Can Thoracic Disc Herniation Be Effectively Treated Using Minimally Invasive Techniques?

11 Can Thoracic Disc Herniation Be Effectively Treated Using Minimally Invasive Techniques?

MIS: Paul W. Millhouse, Troy I. Mounts, and Kristen E. Radcliff
Open: Alexander A. Theologis and Vedat Deviren

11.1 Introduction

The diagnosis and treatment of thoracic disc herniations (TDHs) has always been challenging.1 Although asymptomatic TDHs are quite common,^1,2 the incidence of symptomatic TDHs is low, accounting for 0.1 to 4% of all surgically treated disc herniations.^2,3,4 Herniated thoracic discs present equally in both sexes and can occur medially, laterally, or centrolaterally. The symptoms vary widely and include axial pain, radicular pain, and/or myelopathy. This protean presentation contributes to delays in diagnosis.⁴

The thoracic spine lies in close proximity to the thoracic visceral organs and articulates with the rib cage through costovertebral joints. The thoracic spinal cord is unique due to the watershed blood supply, the limited space available in the spinal canal, and the kyphotic alignment.⁵ Thus, TDHs pose unique technical challenges for surgeons. Therefore, operative intervention for thoracic spine pathology is typically reserved for patients with debilitating symptoms refractory to conservative treatment.⁵ The role of surgery for localized or axial pain in the presence of a TDH remains controversial because the herniation may be an incidental finding.⁶ However, for a symptomatic herniated disc, involving neurologic deficit or intractable pain not responsive to conservative measures, surgery is regarded as the treatment of choice to prevent the sequelae of compressive myelopathy.⁷

Given the anatomical considerations and variations in pathology and patient considerations, a plethora of surgical approaches and techniques have been developed to treat TDH.^1,5 A variety of anterior and posterolateral procedures remain in use today. Traditional anterior thoracotomy exposures provide direct visualization of the disc space but risk injury to the intrathoracic elements. Posterolateral approaches afford greater access to the posterior structures, the potential for circumferential decompression, the ability to place segmental instrumentation for stabilization, and the possibility of providing deformity correction. The disadvantage of posterolateral approaches includes diminished access to the ventral cord and disc space. A dorsolateral path can also entail excessive muscle dissection and removal of bony elements with a potential risk of iatrogenic instability. Minimally invasive procedures have been tailored for use in the thoracic spine and are reportedly associated with decreased soft-tissue injury, less operative blood loss, shorter hospitalization, and quicker recovery.⁵ However, there is a substantial learning curve with the thoracoscopic techniques, as well as a limited field of vision and diminished ability to address potential complications. Muscle-sparing lumbar retractors have been adapted to the thorax recently, but are also associated with some technical difficulty. In this chapter, the authors discuss surgical alternatives, including the advantages and disadvantages of open versus minimally invasive surgery (MIS) for TDHs and propose recommendations for future improvements and considerations for treating this rare disorder.

11.2 Indications

Indications for surgical excision of TDHs include myelopathy, radiculopathy, and severe intractable axial pain.8 Myelopathy is the clearest indication for surgery to prevent further compression and irreversible spinal cord damage.⁹ Among the myelopathic patients, acute myelopathy is the strongest consideration for surgery and best predictor for neurologic improvement.⁸ Another primary indication for surgical intervention is protracted, disabling radicular pain refractory to conservative treatment.⁹ Radiculopathy, defined as axial pain or pain radiating to a dermatomal area of distribution, is a relative indication and often responds to nonsurgical therapy.⁸ Most of the patients with purely axial pain can be managed nonoperatively.⁸ Surgery for axial back pain in the presence of a TDH is controversial. Many TDHs discovered on magnetic resonance imaging (MRI) may be incidental, and additional studies, such as computed tomography (CT) myelography, must be done to correlate the finding with pain.

11.3 Advantages of Minimally Invasive Surgery

After one decides to intervene surgically on a TDH, the next critical question to address is how the decompression will be performed. A multitude of surgical techniques and approaches have been described to treat TDHs. They include anterior/anterolateral (i.e., transthoracic transpleural, transthoracic extrapleural), posterolateral (i.e., costotransversectomy [CTVS]/lateral extracavitary, transpedicular, transfacet/pedicle-sparing), and posterior (laminectomy) approaches. There is no perfect approach, as each technique is associated with unique advantages and disadvantages.

11.3.1 Minimally Invasive Anterior/Anterolateral Approaches

Although traditional open anterior approaches to the thoracic spine allow for access to all types of disc herniations below T4, direct visualization of the dura-disc interval, and decompression of the canal with the least manipulation of neurologic structures, they have been associated with significant perioperative discomfort, difficult ventilation, shoulder girdle dysfunction, wound healing problems, and pulmonary complications (i.e., pulmonary emboli, parenchymal tears, pneumonia).10,11,12,13,14 With the goal to decrease pulmonary complications and improve patient outcomes, MIS techniques performed via an anterior/anterolateral approach were developed. The two most common MIS techniques to address TDHs are thoracoscopy and mini-open anterior approaches and their modifications, including the mini-open lateral approach. One of the most common indications for thoracoscopic spine surgery is thoracic discectomy.12,15,16,17,18 Using thoracoscopy, a discectomy is performed through three or four small portals. As the portals are situated between the ribs, the need to retract and/or resect a rib is eliminated, which decreases the incidence of intercostal neuralgia (ICN) compared to open procedures.¹⁸ Additional intraoperative advantages of thoracoscopy include enhanced visualization to the surgeons and operative team, direct visualization of the ventral spinal surface without the need to take down the diaphragm, and reduced intraoperative blood loss.^{9,19,20,21,22} Many of the intraoperative benefits correlate to postoperative advantages, which include a better cosmetic result, improved shoulder girdle function, less postoperative pain and narcotic use with improved ventilatory excursion, shorter chest tube use, shorter hospital length of stay (LOS), and faster recovery times.^{9,12,15,19,20,21,22} While previous thoracotomy and pleural adhesions have been stated as contraindications to the use of thoracoscopic intervention,¹² it has been used successfully and safely for reoperation of thoracic discs in the presence of pleural adhesions and distorted spinal anatomy after previous thoracotomy.²³ Despite these advantages, there are no prospective studies that compare thoracoscopy to open thoracotomy in myelopathic patients with thoracic disc disease.

Mayer first described the mini-thoracotomy approach (mini-TTA), which represents a middle ground between open and thoracoscopic approaches.²⁴ The approach combines the resection of a rib, single-lung ventilation, and the use of a surgical microscope. This setup provides the unique advantage of direct visualization and magnification of the surgical field. In contrast to thoracoscopy, which involves 2D visualization of 3D pathology, the microscope allows for 3D visualization. The mini-TTA also allows for the most direct and shortest route to the anterior thoracic spine and is associated with a reduction in postoperative thoracic pain and pulmonary complications relative to open approaches.^25,26,27 The mini-TTA has been used with great success and has had few major associated complications in the treatment of TDH that occupy greater than 40% of the spinal canal diameter, also known as “giant thoracic disc herniations” (GTDHs).^24,25

The most recently developed MIS approach to the thoracic spine is the mini-open lateral approach, which represents a modification to the mini-TTA. The strength of this technique is that a standard anterior discectomy and fusion with instrumentation can be performed while avoiding collateral approach-related complications. The mini-open lateral approach can be a retropleural technique, which allows for canal exposure without sacrificing intercostal nerves, avoids the potential risk of intraforaminal radiculomedullary artery occlusion, and optimizes spinal exposure by limiting aorta or vena cava spinal coverage compared to standard open thoracotomy.²⁵ In addition, the use of a retropleural retractor system eliminates the need to deflate the ipsilateral lung, theoretically decreasing the risk of postoperative atelectasis and pulmonary complications and minimizing overall perioperative morbidity. The mini-open lateral approach also does not involve a microscope, avoids rib resection, and allows for performance of the procedure under direct vision with concomitant control of the pathologic process and the essential anatomic structures. As this approach allows for direct visualization and conventional surgical techniques, there is no need for highly trained staff and the learning curve is theoretically also shorter than that for thoracoscopy, although this has not been demonstrated in a clinical series.

11.3.2 Minimally Invasive Posterolateral Approaches

Traditional posterolateral approaches to the thoracic spine are popular among spine surgeons, as they can be performed independently without the assistance from an “approach surgeon.” However, in order to access a TDH via these approaches, extensive muscle dissection and ligamentous detachment are required. The working area required to adequately visualize pathology and manipulate surgical instruments is much larger than the actual area of pathology.26,28,29 Another disadvantage of traditional posterolateral approaches is that their use is limited to the treatment of lateral, soft TDHs, as surgical decompression of the ventral spinal cord is often performed blindly. Thus, in an attempt to minimize soft-tissue disruption, avoid complications associated with anterior thoracic approaches, decrease incision length, and improve visualization, posterolateral MIS approaches to the thoracic spine were developed.

Creation of MIS posterolateral approaches can be credited to Jho and Fessler.²⁸ Since their first descriptions, MIS approaches have been used for the transpedicular,^28,30,31 transfacet/pedicle-sparing,^32,33 and CTVS^34,35 approaches with the aid of tubular retractors for exposure and the endoscope and/or microscope for visualization.^{28,30,31,32,33,34,35} Access via tubular retractors (i.e., 18 and 20 mm) requires incisions that range in size from 2 to 4 cm,^{28,30,31,32,35} which are significantly smaller than the traditionally sized incisions that range from 7 to 10 cm in length. The importance of a smaller incision cannot be overlooked, given it results in minimal muscular and ligamentous trauma, thereby decreasing postoperative pain, hospital LOS, and overall morbidity.^{28,30,31,32,35} For example, an MIS CTVS approach avoids the need for rib resection, pleural retraction, and the violation of the costal neurovascular bundle and pleural cavity. Therefore, it is associated with less intraoperative estimated blood loss (EBL), shorter operative time (ORT), avoidance for the need of a chest tube, fewer blood transfusions, shorter hospital LOS, and lower overall complication rates compared to a traditional open CTVS approach.³⁵

The use of the endoscope is also an important advantage of MIS posterolateral techniques, as it allows for safe access and decompression of areas around the spinal cord that were traditionally thought to be too dangerous to access with an open posterolateral approach. For example, Jho described the use of a 70°-angled endoscope and curved surgical instruments introduced through a transpedicular route to directly visualize and safely decompress the ventral spinal cord.^30,31 The use of a microscope provides the unique advantage of performing thoracic disc decompression using traditional discectomy instruments under 3D visualization.²⁸ In addition, the microscope’s stereoscopic views allow surgeons to drill in a more controlled and safer manner than if the endoscope were used.³⁰ However, the views of an endoscope can be enhanced if a 3D endoscope is used with a high-definition video monitor.30 The advantages described herein for MIS posterolateral approaches have allowed many surgeons to treat a variety of TDHs with excellent outcomes and a limited risk profile.

11.4 Advantages of Open Surgery

Anterior thoracotomy is the gold standard procedure to treat TDH. The greatest risks with intrathoracic procedures (open or MIS) are great vessel, cardiac, lung, and/or esophageal injury. Of these, the most common catastrophic complication involves hemorrhage and death from an aortic or segmental vessel injury. The open anterior exposure offers unparalleled ability to identify, dissect, and control the vessels safely and prevent lethal hemorrhage compared to an MIS approach. If a vascular injury occurs during an MIS procedure, the operative field often is obscured with blood, which makes control of the injury exceedingly difficult. Emergent conversion to an open thoracotomy due to vascular injury and/or extensive bleeding has been reported as an adverse event of MIS techniques.36

The second advantage of open thoracotomy for thoracic discectomy includes a greater appreciation of thoracic and spinal canal anatomy. Following open thoracotomy, the surgeon has excellent visualization of the vertebral bodies, end plates, and disc. If an MIS approach is performed, the surgeon may lose appropriate orientation due to loss of visualization of 3D surface landmarks. This latter complication may be particularly concerning if the patient is malrotated, which is common in the lateral decubitus position. Instrumentation can also be more carefully applied in open cases. Thoracic disc pathology differs from the lumbar spine, where disc herniations frequently are composed of “soft” nucleus pulposis and are easily amenable to treatment through a smaller approach. TDHs are usually not “soft” herniations, as they are extruded, sequestered, and/or calcified. TDHs frequently occur in the setting of ossification of the posterior longitudinal ligament (OPLL). Careful resection of the osteophytes and occasionally the vertebral body end plates is required to adequately decompress the spinal cord. An open approach affords the surgeon the best opportunity to perform a “partial corpectomy” to confidently resect all the osteophytes.

Posterolateral approaches are increasingly common in the treatment of complex thoracic pathology, including tumor metastases and epidural abscess. With each of these approaches, the surgeon can stabilize spinal elements, place segmental instrumentation, perform a partial or complete corpectomy with an expandable cage, and correct a deformity. In contrast to lumbar discectomies, retraction of the thecal sac to access anterior pathology is impractical in the thoracic spine. Instead, the anterior epidural space is accessed through a pedicular zone or extrapedicular corridor, thus avoiding cord manipulation. A transpedicular approach avoids damage to the radicular vessels⁴; however, it involves the resection of the pedicle and lateral vertebral wall.⁴ A transfacet approach has the advantage of sparing resection of the pedicle. Maintaining the structural integrity of the pedicle may reduce postoperative back pain and improve spinal stability.² This technique is not suitable for midline disc herniations.³⁷ Open posterolateral approaches are safer than MIS cannulated discectomy through the lateral recess, given the latter requires thecal sac retraction and the associated risk of spinal cord injury. Due to the complex costovertebral anatomy, endoscopic discectomy through the neural foramen and Kambin’s triangle is not possible in the thoracic spine. Minimally invasive tubular transpedicular decompression is potentially very difficult due to poor visualization from the pedicle’s bleeding cancellous bone. In addition, placement of instrumentation and stabilization through a percutaneous approach is technically demanding.

In summary, in comparison to MIS counterparts, open anterior and posterolateral approaches to the thoracic spine are safer and provide a greater ability to address thoracic spine pathology, visualize and control potential blood vessel injuries, as well as place spinal instrumentation.

11.5 Case Illustration

The patient was a 59-year-old woman with a long history of chronic back pain and prior cervical, thoracic, and lumbar surgeries who presented with persistent back pain and upper abdominal numbness and radicular pain. Pertinent physical examination findings included bilateral lower extremity numbness and weakness (4/5 in proximal muscle groups). After clinical examination and review of appropriate radiographic imaging, it was evident that her symptoms were due to chronic, severe thoracic myelopathy from a large midline, calcified T9– T10 disc herniation ( Fig. 11.1). Given that the patient had exhausted multiple conservative methods of management, surgical intervention was recommended. The goals of surgery were to arrest the progression of myelopathy and to improve her thoracolumbar radicular pain.

11.6 Surgical Technique in Minimally Invasive Surgery

The patient is brought to the operating room. The procedure is performed under general anesthesia. The patient is intubated with a regular endotracheal tube rather than a double-lumen tube. Neuromonitoring modalities, including SSEP (somatosensory evoked potential), MEP (motor evoked potential), and EMG (electromyogram), are placed. The patient is then positioned in the lateral decubitus position with the left-side up. The patient is secured to the table with 4-inch tape and the table is flexed to help open the space between the ribs at the affected level.

The surgical field is widely prepped so that the incision could be extended to a regular thoracotomy if necessary. The junction of the posterior and middle thirds of the desired disc space is marked over the skin using fluoroscopy. A 4-cm incision is centered over the mark. The subcutaneous tissue and the intercostal muscles are divided with electrocautery. The ribs are identified with gentle subperiosteal dissection. The thoracic cavity is entered through the superior edge of the rib overlying the desired disc space in order to avoid the neurovascular bundle at the superior aspect of the intercostal space. Approximately 2 inches of the rib are resected, and with blunt dissection, the pleura is mobilized over the rib cage. The parietal pleura is carefully deflected from the inner thoracic wall. The desired level is verified with fluoroscopy, and the dilators are placed over the affected disc space. The retropleural retractor system is then introduced into the thoracic cavity and the disc space of interest is exposed. After confirmation of the level, the segmental vessels are ligated over the vertebral body. The vessels are mobilized and the anterior border of the spine is exposed along with the posterior border of the disc space and the rib heads. The rib head covering the affected disc spaces is resected. The disc is then removed with a pituitary rongeur, curettes, Kerrison rongeurs, and disc cutters. The anterior and posterior annuli are left intact. After the discectomy is completed, attention is turned to cord decompression. A partial corpectomy may be performed with wedge resections. The disc space is prepared and a cage filled with local bone graft is placed into the disc space. The spine is instrumented with an anterior single-rod construct spanning the index disc space. The wound is irrigated, a small chest tube drain is placed over the extrapleural space, and the wound is closed in layers.

Fig. 11.1 (a) Preoperative sagittal and (b) axial MRI images, and (c) sagittal and (d) axial CT radiographs at affected level.

11.7 Surgical Technique in Open Surgery

The anterior transthoracic approach is usually performed with the assistance of an “approach surgeon.” After adequate general anesthesia, a patient is intubated with a double-lumen endotracheal tube and placed in the lateral decubitus position. A left lateral thoracotomy incision is made over the planned level of exposure in the midaxillary line, extended anteriorly toward the umbilicus and then carried posterior and proximally. The dissection is carried through the skin and subcutaneous tissues to the latissimus dorsi and serratus anterior, which are divided sequentially to expose the thoracic cage and intercostal muscles. The rib is dissected from the surrounding structures using electrocautery and a periosteal elevator. The thoracic cavity can be entered through an intercostal space or by costal resection. The ribs are retracted using a Finochietto retractor. The ipsilateral lung is deflated and retracted anteriorly, although this is not necessary for TDHs in the lower thoracic spine. The pleura overlying the thoracic spine is incised and dissected toward the aorta, which is retracted medially and anteriorly. Fluoroscopy is then obtained to verify the proper level, and the intercostal vessels overlying the disc are then clipped and divided.

A partial corpectomy may be necessary for large TDHs that protrude posterior to the vertebral body and impinge on the ventral spinal cord. These procedures are typically performed through an anterior transthoracic approach to the spine with the patient placed in the lateral decubitus position. The dissection is performed in the same manner as described above. The majority of the disc and the cartilaginous end plate are removed. A partial vertebrectomy is performed by careful dissection of the posterior vertebral cortices, herniated disc material, and posterior longitudinal ligament off the dura.37 The extent of bone resection is dependent on the size and location of the herniation.³⁷ A strut graft or a cage packed with autologous bone graft is then inserted. The spine is instrumented with an anterior single-rod construct spanning the index disc space.

11.8 Discussion of Minimally Invasive Surgery

TDHs are a challenging clinical entity that can be treated via a multitude of surgical approaches. As a result of an unacceptably high risk profile associated with traditional open anterior and posterolateral approaches, MIS surgical approaches were designed and refined over the last 20 years. Currently, MIS anterior and posterolateral approaches to the thoracic spine provide a spine surgeon with unique, safe, and effective options for the treatment of TDHs. However, comparative data of open approaches to MIS approaches for the treatment of TDHs are limited. The literature presented below represents the current evidence of MIS anterior and posterolateral thoracic approaches for the treatment of TDHs. Studies that report on the use of MIS anterior thoracic approaches for thoracic pathology of another nature (i.e., tumor, infection) are omitted from this discussion.

11.8.1 Level I Evidence in Minimally Invasive Surgery

There are no level I studies available.

11.8.2 Level II Evidence in Minimally Invasive Surgery

There are no level II studies available.

11.8.3 Level III Evidence in Minimally Invasive Surgery

Thoracoscopic surgery was originally used by cardiothoracic surgeons to treat tumors, autonomic disturbances, and infectious diseases of the chest.12 In 1993, Mack et al presented the first clinical series of the use of thoracoscopy for patients with thoracic spine disease, which ranged from TDH to spinal deformity.^12,38 Since that initial report, several more homogeneous clinical cohort studies have been published regarding the efficacy of thoracoscopy in the treatment of TDH. Regan et al in 1998 presented a landmark retrospective cohort analysis of 29 patients with 32 TDHs between T5 and L1, and compared the results to 10 patients who had previously undergone treatment for TDH with a traditional anterior approach.¹² The minimum follow-up was 1 year (range, 12–24 months).¹² In comparison to open procedures, the authors found a significantly shorter ORT, EBL, ICU, and hospital LOS with thoracoscopy.¹² In patients treated with thoracoscopy, there was also a significant improvement in disability related to radiculopathy and myelopathy, as assessed by the Oswestry Disability Index.¹² In the thoracoscopy group, narcotic usage was significantly reduced and/or eliminated postoperatively, and 75.8% of patients were satisfied with their outcomes.¹² This study was one of the first series to demonstrate the advantages of thoracoscopy for the treatment of symptomatic TDHs in comparison to open approaches.

A second important study was published in 1998 by Rosenthal and Dickman, who compared thoracoscopy to thoracotomy and CTVS.⁹ The authors studied 55 patients who underwent thoracoscopy for TDHs between T3 and T12, of which 36 patients had myelopathy and 19 patients had radiculopathy.⁹ Thirteen of the thoracoscopic procedures were performed after a previously unsuccessful attempt at decompression via a transpedicular, transfacet/pedicle-sparing, CTVS, or thoracotomy approach.⁹ The thoracoscopy patients were compared to 18 patients who underwent thoracotomy and 15 patients who underwent CTVS, all of whom had similar characteristics.⁹ The authors found that the patients treated with thoracoscopy in comparison to thoracotomy patients had at least an hourshorter ORT, 350 mL less EBL, 18 mg/day less narcotic use, 2-day shorter chest tube use, and a shorter hospital LOS by 10 days.⁹ In comparison to the CTVS group, thoracoscopy had 75 minutes shorter ORT and an equivalent EBL, chest tube use duration, narcotic usage, and hospital LOS.⁹ Importantly, 61% of the patients with myelopathy and 79% of patients with radiculopathy treated with thoracoscopy had a full neurologic recovery.⁹ These results suggested that patients with recurrent TDHs treated with thoracoscopy had good outcomes and significantly less intraoperative and in-hospital morbidity compared to open approaches.

Another, more recent retrospective cohort analysis was performed by Oppenlander et al, who compared outcomes of patients with TDHs treated with thoracoscopy, thoracotomy, and posterolateral approaches.³⁹ The authors’ cohort consisted of 56 patients who underwent 62 procedures for 130 TDHs.³⁹ A thoracotomy was performed in 23 patients, thoracoscopy in 26 patients, and a posterolateral approach in 13 patients.³⁹ The approach utilized was based on the disc’s characteristics, that is, patients who underwent thoracotomy were more likely to have multilevel, calcified, central, giant, and/or intradural herniations.³⁹ Presenting symptoms persisted for an average of 28 months (range: 0.1–180 months) and included myelopathy (82%), radiculopathy (64%), and mixed symptomatology (43%).³⁹ Patients treated with thoracoscopy in comparison to thoracotomy were found to have a lower EBL, shorter hospital LOS, fewer instrumented fusions, and an equivalent requirement for blood transfusions.³⁹ While the analysis by Oppenlander et al demonstrated several advantages of treating TDHs with thoracoscopy, the advantages should be critically considered in the setting of the open approaches being used to treat more difficult pathology.

The first study to compare thoracoscopy to another MIS approach was presented by Bartels and Peul in 2007, as a retrospective cohort analysis.⁴⁰

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