This work presents an analysis and comparison of the efficacy of two methods for pedicle screw placement during posterior spinal fusion surgery. A total of 100 screws (64 manual and 36 power driven), all placed utilizing a surgical navigation system, were analyzed and compared. Final screw placement was compared to initial surgical plans using the navigation system, and the final screw locations were analyzed on the basis of angular deviation from these planned trajectories as well as screw translation within a critical reference plane. The power driver was found to insignificantly decrease the resulting angular deviation of these pedicle screws with a mean deviation of 3.35 degrees compared to 3.44 degrees with the manual driver (
Spinal fusion procedures are used to treat a variety of spinal conditions, including injuries to the vertebrae, spinal instability due to infections or tumors, and abnormal spinal curvature. Spinal fusions with instrumentation are typically able to relieve pain in the lower back and lower extremities and correct abnormal spinal geometry [
Posterior spinal fusion (PSF) is a procedure in which bone growth is promoted between two or more vertebrae, eventually joining them into a single bone. This procedure may be performed for a variety of reasons, including damage to the spine such as degenerative changes, fracture or damage to an intervertebral disc, spinal deformations such as scoliosis or kyphosis, spinal instability due to infection or tumor removal, or relief of pain due to abnormal bone growth as in spinal stenosis [
During a PSF procedure, pedicle screws are inserted into the vertebra pedicle (Figure
Transverse cross section of vertebra. In spinal fusion procedures, pedicle screws are inserted through the pedicle and into the vertebral body.
Pedicle screws and support rods are shown in this image. Also note the location of the bone graft between vertebrae.
Due in part to the critical nature of the spine and the nervous tissue protected by the spinal column, PSF procedures involve a range of risks, including the potential for nervous tissue damage. In particular, complications may occur due to inaccurate pedicle screw placement that results in a medial breach or perforation of the pedicle. In addition, poor screw placement may allow for premature loosening of screws, potentially resulting in a failure to fuse. For these reasons, significant emphasis is given to the careful training of surgeons in the correct placement of pedicle screws. Expert knowledge of the spinal anatomy and relevant anatomical landmarks in tandem with guided training and experience is paramount for surgeons to accurately place pedicle screws.
While accurate pedicle screw placement can be achieved by skilled surgeons without the use of imaging or navigation tools [
Even with the use of navigation aids for pedicle screw placement, spinal fusion procedures still pose significant technical challenges to surgeons and require intense physical effort from the surgeon throughout the procedure. In addition, as prolonged operative time has been shown to increase the risk of infection and other complications for many minimally invasive procedures, a reduction of operative time stands to significantly benefit patients [
Given the importance of minimizing operative time as well as overall surgeon effort during spinal fusion procedures, the use of power-assisted drivers is being adopted. The power driver used in this study has been designed to decrease surgeon fatigue during screw insertion. In addition, according to the manufacturer, this driver may offer significantly reduced procedure times, demonstrating a 51% reduction in the time required for the tapping of the pedicle and a 55% reduction in the time required for inserting pedicle screws as described in a study conducted by the manufacturer [
While the development of the power driver may aid in the advancement of PSF, to the authors’ knowledge, no comparative accuracy study between a power and manual driver has been conducted. This work aims to provide quantitative data comparing the two methods of pedicle screw insertion.
The study methodology is divided into five sections. The first section briefly describes the study design. The second section describes the collection of data using the StealthStation and O-arm imaging system (O-arm Surgical Imaging, Medtronic Inc., Dublin, IE) for both manual and power procedures. These systems are used for all patients throughout this study and use intraoperative 3D scans and optical tracking to provide pseudo-real-time feedback to the surgeon with regard to tool position relative to patient anatomy. The third section describes the postprocessing steps that were taken after the procedure to determine the final screw placement. Finally, the last two sections detail the two metrics used to assess procedure performance: angular deviation and reference point deviation. All research was conducted in accordance with a test protocol approved by the University of Colorado Anschutz Medical Campus’s Institutional Review Board (COMIRB Protocol 11-0990).
After local IRB approval, a consecutive series of patients undergoing posterior spinal fusion were prospectively recruited, and those agreeing to participate were consented for inclusion in the study. Patients meeting the following inclusion criteria were eligible for enrollment: (1) aged 18–75, (2) scheduled to undergo thoracic and/or lumbar posterior spinal fusion procedures with pedicle screw placement using O-arm and StealthStation at the University of Colorado Hospital, and (3) planned fusion involved at least 2 motion segments. Exclusion criteria were (1) pregnancy and (2) revision surgeries with previous pedicle screw instrumentation. All procedures were performed by two surgeons with more than 10 years of clinical practice, including regular use of navigation during pedicle screw placement. The power driver was available to the surgeons for a limited time during the study, so all subjects enrolled during that time period had screws placed with the power driver. Patients enrolled outside of that time frame had pedicle screws placed with a manual driver.
Once general anesthesia was administered to the patient, access to surgical space was achieved via open incision. A reference arc is rigidly attached to one of the patient’s vertebra (typically on the spinous process of one of the vertebrae to be included in the fusion) and the imaging system is moved into position to take the first 3D scan with the 3D camera system in place. This scan captures the spinal region to be fused and locates the reference arc with respect to the patient’s spine (Figure
Schematic of reference arc attached to patient. Reflective optical markers are appended to each tool to facilitate localization by the navigation system.
Following the reference arc attachment and initial 3D scan, tool positions and orientations are tracked via the surgical navigation system and overlaid onto the initial 3D scan. This visualization allows for pseudo-real-time navigation of the tools with respect to the patient’s anatomy with an accuracy of approximately 1 to 1.4 mm [
Before each screw is inserted, several initial steps are taken by the surgeon. First, a surgical awl is inserted into the pedicle. The awl, which is tracked via the navigation system, is positioned with its tip at a depth within the pedicle isthmus which the surgeon believes most susceptible to a medial breach. This initial “Entry” trajectory depth position is saved by the technician operating the navigation system (Figure
Screenshot taken from the StealthStation navigation system. The display that a surgeon has access to during a procedure is shown. The location of each vertebra, the screw trajectory coordinates, and the anatomical planes are marked in this figure, but are not during the procedure. This image was obtained from the workstation’s spinal software.
With these initial steps complete, and the Entry and Plan trajectories saved, the surgical awl is removed and screw insertion can begin (in rare instances of hard bone, nonnavigated tapping, up to a max depth of 20 mm was also performed). For each patient, all screws are inserted with either a manual- or power-assisted driver, with the surgeon using the same type of driver for all screws in a given patient. Both tools include passive reflective markers so they can be optically tracked by the navigation system throughout the insertion. When the screw is tightened into its final position, the trajectory of the driver is saved in the navigation system as the “Screw” trajectory. These steps, including the insertion of the awl and screw insertion, are then repeated for all subsequent screws.
After all screws have been inserted, a second 3D scan is taken which serves to capture the final locations of all screws relative to the reference arc. Once this scan has been completed and saved, the reference arc is removed. In total, 64 manually driven and 36 power-driven screws were inserted in 11 patients throughout this study.
Following each spinal fusion procedure, O-arm scans were examined to assess final screw placement. To do this, the two 3D scans (the first of the Plan and Screw trajectories and the second which shows the final screw positions) must be carefully aligned and merged using the second scan as the reference image. An initial merge of the scans is done automatically via StealthStation’s Cranial Suite; however, this merge is not accurate enough to allow for good analysis, and it is not possible to export these images to 3rd party software for custom merging. Thus, a final, manual merge is completed by a single user trained by Medtronic in the correct operation of the StealthStation. This user must visually align the two scans until no discernable difference exists in any of the three imaging planes (coronal, sagittal, and transverse). Several tools, including zoom, color, and transparency adjustment, are built into the Cranial Suite to allow for the effective and accurate alignment and merging of the two scans. While all postprocessing was performed by a single user, no statistically significant difference was seen when comparing this primary user to a trained, secondary user.
Once the scans have been merged, the final position of each screw must be drawn (Figure
Actual trajectories obtained from the StealthStation (represented by the colored, dashed lines), which were drawn in the software by the workstation user. (a) shows the coronal frame, (b) shows the sagittal frame, and (c) shows the transverse frame. This image was obtained from the workstation’s Cranial Suite software.
When all data have been exported from the navigation system, they are imported into MATLAB (MathWorks, Natick, MA) for analysis. The Actual trajectory’s entry and terminal points are multiplied by the transformation matrix corresponding to the merge, and all trajectories are then drawn and visualized in MATLAB to correlate imported data to StealthStation data and as a visual verification to ensure no unexpected errors occurred while drawing trajectories or exporting the data. At this point, all data are ready for final comparison and analysis.
A common metric used to assess the success of pedicle screw placement is the angular deviation between the final screw location (Actual trajectory) and the intended screw location (Plan trajectory). Given the assumption that the Plan trajectory and the Actual trajectory should have nearly identical entry locations, it is expected that translational deviations between trajectories will be very small. Thus, small angular deviations between these trajectories should be indicative of precise screw placement. As the initial hole created by the awl serves as the entry point for the screw, this assumption should be valid in most cases. It should also be noted that while the above does make the assumption that the vectors share a common origin, this does not affect the calculated angular deviation.
Once the Actual trajectory has been transformed into the Plan/Screw trajectory frame, the angular deviation between any two of the three trajectories can be calculated using (
In addition to calculating the angular deviation between the Actual and Plan trajectories, angular deviations were also calculated between the Actual and Screw trajectories as well as the Screw and Plan trajectories.
While the angular deviation calculation is a commonly used calculation for determining accurate screw placement, this calculation does not assess any translational differences that may occur between trajectories. More importantly, while angular deviation provides a useful metric for determining overall alignment of pedicle screws, it does not necessarily indicate how close these screws came to breaching the pedicle isthmus. Because a pedicle breach (especially a medial pedicle breach) represents a key failure criterion when assessing pedicle screw placement and because any movement of the patient or reference arc may result in unintended translational errors, an additional metric was employed to compare screw placement within a critical region of the pedicle isthmus (Figure
The critical region is determined via the Entry trajectory that is recorded initially as the awl is pressed into the pedicle isthmus. This trajectory ends with the tip of the awl at a point which we will refer to as the critical depth (CD) within the isthmus. This point does not necessarily represent the center of the isthmus, but is estimated to be close to the depth within the isthmus where the risk of perforation is the highest.
Given point CD, the closest point along the line defined by the Plan trajectory vector to CD can be obtained. This new point is used as the reference point (RP), and points RP and CD can then be used to define a plane within the pedicle which we will refer to as the critical region (Figure
Visualization for determining the reference point deviation (
As shown in (
Once point RP has been determined along the Plan trajectory, it is then used to compare with the Screw and Actual trajectories (Figure
A screenshot taken from the StealthStation that illustrates the Entry vector (green) and Plan vector (red). Vectors have been outlined and center lines have been added to more clearly show actual deviations. The zoomed section shows the reference point (RP) as measured from the original terminal point of the entry vector.
In addition, because the Screw trajectory is also navigated, the final reference point deviation of the Actual trajectory from this additional reference point was also considered. While this trajectory is not shown in Figure
The distance from the marked reference point along the Plan trajectory to the Actual trajectory (i.e., the Euclidean distance) is calculated according to
All computations were performed in MATLAB with reference points, trajectories, and distances drawn graphically to provide visual verification.
Results are divided into two sections. The results of the angular deviation analysis are presented, followed by the results of the reference point analysis. It should be noted that unequal sample sizes between the two groups and skewed data distributions were anticipated (and observed) in the results of this study. For this reason, a permutation method was implemented to determine the
Angular deviations for all screws were calculated across both the manual and power groups. These results are shown in Figure
Angular deviations are shown for manual and power drivers. Median lines are shown within boxes; hash marks represent the outer quartile range. Outliers are shown in red. Mean values for the manual driver are 3.44, 2.13, and 2.94 degrees from left to right. Mean values for the power driver are 3.35, 2.08, and 2.88 degrees from left to right.
Reference point deviations for all screws were calculated for both the manual and power groups. The results of this comparison are shown in Figure
Reference point deviations are shown for manual and power drivers. Median lines are shown within boxes; hash marks represent the outer quartile range. Outliers are shown in red. Mean values for the manual driver are 1.54, 2.30, and 1.94 mm from left to right. Mean values for the power driver are 2.45, 3.05, and 2.26 mm from left to right.
The results of this study suggest that there is some small performance lost via use of the power driver relative to the manual pedicle screw driver. While angular deviations from the Plan trajectory were statistically insignificant with means in favor of the power driver, the statistically significant translational errors in the critical region indicate that the power driver was less able to follow the Plan trajectory accurately through the pedicle isthmus, potentially increasing the risk of perforation in this region. However, in this study, no perforations were observed.
This study presents a novel metric for assessing pedicle screw placement performance, utilizing a reference point within the pedicle to compare trajectories in 3D. While previous work has relied on the more simplistic review of X-rays and CT scans, to our knowledge, no previous work has attempted to use a metric such as reference point deviation in assessing screw placement.
The conclusions drawn from this study are based entirely on the metrics presented here. It should be noted however that this paper is not intended to present a risk–benefit analysis of the manual versus power drivers but rather to provide an analysis of two important performance metrics: angular deviation and reference point deviation.
While the reference point deviation metric has, to our knowledge, not been used previously, the angular deviation metric has been used in other studies to access the accuracy of pedicle screw placement using 3D navigation [
One method which is often cited to access successful screw placement is the Gertzbein-Robins criterion, which defines success based on the distance in mm by which a screw breaches the pedicle [
Many factors may influence whether a surgeon chooses to use a particular tool, and as different tools may entail different risks and benefits, it is important to recognize the limitations of these tools with respect to specific patients and procedures. As noted previously, the power driver used in this study has a number of potential advantages that may offset some of the performance risks noted here. In particular, the projected, decrease in procedure time may offer overwhelming benefits to both the patient and the surgeon. The decreased risk of infection that often stems from shorter, minimally invasive procedures may, in effect, be a much more important factor than any observed decrease in critical region accuracy.
In addition, the long-term impact of this driver on the surgeon is an important factor to consider. Manual tools used in minimally invasive procedures often require intense physical exertion from the surgeon over long periods of time. This exertion can result in localized muscle fatigue in the short term, potentially impeding a surgeon’s ability to perform procedures quickly and accurately. Perhaps most importantly, repeated exertion at the level required by these tools and procedures may lead to repetitive stress injuries to surgeons over time, further impeding their endurance in the operating room.
Improved ergonomics and decreased insertion time may thus offer significant benefits to the surgeon and patient; however, length of procedure and surgeon fatigue were in no way evaluated in this study. To our knowledge, no independently conducted study currently exists demonstrating these impacts with the power driver used here. Thus, despite the results of this study, further work is needed to determine whether the overall performance of the power driver significantly impacts patient outcomes for better or worse.
The authors declare that they have no conflicts of interest.