Techniques to Decrease the Invasiveness of Thoracic Deformity Correction

31 Techniques to Decrease the Invasiveness of Thoracic Deformity Correction

Brenton Pennicooke, Robert Nicholas Hernandez, and Michael S. Virk

Summary

This chapter summarizes techniques and technologies that can be used to minimize the invasiveness of surgery for pathologies in the thoracic spine. Given the complexity of the surgical goals, this is clearly a frontier in MISS and we can expect significant innovation to occur in these areas.

Keywords: minimally invasive thoracic spine surgery MIS thoracic deformity correction thoracic spine surgery thoracic spinal deformity

31.1 Introduction

Traditional open surgical approaches to the thoracic spine for the correction of thoracic deformity include posterior techniques (posterolateral, transpedicular, costotransversectomy, lateral extracavitary) and anterior techniques (thoracotomy). An approach that allows access to both the posterior elements to achieve decompression and to the anterior vertebral column for circumferential release is required for correction of major deformities. Access to the anterior column via a posterior-only approach is commonly performed, but can result in neurologic injury as it requires reaching around the spinal cord to access the anterior column. Traditionally, thoracic deformities are corrected with extensive exposures that allow full view of and access to the bony anatomy of the vertebral column. Bony surface area is also required for arthrodesis and the longevity of the construct. However, such wide exposures are associated with high morbidity and complication rates due to blood loss, operative time, and postoperative hospital stays. The International Spine Study Group recently published a retrospective review that identified a 78% overall complication rate and 61% major complication rate associated with three-column osteotomy in patients with adult spinal deformity.1

Minimally invasive spine surgery (MISS) is a rapidly advancing field that has shown some benefits over traditional open approaches for the treatment of spinal pathology, particularly in the lumbar spine.²^,³^,⁴^,⁵ This chapter aims to describe techniques spine surgeons can employ to decrease the level of invasiveness of thoracic deformity correction with an emphasis on minimally invasive surgery (MIS) techniques.

31.2 Computer-Assisted Navigation/Robotics/Planning Software

The advent of computer-assisted navigation (CAN) has and continues to advance the field of MISS. Because of the smaller incisions and ports employed by MISS procedures, surgeons cannot rely on traditional anatomical landmarks and orientation reference points, particularly in patients with deformity. CAN facilitates precise anatomical localization when landmarks remain obscured by overlying muscle and soft tissue. Surgeons can readily identify their position and desired targets. Modular reference arrays can be attached to essentially all intraoperative instruments and calibrated for use during the procedure. Such devices include drills, pedicle probes, taps, drivers, osteotomes, and interbody device/cage introducers. Pedicle screws and cages can be sized virtually and projected over the intraoperatively acquired imaging to optimize dimensions during the planning phase as well as placement location. CAN reduces the incidence of pedicle breaches, particularly in patients with small thoracic pedicles. Thus, CAN allows surgeons to safely access and instrument the spine, as well as decompress the neural elements, through smaller incisions and ports when the surrounding anatomy cannot be directly visualized. Multiple studies have demonstrated that the use of CAN results in a lower thoracic pedicle screw breach rate compared to fluoroscopy-guided pedicle screw placement. Two meta-analyses reported the thoracic pedicle screw breach rate with the use of CAN to be 5.4 to 8.7% and the breach rate with fluoroscopic guidance to be 15.1 to 22.9%.⁶^,⁷ Moreover, CAN facilitated faster screw insertion, lower complication rate, and less intraoperative blood loss, but a longer overall operative time compared to fluoroscopy.⁶ With increased repetitions leading to greater experience and smoother workflow, overall operative time generally decreases. Ultimately, experienced users will have less misplaced screws to revise intraoperatively and patients will not leave the operating room until optimal screw placement is confirmed radiographically, thus decreasing revisions for screw misplacement.

Advancements in intraoperative computed tomography (CT) scans has increased the utility and accuracy of using CAN for the placement of pedicle screws and osteotomies using minimally invasive techniques. The intraoperative CT with CAN increases pedicle screw accuracy to 99% compared to 94.1% accuracy with free-hand techniques.⁸^,⁹ Additionally, intraoperative CT allows for the merger of previously acquired imaging studies, magnetic resonance imaging (MRI) or vascular imaging, for a more detailed and nuanced assessment of spinal anatomy. For example, the intraoperative CT can be merged with an MRI to localize specific areas of neural element compression, which require a direct decompression, or to identify vascular structures. The integration of CAN with robotic pedicle screw placement and associated planning software has allowed for further technological advancements in minimally invasive techniques. Planning software is used for preoperative planning of screw placement and osteotomies; such planning can be used in a predictive fashion to estimate correction based on the size and location of osteotomies. It can also facilitate more accurate rod placement by putting pedicle screw tulip heads in the optimal alignment. It may be possible to save considerable time intraoperatively by well-planned rod contouring and passing through the use of software, particularly for long constructs. Most recently, CAN is being integrated with augmented reality (AR), which is a technology that superimposes a computer-generated image on a user’s view of the real world. AR provides the surgeon with real-time evaluation of deep bony spine structure prior to any incision being made.¹⁰^,¹¹ Though it has not been extensively utilized in the operating room, there is potential for AR to be used for intraoperative planning of MISS procedures in the future.

31.3 Minimally Invasive Approaches to Three-Column Osteotomies and Expandable Cages

Posterior vertebral osteotomies can provide a significant amount of deformity correction in the sagittal and coronal planes and are, therefore, a mainstay technique for deformity surgeons. However, using MIS techniques for osteotomies can be challenging and does not allow for the same degree of deformity correction as open osteotomies.12 The advent of expandable cages has augmented the ability to achieve a correction through a small corridor to the anterior column. Corridors to the anterior column include direct anterior approach, anterolateral (retropleural) approach, posterolateral (lateral extracavitary or costotransversectomy) approach, and posterior (transpedicular) approach; and each utilizes either expandable retractors or thoracoscopic techniques to allow minimal violation of surrounding tissue en route to the targeted thoracic vertebra; however, each has specific advantages and disadvantages.¹³

31.3.1 Anterior (Transthoracic)

Earlier, direct anterior access to the thoracic spine for deformity correction required a large thoracotomy. This is an invasive procedure, requiring a chest tube to be placed, and is associated with a number of complications including hemothorax, pulmonary contusion, and pleural effusions.¹⁴^,¹⁵^,¹⁶ Nevertheless, video-assisted thoracoscopy techniques have been developed, which minimize the size of the incision and tissue dissection needed to access this corridor. Though technically challenging with a steep learning curve,¹⁷^,¹⁸ some authors have suggested that thoracoscopic techniques have been equivalent to open thoracotomy for spinal cord decompression and instrumentation of the anterior vertebral column.¹⁹^,²⁰

31.3.2 Anterolateral (Retropleural)

Scheufler illustrated the use of expandable cages in a series of 38 patients who underwent a vertebrectomy for kyphoscoliosis correction²¹ using an MIS anterolateral retropleural approach and a combined lateral extrapleural/extraperitoneal thoracolumbar approach to achieve 19.3 degrees of kyphotic correction, on average. These lateral approaches for a vertebrectomy with the placement of an expandable cage can utilize laterally projecting screws that engage the inferior end plate and superior end plates of the corresponding levels above and below the expandable cage, as illustrated in Fig. 31.1. Lateral trajectory screws do not provide equivalent reduction in range of motion as the traditional pedicle screws but prevent the expandable cage from translating laterally through the approach corridor.²² Thus, expandable cages frequently require posterior percutaneous screw placement to augment fixation until fusion is obtained. This involves repositioning the patient from the lateral position to a supine position. Uribe et al further describes an MIS anterolateral retropleural approach in a study employing a cadaveric model and four patients.²³ These minimally invasive approaches that aim to keep the pleura intact preclude chest tube placement postoperatively. However, a high rate of pleural violation has been reported, which remains a significant limitation of this minimally invasive approach.²⁴

Fig. 31.1 Cadaveric photograph of a lateral approach to the lower thoracic spine (top) and placement of a corpectomy cage (bottom) for a patient in a left lateral decubitus position. (Reprinted with permission from Operative Neurosurgery, Oxford University Press.)

31.3.3 Posterolateral (Lateral Extracavitary)

The use of an expandable cage for thoracic deformity correction requires a corpectomy or complete vertebrectomy. MIS techniques allow for direct lines of approach to the intended vertebra and, in certain approaches, permit a more favorable angle to the targeted vertebra than equivalent approaches during an open procedure. In a cadaveric study, it was shown that minimally invasive corpectomy can be performed posterolaterally using a starting point 3, 6, and 9 off midline, as illustrated in Fig. 31.2.²⁵ Kim et al described this posterolateral approach using a minimally invasive technique in four clinical cases, reporting that they could safely remove 93% of the bone over the ventral canal and 80% of the corresponding vertebral body for implantation of an expandable cage.²⁶ A similar approach was utilized by Musacchio et al, who also utilized a second port for a contralateral transpedicular approach to allow for a circumferential decompression.²⁷

Fig. 31.2 Computed tomography (CT) with navigation showing the trajectories for a posterolateral minimally invasive corpectomy using a starting point 3, 6, and 9 cm off midline (top). In particular, a trajectory starting 9 cm off midline allows for a complete resection of the vertebral body to the contralateral pedicle (bottom). (Reprinted with permission from Clinical Spine Surgery, Wolters Kluwer Health, Inc.)

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