Article Text

Original research
Combining C-arm CT with a new remote operated positioning and guidance system for guidance of minimally invasive spine interventions
1. Christoph Czerny1,
2. Katrin Eichler2,
3. Yann Croissant2,
4. Boris Schulz2,
5. Gernot Kronreif3,
6. Renate Schmidt4,
7. Martin von Roden5,
8. Christof Schomerus6,
9. Thomas J Vogl2,
10. Ingo Marzi1,
11. Stephan Zangos2
1. 1Department of Trauma Surgery, University Hospital Frankfurt, Goethe-University Frankfurt, Frankfurt am Main, Germany
2. 2Department of Diagnostic and Interventional Radiology, University Hospital Frankfurt, Goethe-University, Frankfurt am Main, Germany
3. 3Austrian Centre for Medical Innovation and Technology—ACMIT, Wiener Neustadt, Austria
4. 4iSYS Medizintechnik GmbH, Kitzbuehel, Austria
5. 5Siemens AG, Healthcare Sector, Forchheim, Germany
6. 6Fachbereich Medizin der Goethe-Universität, Dr Senckenbergische Anatomie, Frankfurt am Main, Germany
1. Correspondence to Dr S Zangos, Department of Diagnostic and Interventional Radiology, University Hospital Frankfurt, Goethe-University, Theodor-Stern-Kai 7, Frankfurt am Main D-60590, Germany; s.zangos{at}em.uni-frankfurt.de

## Abstract

Objective To report our experience using C-arm cone beam CT (C-arm CBCT) combined with the new remote operated positioning and guidance system, iSYS1, for needle guidance during spinal interventions.

Methods A C-arm CBCT with a flat panel angiography system was acquired (Artis Zeego; Siemens Healthcare Sector, Forchheim, Germany). Reconstruction of CT-like images and planning of the needle path were performed using a common workstation. The needle holder of iSYS1 acted as a guide during insertion of Kirschner (K) wires. 20 percutaneous K wires were placed in the pedicles at T2–T3, T7–T12, and L1–L2 in a cadaver specimen. Postprocedure C-arm CBCT scans were obtained to confirm the accuracy of the K wire placement.

Results All K wire placements were successfully performed. Mean planning time with Syngo iGuide was 4:16 min, mean positioning time of iSYS1 was 3:35 min, and mean placement time of the K wires was 2:22 min. Mean total intervention time was 10:13 min per pedicle. A mean deviation of 0.35 mm between the planned path and the placed K wire with a mean path length of 6.73 cm was documented.

Conclusions Our results demonstrate the potential of combining C-arm CBCT with iSYS1 for safe and accurate percutaneous placement of pedicle K wires in spinal interventions.

• Intervention
• Spine
• guidewire
• CT

## Introduction

Treating vertebral fractures is a meticulous procedure for every spinal surgeon. The close proximity of large vessels, spinal nerves, and the spinal cord can lead to serious complications in posterior instrumentation, especially in the upper thoracic spine. Different techniques have been developed to increase the precision of pedicle screw placement.

The current standard is to display the pedicles in anterior to posterior and lateral view under fluoroscopy. However, these views are extremely limited in the upper thoracic spine, and thus pedicle perforation rates are 6.2–50.6%.1–3

CT guidance can be used during spinal interventions to reduce the complication rates. Ploss et al demonstrated that after placement of CT guided Kirschner (K) wires within the pedicles by an interventional radiologist, the perforation rate of cannulated pedicle screws was decreased significantly compared with the conventional fluoroscopy technique.4 The well known disadvantages of CT guidance are radiation exposure to patients and operators, limited imaging plane orientation for needle placement, limited needle length due to a small gantry bore, and a lack of real time visualization.5

Today, the most established technique, apart from the conventional fluoroscopy technique, is the CT based navigation method. Due to improvement in this procedure, the rate of misplacement of pedicle screws has been significantly decreased.6–8 The use of a navigation system is associated with expensive equipment, including a carbon fiber operating table, a three-dimensional C-arm imaging system, and the navigation system itself. These costs make the use of navigation systems unattractive for centers with a small number of spinal injury patients. Moreover, the safe and fast use of a navigation system requires continuous application and training of the operator.

Computer assisted procedures are widely established in spinal surgery and mainly used for transpedicular screw insertion into the thoracolumbar spine due to the resulting improved precision.9 Most of these procedures are based on optoelectronic navigation systems that enable real time instrument guidance on a computer generated anatomical map.

In 2008, C-arm mounted cone beam CT (C-arm CBCT) was introduced as a new imaging modality that uses flat panel detector technology to combine quick tomographic and fluoroscopic image acquisition at submillimeter spatial resolution for different interventional procedures.10 The isotropic dataset can be immediately postprocessed and offers a CT-like contrast for planning interventions. By combining the three-dimensional imaging information and dedicated software for image guided interventions (Syngo iGuide; Siemens Healthcare Sector, Forchheim, Germany), double oblique intervention paths can be planned, reassessed, and superimposed onto live fluoroscopy images for various interventional procedures.

The clear advantages of fluoroscopic guided interventions over standard CT interventions are the ability to visualize the procedure in slant projections and better access to the patient during the intervention. The disadvantage of this technique is the difficult introduction of the instruments under fluoroscopic guidance, which can result in an increased radiation load for the operator.

A previously published phantom study demonstrated that the new compact robotic device, iSYS1 (Medizintechnik GmbH, Kitzbuehel, Austria) allows highly accurate needle guidance in a reasonable timely manner in single as well as double oblique trajectories.11 The purpose of the current cadaver study was to evaluate the performance of the proposed robotic device for needle interventions during spinal interventions in terms of accuracy and required time in a dedicated C-arm based CBCT suite.

## Methods

### Robotic device

The robotic device used for this study, iSYS1, is a small positioning and guidance system that can be remotely operated during fluoroscopic guided interventional procedures. The robot can be prepositioned with a 7 degrees of freedom passive holding arm connected to the examination table via a dedicated table adapter. A control unit, powered by a standard 220 V outlet, was attached to a holding platform mounted on the side rail of the examination table and connected to the robot by two cables. By using the wired joystick control, the user could move the robot in two axes (range of motion for each axis±2 cm) and rotate it in two directions (range of motion for each direction±35°) to adjust the needle position and angulation of the needle guide adapter connected to the robot. An insert including two radio-opaque marker rings located on top of each other was used to adjust the positioning unit with fluoroscopic imaging in the frontal direction to verify the position and orientation of this needle guide adapter. By using different needle guide inserts, the positioning unit is capable of holding and guiding different instruments of different sizes.

### Procedure

A cadaver was placed prone on the operating table. A state of the art floor mounted C-arm CBCT device (Artis Zeego; Siemens Healthcare) with a 30×40 cm flat panel detector was used for the study. Under CBCT and iSYS1 control, a total of 20 K wires (Synthes, Umkirch, Germany) were placed transpedicularly into 10 vertebral bodies (T2, T3, T7–T12, L1, and L2) using a posterior percutaneous approach. Each pedicle was treated as a separate procedure (figure 1).

Figure 1

Experimental setup showing the positioning of the cone beam CT device and the new remote operated positioning and guidance system. Fine positioning of the robotic device is performed by remote control operation. The correct position is verified by live fluoroscopic image.

The planning three-dimensional dataset was acquired using the standard DynaCT Body 8sDR examination protocol on the Artis Zeego system. During this acquisition, the C-arm rotates for 8 s on a 200° circular trajectory from RAO170° to LAO30°. In total, 397 projection images are acquired with a 0.36 nGy detector entrance dose. The tube voltage is preset to 90 kV but is modulated together with the tube current during the rotational run to keep the detector entrance dose constant. From these 397 projection images, a three-dimensional dataset with isotropic voxels of 0.5 mm is automatically reconstructed on the connected workstation (Syngo XWP) using a filtered backprojection (Feldkamp) algorithm.

This three-dimensional dataset was then loaded into the software application Syngo iGuide, which allows the user to plan the path by using thin slice multiplanar reformatted images. After selecting both the target and ‘skin’ entry point, the software displayed the calculated needle path and length in the sagittal and transversal directions as well as in a volume rendered image technique for topographical overview. The calculated pathway was confirmed after control of the access way. Syngo iGuide then superimposed this path on the fluoroscopic images on the monitor in the interventional suite. The C-arm and examination table were positioned in a central anterior–posterior projection (‘bull's eye view’) to the superimposed puncturing path. The time required to bring the C-arm and examination table into position was noted.

Next, the robotic arm of the iSYS1 system was manually approximately positioned to the insertion point with the passive holding arm. By using the joystick control and the marker rings overlaid on the bull's eye projection, the robotic arm was then fine adjusted to achieve the exact puncturing angle (figure 2). The duration needed for positioning the robotic device was noted again. Using the needle guide insert attached to the robotic device as a precise guide, the K wire was inserted and advanced under fluoroscopic guidance (lateral and oblique projections) by the physician using a drill (Colibri; Synthes, Umkirch, Germany). The time from inserting the K wire until reaching the target destination was also measured.

Figure 2

After fine positioning, the angulation of the robotic device is adjusted and verified by live fluoroscopic image. (A) A perfect position of the adapter, verified with the iGuide software. The adapter is then used for guidance of the K wires (B).

### K wire placement confirmation

After placement of each K wire, another C-arm CBCT was acquired to confirm its position. This postprocedural C-arm CBCT was automatically fused with the planning volume using a 3D/3D registration software (Syngo InSpace 3D3D fusion) on the Syngo XWP. After the accuracy of the 3D/3D fusion was confirmed by checking the alignment of the vertebra in which the K wire had been placed, deviation of the actual K wire from the planned path was measured on the workstation (figure 3).

Figure 3

Accuracy of the iSYS1 aided K wire placement in a posterior percutaneous approach documented on the postprocedure cone beam CT images.

### Statistics

Descriptive and analytical statistics were performed and frequencies were calculated using SPSS software.

## Results

All of the procedures were successfully performed without technical problems. The intervention path planning was performed with the Syngo iGuide software, with a mean planning time of 4:16 min (range 2:20–6:25 min) (table 1). For each planned path, the mean time to place the C-arm device, iSYS1, and patient table was 3:35 min (range 1:30–8:19 min). The mean time to place the K wire, starting at perforation of the skin and ending when the destination point was reached, was 2:22 min (range 0:43–6:40 min). The mean total duration of an intervention of one pedicle was 10:13 min (range 6:25–17:11 min) (figure 5).

Table 1

Parameters and time duration of the cadaver punctures

Using a combination of CBCT and the iSYS1 system, all K wires were successfully positioned within the pedicle without penetrating the cortical wall. The mean distance from the skin to the target point was 6.73 cm (range 5.17–8.42 cm) (table 1). A mean deviation of 0.35 mm (range 0–1.3 mm) in the x axis and 0.7 mm (range 0–2.1 mm) in the z axis from the planned path was observed (figure 4).

Figure 4

Under cone beam CT and iSYS1 control, 20 K wires were placed transpedicularly into 10 vertebral bodies (T2, T3, and T7–L2) using a posterior percutaneous approach. A mean deviation of 0.35 mm (range 0–1.3 mm) in the x axis and 0.7 mm (range 0–2.1 mm) in the z axis from the planned path was observed.

## Discussion

Pedicle screw placement is technically demanding, particularly in patients with traumatic injury. It requires meticulous attention to both surgical technique and patient anatomy. A misplaced screw may cause vascular or neurologic damage. Intraoperative imaging with fluoroscopy is commonly used by surgeons to confirm the anatomic starting point for screw entry and also to assess screw trajectory, depth, and position.12

However, depending on the skill and experience of the surgeon, the rate of screw misplacement with the conventional free hand technique pedicle screw has been reported to be as high as 40%.13 ,14 Different techniques have been developed to decrease the rate of screw malpositioning, thereby avoiding serious adverse events. Thus far, C-arm CT imaging is predominantly used for the precise guidance of endovascular or intra-arterial therapy. A novel combined three-dimensional navigation C-arm system allows cross sectional and fluoroscopy con­trolled interventions. Different studies have reported successful CT image guided navigation with C-arm systems in vertebroplasty. In the future, complex interventions will likely be simplified and safer due to C-arm CT based navigation systems, and radiation exposure will be simultaneously reduced.15

In this cadaver study, we investigated the use of the new commercially available remote operated positioning and guidance system, iSYS1, in combination with a C-arm CBCT system for spinal intervention. Through the combination of the intervention planning software (Syngo iGuide) and iSYS1, we were able to position K wires precisely within the pedicles without penetrating the cortical wall in a mean time of 10:13 min per pedicle, with marginal deviations. Compared with the average procedure time for each level of 64.8 min reported by Tam et al,16 it is likely that the combination of iGuide and iSYS1 was responsible for decreasing the total procedure time in our study. Although we did not inject cement, the setups and approaches used in the two studies were comparable.

Several studies have demonstrated that computer navigation increases not only the accuracy of pedicle screw placement but also insertion time.17 ,18 Due to the complexity of such navigation systems, the preparation time of pedicle screw is prolonged compared with the conventional technique. In contrast, other groups have observed a decreased operative time with computer assisted navigation systems compared with conventional fluoroscopic techniques.19 ,20 However, reliable data showing that the use of a navigation system decreases the intervention time have not yet been published.

Tam et al16 reported their experience with Syngo iGuide in fluoroscopic guided vertebroplasty. In most cases, they used a unilateral extrapedicular approach. They reported a mean deviation of 4.5 mm from the target. A deviation of 4.5 mm is not sufficient to place K wires within the narrow pedicles of the thoracic spine. Such a deviation would lead to misplacement and failure of pedicle screws in posterior instrumentation.

Ploss et al4 demonstrated that a posterior instrumentation after presurgical placement of K wires by a radiologist is an accurate, reliable, and safe method for the treatment of vertebral body fractures. However, this method depends on a highly experienced radiologist and is time consuming. They reported that using this method in a CT suite prolonged the complete procedure by 67±25 min compared with the standard procedure. Although this method is accurate and safe, it is unfortunately not feasible for the majority of hospitals.

Navigation of pedicle screws is becoming common and decreases the rate of misplacement and injury to spinal nerves, spinal cord, and vessels.21 To use a navigation system in spinal interventions, expensive equipment is needed. Moreover, in addition to the equipment, our own experience suggests that a continuously trained operator is required to perform demanding spinal interventions. However, the major disadvantage of navigation is that there is no opportunity to confirm the real position of a pedicle screw during placement.

The amount of radiation exposure of the patient and the attending personnel depends on the experience and skill of the surgeon. In the free hand technique, the entry point of the pedicle screw is localized by real time fluoroscopy imaging. In contrast, the combination of the C-arm CBCT and iSYS1 allows this difficult and demanding step to be performed without additional radiation exposure. To our knowledge, no published data exist that describe the radiation exposure during a similar spinal intervention using the conventional free hand technique compared with a navigation system.

As cost efficiency and operational capacity are significant considerations, iSYS1 could be a favorable alternative. To establish a navigation system for spinal intervention, approximately US$750 000 is required. This includes a carbon fiber table, three-dimensional C-arm, and the navigation system itself. If an angiography/interventional suite with a C-arm CBCT already exists, an investment cost of$200 000 is required for the iSYS1. However, the iSYS1 can be used for different types of interventions by radiologists and surgeons and thus may serve multiple purposes. An additional benefit of iSYS1 is its compatibility with all existing CBCT systems and CT devices, without further modifications of the existing systems.

With the combination of a C-arm CBCT and iSYS1, we achieved a mean deviation of 0.35 mm in the x axis and 0.71 mm in the z axis. This precision, together with an expected decreased radiation exposure to the attending staff and a mean intervention time of 10 min per pedicle, make the combination of C-arm CBCT and iSYS1 a promising technique to prevent misplacement of pedicle screws and decrease the rate of injuries to vascular and neuronal structures.

This technique is simple to use and could assist less experienced operators with the transpedicular approach in spinal interventions, such as pedicle screw placement, vertebroplasty, or kyphoplasty.

In a recent study, Shin et al22 reported that 87.7% of fluoroscopy guided pedicle screws and 91.9% of navigation guided pedicle screws in the thoracic and lumbosacral spine from T9 to S1 did not violate the cortex. The similar high success rates demonstrate that either method can be safe and reliable when performed by an experienced operator. However, the advantage of iSYS1 or a navigation system is more relevant in the more challenging upper thoracic spine where the rate of pedicle screw misplacement is 21%.23 ,24 Moreover, Ughwanogho et al showed that CT guided navigation of thoracic pedicle screws increases the accuracy of placement and significantly decreases the rate of screw removal compared with the conventional freehand technique. Using CT guided navigation, their rate of potentially unsafe placed thoracic pedicle screws in patients with adolescent idiopathic scoliosis was only 3%, an outstanding result; using non-navigated placement, their rate of misplaced screws was 9%. However, these results demonstrate that even highly experienced operators can improve their success rate with the aid of a guidance system for pedicle screw placement.25

In our experiment, we observed several problems that influenced the precision of K wire placement. Most of these issues were discussed and reported in our previous work.11 In contrast with the work of Schulz et al, we encountered stiff structures, which can redirect an inserted K wire. In a few cases, the deviation from the planned path was realized while drilling the K wire. To decrease this deviation in the future, we will use a drill to open the first cortical wall.

These reported results represent our first experience with Syngo iGuide and iSYS1 in spinal interventions. After a few successfully placed K wires in the lumbar spine, the time required per pedicle decreased and precision increased. However, in the upper thoracic spine, T2 and T3, the required time increased (figure 5).

Figure 5

Time required for transpedicular K wire placement per pedicle decreased during the experiment. The mean planning time was 4:16 min, the mean positioning time was 3:35 min, and the mean placement time was 2:22 min.

As a cadaver was used, there was no significant movement of the target during the intervention. For real patients, this procedure will be performed under general anesthesia. Due to a ventilation hold after preoxygenation, the respiration movement can be minimized during the 8 s rotation program and during the first step of placing the K wires into the pedicles.

Minimally invasive techniques are known to preserve soft tissue and decrease infection rates. Different manufacturers offer pedicle screws for a minimally invasive posterior approach. The combination of such pedicle screws and CBCT together with iSYS1 could provide a fast, safe, and efficient treatment of vertebral fractures.

Moreover, due to the variable needle guide adapter, different instruments can be used with iSYS1. This property allows for the potential use of iSYS1 in different types of spinal intervention, such as kyphoplasty or vertebroplasty. The high precision of iSYS1 could be a benefit in all interventions where cement will be applied. An optimal placement and unviolated cortical wall are important to decrease cement leakage.

## Conclusion

iSYS1 is a promising robotic device for use in various spinal interventions, decreasing the radiation exposure to the attending staff and increasing precision. However, further experiments, especially in vivo studies, are needed to establish this application in the clinical routine.

## Acknowledgments

iSYS1 was kindly provided by iSYS Medizintechnik GmbH, Kitzbuehel, Austria. The specimen was provided by Fachbereich Medizin der Goethe-Universität, Dr Senckenbergische Anatomie, Frankfurt am Main, Germany.

## Footnotes

• Contributors All authors participated in the conception and design of the article or revising it critically for important intellectual content, and final approval of the version to be published.

• Competing interests RS is an employee of iSYS Medizintechnik GmbH, Kitzbuehel, Austria. GK is involved in the development of iSYS1. MvR is an employee of Siemens AG, Healthcare Sector, Forchheim, Germany.

• Provenance and peer review Not commissioned; externally peer reviewed.

## Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.