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Anterior and Lateral Approaches for Thoracic Discectomy: Endoscopic and Mini-Open Techniques
Introduction
Thoracic disc herniation (HTD) was first described by Key in 1838 and has since proven to be challenging to both diagnose and treat properly. The surgical management of HTDs has evolved over the past 50 years. Symptomatic HTD is a disease of middle to late adult life and is often not the result of a traumatic event. , HTDs occur in approximately 12% to 37% of the population, affecting men and women with an approximately equal distribution. While many patients are found to have HTDs on imaging, the incidence of symptomatic thoracic disease herniation is estimated at 1 in 1,000,000. Herniations in the upper thoracic spine are exceedingly rare, with a majority of symptomatic lesions occurring at the interspaces of T10–L1 due to their mobility and potential for accumulating degenerative changes. , Nearly 94% of these are midline or paramedian, requiring an anterolateral approach for management. ,
Initially, the midline laminectomy was the mainstay of treatment for HTDs; however, disastrous neurologic complications led to its abandonment. , Although this approach was simple, it was afflicted by many limitations, including access to centrally located lesions and calcified lesions, which required manipulation of the spinal cord for access. For these reasons, alternate techniques utilizing anterior and lateral approaches were developed with the goal to achieve the same decompression while reducing approach-related trauma and morbidity.
Interestingly, while anterior approaches to the thoracic spine have been described as early as 1928, the approach essentially remained unused until the 1950s when Hodgson and Stock reported their technique for treatment of a tuberculous lesion. , In the early 1960s, dorsolateral techniques via a costotransversectomy and lateral extracavitary approaches were used to reach difficult-to-access ventral lesions with encouraging results. The transthoracic thoracotomy approach was first reported independently in 1969 by Perot and Munro as well as Ransohoff and colleagues, and it has since paved the way for the development of minimally invasive thoracic access techniques for the management of symptomatic HTDs. , ,
Beginning with the introduction of the microscope by Yasargil to minimize surgical exposure and morbidity for the lumbar discectomy, minimally invasive or minimal access surgery was developed to address the approach-related morbidity associated with open surgery. We now use minimal access technology to treat thoracic disc disease as well as spinal deformity, trauma, and spinal neoplasms. This is being accomplished by designing procedures that require smaller incisions, result in less soft tissue disruption, and involve limited surgical corridors, utilizing technological advances in illumination, magnification, instrumentation, and endoscopy. Additionally, the expansion of video technology in the early 1990s led to improvements in endoscopic spine surgery by providing a high-quality image with smaller equipment. At present, such approaches to the spine have been well described for management of thoracolumbar fractures and osteomyelitis, in addition to HTD, with the advantage of reductions in recovery time, length of hospital stay, and approach-related morbidity.
Herein, we describe minimally invasive anterior and lateral approaches to the thoracic spine including endoscopic and mini-open techniques, with focus on their indications, advantages, disadvantages, and associated complications.
Anatomic Considerations
Anterior and lateral approaches to the thoracic spine require adequate knowledge of both the spinal and extraspinal anatomy in order to achieve a safe and successful discectomy. The surgical anatomy includes the external anatomy of the chest, the intrathoracic viscera, vascular anatomy, the contents of the posterior mediastinum, the ribs, the vertebrae, and neural elements. The thoracic spine plays an integral role in the support of the axial skeleton, as it is a rigid transition zone between the mobile cervical and lumbar regions owing its unique stability to the surrounding thoracic rib cage.
Spinal stability is provided by both the structure of the vertebral body and by the joints. The thoracic spine has two types of joints. The intervertebral discs are symphysial joints, and primarily play a role in load bearing and shock absorption. The facets are zygapophysial joints and, in contrast, are lined by a synovial membrane. Unlike the lumbar and cervical spine, the thoracic spine also shares articular surfaces with the ribs at the vertebral body and transverse processes. The relationship of each rib to its corresponding vertebral body changes along the rostral-caudal axis of the spine. Ribs 1, 10, 11, and 12 articulate with a single vertebral body via a single, complete facet located at the lateral aspect of the vertebral body. Ribs 2 through 9 articulate with their like numbered vertebrae as well as the vertebrae below, via paired demifacets, located initially on the lateral aspect of the vertebral body, then moving dorsally toward the more caudal segments. The neurovascular bundle runs on the undersurface of each rib, and for this reason, skin incision is directed toward the superior border of the caudal rib at the desired level.
The thoracic spine lies in the posteromedial compartment of the mediastinum and is covered by the parietal pleura. The parietal pleura also drapes over the great vessels, trachea, esophagus, and spinal column. Because the parietal pleura conceals much of the relevant anatomy, palpation of the rib head is an important orienting maneuver during transthoracic approaches to the spine. Resection of the rib allows access to the pleural space. Deflation and retraction of the lung then permits a clearer identification of the intrathoracic structures. In particular, identification of the pleura and lungs bilaterally; the heart, trachea, and esophagus medially; the diaphragm inferiorly; the aorta, hemiazygos vein, and liver on the right; and the thoracic duct, vena cava, and azygos vein on the left is crucial for complication avoidance. This is also important in choosing the desired approach to an HTD. The preferred side of approach in the upper thoracic spine is typically through a right thoracotomy to avoid the heart as well as the carotid and subclavian arteries. The aortic arch typically limits visualization on the left as well. The left side is usually preferred to avoid the liver and inferior vena cava.
The posterior intercostal arteries of the first two thoracic vertebral segments branch from the superior intercostal artery, which arises from the costocervical trunk of the subclavian artery. The lower posterior intercostal arteries, however, arise segmentally from the aorta. The descending aorta begins on the left side of the fourth thoracic vertebrae and descends ventromedially. The origins of the segmental vessels are positioned on the right side of the aorta in the upper thoracic spine; however, as you progress caudally they orient more dorsomedially. The segmental vessels lie transversely across the midportion of the vertebral body deep to the parietal pleura. Once the parietal pleural is incised and dissected, the segmental vessels are identified. Care should be taken not to inadvertently injure the segmental vessels, which may bleed profusely if not ligated in a controlled fashion.
The thoracic pedicles are short, and their height and radius increase from T1 to T12. , The nerve root is numbered according to the pedicle located immediately superior to it. Throughout the thoracic spine, the angle between the pedicle and midsagittal plane changes depending on the level. At T1, the angle between the pedicle and the midsagittal plane is wide, but by T12, the pedicles are parallel to the midsagittal plane. The thoracic pedicles are shorter and thinner than their lumbar counterparts, making them more susceptible to breach during screw placement. The relationship between the transverse process and the pedicle is variable in the thoracic spine as well. In the midthoracic spine (T7–T9), the transverse process is oriented more inferiorly relative to the pedicle. As you move superiorly and inferiorly from this segment, the center of the pedicle becomes more aligned with the center of the transverse process. For spinal canal decompression, it is necessary to identify the ipsilateral pedicle, which requires the removal of the rib head in the middle and upper thoracic spine. Below T11, however, the rib head may be preserved, as it inserts below the disc space. The neural foramen is then identified at the base of the pedicle and the lateral spinal canal may be exposed by removing the pedicle.
Patient Evaluation
In evaluating a patient with a herniated thoracic disc, great care must be taken to accurately correlate imaging findings with clinical presentation. It is important to focus on physical examination findings, as these may be essential in differentiating cervical, thoracic, and lumbar origin of disease. There is a diverse array of signs and symptoms associated with HTD, and as such, there are many other clinical entities that produce similar clinical findings. ,
The incidence of a herniated disc is quite high in the general population, and radiographic presence alone is not a sufficient indication for surgical intervention. In fact, a landmark paper by Wood and colleagues revealed that in a population of 90 asymptomatic individuals, 73% had positive anatomical findings at one level or more. Similarly, Awwad and colleagues reviewed computed tomography (CT) myelograms of 433 asymptomatic individuals and discovered thoracic disc disease in 11% to 13% of their population, with no correlation existing between imaging characteristics and development of symptomatic disease. Because of the ambiguity in the clinical presentation of HTD, diagnosis is often delayed, resulting in progressive deterioration of the patient’s condition.
The signs and symptoms associated with HTD often depend on the location of the herniation, as well as the size of the lesion, duration of compression, and degree of vascular compromise. Most frequently, patients with HTDs will present with unilateral radicular pain or pain radiating in a bandlike distribution from the upper or middle back to the chest. The T10 dermatomal level is most often reported, regardless of the level of the disc herniation. This is occasionally accompanied by numbness, paresthesias, claudication, or an electric shock–like pain that radiates down the back toward the legs. Gait instability, including the presence of splinting, altalgia, circumduction, or foot drop, may be present secondary to lower extremity weakness or sensory disturbance. A sciatic list may indicate contralateral disc herniation, while lateral bending may indicate ipsilateral disease. Symptoms often progress from thoracic pain to sensory disturbance, weakness, and finally bowel and bladder dysfunction, which may be present in up to 15% to 20% of patients with symptomatic disease. , , Patients may or may not demonstrate symptoms of myelopathy, including hyperreflexia, sustained ankle clonus, or upgoing toes on Babinsky testing. Rarely is one leg weaker than the other; however, if present, this may elude to the side more affected by a disc herniation. Midline or paramedian herniations have the greatest propensity for producing myelopathic symptoms, while lateral herniations more frequently result in radicular pain.
Imaging
Careful review and understanding of radiographic studies are essential to identifying the appropriate management and surgical approach for HTDs. In addition to assessment of the disc herniation, the evaluation of bone quality, ligament integrity, degree of spinal cord compression, and anatomic alignment should be performed for the purpose of surgical planning. Furthermore, in many cases a cervical spine magnetic resonance imaging (MRI) is warranted to rule out potential cervical disease contributing to myelopathy. While plain radiographs may be useful for localizing the involved level of disease, they are not ideal for evaluating herniated discs as they are unable to directly identify the neurocompressive pathology. CT and MRI remain the modalities of choice for evaluation of HTDs. In particular, coronal, axial, and sagittal reconstructions demonstrate the morphology and level of an HTD. Due to its superior contrast resolution, MRI affords the ability to evaluate the spinal cord, neural elements, intervertebral discs, epidural contents, paraspinous anatomy, and soft tissues. Disc herniations have an intermediate signal intensity on T1-weighted images and appear as an area of low signal density on T2-weighted images. CT in combination with myelography assists in the determination of the type and level of herniation but also importantly clarifies the osseous anatomic features of disc herniations. In particular, giant thoracic discs are often calcified or ossified and tend to erode the dura. Intradural extension of thoracic discs occurs in up to 12% of cases, and is useful to identify preoperatively in order to avoid inadvertent creation of a cerebrospinal fluid (CSF) leak. , CT myelography is useful in identifying intradural disc herniations and seems to have a greater sensitivity to detect this than MRI.
Indications
Considerable debate has been focused on the appropriate management of the symptomatic patient. Similar to disc herniations of the lumbar spine, symptomatic patients without evidence of cord compression are recommended to undergo a trial of conservative therapy for a period of 4 to 6 weeks, including nonsteroidal antiinflammatory medication, physical therapy, oral steroids, intercostal blocks, and/or epidural steroid injections. In general, myelopathy related to compression of the thoracic spinal cord by a herniated thoracic disc is a strong indication for surgery. There is no consensus on the ideal treatment paradigm for patients with symptomatic HTD; however, several authors have described their experience. Anand and Regan proposed a grading system to group patients according to their initial presenting symptoms and pain visual analog score (VAS), and found that long-term patient satisfaction and long-term improvement in Oswestry functional score was best seen in patients with myelopathy. It has been suggested, however, that patients with a nonsevere, static, or improving deficit and tolerable pain may be considered for nonoperative management. For those patients without myelopathy, surgery should be recommended for those patients with chronic, disabling symptoms. In general, for large, centrally located discs, calcified discs in low-medical-risk patients, and in patients with significant spinal cord compression and myelopathy, an approach should be selected that allows for direct visualization of the herniation and excision without excessive manipulation of the already compromised spinal cord. , , , Various authors have attempted to simplify the approach to patients by applying different algorithms to evaluation and surgical decision making ( Figs. 144.1 and 144.2 ). ,
Results of biomechanical studies of the thoracic spine after discectomy have shown increased range of motion as well as significant distress on the neutral zone, suggesting instability; however, conflicting results are presented in the literature. , Additionally, comparisons of clinical outcomes of thoracic fusion and nonfusion after discectomy have shown equivocal findings. Anand and Regan documented significant improvement in patient Oswestry disability score in patients who received fusion in addition to thoracic discectomy for axial back pain; however, when compared to those who did not undergo fusion, no significant improvement was observed. This is supported by clinical outcomes in the studies by Currier and colleagues and Bisson and colleagues, which both suggest the safety and efficacy of transthoracic discectomy and fusion. ,
Fusion may be necessary for giant calcified HTDs, or for procedures that require significant vertebral body drilling or with a prominent thoracic kyphosis. , Some also advocate for fusion in patients with a significant component of axial back pain. , The superiority of discectomy and fusion over discectomy alone, however, remains to be proven clinically, and the decision to pursue interbody fusion following discectomy should be individualized.
Endoscopic and Mini-Open Anterior and Lateral Surgical Approaches
Video-Assisted Thoracoscopic Resection of Symptomatic Herniated Thoracic Discs
Video-assisted thoracoscopic surgery (VATS) for anterior spinal exposure and treatment of a variety of spinal pathologies, including thoracic disc excision, corpectomy, and anterior release, has been well described. , , , , , , , , The VATS approach satisfies the criteria of a minimally invasive approach that, in experienced hands, allows for shorter operative times, less blood loss, less postoperative narcotic use, and less morbidity when compared to conventional posterolateral approaches. , Additionally, the VATS approach is associated with less blood loss, shorter hospital stay, fewer pulmonary complications, and a lower incidence of persistent intercostal neuralgia when compared to open thoracotomy. , , Contraindications to the thoracoscopic approach include severe pulmonary disease, and cardiovascular or other systemic disease that would preclude open thoracotomy, as that is the bailout procedure should there be a complications during thoracoscopy. Additional contraindications include prior thoracotomy on the side of access (due to the risk of pleural adhesions), prior chest trauma, history of pleural empyema, inability to tolerate single-lung ventilation due to pulmonary or cardiac disease, morbid obesity (due to poor visualization and significantly increased working distances), a disc located above the T3/4 or T11/12 level (due to increased difficulty with surgical access and exposure) or other general medical or surgical contraindications (Video 144.1). , ,
Anesthetic Considerations and Positioning
General endotracheal anesthesia is induced and either a double-lumen endotracheal tube or a single-lumen tube and endotracheal blocker is used to allow for single-lung ventilation for maximal surgical exposure. Position of the endotracheal tube should be confirmed with bronchoscopy prior to the procedure to ensure adequate placement. A Foley catheter and arterial line are routinely placed. The senior author routinely uses electrophysiological monitoring, including somatosensory evoked potentials and motor evoked potentials; therefore total intravenous anesthesia is used. ,
In general, the anatomy and position of the disc and spinal cord are considered when planning the side of access. For eccentrically located discs that displace the spinal cord toward one side of the spinal canal, the side closest to the herniated disc is chosen to avoid unnecessary manipulation of the spinal cord. , In general, a left-sided approach is preferred for access to the thoracolumbar junction, as it avoids obstruction of the surgical field by the liver, and a right-sided approach for middle to upper thoracic spine, to avoid the great vessels and heart. , , , Despite these general considerations, it is important to consider the individual patient’s vascular anatomy to avoid complications related to anatomic variations.
The patient is placed in a lateral decubitus position on a radiolucent table with the access side turned upward and the ventilated lung in a dependent position. The legs are slightly flexed, an axillary roll is placed under the dependent arm, and the superior arm is placed in an extended position on an arm holder to bring it out of the operative field. A C-arm fluoroscopy unit is brought into the field and used to ensure the patient and the spine are perpendicular to the operating table. Once the patient is positioned, the C-arm is used to identify the level of the disc herniation, and the involved vertebral bodies, disc spaces, anterior spinal line, and posterior spinal line are marked on the skin. The senior author utilizes a four-portal technique with the main working portal centered over the disc space and two to three additional ports for suction/irrigation, retraction, and the thoracoscopic camera. The camera port can be placed two to three intercostal spaces caudal to the working portal for cases involving the middle to upper thoracic spine, or cranially for cases involving the thoracolumbar junction ( Fig. 144.3 ). Adequate preparation of the surgical field is done to allow for conversion to open thoracotomy should it be necessary.
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