Full Endoscopic Transforaminal Thoracic Discectomy Operative Technique

Abstract

Thoracic disc herniations are a degenerative pathology of the thoracic spine wherein a portion of nucleus pulposis herniates into the epidural space, potentially causing spinal cord or nerve root compression. Traditional surgical treatment for patients with thoracic disc herniations requires relatively invasive anterior or posterolateral approaches that involve extensive muscular dissection and removal of bone in order to access and remove the disc herniation without causing undue compression of the spinal cord. Full endoscopic thoracic discectomy is a minimally invasive technique which allows for the resection of thoracic disc herniations through a small (1 cm) incision, minimizing collateral tissue trauma and obviating the need for the extensive muscle dissection and bony removal required for traditional surgical approaches. In this article, we describe in detail the operative technique for full endoscopic thoracic discectomy and discuss the pearls and pitfalls of the technique. We also provide a review of the outcomes and complications as seen in the literature.

Introduction

Symptomatic thoracic disc herniations (TDHs) are a relatively rare (0.25%-0.75% of all symptomatic spinal disc herniations)¹ spinal pathology wherein a portion of the nucleus pulposis herniates through the annulus fibrosis of the intervertebral disc, causing spinal cord or nerve root compression. TDHs are most commonly seen at the T7/8, T8/9 and T11/12 levels². Patients with TDHs can present with back pain, thoracic radiculopathy and/or myelopathy².

Surgery for patients with TDHs traditionally involves a relatively invasive approach, which is tailored according to the location of the disc herniation within the spinal canal. Posterolateral approaches (transpedicular, lateral extracavitary, costotransversectomy)^3,4,5 are often preferred for paracentral disc herniations, while anterior approaches (transthoracic or retropleural)^6,7 approaches may be required for central disc herniations. These surgical approaches typically require a relatively large amount of muscle dissection and bony removal in order to access the disc herniation. Complication rates associated with these traditional approaches vary (7.1%-24%)^4,8,9 and can include neurological deterioration (2%-5%)¹⁰, durotomy/CSF leak, intercostal neuralgia, and pulmonary complications associated with anterior approaches.

Full endoscopic spine surgery is an ultra-minimally invasive technique for the treatment of spinal pathology that utilizes a small (<1 cm) incision and an endoscope to access the epidural space via the transforaminal or interlaminar route with a minimal amount of collateral tissue damage. Access via the transforaminal route requires only a small amount of the ventral, non-articulating portion of the superior articulating process to be removed. Outcomes of full endoscopic spine surgery for the treatment of lumbar disc herniations have been shown to be safe and effective¹¹. When utilized in the context of TDHs, full endoscopic spine surgery allows access to the ventral epidural space of the thoracic spine without the need for extensive soft tissue dissection and bony removal seen with traditional approaches. A number of small literature series have documented the safety and efficacy of full endoscopic spine surgery for the resection of thoracic disc herniations, many of which show that the procedure can be performed on an outpatient basis^12,13,14.

This study demonstrates the technique for full endoscopic thoracic discectomy and provides a review of the outcomes of this technique seen in the literature.

Protocol

The protocol follows the human care guidelines of the Houston Methodist Hospital Institutional Review Board. A single patient was included in the study. Inclusion criteria: patient aged >18 years of age with a thoracic disc herniation causing symptoms of myelopathy and/or radiculopathy, a patient without medical comorbidities prohibiting 3-4 h of general anesthesia or prone positioning. Informed consent was obtained from the patient prior to participation in the study.

1. Preoperative planning

1. Obtain a preoperative MRI to evaluate the morphology of the disc herniation.
2. Obtain a preoperative CT scan to evaluate for calcification of the disc herniation as well as full understanding of local spinal and rib anatomy. A CT will also be helpful for the identification of localizing anatomy at both the index and adjacent levels (i.e., adjacent level unique osteophytes).
3. If transitional anatomy is present or if the operative level cannot be safely and accurately localized with the preoperative MRI and CT, obtain preoperative whole spine X-rays (36-inch films) to evaluate for transitional anatomy and improve the accuracy of intraoperative localization.
   NOTE: The placement of an intrapedicular or intravertebral localization marker (e.g., metallic bead or coil) pre-operatively by interventional radiology may be considered to avoid wrong-level surgery.

2. Surgical details

1. If the procedure is to be performed under general anesthesia, utilize total intravenous anesthesia (TIVA) to facilitate neurophysiologic monitoring and, in cases of spinal cord compression and/or myelopathy, utilize invasive arterial blood pressure monitoring to maintain a mean arterial pressure >85.
   1. If the procedure is to be performed under local anesthesia with conscious sedation, neurophysiological monitoring and invasive arterial blood pressure monitoring are not required. Techniques for both local anesthesia with conscious sedation and general anesthesia administration for endoscopic spine surgery have been described in detail elsewhere¹⁵.
2. Place the patient prone on a radiolucent spinal surgery bed. For lesions above T7/8, tuck the patient's arms at the side to allow for lateral fluoroscopic imaging (see Table of Materials).
3. Sterilize/prepare the surgical area with a surgical prep and drape the surgical field with sterile drapes.
4. Localize the operative level with fluoroscopy and make a mark on the skin at the operative level.
   1. Using fluoroscopy in an anterior-posterior (AP) view, count up from the lowest rib for lower thoracic operative levels - or count down from the highest rib for higher thoracic operative levels - to localize the operative level.
5. Place a shallow subdermal needle at the caudal pedicle of interest to serve as a reference marker for the lateral portion of the targeting.
6. Draw a trajectory line on the AP view, targeting the area of interest.
   NOTE: The trajectory for the approach of a thoracic disc herniation typically projects from a cranial to caudal and lateral to medial direction in order to avoid the exiting nerve root within the neural foramen at the level of interest.
7. Mark the distance from the midline to the entry point for the endoscope, which is generally 6-8 cm off the midline in the thoracic spine.
8. Make a 1 cm incision at the skin entry point using a scalpel.
9. Using lateral view fluoroscopy, advance a targeting biopsy needle (see Table of Materials) toward the index facet joint. Due to the approach angle dorsal to the rib apex and the medial to lateral downward sloped facet joints in the thoracic spine, the biopsy needle will likely land on the lateral aspect of the inferior articulating facet.
10. Once the biopsy needle is docked on the facet joint at the index level, advance the needle into the facet joint several millimeters to ensure adequate engagement and securement.
11. Exchange the biopsy needle for a guidewire (see Table of Materials) and return the fluoroscopy to the AP view.
12. Under AP fluoroscopy, place sequential dilators over the guidewire and ream the lateral aspect of the facet joint (superior articulating process) to enlarge the neural foramen, taking care not to cross the medial pedicular line (i.e., the lateral boundary of the canal).
    NOTE: Sequential dilators are a set of three progressively large tubular retractors that dilate soft tissue surrounding the biopsy needle/K-wire to the caliber required for the introduction of an endoscopic cannula.
13. Once the reaming is complete, place the endoscopic cannula (inner diameter 6.5 mm; outer diameter 7.5 mm, see Table of Materials) over the largest dilator and advance the cannula down to the neural foramen.
14. Remove the guidewire and dilators, leaving the endoscopic cannula in place.
15. Insert the endoscope into the endoscopic cannula.
16. Using a 3.5 mm diamond burr on an endoscopic drill (see Table of Materials), drill away a portion of the remaining superior articulating process in order to allow for visualization into the lateral recess of the spinal canal.
17. Use the endoscopic dissector probe to identify the lateral recess, disc space, disc herniation, and thecal sac (Figure 1).
18. Remove any easily accessible free fragments of herniated disc material with grasping forceps.
19. For large and/or calcified disc herniations in particular, use the 3.5 mm diamond burr to drill a small cavity into the vertebral body cranial, caudal and ventral to the disc herniation (i.e., a limited corpectomy) to facilitate the manipulation of disc herniation fragments away from the spinal cord. Cut the annulus/posterior longitudinal ligament above and below the disc herniation in order to free the disc herniation prior to mobilizing and removing it.
    NOTE: As decompression progresses, the thecal sac will drop into view. The thecal sac will be seen to pulsate when adequate decompression is achieved. The endpoint of the surgery is when the thecal sac is visualized in anatomic configuration, pulsating.
20. After the disc herniation has been removed and the spinal cord is decompressed, withdraw the endoscopic cannula and close the 1 cm skin incision with deep dermal buried 3-0 monocryl sutures (see Table of Materials). Seal the skin closure with a dermal glue adhesive.

3. Postoperative care

NOTE: If the patient did not have severe neurological deficits/functional deficits pre-operatively and their pain is well controlled, they are discharged home the same day.

1. Discharge patients with Acetaminophen 300 - Codeine 30 (1-2 tablets to be taken every 4-6 h PRN pain) and methocarbamol 500 mg (1 tablet every 6-8 h PRN muscle spasms).
   NOTE: Patients with severe preoperative neurologic deficits may be evaluated for discharge to inpatient rehabilitation.
2. Instruct patients to keep the incision dry for one day, after which they can shower normally.
3. Instruct patients not to submerge the wound in water for at least 6 weeks.
4. Instruct patients to avoid strenuous activities and minimize bending, lifting greater than 10 pounds, or twisting of the torso for 6 weeks postoperatively.

Representative Results

A 74-year-old male with a history of type-2 diabetes and hypertension presented with 3-4 months of mid-back pain and lower back pain along with progressive weakness in his proximal bilateral lower extremities. On physical exam, the patient was noted to have 4/5 strength in his bilateral psoas muscles, as well as 3+ patellar reflexes. MRI and CT imaging demonstrated a large, calcified T11/12 disc herniation with spinal cord compression and myelomalacia (Figure 2). Due to the patient's progressive myelopathy symptoms and the degree of spinal cord compression seen on imaging, surgical intervention was recommended.

The patient underwent full endoscopic thoracic discectomy via a transforaminal approach. Operative time was 2 h 56 min. Estimated blood loss was minimal. The patient was neurologically stable postoperatively and was discharged home the same day. Over the course of the next several weeks, his proximal leg strength improved substantially. Postoperative MRI demonstrated near-complete resection of the T11/12 disc herniation with residual myelomalacia seen within the spinal cord at that level (Figure 3).

Currently, data regarding outcomes of full endoscopic transforaminal thoracic discectomy are drawn primarily from retrospective case series and case reports, but the results of these studies indicate that postoperative outcomes are largely favorable. Bae et al.¹² published a series describing the outcomes of 92 patients undergoing transforaminal full endoscopic thoracic discectomy under local anesthesia. All patients displayed a significant reduction in pain (Visual Analogue Scale, VAS) and disability (Oswestry Disability Scale, ODI) after surgery. Complications included neurological deterioration in 1 patient (1.1%), transient lower extremity parasthesias in 3 patients (3.3%), and recurrent disc herniations in 2 patients (2.2%). A series by Nie and Liu¹³ described the outcomes of 13 patients with thoracic disc herniations treated with transforaminal endoscopic discectomy. All patients experienced a substantial improvement in VAS and ODI postoperatively. One patient developed a positional headache postoperatively that resolved with an epidural blood patch and another patient developed a recurrent disc herniation at 8 months postoperatively. A series by Houra et al.¹⁴ described the long-term outcomes of 16 patients undergoing transforaminal endoscopic discectomy for thoracic disc herniations. The large majority of patients experienced substantial improvements in postoperative pain and disability scores, and no complications were reported.

Figure 1: Intraoperative fluoroscopy images in a patient undergoing transforaminal endoscopic thoracic discectomy. (A) AP and (B) lateral images demonstrating the initial trajectory of the endoscopic transforaminal approach. (C,D) AP images demonstrating the use of the ball-tipped dissector probe to orient the surgeon, with (C) demonstrating cranial extension of the probe into the ipsilateral lateral recess cranially and (D) demonstrating extension of the probe to the midline. Please click here to view a larger version of this figure.

Figure 2: Preoperative MRI and CT. Preoperative images of a 74-year-old male presenting with back pain and signs/symptoms of thoracic myelopathy. MRI Thoracic Spine T2-weighted images (A, sagittal; B, axial) demonstrate a large central disc herniation at T11/12, causing spinal cord compression. Preoperative CT images (C, sagittal, D, axial) demonstrate calcification within the disc herniation. Please click here to view a larger version of this figure.

Figure 3: Postoperative MRI. Postoperative MRI sagittal T2-weighted image of a 74-year-old male who underwent T11/12 endoscopic discectomy for a large, calcified disc herniation. Near-complete resection of the disc herniation is seen, with residual intrinsic T2 signal abnormality within the spinal cord (arrow). Please click here to view a larger version of this figure.

Discussion

Thoracic disc herniations, though relatively rare, can be a source of uncontrolled pain and neurological deficits that substantially impact patients' quality of life². Traditional surgical treatments of thoracic disc herniations are relatively invasive and associated with substantial postoperative morbidity^4,8,9. Full endoscopic transforaminal thoracic discectomy provides an ultra-minimally invasive method for the surgical treatment of TDHs, allowing for complete resection of thoracic discs through a 1 cm incision and with minimal associated muscle dissection or bony removal, minimizing postoperative pain and the need for prolonged hospital stays. Because full endoscopic spine surgery requires only minimal bony removal, there is typically no need for fusion or instrumentation in patients undergoing full endoscopic thoracic discectomy, as there may be after some traditional anterior or posterolateral approaches.

Patients with soft thoracic disc herniations in central, paracentral, foraminal, and/or extraforaminal locations, with or without cranial/caudal migration, are all candidates for full endoscopic transforaminal thoracic endoscopic discectomy. Partially calcified herniations or herniations with peripheral calcification, but still with largely soft contents on preoperative CT studies are also candidates for this approach. Densely calcified disc herniations represent a challenge for the full endoscopic technique and can result in incomplete/insufficient decompression, but they are not strictly contraindicated.

Localization of the operative level is key to any surgical intervention in the spine, and localization in the thoracic spine may be more challenging than in the cervical or lumbar spine. Keys to successful localization include (1) sufficient preoperative imaging demonstrating the level of the pathology in relation to the cervical and/or lumbar spine and evaluating for any variant/translational anatomy, (2) evaluation of preoperative imaging for unique anatomical features (e.g. nearby osteophytes, fractures) that can assist with localization during surgery, and (3) consideration of preoperative placement of a pedicle marker at the level of interest by a radiologist in cases wherein the risk of wrong-level surgery is considered relatively high. Additionally, a thorough understanding of the anatomy of the neural foramen is required for all transforaminal endoscopic spine surgeries, as visualization from an endoscopic approach is relatively limited compared with traditional open approaches, and visuospatial disorientation may lead to inadvertent compression of nerve roots or the spinal cord. When performing the initial targeting, it is important to utilize a trajectory that allows access to the ventral epidural space while also avoiding inadvertent breaching of the pleural cavity. Preoperative axial MRI images at the level of pathology may assist with the calculation of an initial approach trajectory. Finally, while some authors have described the use of local anesthesia for full endoscopic transforaminal thoracic discectomy¹², we prefer to use general anesthesia with full neuromonitoring and an arterial line, as unexpected patient movement during such a surgery poses risks of spinal cord compression.

The primary limitations of full endoscopic transforaminal thoracic discectomy are (1) the reliance of the technique on endoscopic equipment, which may not be at the disposal of every spine surgeon, and (2) the learning curve associated with full endoscopic spine surgery in general. Surgeons who are new to the practice of full endoscopic spine surgery may be best suited to avoid full endoscopic resection of thoracic disc herniations in their early endoscopic training. Calcified thoracic disc herniations may be considered another relative contraindication, as these are often adherent to the ventral dura, but they can nevertheless be removed safely in many cases, as described in the representative case above.

Materials

Name	Company	Catalog Number	Comments
#10 blade scalpel
3-0 Monocryl suture	Ethicon	Y427H
40 x 2 mm Guglielmi Detachable Coil	Boston Scientific/Target	M0013612040
C-arm/fluoroscope	GE Healthcare
Dermabond Topical Skin Adhesive	Ethicon	DNX6
Endoscope	Tessys, Joimax gmbh, Karlsruhe, Germany
Endoscopic Cannula	Tessys, Joimax gmbh, Karlsruhe, Germany
Endoscopic Drill/"Shrill"	Joimax gmbh, Karlsruhe, Germany
Endoscopic Irrigation Tubing	Joimax gmbh, Karlsruhe, Germany
Endoscopic Tower	Joimax gmbh, Karlsruhe, Germany
Guidewire/K-wire	Joimax gmbh, Karlsruhe, Germany
Jamshidi needle	Joimax gmbh, Karlsruhe, Germany		Biopsy needle
Lead apron
Normal saline
Radiolucent operating table	Mizuho OSI Jackson Modular Surgical Table
Surgical drapes	Joimax gmbh, Karlsruhe, Germany
Surgical prep
Tessys Endo-Flexprobe	Joimax gmbh, Karlsruhe, Germany	TEFP32020