Background

Minimally invasive surgery (MIS) of the spine has become increasingly popular with the advent of advanced intraoperative imaging, electromyographic monitoring, and innovative minimally invasive biotechnology [1]. The goals of minimally invasive spine surgery are to theoretically decrease tissue damage and therefore to provide improved morbidity, faster recovery, and improved functional outcomes [2]. Although theoretically beneficial, there are limited high-quality studies comparing minimally invasive and open procedures. Nevertheless, evidence for improved outcomes for both open and minimally invasive procedures has been described. The decision to perform an open versus minimally invasive approach therefore depends on surgeon experience, preference, system availability, and patient preference [3–5].

The anterior approach to thoracic spine has unique benefits and limitations. This approach provides excellent access to the anterior aspects of the thoracic spine and limits manipulation of the spinal cord [6]. Additional reported benefits specific to the minimally invasive approach include avoiding the use of rib resection or retractors, reduced blood loss, and diminished postoperative pain. However, the minimally invasive approach also required increased anesthetic monitoring due to single lung ventilation, and it is a technically demanding procedure with a steep learning curve [7].

Indications

A thorough understanding of the indications for the anterior thoracic approach to the spine is critical to ensure optimal surgical outcomes. Reported indications for this approach include anterior thoracic spine fractures classified as AO Classification type A1.2, A1.3, A2, A3, B, and C with significant curvature displacement of 20° or more in the sagittal or AP plane, thoracic disc herniation, discoligamentous segmental instability, degenerative stenosis or deformity, osteomyelitis/tuberculosis, and tumor [7, 8]. Of note, approximately two-thirds of spinal metastases are found anteriorly in the vertebral bodies and pedicles. The anterior approach enables direct decompression and restoration of stability as laminectomy alone is not adequate for anteriorly located pathology [6]. Contraindications are similarly important to ensure optimal outcomes and include previous chest trauma or surgery, adhesions, infection, or comorbidity that would make single-lung ventilation dangerous [8].

Approaches/Techniques

Multiple approaches have been described to perform anterior thoracolumbar decompression and fusion. The variations largely hinge on avoiding vital structures during the approach. Traditionally, an open transthoracic approach was performed [9]. This technique involves placing the patient in the lateral decubitus position, with the hips and knees flexed to relax the ipsilateral psoas. Fluoroscopy is used to identify the targeted vertebral level. An oblique incision 4–6 inches in length is centered over the rib two at the desired surgical level. A right-sided approach is performed between the 3rd and 10th thoracic levels to avoid the great vessels, whereas a left-sided approach is employed to address the 11th thoracic through the 1st lumbar level. After incision through subcutaneous tissue, a thoracotomy is performed. Self-retaining retractors are placed and the lung is retracted carefully. The caudad rib is traced and the base excised subperiosteally taking care to divide the costovertebral ligaments. The corresponding intercostal nerve is identified and traced to confirm the correct level for disc excision. Once this level has been identified, the parietal pleura is then reflected, taking care to identify the segmental vessels that lie in the fatty tissue midway between the vertebral bodies above and below the level of the desired disc. The parietal pleura is then split longitudinally and segmental vessels ligated and divided. Subperiosteal dissection is performed to delineate the adjacent vertebral bodies, as well as the pedicle of the caudad vertebral body. The isolated intercostal nerve is again traced to identify the appropriate foramen. The inferior pedicle is removed, exposing the underlying dura and revealing the herniated disc. This window allows visualization of the lateral disc space. An annulotomy is performed and the mid-lateral portion of the disc is removed with a pituitary rongeur. The posterior annulus is addressed last and is bluntly freed from the dura using a penfield. Extruded or herniated disc material is then pulled into the cavity created by already removed disc material. After the disc has been addressed, the PLL is then bluntly dissected off the cord and removed. After discectomy, a cage or bone graft can be placed if fusion is desired. The windows created in the parietal pleura and thoracic cavity are closed with watertight layer [9].

In addition to an open approach, multiple minimally invasive approaches have been described. The first is a posterolateral extracavitary technique [10]. In this approach, the patient is positioned prone on a Jackson frame with the abdomen free. Neuromonitoring with SSEPs and MEPs are employed during the case. Fluoroscopy is used to identify the desired level, and a K-wire is placed percutaneously down the rib angle to the transverse process of the caudad verterbral body. A 2-cm vertical incision is made through fascia. A finger is then used to bluntly dissect and dilate muscle fibers to the transverse process and facet of the target level. Progressive dilators are placed to form port, typically up to 22 mm, and secured to the surgical able using a mounted arm. Biplanar fluoroscopy is again used to identify correct level. Ideally, the target disc should be parallel to and in the center of the working access of the portal on the lateral radiograph. On the AP, the lateral aspect of the pars interarticularis should lie in approximately the 20% horizontal meridian of the portal, ensuring that the trajectory is lateral and oblique enough to minimize the need for spinal cord retraction. Using an operating microscope, a combination of cautery and rongeurs are used to free the remaining soft tissue from the inferior transverse process-facet complex. A drill is used to remove the transverse process and expose the intertranverse ligament, which is opened sharply to access the underlying disc space. The lateral aspect of the lamina and the pars overlying the neural foramen are decompressed from lateral to medial, and the cephalad portion of the inferior pedicle is flattened with a drill to allow better access to the disc space. The ligamentum flavum is dissected off the underlying nerve root and lateral cord. Decompression of the flavum allows a near-lateral view of the spinal cord and disc space is obtained, highlighting any disc fragments. An annulotomy is performed allowing access to the disc space. Discectomy then performed, and endplates are curetted with placement of interbody cage if fusion is desired. Position of the cage is confirmed on fluoroscopy. It is important to slowly remove your retractors with cautery available as bleeding can be encountered during closure [10].

A lateral minimally invasive approach has also been described via a transthoracic window [11]. The patient is similarly placed in a lateral decubitus position with the bed broken at the affected level. The junction between the posterior and middle thirds of the disc space is marked on the skin under fluoroscopy. A 3–5 cm incision is centered over the mark which is perpendicular to the direct posterior approach. The subcutaneous tissue and intercostal muscle is divided, allowing access to the thoracic cavity. The cavity is entered over the superior edge of the rib is that overlying the affected disc space in order to avoid the neurovascular bundle. For a single level, dissection between the adjacent ribs and intercostal muscle is performed and pleural access is provided through blunt dissection. For a multi-level case, a small portion of the rib must be resected to allow adequate access. A dilator is used in the plane of the disc space to access posterior to the thoracic cavity, stopping at the junction of the rib head and vertebral body. Decompression of the disc space is then performed in similar fashion. This approach can also allow a transpleural window. During this approach, the parietal pleura is divided longitudinally. The rib head overlying the posterolateral corner of the disc is identified and removed, allowing access to the disc space. Standard closure is performed and a chest tube is placed if a transpleural window is employed [11].

In addition to open procedures, a thoracoscopic approach for discectomy has also previously been described [12]. General anesthesia is performed using a double lumen ET tube to allow for collapse of the ipsilateral lung. The patient is also placed in a lateral decubitus position. AP and lateral fluoroscopy used to localize endoscopic ports. Three or four ports are typically needed: one on the posterior axillary line, and an additional two ports on the anterior axillary line. The first port placed blindly above the superior aspect of the rib above, and the remaining two ports triangulated 8–10 cm apart, centered over the affected level. A Steinman pin is placed for spinal level localization. Lung retraction performed by rotating the surgical table anteriorly by 30°, with resection of pleural adhesions as needed. The parietal pleura over the proximal 2 cm of the rib head adjacent to the desired level is resected, and the proximal 2 cm of the rib is resected using a burr, exposing the lateral pedicle, neural foramen, and disc. The pedicle is removed using a drill, as well as small portions of the vertebral bodies adjacent to the affected disc. The target disc fragments are removed endoscopically and the spinal canal is decompressed [12].

Postoperative Care

Following extubation, the patient is transferred to the intensive care unit for monitoring. Antibiotics, analgesia, and drain removal protocol may differ based on the institution. Similar to other spinal procedures, deep venous thrombosis prophylaxis is generally mechanical. Due to the nature of single lung ventilation, the patient should be instructed to utilize incentive spirometry. The patient should ambulate early with a skilled physical therapist and postoperative standing X-rays should be taken [8]. Patients should be restricted from bending, twisting, or lifting for 4–6 weeks while the fusion forms. Bracing is not generally required.

Outcomes

Certain complications are unique to the anterior thoracic approach to the spine. Due to the intrathoracic nature of the approach, pulmonary effusion, or hemo/pnuemothorax are possible. As mentioned previously, incentive spirometry is critical to decrease atelectasis and subsequent pneumonia. Additionally, vascular or lymphatic structures such as the aorta, vena cava or thoracic duct, is possible. This approach should only be undertaken at a facility where a thoracic surgeon is available. As with other approaches to the spine, a low threshold for neurologic injury must be maintained. Evidence of Horner’s syndrome, changes in the neuromonitoring during the case, and postoperative neurologic deficits should raise suspicion for hematoma, dural injury, hardware malpositioning, or graft dislodgment. Radiography including MRI, CT, and X-ray should be obtained quickly to evaluate these possible etiologies [8].

In a case series of 121 patients treated with thoracoscopic resection of symptomatic herniated thoracic discs over 14 years, Wait et al. demonstrate improvements in myelopathy, radiculopathy, and back pain of 91.1%, 97.6%, and 86.5%, respectively, at a mean follow-up of 2.4 years [13]. Additionally, 97.4% reported they would undergo the same operation again. The thoracoscopic group was also reported to have shorter hospital stays, shorter chest tube duration, less estimated blood loss, fewer transfusions, and less risk of intercostal neuralgia compared to an unmatched thoracotomy cohort. These authors also report an initial complication rate of 28.3% in the first 6 years of the study which improved to 5.3% in the following 9 years. These complications included pleural effusion, durotomy, reintubation for respiratory distress, delayed fusion, and reoperation for residual disc [13].

Khoo et al. demonstrated that a MIS approach to the thoracic spine for discectomy and interbody fusion produced similar radiographic and clinical outcomes to an open approach at 1-year follow-up in 13 MIS patients compared to a matched cohort. [10] The MIS group had statistically significant improvements in estimated blood loss, operative time, duration of ICU stay, transfusion incidence, and overall length of stay [10].

Thoracic disc herniation is rare, with the incidence at 0.15 to 1.8%. In a small retrospective series of 12 patients with thoracic disc herniation, Ohnishi et al. describe an anterior open approach to the spine [14, 15]. These authors report results as excellent in two patients, good in two, fair in six and unchanged in two using the Japanese Orthopaedic Association score for thoracic myelopathy. No patient was classified as worse. They report pneumonia, chylothorax, and incisional pain as complications that resolved postoperatively [14].

Utilizing an anterior manubrium splitting and an extrapleural approach to the thoracic spine in 33 patients with a follow-up average of 8 years and 2 months, Fujimura et al. reported outcomes of thoracic myelopathy due to ossification of the posterior longitudinal ligament [16]. The authors report that the Japanese Orthopaedic Association score for thoracic myelopathy improved significantly at 1 year postoperatively, remained consistent through 5 years postoperatively, and decreased significantly at final follow-up. Postoperative complications included three cases of deterioration of thoracic myelopathy and four cases of extrapleural cerebrospinal fluid leakage [16].