Keywords
Thoracic disc herniationCentrolateralTranspedicularCostotransversectomyLateral extracavitaryLateral retropleuralTransthoracicIntroduction
Disc herniation is a fairly common condition with a frequency of 40 to 50 per 100,000 people. Conversely, symptomatic thoracic disc herniation (TDH) is significantly less common. Modern imaging developments in computed tomography (CT) and magnetic resonance imaging (MRI) have revealed an incidence of asymptomatic TDH to be estimated at 11% to 37% [1]. Nevertheless, symptomatic TDH continues to be rare, with an incidence of 1 to 1,000,000 in the general population [2]. TDH accounts for 0.25% to 0.75% of all protruded discs [3]. It is most commonly seen in mid to late adult life, with the peak of 80% occurring between 40 and 50 years of age [4]. There is no significant difference in gender [5].
Surgical procedures for symptomatic TDH represent only 0.15% to 4% of all surgeries for intervertebral disc herniation [6]. TDH has been reported at every level. However, in 75% of cases, the TDH is below the T7–T8 disc due to this being the more mobile portion of the thoracic spine and due to weakness of the posterior longitudinal ligament (PLL) at this level [4]. Only 4% of TDHs are located above T3–T4. The T11–T12 disc is the most vulnerable to TDH, with a peak of 26% occurring at this level [7]. Centrolateral disc herniations are most common, making up 94% of cases, while 6% of cases are lateral [7]. A history of trauma may be elicited in 11–25% of cases [8].
Presentation
The diagnosis of symptomatic TDH can often be missed due to mismatch between the symptoms and location of the herniation [9]. Up to 90% of patients may describe insidious onset of symptoms [8]. The most common presenting symptoms for TDH include pain, sensory disturbances, myelopathy, and lower extremity weakness [8]. Pain is present in 57% of the cases at presentation [10]. The classic character of thoracic radicular pain from a TDH is a stabbing pain that begins posteriorly and radiates unilaterally or bilaterally around the chest wall in a dermatomal distribution [11]. Radicular pain may often occur in association with a concurrent myelopathy; it may also occur in isolation with no spinal cord injury. The pain usually occurs with no loss of motor or sensory function at the level of the spinal root because the innervation of adjacent intercostal nerves overlaps [11]. By the time of diagnosis, 90% of patients have signs of spinal cord compression [10]. An early and accurate diagnosis, coupled with improvements in surgical approaches, offers a much better prognosis for patients with TDH [10].
Non-operative Management
Isolated thoracic radicular pain will often respond to a regimen of restricted activity, a hyperextension brace, nonsteroidal anti-inflammatory agents, oral steroids, and/or epidural steroids [11]. Patients who fail a full course of non-operative management, which includes 3 to 6 months of therapy, may be considered for surgical treatment of the TDH [11].
Indications for Surgery
The strongest indication for surgery is injury to the spinal cord caused by compressive lesions. When myelopathy is present, the goals of surgery are to prevent further irreversible damage to the spinal cord and to improve function [11]. The more severe and the longer the patient has experienced the myelopathy at presentation, the less likely the patient is to recover neurological function [11].
Herniated thoracic discs requiring surgery are rare [7]. Indications for surgery include intractable pain, which usually presents in a radicular or band-like distribution. Progressive myelopathy and axial back pain are also indications for surgical treatment [4]. Less common symptoms of TDH include symptomatic syringomyelia originating at level of disc herniation. Over the past decades, the treatment of TDH has changed profoundly. There has been a considerable improvement in the surgical treatment of TDH, with an over 80% success rate for surgical approaches other than decompressive laminectomy [10].
Evaluation
MRI is the mainstay of diagnosis for TDH. As discs frequently become calcified, a CT is usually necessary in order to determine if the disc is soft or calcified. A calcified disc may affect the choice of surgical approach. The CT may also be useful in demonstrating bony detail to help determine if instrumentation will be required.
Surgical Considerations
Preoperative preparation is especially critical in cases of TDH. Efforts should be made to characterize whether the disc is hard or soft, or if it is eccentric to one side or central in location, as these data points will influence the surgical approach. The placement of fiducials with subsequent CT myelogram can assist the surgeon in correctly identifying the level of interest intraoperatively, thus avoiding wrong-level surgery. Once all preoperative studies have been obtained and reviewed, the surgeon can determine the best surgical corridor to safely decompress the neural elements.
The approaches to the thoracic spine can be arranged in a circumferential manner from a transpedicular approach to a transthoracic approach. We will discuss each approach in detail below.
Posterior Approaches
Transpedicular Approach
The transpedicular approach is the most commonly described procedure for thoracic discectomy. It involves the unilateral removal of the pedicle and facet, and it provides the most direct of the posterior approaches to the disc space. It allows access to the lateral aspect of the spinal canal. The advantages of this technique include the ability to preserve the radicular arteries, as well as lack of manipulation of spinal cord. Its major disadvantage is limited visualization of the spinal dura.
The patient is positioned prone on gel rolls or a Wilson frame. A midline vertical incision is made centered over the level of interest. Sharp dissection is performed through the thoracodorsal fascia with monopolar electrocautery until the spinous process is reached, followed by a unilateral subperiosteal dissection with a combination of electrocautery and Cobb elevators along the bony elements to expose the lateral aspect of the facet.
After the soft tissue exposure is completed, the pedicle of the inferior vertebral level is drilled. It is entered below the edge of the inferior facet of the superior vertebra. A laminotomy is performed prior to drilling the pedicle using a high-speed drill and Kerrison rongeurs to visualize the lateral aspect of the thecal sac and avoid injury to it. Once the exiting nerve root is visualized and protected, the pedicle is drilled to the depth of the vertebral body and disc space. One can further protect the nerve root by drilling the inferior portion of the pedicle first, using the superior and medial cortex of the pedicle to shield the nerve root, which are then removed with curettes and rongeurs.
Removal of the pedicle allows the surgeon to visualize the lateral disc space. The annulus is then incised, and the discectomy performed with pituitary ronguers and curets. After satisfactory decompression, hemostasis is achieved, and closure is done in standard layered, interrupted fashion.
Costotransversectomy Approach
The costotransversectomy entails a more lateral working corridor than the transpedicular approach and thus provides more access to the anterior spinal canal. It is a relatively more invasive approach that involves resection of medial aspect of rib and removal of transverse process.
The patient is positioned prone or partly lateral. The skin incision can be curvilinear toward midline or straight paramedian and centered over the level of interest extending two to three levels above and below. The muscle layers are elevated medially with a combination of sharp and blunt dissection to expose the ribs and transverse processes. The rib associated with the inferior vertebral body is identified and disarticulated from the transverse process and vertebral body and then removed with rongeurs. The associated transverse process is also removed.
After removal of the appropriate bony elements, the pleura is visualized and retracted anteriorly. The neurovascular bundle at that level, as well as the corresponding pedicle, is also identified prior to approaching the disc space of interest. The pedicle is partially drilled to maximize exposure of the disc herniation. The annulus is incised, and the discectomy is performed from a lateral to medial trajectory with curettes and pituitary ronguers. Prior to closure, the pleura must be inspected for tears, which can be visualized by irrigating during positive pressure ventilation. Any tears may be repaired, or a chest tube placed based on the surgeon’s discretion. Closure is performed in usual fashion.
Lateral Extracavitary Approach
The lateral extracavitary approach provides the most lateral exposure and thus medial and ventral access of the posterior approaches to thoracic discectomies. It is limited, however, above the T5 level by the scapula. It also requires the largest incision and most signification dissection.
The patient is positioned prone or partly lateral, and the incision can be curvilinear toward midline or a hockey stick centered over the affected level. The dissection is similar to that of a costotransversectomy, but a more extensive resection of the inferior rib is necessary. This trajectory allows for more visualization of the anterior canal. A laminotomy may be performed for greater exposure. The exiting nerve root is identified and can either be spared or sacrificed and used to mobilize the thecal sac. The discectomy and closure are performed as previously described. This approach can also be used for more extensive vertebral body resections and interbody instrumentation.
Anterior Approaches
Lateral Retropleural Approach
The lateral retropleural approach is advantageous for patients with central disc herniations. It avoids the need of more extensive posterior surgical approaches, as there is no longer a need to work “around” the spinal cord. However, greater risk of injury to the lung and great vessels exists. In the lower thoracic spine, consideration must be given to the diaphragm and its attachments.
Surgical Technique
The lateral retropleural approach invokes a similar surgical strategy with the lateral transpsoas approach. After intubation and administration of general endotracheal anesthesia, the patients are placed in the lateral decubitus position, and secured to the operating table once carefully ensuring the proper padding of all pressure points. Neuromonitoring leads are placed to obtain somatosensory-evoked and motor-evoked potentials. Intraoperative fluoroscopy is then used to identify the level of interest. An incision is then planned over the disc space of interest (either above or between rib(s) for single level surgery). After careful dissection of the intercostal muscle layers, the parietal pleura is incurred. The parietal pleura is then carefully swept off of the rib cage using blunt digital dissection. With the aid of serial dilation, a lateral retractor is introduced into the retro pleural space, centered over the targeted disc space. Once fixed in place, the retractor is deployed, and exposure of the herniated disc can commence. In cases of large herniated fragments, it may be necessary to remove a portion of the rib head corresponding to the lower vertebral body (i.e., T9 rib head in a T8/T9 disc herniation). Additional exposure can be gained by removal of a portion of the vertebral bodies and/or the corresponding pedicle in order to provide earlier identification of the posterior longitudinal ligament and dura prior to attempting to remove the disc fragment.
After adequate exposure is obtained and the dura is identified, central debulking of the intervertebral disc is performed. This creates a space into which the herniated component can be manipulated before it can be safely removed. All motion(s) should be away from the thecal sac to avoid injury to the underlying spinal cord. This can be carried out by the use of straight and down-going curettes.
Once adequate removal of disc is completed, it can be determined whether or not fusion is required. In the lateral approach, this can range from anterolateral plating to interbody fusion based on the level or pathology and degree of instability.
Prior to removal of the lateral retractor and closure, it is necessary to ensure that there has not been any violation of the parietal pleura. In the event of an injury to the pleura, one should be prepared to place a chest tube to treat the pneumothorax.
Transthoracic Approach
This approach is similar to the lateral retropleural approach in that it allows access to central disc herniations without having to work around or manipulate the thecal sac. However, this approach typically requires the assistance of an approach surgeon to retract the lung in order to gain access to the thoracic spine.
Surgical Technique
The patient is intubated using a double lumen endotracheal tube and placed in the lateral decubitus position. All pressure points are padded, and intraoperative fluoroscopy is used to identify the level of interest. Neuromonitoring leads are placed to obtain somatosensory-evoked and motor-evoked potentials. Exposure is achieved by a thoracic surgeon. Any rib taken during the approach may be saved and morselized into autograft if fusion is planned.
Once access to the spine is obtained, the discectomy is carried out in similar fashion to that described above. Bony work is completed first to provide adequate exposure of extruded fragment. This includes removal of the proximal pedicle and posterior portions of the adjacent vertebral bodies. Central debulking is then carried out to make room for manipulation of the extruded fragment. The PLL and dura are then identified to establish a plane lateral to the herniated disc fragment. Using curettes (straight and down-going) and Kerrison rongeurs, the disc fragment is then carefully removed, paying special attention to avoid any motion in the direction of the thecal sac, which could injure the underlying spinal cord. Upon completion of the decompression, the decision can be made to augment stabilization with a lateral plate, place an interbody graft, or rely on the integrity of the rib cage.
Closure is performed by the approach team. A chest tube is left in place to treat the iatrogenic pneumothorax. Serial plain films of the chest are obtained to confirm resolution of the pneumothorax.
Complications
TDHs have a particular progression, primarily concerning for risk of medullary compression. Surgery for TDH historically has had a poor reputation due to technical difficulty and risk of potentially serious and hard-to-treat complications [1]. Thoracic discectomy has a complication rate of 15% to 30% [6]. A meta-analysis of 545 patients who underwent surgical treatment for TDH found a 24% complication rate, which included 6% lung complications and 6% intercostal neuralgia [11]. Another analysis of 13,387 patients that underwent surgical treatment for TDH with myelopathy reported the rate of developing a complication post-operatively was 14.5% [12]. In one study, the overall complication rate for the thoracoscopic approaches was 15% versus 23% early in their practice, suggesting there is a required learning curve associated with TDH surgical procedures [8].
Other concerning complications in TDH surgery include cerebrospinal fluid (CSF) leak, spinal cord injury, and misidentification of surgical level.
The incidence of CSF leakage has been reported to range from 0% to 15% [13]. There are two courses of a CSF leak during thoracic disc surgery: iatrogenic or intradural disc herniation [13]. Intradural TDHs have been reported in up to 12% of cases, most of which involved discs that were densely calcified [14]. One study found that 7% of TDH had intradural extension at the time of surgery [15]. Management of CSF leaks typically involves a combination of primary or graft closure, fibrin glue application, and lumbar drain placement [13].
A high rate of neurological deterioration from spinal cord injury was observed in surgical treatment of TDH when thoracic laminectomy was the procedure performed [13]. It has been proposed that spinal cord manipulation required for removal of the disc ventral to the spinal cord may induce mechanical injury [13]. This manipulation may also interfere with blood supply to the spinal cord [13]. Tethering of the spinal cord, caused by minor kyphotic deformities resulting from the laminectomy, over incompletely removed disc or osteophyte can lead to neurologic deficit [16]. However, paresis and paralysis as operative complications have become relatively rare since laminectomy has become essentially abandoned [13].
Accurate intraoperative localization of thoracic vertebral levels remains a concern in thoracic spine surgery [17]. A 2008 survey reported that 50% of spine surgeons reported a wrong level surgery during their career [18]. Overlying scapular shadows, variation in the number of rib-bearing vertebrae, and osteopenia are all factors that complicate accurate intraoperative localization of specific thoracic vertebral levels [17]. Counting spinal levels and ribs on pre- and perioperative imaging, as well as identifying osteophytes and landmarks, can help avoid such complications [13]. In addition, preoperative placement of radiopaque markers using fluoroscopy or CT at the pedicle of interest may aid in avoiding this complication [19].
In a comparison of anterior/anterolateral decompression and spinal fusion (ASF), posterior/posterolateral decompression and spinal fusion (PSF), and disc decompression/excision without fusion (DDE), ASF had the highest complication rate at 24.2%, followed by PSF at 15.5% and DDE at 10.4% [12]. Patients undergoing ASF had 1.1% mortality, while DDF had 0.39% mortality and PSF 0.56% mortality [12]. Over the period of this analysis, the preferred treatment shifted substantially from DDE (performed in 30% of the patients in 2000) to PSF (performed in almost 50% of all patients by 2010) [12].
