Cone Beam Intraoperative Computed Tomography-based Image Guidance for Minimally Invasive Transforaminal Interbody Fusion

Summary

The purpose of this article is to provide image-guidance for minimally invasive transforaminal interbody fusion.

Abstract

Transforaminal lumbar interbody fusion (TLIF) is commonly used for the treatment of spinal stenosis, degenerative disc disease, and spondylolisthesis. Minimally invasive surgery (MIS) approaches have been applied to this technique with an associated decrease in estimated blood loss (EBL), length of hospital stay, and infection rates, while preserving outcomes with traditional open surgery. Previous MIS TLIF techniques involve significant fluoroscopy that subjects the patient, surgeon, and operating room staff to non-trivial levels of radiation exposure, particularly for complex multi-level procedures. We present a technique that utilizes an intraoperative computed tomography (CT) scan to aid in placement of pedicle screws, followed by traditional fluoroscopy for confirmation of cage placement. Patients are positioned in the standard fashion and a reference arc is placed in the posterior superior iliac spine (PSIS) followed by intraoperative CT scan. This allows for image-guidance-based placement of pedicle screws through a one-inch skin incision on each side. Unlike traditional MIS-TLIF that requires significant fluoroscopic imaging during this stage, the operation can now be performed without any additional radiation exposure to the patient or operating room staff. After completion of the facetectomy and discectomy, final TLIF cage placement is confirmed with fluoroscopy. This technique has the potential to decrease operative time and minimize total radiation exposure.

Introduction

The TLIF is one of several options available when considering interbody fusion for degenerative disc disease and spondylolisthesis. The TLIF technique was initially developed in response to complications associated with the more traditional posterior lumbar interbody fusion (PLIF) approach. More specifically, the TLIF minimized retraction of neural elements, thereby reducing the risk of nerve root injury as well as the risk of dural tears, which can lead to persistent cerebrospinal fluid leak. As a unilateral approach, the TLIF technique also affords better preservation of the normal anatomy of the posterior elements¹. The TLIF can be performed either open (O-TLIF) or minimally invasive (MIS-TLIF), and MIS-TLIF has proven to be a versatile and popular treatment for lumbar degenerative disease and spondylolisthesis^2,3,4. Compared to the O-TLIF, the MIS-TLIF has been associated with decreased blood loss, shorter hospital stay, and less narcotic use; patient-reported and radiographic outcome measures are also similar between open and MIS approaches, thus suggesting the MIS-TLIF is an equally effective but potentially less morbid procedure^{5,6,7,8,9,10,11}.

However, a frequent limitation of the traditional MIS technique is the heavy reliance on fluoroscopy which exposes the patient, surgeon, and operating room staff to non-trivial radiation doses and fluoroscopy time ranging from 46-147 s¹². More recently, however, the use of intraoperative CT-guided navigation has been studied, with several different systems available and described in the literature including the O-arm/STEALTH, Airo Mobile, and Stryker Spinal Navigation Systems.^13,14 This type of navigated technique has been shown to result in accurate pedicle screw placement while also minimizing the radiation risk to the surgeon^{15,16,17,18,19}. In this article, we present a novel technique for MIS-TLIF that utilizes image-guidance-based pedicle screw placement followed by cage and rod placement with traditional fluoroscopy. This strategy has the potential to increase the speed and accuracy of the pedicle screw placement while minimizing the radiation exposure to both the patient and operating room staff.

Protocol

All procedures and research activities were performed with institutional review board approval (CHR #17-21909).

1. Pre-operative Preparation

1. Induce general anesthesia in the patient, and position the patient prone on the Jackson table with chest bolster and hip pads.
2. Prep and drape the patient's back in the usual sterile fashion.

2. Surgical Procedure

1. Make a small stab incision using a #15-blade over the PSIS contralateral to the side of the planned TLIF.
2. Place a biopsy needle through the stab incision into the ilium to harvest bone marrow aspirate (Figure 1A). Drive the navigation reference frame into the PSIS in a trajectory that places the reference arc inferior and medial, thereby avoiding interference with the standard trajectory of an S1 pedicle screw (Figure 1B).
3. Cover the wound with a sterile drape with the reference arc exposed and perform an intraoperative CT scan.
4. Plan pedicle screw trajectories using the navigation system (Figure 1C); they are generally 3.5 cm lateral to the midline through a one-inch incision on each side for single level fusion (1.5 inch for two levels, and 1.75 inch for three levels).
5. Use a navigated drill guide and 2-3 mm bit and high-speed drill to cannulate the pedicles and utilize K-wires to mark these trajectories.
6. Place the cannulated pedicle screws with reduction towers over the k-wires on the side opposite the TLIF.
7. Determine the trajectory along the disc space using the first tubular dilator which is oriented using the navigation system (Figure 1D). Place additional dilators followed by the TLIF retractor, which is connected to a self-retaining arm mounted to the bed.
8. Confirm the retractor positioning via navigation.
9. Perform the laminotomy, flavectomy, and facetectomy in standard fashion under the microscope.
   1. Use a high-speed drill to perform the laminotomy and facetectomy; if just a laminotomy is desired, avoid drilling into the facet joint in order to preserve the structural integrity of the posterior column.
   2. Ensure that the lateral border of the laminotomy is the medial aspect of the facet joint, while the medial border of the laminotomy should be the medial edge of the lamina. Utilize a Woodson elevator to dissect the ligamentum flavum off the dura. Once this is achieved, use a 2 or 3 mm Kerrison rongeur to remove the ligamentum flavum.
      NOTE: Navigation allows for maximal safe decompression without violation of the pedicle (Figure 1D, E).
10. If contralateral decompression is needed, angle the retractor across the midline and remove the underside of the contralateral lamina, ligamentum flavum, and hypertrophic facet capsule using a 2 or 3 mm Kerrison rongeur.
11. Use the navigation again to identify the trajectory along the disc space to facilitate a safe and thorough discectomy.
12. Prepare the disc space with shavers and distractors.
13. Upon completing the discectomy, use intermittent fluoroscopy to visualize the degree of distraction required during the interbody cage trial placement to ensure preservation of the endplates (Figure 2A).
14. Mix the allograft cellular bone matrix with the autologous bone marrow aspirate harvested at the beginning of the operation and carefully pack it into the disc space.
15. Insert the interbody cage (polyetheretherketone [PEEK]), and confirm its position via lateral and anterio-posterior (AP) fluoroscopy (Figure 2B).
16. Once TLIF has been completed, place the remaining pedicle screws.
17. Carefully drive a pre-bent rod through the screw heads below the dorsal lumbar fascia. Use periodic fluoroscopy to confirm adequate rod length.
18. Gently compress the rods to induce lordosis before securing them with locking set screws.
19. Obtain a final fluoroscopy prior to closure.
20. Close the thoracodorsal fascia with an 0 polyglactin 910 suture, close the subcutaneous tissue with 3-0 polyglactin 910, and approximate the skin edges with skin closure strips. Apply a water tight dressing.

3. Post-surgical Care

1. Ambulate patients on postoperative day 1 with a soft lumbar brace, and obtain standing 36-inch X-rays prior to discharge (Figure 2C).
2. Provide patients a patient-controlled analgesia (PCA) pump with morphine or hydromorphone overnight and ambulate on postoperative day 1.
3. Transition patients to oral pain medications on the first day and discharge on postoperative day 2-3 with follow-up in 6 weeks.

Results

Fifty patients underwent surgery with this technique under a single surgeon (AC). The average age was 53 years (range 29-84 years) with 30 women and 20 men. Patients presented with the following pathology: spinal stenosis (n=45), spondylolisthesis (n=29), facet cysts (n=5), degenerative scoliosis (n=3), and cauda equina syndrome (n=1). Symptoms were back and leg pain in 42 cases, back pain alone in 2 cases, and lower extremity radiculopathy in 6 cases. In 10 cases, patients had undergone ...

Discussion

There are several critical steps to the procedure described. The first critical step is the process of registration. The reference arc must be placed in solid bone and should be oriented appropriately to avoid interfering with the S1 pedicle screw placement if needed. The second critical step is maintaining accuracy of the navigation after an intraoperative CT scan is performed, which can be done by identifying normal anatomic structures and confirming the correct positioning. The accuracy should be periodically verified...

Materials

Name	Company	Catalog Number	Comments
O-arm intraoperative CT	Medtronic, Minneapolis, MN
Stealth Navigation System	Medtronic, Minneapolis, MN
Jamshidi Needles			for bone marrow biopsy
Cefazolin			antibiotic.
Vicryl Sutures
Steri-Strips			for skin closure
Telfa dressing
Tegaderm			for dressing
Jackson table
15-blade
High-speed bone drill
Tubular dilator
K-wires
Reduction towers
TLIF retractor
2 or 3 mm Kerrison rongeur
Woodson elevator
Disc shaver and distractor
Fluoroscopy
Allograft cellular bone matrix
Interbody cage
Rod
Soft lumbar brace
X-ray
Patient-controlled analgesia pump