors may be used for specialized applications such as stereo-tactic electroencephalography, SEEG.) They serve as a rigid reference point for image registration and rigid attachment for the MTP later on. By necessity, the fiducials must remain fixed in the same location between procedures. The bony anchors went through several transformations from externalized MRI-detectable posts and caps to the current internalized bone posts that are buried completely under the scalp. The anchors are placed into the outer table of the skull through simple stab incisions and closed with a single suture or staples. CT scan is obtained immediately following the procedure while the patient is still under general anesthesia or immediately after the procedure. The CT is then registered to any additional imaging that has been acquired. A high-resolution MRI is typically used and can be obtained under the same general anesthesia (if used) for outstanding motion-free images. This enables the acquisition of higher quality imaging free from motion artifact. Patients are usually discharged home with instructions to keep the anchor sites clean.

Surgical planning follows similar steps as for traditional frames where the CT and MR images are coregistered, identifying the target locations and selecting the optimal entry points. However, instead of generating coordinates, the planning software creates a customized MTP design. The design file is sent to the manufacturer and the MTP is delivered to the hospital within a few days. Several compatible planning software are available to generate the design files such as Voxim, WayPoint planner, and StimPilot.

Stage II is usually performed about a week after stage I. Most commonly, this stage is done under local anesthesia, with intravenous sedation. The bone marker incisions are opened and the MTP is rigidly connected to the bone anchors using couplers with submillimetric tolerance. This obviates the need to lock the patient’s head to the operating table. A guide is used to mark the entry point on the scalp and skull through the ring opening of the MTP. The steps afterwards from burr hole creation, microelectrode mapping, electrode implantation and macrostimulation are done in a standard fashion.

The STarFix system offers some distinct advantages and disadvantages. First, both trajectories can be mounted and mapped simultaneously through separate microdrives. This can potentially save significant time as both sides are explored and recorded at once. Second, although the frame is nondeformable around the planned trajectory, the trajectory can be adjusted using various offset adapters for the drive assembly allowing for a maximum offset of 11 mm from the central target in all directions which is typically sufficient for any deep brain stimulation (DBS)-type procedure. No final confirmation fluoroscopic imaging is used due to the lack of stable reference imaging with the patient’s head not being locked to the operating table. Moreover, the STarFix system is compatible with most micro-drives and cannula systems, although the frame height from the skull is different from the Leksell frame and that difference needs to be accounted for when the microelectrodes and cannulas are mounted to calculate the correct distance to target.

The STarFix system was approved by the Food and Drug Administration (FDA) in 2001 and the largest reported experience comes from Vanderbilt where it has been adopted since 2002. The largest case series of 265 patients covered cases performed from 2002 to 2008 using several iterations of the system including its current mature form.5 The system showed high accuracy with a targeting error of 1.99 ± 0.9 mm across 75 patients. The targeting error was further reduced to 1.24 ± 0.4 mm when accounting for brain shift. The case series demonstrated the safety of the system with less than 0.2% complication rate across the entire cohort. Specifically, 0.1% of patients had dislodgment of the bony fiducial. However, this occurred in earlier versions of the system that had externalized posts and caps in patients with severe dyskinesia. This complication is not seen in the current internalized version of the bony fiducials. There was one case of bone marker infection (0.004%) that was simply treated by removal of the fiducials and a short course of antibiotics. Also, the custom MTP for one patient (0.004%) could not couple to the bone anchors. That was traced back to anchor localization error during planning rather than a manufacturing error.

The clear benefit of the STarFix system is the superior patient comfort it affords by allowing patients to move freely especially in case of patients with severe tremors or dyskinesias. This can help a lot of patients overcome the anxiety of being in a rigid frame for extended periods of time. There is no limitation on head size to fit inside the frame. Finally, it allows for simultaneous bilateral cells microelectrode mapping which increases the speed and efficiency of the operation and may open many opportunities for scientific research.

A recent innovation on the STarFix platform is the advent of the Microtable. This fixture employs the same basic strategy of bone marker insertion and subsequent fixture application. The fixture is a Lexan plate that has holes of various depths drilled into it to hold legs of different length ( Fig. 2.3). The resulting geometry can reproduce any single stereotactic trajectory with accuracy equivalent to the STarFix platform. The advantage of the Microtable is that the fixture can be created in just a few minutes and therefore be available for same day surgeries. To date it has been utilized in more than 20 surgeries and publications on safety and accuracy are anticipated in the coming year. (M Fitzpatrick, personal communication).

2.2.2 Nexframe

The Nexframe (Medtronic Inc, Minneapolis, MN), while in the same category as the STarFix system, is the only true frameless system available currently (in the sense that there is no rigid attachment between the reference points and the trajectory) ( Fig. 2.4). It also uses bone fiducial markers for image registration, but they are used as markers for optical tracking to manually register and align the trajectory during surgery⁶ and are not incorporated in the fixture itself. It utilizes principles similar to infrared-guided biopsy probes. However, it relies on rigid registration markers and more tightly controlled guide tower allowing for the precision that is required for DBS lead implantation. The Nexframe tower is a standardized frame that is adjusted during targeting, so it does not require the overhead in time required to manufacture the STarFix frame. Also, it is comprised of disposable components thus eliminating the need to be recalibrated after repeated use like traditional frames.