Minimally Invasive Approaches to the Thoracic Spine

A variety of differing surgical approaches have been proposed and implemented in the treatment of thoracic spine pathology, with the latest advancements geared toward minimally invasive options. ¹^, ²^, ³^, ⁴^, ⁵^, ⁶ The direct open posterior approach can be used in cases of purely dorsal disease but otherwise is unfavorable in the thoracic region because of the requirement for retraction of the thoracic cord instead of cauda equina nerve roots. ⁴^, ⁵^, ⁶^, ⁷ The thoracic cord is especially sensitive to minimal retraction, and this has been postulated to lead to the relatively poor outcomes traditionally seen with posterior approaches to more central and ventral pathology. ¹^, ⁴ These poor outcomes have led surgeons away from open direct posterior approaches to posterolateral approaches, including both costotransversectomy and transpedicular trajectories, which use more extensive bone removal to minimize manipulation of neurologic structures and have thus been shown to be safer than a direct posterior approach. These posterolateral approaches, however, result in removal of supportive bone structures that often necessitates fusion for prevention of postoperative instability, and can also lead to increased postoperative pain and morbidity. Open anterior and lateral approaches have also been used and are associated with complications related to the approach through the thoracic cavity such as risk of injury to vital thoracic structures and vessels: pulmonary contusion, hemothorax, chylothorax; intraoperative and postoperative difficulty with ventilation; shoulder girdle dysfunction; and difficulty with wound healing. ⁴

35.1.1 Minimally Invasive Approaches

Minimally invasive surgery (MIS) options were developed in an effort to decrease the morbidity related to the open approaches as described herein, without compromising the surgical goal of decompression. ⁵ The first minimally invasive options were adaptations of thoracoscopic and video-assisted thoracoscopic techniques (VATS) as used by thoracic surgeons for the anterior approach in an effort to avoid open thoracotomy. These approaches, although useful, still carry the risks associated with entering the thoracic cavity and have a steep learning curve for spine surgeons who are generally not familiar with VATS. These limitations likely account for the lack of widespread adoption of these options. ⁴^, ⁸

Minimally invasive decompression options include endoscopic lateral retropleural or extracavitary and minimally invasive transpedicular and thoracic microendoscopic decompression (TMED). ⁴ TMED is a modification of the lumbar microendoscopic technique. Benefits of this approach include sparing most of the pedicle, which must be removed in the transpedicular approach, and avoidance of rib resection, required in the lateral retropleural approach. ⁴^, ⁵^, ⁶ The use of the endoscope is not required for visualization during this approach, and a similar approach using tubular muscle retractors can be used for a variety of thoracic pathologies, with the use of loupe, microscope, or endoscopic visualization. ⁵^, ⁶ Once a laminectomy is performed through either a direct posterior approach or a more lateral transpedicular approach, depending on the angle of pathology, both ventral and dorsal decompression can be achieved, as well as durotomy and resection of intradural lesions. Tredway et al ⁹ successfully adapted a differing minimally invasive unilateral laminotomy approach for resection of intradural extramedullary lesions in both the cervical and thoracic spine. This adaptation led to further advancement in the treatment of thoracic pathologies involving more than one spinal segment and the generalizability of the nonendoscopic minimally invasive approach. This technique was further expanded by Smith et al, ⁶ who treated lesions extending over multiple levels through the use of an MIS hemilaminotomy posterior approach at the rostral and caudal end of the pathology. These lesions were uniquely able to be manipulated and removed through a rostral and caudal exposure; however, further adaptation of the MIS technique demonstrates the variety of pathology that can be treated posteriorly.

The lateral retropleural approach can allow easier access to for vertebral body decompression or midline ventral compressive lesions and can be performed in a fashion quite similar to lateral lumbar interbody fusion approaches using the same retractor system with long retraction blades. For cases of trauma or instability relating to tumor or approach, instrumentation can be achieved through the use of percutaneous screw placement with fluoroscopic or navigation guidance. A summary of the trajectories of the various approaches is shown in ▶ Fig. 35.1.

Fig. 35.1 Axial computed tomographic image in thoracic spine showing the trajectories of different approaches to the thoracic spine. The costotransversectomy approach is an open approach with the trajectory shown compared with various minimally invasive approaches. TMED, thoracic microendoscopic decompression.

35.2 Preoperative Prepration

35.2.1 Level Identification

One of the most important steps, irrespective of the technique used, is appropriate identification of the surgical level. Identification of the surgical level in the thoracic spine is more difficult than in the cervical or lumbar spine, where counting levels facilitates knowledge of the appropriate level. This is due to the distance of the thoracic spine from the skull or sacrum, individual variance in regional anatomy and the number of ribs which could be used for counting, and poor fluoroscopic penetration in upper thoracic levels, especially in patients with increased subcutaneous fat. We have found that careful preoperative examination of ribs and levels, combined with careful fluoroscopic intraoperative counting, allows identification of the appropriate level. Other described adjuncts for level identification include percutaneous placement of radiographic skin markers, percutaneous placement of a radio-opaque marker at the periosteum of the pedicle of interest, percutaneous injection of methylene-blue dye, and even preoperative vertebroplasty; however, none of these adjuncts has gained widespread use. ¹⁰ Depending on the procedure being performed, intraoperative neuronavigation can help with the identification of level but requires an intraoperative computed tomographic scan and is not usually of benefit in cases without instrumentation placement. At our center, we rely on anatomical landmarks and level counting with both lateral and anterior–posterior fluoroscopic views. Preoperative chest or thoracic spine radiographs are useful for determining the number of ribs visualized on anterior–posterior films.

35.2.2 Thoracic Spinal Cord Perfusion

The arterial supply to the thoracic spinal cord has less collateral supply than exists in either the cervical or lumbosacral regions, with most of the thoracic cord, aside from where radicular branches enter, existing as watershed territory. This results in an increased risk of both ischemia and subsequent infarction when the already limited blood supply is compromised through compression or other injury. The anterior spinal artery in the thoracic spine is dependent mostly on a single radicular artery known as the artery of Adamkiewicz, which is present on the left side 80% of the time and arises between the T9 and T12 nerve roots 75% of the time. When operating in this area, and there is potential for root sacrifice, a spinal angiogram or spinal vascular imaging may be of use to identify the location of the artery of Adamkiewicz as it is not readily identifiable intraoperatively and could be inadvertently sacrificed. ¹¹^, ¹²

Spinal perfusion pressure (SPP) is similar in concept to cerebral perfusion pressure and thus is equal to the patient’s intrathecal spinal fluid pressure subtracted from the mean arterial pressure (MAP). In cases of relative thoracic compression, such as from a thoracic disk herniation (TDH), the increased pressure is transmitted to the neural elements focally in the area of compression and this results in increased interstitial fluid pressures which act focally to decrease tissue perfusion. In this scenario, the SPP focally equals the MAP minus the interstitial fluid pressure of the thoracic spinal cord. The actual blood flow is related to both this measure of SPP and also the resistance of the arterial vessels in the spinal cord, providing the following equation: spinal cord blood flow = SPP/spinal cord vascular resistance. When examining this equation in the scenario of TDH and focal compression, the interstitial pressures result in an increase in SPP, and because of the decrease in blood flow, there is a compensatory vasodilation to decrease the spinal cord vascular resistance. This compensation is thought to occur through smooth muscle relaxation at the level of the precapillary sphincter of the penetrating arteriole into the spinal cord parenchyma. This compensatory mechanism is limited, and by raising MAP, we attempt to help with the relative decrease in blood flow before the mechanism is exhausted, resulting in ischemia and ultimately infarction. ¹¹^, ¹²

35.2.3 Mean Arterial Pressure Goals

The MAP goals are therefore critical in coordination of surgery with anesthesia. Zuckerman et al ¹¹ examined the intraoperative MAP goals in patients with TDH who had changes in their intraoperative somatosensory evoked potentials (SSEPs) or motor evoked potentials (MEPs) (both were used). Because of the increased pressure from the TDH, during general anesthesia, it is common practice to raise the patient’s MAP in an effort to maintain adequate arterial spinal perfusion. Zuckerman et al examined three patients who had decline in their monitoring associated with decrease in MAP at patient induction under general anesthesia, and with subsequent increases in MAP they had improvement in monitoring in two-thirds of the cases reviewed.

The goal of MAP increase is to maintain the spinal cord blood flow at a rate of 10 mL/ 100 g of tissue per minute, thought to be the minimal amount of flow required to avoid infarction. Multiple studies have associated intraoperative hypotension in spine surgery with adverse events. The surgeries varied in these studies and included cervical procedures, deformity correction, and thoracic diskectomy. Based on this literature, the MAP goal is probably best based on the patient’s baseline values of MAP, with Zuckerman et al recommending MAP at 110% or greater of the baseline value until cord decompression is achieved. Previous goals have ranged between > 70 mm Hg and > 90 mm Hg. Preinduction placement of an arterial line in patients with severe compression who are thought to be at risk of ischemia helps to monitor MAP closely during the high-risk time period of induction. It is also useful to assess the patient’s preoperative volume status and optimize their volume status as much as possible. ¹¹

35.3 Operative Procedure

35.3.1 Thoracic Minimally Invasive Endoscopic Diskectomy

The TMED ²^, ⁴^, ¹³ procedure is done with the patient in the prone position and under general anesthetic. A radiolucent Wilson frame or a Jackson table with appropriate chest and hip pads facilitates use of fluoroscopy during the case. Arms can be tucked with sheets for upper thoracic cases and positioned on arm boards for lower thoracic cases, with care to appropriately pad the elbows, and especially the ulnar nerve, as well avoiding extension of the arms greater than 90 degrees. It is the practice of the senior author to obtain continuous SSEPs as well as MEPs throughout the procedure.

Once the appropriate level has been identified and marked as described already herein, an incision is made 1.5 to 2 cm lateral to the midline. Through the incision, a Kirschner wire (K-wire) is inserted at the rostral side of the caudal transverse process of the level of interest. The first tubular dilator is then placed over the K-wire under fluoroscopic guidance. The K-wire is then removed before placement of the remaining dilators. This ensures maximum safety during the dilation process. After dilation is complete, the tubular retractor is placed over the dilators and fixed to the rigid retractor arm, attached to the operating room table. Through the tubular retractor, a microscope, loupes, and a headlight or an endoscope with a 30-degree lens can be used for visualization. When using the endoscope, it is useful to orient the scope such that medial is located at the top of the monitor and lateral at the bottom bringing the rostral–caudal axis along the horizontal.

Remnant muscle and soft tissue at the bottom of the tubular retractor are then dissected away using monopolar cautery and can be removed from the field with a pituitary rongeur. With this small amount of soft tissue removal, the interlaminar space is identified. A hemilaminotomy can then be performed using either a Kerrison punch or a drill. Lateral exposure is continued until approximately half of the medial facet is removed. This enables access to the medial epidural space without retracting the lateral dural edge. The anulus of the disk is then incised, and the disk fragment can be removed with a combination of curets and rongeurs. As a result of the lateral trajectory, minimal to no manipulation of the thecal sac is required for entry into the disk space. Laterally placed disk herniations are readily identified, and for more medial fragments, they can be dissected away from the thecal sac, underneath the anulus, with down-pushing curets into the disk space, where they are then safely retrieved. Drilling of the portion of the pedicle overlying the disk space can help to improve exposure of the disk space if required. An example of the intraoperative visualization is shown in ▶ Fig. 35.2.

Fig. 35.2 Intraoperative view through tubular retractor in the thoracic microendoscopic decompression approach (TMED). Anatomical labels show the rostral (R), caudal (C), medial (M), and lateral (L) orientation. The dorsal thecal sac is visible with a suture in place. The lateral remnant portion of the thoracic facet is labelled with F.

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