Navigated Spinal Instrumentation Utilizing a “Hybrid” Technique to Compensate for Intraoperative Anatomic Changes: A Retrospective Review of a New Technique

Original Research: Hybrid O-C-O Technique for Lumbar Spinal Fusion

Abstract

Background
In an effort to reduce the risk of a malpositioned spinal implant, surgeons have increasingly adopted image-guided techniques for implant placement, both fluoroscopic and computerized tomography-based (CT-based). The radiation exposure to the patient of both techniques can be substantial, and each has its own limitations. Relevant to our work, CT-based techniques cannot provide real-time images and therefore cannot compensate for intraoperative anatomic changes.

Objective
After describing the O-arm, C-arm, O-arm (O-C-O) technique, this study also seeks to analyze the radiation exposure to the patient during this “hybrid” approach (hybrid O-C-O) in comparison with alternate image guided methods.

Methods
We performed an Institutional Review Board-approved (IRB), retrospective chart review of all patients who underwent a single level minimally invasive, transforaminal lumbar interbody fusion (TLIF) utilizing this technique over an 18-month period. Radiation exposure to the patient was reviewed as well as accuracy of implant placement.

Results
Thirty-three patients were identified based upon the study criteria. Mean surgical time was 162 minutes and estimated blood loss averaged 85 cc. Regarding the O-arm, the average dose length product (DLP) per scan was 49.33 mGycm delivering an average effective total radiation dose per patient of 0.78 mSv/scan. Average fluoroscopy time was 48 seconds/case; yielding 0.34 mSv/case. Of the 132 pedicle screws placed in the study population, 100% were defined as a Grade I based upon the 2 mm increment grading system.

Conclusion
The hybrid O-C-O technique is an effective, radiation-sparing method for surgeons to compensate for certain intraoperative anatomic changes that are routinely encountered during minimally invasive, image-guided spinal surgery.

Introduction

Background
Since the initial description of vertebral screw placement by King in 1944, spinal instrumentation has experienced a dramatic growth in surgeon adoption, implant development and technique advancement.1 Current generation spinal implants (eg, pedicle screws and interbody cages) play a critical role in the surgical management of spinal fractures, tumors, trauma, spinal deformity and degenerative conditions. Despite their important role in the surgical care of spinal disorders, the use of spinal implants carries a number of challenges, including: implant failure, infection, material allergy and implant malpositioning.

In an effort to mitigate the risk of implant malpositioning, and the subsequent problems of neural injury and poor fixation, spinal surgeons have increasingly adopted image-guided techniques for implant placement. Among the most popular techniques are fluoroscopic and CT-guided navigation. Both techniques have been shown to provide a high level of accuracy with regards to implant placement, with some data suggesting higher rates of accurate pedicle screw insertion with navigation-based insertion techniques.2,3 However, the radiation exposure of both image-guided techniques can be substantial, and each technique has its own unique limitations.

Fluoroscopy has unique limitations including the surgeon’s need to wear lead shielding, radiation exposure to the patient and treatment team, and the inability to provide mutliplanar images (particularly in the axial plane). CT-based techniques eliminate many of these issues but are unable to provide real-time images and cannot compensate for shifts in patient anatomy during surgery. This presents unique difficulties in cases where there is reduction of a spondylolisthesis or deformity, or significant change in disc height—not uncommon situations in spinal surgery.

Objective
To overcome the limitations of navigated spine surgery with intraoperative anatomic changes, we have adopted the “hybrid O-C-O” (O-arm, C-arm, O-arm) technique. This technique uses an initial CT image transferred to a navigation system, and then the selective use of a small number of fluoroscopic images to supplement the CT images during and after the changes in anatomy. This technique is an alternative to obtaining repeat navigation scans during the procedure, with a goal of minimizing radiation delivery to the patient while maintaining the benefit of 3D navigation throughout the procedure. After describing the implementation of the hybrid O-C-O technique in navigated, minimally invasive lumbar interbody fusion, this study also seeks to analyze the radiation exposure to the patient during this “hybrid O-C-O” approach in comparison with alternate image guided methods.

Methods

Study Design/Setting
We performed an IRB-approved, retrospective, cohort study. A chart review of all patients who underwent a single level minimally invasive TLIF utilizing the hybrid O-C-O technique over an 18-month period at a single center was performed. As patient data was deidentified, individual patient consent was not sought nor required by the IRB. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines were utilized during preparation of this manuscript. Patients were excluded from the study group that did not undergo a significant intraoperative anatomic change, defined as either a 50% or greater increase in disc height or reduction of a Grade I or higher spondylolisthesis.

Technique
The hybrid O-C-O technique is defined as follows: an initial intraoperative high-definition CT scan is obtained with the O-arm and transferred to the StealthStation™ Surgical Navigation System (Medtronic, Memphis, TN) for initial intraoperative guidance. Percutaneous pilot holes are fashioned in all four pedicles, but screws are implanted only on the side contralateral to the decompression and interbody fusion device placement. Once significant intraoperative modification has been made to the anatomy (eg, spondylolisthesis reduction or intradiscal height restoration), a fluoroscope is brought into the surgical field to help compensate for anatomical changes and, in combination with the navigation images, assist with remaining implant placement.

A navigated, straight thoracic pedicle probe is used to locate the previously tapped pedicle tracts using a combination of the data available from the navigation system and the lateral fluoroscope (Figure 1). The majority of patients experience anatomic change isolated to the sagittal plane.

intraoperative photos, surgeon demonstrates hybrid O-C-O techniqueFigure 1. Intraoperative photos (Left) demonstrate the equipment setup for a minimally invasive TLIF using the hybrid O-C-O technique; (Right) the surgeon probing for the pedicle tract with the navigated straight, thoracic probe. Images © Used with Permission, Joshua M. Ammerman, MD, and SpineUniverse.com.

As such, the medial-lateral location on axial views obtained via navigation remain highly accurate, but the superior-inferior location on sagittal navigation accuracy is lost due to alteration in disc height. The navigated thoracic pedicle probe is then used to locate the prior tract using the navigation data for medial-lateral location and using spot lateral fluoroscopic images for superior-inferior location (Figure 2).

intraoperative axial navigation image, lateral fluoroscopic imageFigure 2. (Left) Intraoperative axial navigation image and (Right) lateral fluoroscopic image. The surgeon is using the combined information from the 2 images to locate the previously created tract in the pedicle. Images © Used with Permission, Joshua M. Ammerman, MD, and SpineUniverse.com.

The thoracic probe also allows tactile confirmation of pedicle location. Once it is in place, the trajectory is then saved on the navigation system, and that newly saved trajectory is then used for screw placement (Figure 3). It is then our practice to obtain a final O-arm spin prior to skin closure to confirm appropriate implant position, however this is a step left to individual surgeon discretion.

intraoperative navigation: sagittal, pedicle tract, disc height restoration imagesFigure 3. Intraoperative sagittal navigation images demonstrate (Left) the pedicle probe in the pedicle tract prior to disc height restoration, (Middle) after disc height restoration and (Right) comparing the relative change in the tract on navigation. Images © Used with Permission, Joshua M. Ammerman, MD, and SpineUniverse.com.

Measurement of Radiation Delivery
In all cases, O-arm images were obtained utilizing the high-definition 3D setting, in combination with a 20 cm field of view. Effective patient dose, for CT/O-arm, was obtained by multiplying the DLP reported by the device by the tissue weighting factor for the anatomic region of interest (k-space), approximately 0.015 for the lumbar region.4 The tissue weighting factor is a relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue. It accounts for the variable ratio sensitivities of organs and tissues in the body to ionizing to radiation.

For all patients undergoing fluoroscopy, the time (reported in seconds) of fluoroscopy was recorded. In an effort to understand the relative radiation exposure to the patient when comparing the O-arm to fluoroscopy; the manufacturer’s dosimetry data was reviewed, stratified by patient size, with consideration given to dose both at the surface and at the center of the body (Table 1).5

Table 1: Entrance and Center Dose Per Minute:SecondTable 1. Entrance and center dose per minute:second (min:sec) relative to mGy per O-arm spin stratified by patient size. Note: Values are based on a 20 cm field of view (FoV) and a high-density 3D (HD3D™) scan setting for the O-arm. Table © Used with Permission, Joshua M. Ammerman, MD, and SpineUniverse.com.

Assessment of Implant Placement Accuracy
The 2 mm increment grading system as described by Raley and Mobbs was used to assess the accuracy of pedicle screw placement when reviewing post-insertion O-arm images.6

Results

Descriptive Data (Patient and Surgical Characteristics)
Thirty-three patients were identified based upon the study criteria (n=15 male, n=18 female; mean age 71 (range 43-84, SD+11.8). The index surgical level was L5-S1 in 15 patients (45%), L4-L5 in 15 (45%), L3-L4 in 3 (10%). Mean surgical time was 162 minutes (range 100–240, SD+37.7) and estimated blood loss averaged 85 cc (20-250, +56). No surgical site infections were identified in the study population. In all patients, 4 percutaneous pedicle screws and 2 connecting rods were implanted along with a single “bullet” shaped PEEK (polyetheretherketone) interbody fusion cage device.

Radiation
With regards to the O-arm, the average DLP per scan in the study was 49.33 mGycm (range 30.9-60.17, SD+13.5) delivering an average effective total radiation dose per patient of 1.55 mSv/case (range 0.66-2.63, SD+0.4) or 0.78 mSv/scan.

Average fluoroscopy time was 48 seconds/case (range 0-154.9, SD+50.1) yielding 0.34 mSv/case. Individual patient radiation exposure can be found in Table 2.

Table 2: Individual Patient Radiation ExposureTable 2. Individual, per case, patient DLP values for each O-arm scan along with total effective dose from O-arm scans and amount of fluoroscopy time. Table © Used with Permission, Joshua M. Ammerman, MD, and SpineUniverse.com.

Pedicle Screw Accuracy
In all cases, a second O-arm spin was obtained prior to removal of the screw towers and skin closure. Of the 132 percutaneous pedicle screws placed in the study population, 100% were defined as a Grade l based upon the 2 mm increment grading system. All interbody cages were placed within in the disc space without any posterior protrusion into the spinal canal. No patient was returned to the operating room for repositioning of either a pedicle screw or an interbody device.

Discussion

Minimally invasive spine surgery techniques have progressed with associated improvements in patient outcomes when compared to open procedures.7,8 However, minimally invasive spine techniques require image-guidance for surgeon orientation, as opposed to anatomic landmarks visible in traditional open surgery. The progression from fluoroscopic-guided spinal surgery to CT-based navigation-guided spinal surgery has repeatedly demonstrated high rates of accuracy for implant placement in conjunction with reduced surgical time.9-12 While radiation exposure to the surgical team has been significantly reduced with these techniques, radiation exposure to the patient remains a topic of discussion and concern. Furthermore, CT-based image-guided spine technology currently is not able to account for intraoperative changes in patient anatomy.

The addition of limited fluoroscopy to a navigated procedure permits the surgeon to gain the advantages of spinal navigation while retaining the ability to assess whether the desired degree of anatomic change has been achieved during surgery (eg, sagittal plane correction or spondylolisthesis reduction). In addition, this permits the surgeon to compensate for anatomic spinal changes, particularly in the sagittal plane, to permit additional implant placement accurately.

Interpretation

With CT-based navigation, a simple solution to this limitation is to obtain a new scan whenever intraoperative anatomy changes. However, each scan is associated with a not-insignificant radiation delivery—in our study, an average of 0.78 mSv per scan. The impetus of this technique and analysis was to attempt to avoid a third intraprocedural scan via the addition of limited fluoroscopy. Based upon the work of O’Donnell et al, one second of fluoroscopic time during lumbar spinal fusion generates an approximate effective radiation dose of .007 mSv.13 Therefore, if we compare the average total radiation delivery in this study’s hybrid O-C-O technique (1.89 mSv) to that of 3 O-arm spins without fluoroscopy (2.3 mSv), the hybrid O-C-O technique saves 0.41 mSv/case (18%).

The analysis of the radiation data yields a comparison of 110 seconds of fluoroscopy being approximately equivalent (in terms of effective patient radiation exposure) to a single O-arm scan. This is a worthwhile conversion to keep in mind, as—if the surgeon is able to complete instrumentation placement with fluoroscopy under this amount—it is a benefit to the patient in terms of spared radiation exposure. If the surgeon anticipates that it will take more fluoroscopic time than this to complete the surgical maneuver, then proceeding with an additional O-arm scan would be appropriate.

As previously mentioned, some surgeons do not opt to obtain a final O-arm spin to confirm implant location. While we do obtain a final spin as standard protocol to avoid leaving the operating room with any malpositioned implants, this technique could be further modified to not include the final O-arm spin, which would further decrease patient radiation while similarly benefiting the patient by avoiding the intraprocedural O-arm spin required to adjust for anatomic changes.

Study Limitations/Generalizability

All O-arm scans in this study were performed in the “high definition setting,” (HD) as opposed to the “standard definition” (SD) setting. Utilizing the SD setting would result in a smaller amount of “equivalent” fluoroscopy time involved in the calculus of whether to obtain an additional O-arm scan or to proceed with fluoroscopic guidance. Further, when considering patient radiation exposure, it is important to understand that fluoroscopy delivers the greatest dose to the patient’s surface and hence, the radiation risk is highest at the skin entry. This is in contrast to CT imaging, where each beam is focused at the target center, passing through a different surface location and hence, the radiation risk is highest at the target.

This was a patient-centric study, in terms of the analysis of radiation exposure, without consideration given to the radiation exposure to the treatment team. Our practice and the practice of many other treatment teams is to leave the operating room and move to a substerile space during O-arm image acquisition, which essentially reduces surgical team radiation exposure to zero. The radiation exposure to the treatment team is clearly higher when any fluoroscopy is incorporated into the procedure, such as in our hybrid O-C-O technique.

As previously discussed, this technique works well in patients with intraoperative anatomic changes in the sagittal plane. For procedures where there is more complex intraoperative anatomic change (ie, derotation or reduction of lateral listhesis), navigation based axial locations would no longer be accurate and an alternative technique would be required.

Conclusions

The hybrid O-C-O technique is an effective method for surgeons to compensate for certain intraoperative anatomic changes that are routinely encountered in minimally invasive, image-guided spinal surgery. The hybrid O-C-O technique allows accurate instrumentation placement, while attempting to reduce radiation delivery to the patient associated with additional navigation scans.

Disclosures
Drs. Ammerman and Wind are paid consultants for Medtronic. Mr. Nguyen has nothing to disclose.

Funding
None of the authors, nor the study itself received any funding for the performance of this study.

Updated on: 11/26/19
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