Interbody Fusion Strategies in Thoracic and Sacral Overlap Diseases





Introduction


The thoracolumbar junction is subject to unique biomechanical forces owing to its existence as a transition zone. The thoracic spine is relatively rigid, largely in part to the costovertebral joints as well as coronally oriented facet joints. In contrast, the lumbar spine is more flexible because there are no rib articulations and more sagittally oriented facet joints. Importantly, this relationship may change with aging, as mild to moderate disk degeneration typically increases segmental lumbar motion. However, severe disk degeneration is often accompanied by subchondral sclerosis, ankylosis, and osteophyte formation, which limit lumbar mobility, thus reducing the motion difference of the thoracolumbar spine. In addition, segmental lordosis decreases as the spine progresses toward the apex of kyphosis, underscoring its importance with regard to sagittal balance.


The lumbosacral junction is also subject to “overlap” disease. Historically, lumbosacral fusion has been associated with a high rate of pseudarthrosis. The advent of interbody fusion and the use of pelvic fixation, with either iliac screw fixation or sacral alar iliac screws (S2AI), has improved the rate of arthrodesis across the lumbosacral (LS) junction.




Indications: Thoracolumbar Overlap Disease


Fusion


Bony fusion of this region is often required to maximize patient outcomes. The thoracolumbar junction is subject to higher biomechanical stresses that may lead to increased nonunion at region of the spine. As such, many surgeons support interbody devices to augment fusion rates.


Spondylolisthesis, although more common in the lumbar spine, is a common indication for instrumented fusion. Although the authors’ are unaware of any reports of interbody fusion rates at thoracolumbar junction owing to this pathology, lumbar interbody fusion rates have been shown to be equivalent or better than posterolateral fusion in the lumbar spine. By extrapolation, one may consider this technique to achieve similarly high bony fusion.


Tumor/Infection


Both tumor and infection may involve variable destruction of vertebral bodies and endplates. Malignancies, of which metastases are the most common, typically involve the vertebral bodies and spare the disk. Therefore, vertebral body cage constructs are more commonly needed than interbody cages. However, there may be isolated instances in which interbody cages are helpful if there is disk destruction with relative sparing of the body and endplates.


Spondylodiscitis is often amenable to interbody devices owing to anatomic involvement of this disease. Most commonly, the vertebral body becomes seeded via arterial spread and the disk subsequently is infected through local extension. Because of this, there is often significant disk space collapse, sclerotic endplate changes, and infected body, but with variable degrees of body destruction. If debridement of the collapsed disk space is all that is surgically needed, then an interbody device may be ideal both to restore vertebral body height and to assist in fusion.


Deformity


Adult spinal deformity is often treated with decompression and thoracolumbar fusion to address neurologic symptoms and provide postural correction. An understanding of relevant radiographic parameters and goals of deformity correction surgery are critical in minimizing complications and maximizing patient outcomes. Sagittal imbalance is the key driver of pain and disability in the adult patient with deformity. Regional alignment, global balance, and spinopelvic parameters are key measurements that must be taken into account when evaluating adjacent segment disease (ASD). In general, the accepted goals of deformity surgery are sagittal vertical axis less than 5 cm, pelvic tilt (PT) less than 15 degrees, and pelvic incidence minus lumbar lordosis plus or minus 10 degrees ( Fig. 22.1 ). It is important to note that these goals likely change in the aging population, and over correction often leads to postoperative complications, most notably instrumentation failure and proximal junction kyphosis/failure.




Fig. 22.1


Realignment objectives in the sagittal plane. Alignment parameters change with age and are likely different in the elderly population.

From Schwab F, Patel A, Ungar B, et al. Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine 2010;35(25):2224–2231.


Sagittal balance has major implications on patient reported outcomes and thus patients with a kyphotic deformity at the T12-L1 level may benefit from interbody support to maximize lordosis or make a correction at that level. Importantly, the type of technique may have implications on focal alignment. Whereas threaded interbodies have been shown to be kyphogenic, vertically oriented cages typically result in lordosis, albeit often under 5 degrees of change. More promising, direct lateral interbody fusion has been shown to result in 9 degrees of lordosis creation.


Proximal junctional kyphosis ( Fig. 22.2 ), a known complication of multilevel instrumented constructs, may be amenable to interbody instrumentation to help restore lordosis and provide anterior column support. Importantly, the integrity of the associated endplates must be assessed if a bony proximal junctional kyphosis is suspected. If damaged, interbody cages have a high rate of subsidence. Lateral interbody fusion is also often useful to address nonunion at the site of a previous pedicle subtraction osteotomy ( Fig. 22.3 ).




Fig. 22.2


A 63-year-old female patient with severe back and leg pain underwent a multilevel lumbar decompression with posterior column osteotomies and T10-ileum to address her stenosis and adult spinal deformity. She developed acute proximal junctional failure that required an extension of the fusion to T4.



Fig. 22.3


A 67-year-old male patient underwent a L2 pedicle subtraction osteotomy for sagittal plane imbalance. He developed a nonunion at the osteotomy site and required a revision L1-2 and L2-3 direct lateral interbody fusion.


Fracture


Certain thoracic injuries may be conducive to transforaminal interbody fusion. In a series of lower thoracic and thoracolumbar fracture dislocations, lordosis and high fusion rates were achieved. Without adequate endplate and vertebral body structural integrity, implant subsidence is common. Therefore, using interbody instrumentation to maintain disk space height as a primary endpoint has variable efficacy, even in the setting of pedicle screw instrumentation. However, interbody placement to assist in fracture healing may be beneficial.


Disk Herniation


Surgical management of thoracic and thoracolumbar disk herniations varies based on location. Laterally based herniations may be addressed with unilateral transpedicular decompressions, whereas more centrally based disks often need more extensile approaches, such as costotransversectomy, lateral extracavitary, or thoracotomy. More recently, a minimally invasive lateral approach for thoracic and thoracolumbar disk herniations has recently been described. Regardless of the approach, fusion is often indicated if enough bone was resected to cause instability. Because of the disk void after fragment removal, interbody fusion may be an ideal choice.


The type of interbody instrumentation is best dictated by the surgical approach used. From a posterior approach, both unilateral and bilateral transforaminal implants have been described. Lateral cage placement is ideal with either a formal thoracotomy or minimally invasive lateral approach. Regardless of the choice, an interbody instrument may be a good option to assist in disk height restoration and increase the fusion surface area.


Limitations


Accessing the anterior column or disk space at the thoracolumbar junction is often challenging owing to its unique anatomy and the presence of the spinal cord or conus medullaris. Unlike in the lumbar spine, the thecal sac cannot be retracted at the thoracolumbar (TL) junction. The traditional way to access the disk space at the TL junction is via a thoracotomy. Owing to the associated risks and morbidity with that approach, less invasive procedures have been developed. The direct lateral approach to the TL junction is a facile way to access the disk space and implant an interbody device for deformity correction and/or fusion. This can be done through a traditional retroperitoneal approach, or via a minimally invasive lateral approach (e.g., direct lateral anterior lumbar interbody fusion, eXtreme lumbar interbody fusion). A more traditional transforaminal decompression and interbody fusion is another option at the TL junction. For the purposes of this chapter, we describe the direct lateral approach.



Surgical Technique


Step 1: Following standard single lumen intubation, the patient is carefully placed in the lateral decubitus position on a radiolucent table. Our preference is to use a sand bag and tape to hold the patient in place during surgery. It is critical to have a perfect anteroposterior (AP) and lateral x-ray of the disk space. The operative table should be adjusted so that the disk space is orthogonal to the floor.


Step 2: The incision is then marked out using intraoperative fluoroscopy. An oblique 2- to 3-cm incision is then made over the disk space and the muscle fibers are sharply incised overlying the corresponding rib. For level 1 diskectomy and fusion, rib resection is typically not required. The chest cavity is then entered and the lung is swept anteriorly with the surgeon’s finger. At T12-L1 it is often necessary to cross the diaphragm, which should be done at the junction of the chest wall to allow for repair. A specialized retractor system with light source and neuromonitoring is then placed centered over the disk space. This is confirmed using AP and lateral intraoperative fluoroscopy.


Step 3: The disk and endplate cartilage is then removed using a combination of curettes, endplate shavers, and pituitary rongeurs. Trial-sized cages are then inserted into the disk space. Care should be taken not to over distract the disk, as this may increase the risk of subsidence. Once the disk space is satisfactorily prepared, the contralateral annulus is released using a Cobb elevator. This will allow for symmetric distraction, and assist in deformity correction if desired.


Step 4: The interbody cage is then filled with the bone graft substitute and/or biologic of choice and inserted into the disk space. Proper positioning of the implant is confirmed using AP and lateral fluoroscopy.


Step 5: Posterior pedicle screw fixation is then placed. The authors do not perform stand-alone direct lateral interbody fusions at the TL junction.


Step 6: The wound is then closed in layers using absorbable suture. If necessary, the insertion of the diaphragm is repaired to the chest wall. A chest tube is often placed if the pleural space was entered during the exposure.



Postoperative Care


Postoperative care of patients undergoing fusion across the thoracolumbar junction depends largely on the nature of the preoperative pathology, the surgical approach, the extent and morbidity of the procedure, and the incidence of intraoperative complications.


For anterior and lateral procedures requiring deflation of the ipsilateral lung, chest tubes are commonly left postoperatively to reduce the incidence of persistent pneumothoraxes, potentially obstructive serous and/or bloody fluid collections, and potentially to reduce strain on the healing diaphragm and parenchymal tissues.


Patients are mobilized with therapy services as soon as possible. It is rare that we use an orthosis in this patient population.


Complications


Potential for intraoperative complications at the thoracolumbar junction is high, owing to the complex and vulnerable anatomy of the abdominal-thoracic junction. The aorta lies just anterior and to the left of midline at the T12-L1 level, while the azygos vein lies to its right, along with the splanchnic nerves and thoracic duct. The inferior vena cava lies more anteriorly at this level, penetrating the diaphragm as high as at the T8 level and descending relatively anterolateral to these accessory veins and lymphatic structures. Much of this delicate and unforgiving anatomy may be unfamiliar to the orthopedic or neurologic surgeon who should employ added caution, vigilance, and patience during these procedures.


The thoracic duct is a thin-walled vesicular structure that returns lymphatic fluid from the abdominal viscera and lower extremities into the main circulation. The thoracic duct originates from the confluence of the lymphatic lumbar and intestinal trunks, known as the cisterna chyli , which is located midline at the mid-to-low lumbar spine. The thoracic duct ascends alongside the aorta as it pierces the diaphragm (T12 level), then ascends immediately anterior to the vertebral bodies and discoligamentous complexes on the right side of the body. Here it is especially vulnerable to transection or indirect tearing via overly aggressive diskectomy or distraction techniques. The thoracic duct terminates at the junction of the left internal jugular vein and the subclavian vein (approximately the T5 level). Injury to the thoracic duct can result in lymphatic spillage—up to 4 liters daily—resulting in a life-threatening chylothorax, which requires drainage and typically operative repair or ligation.


Surgeons accessing the thoracolumbar region must possess an intimate understanding of the anatomy of the diaphragm and its traversing structures. The diaphragmatic crura are a sling of tendons that ascend from the level of the L1 body and travel up and around the aorta and esophagus, which typically pierce the diaphragm at the T12 and T10 levels, respectively. The crura have left and right tendons that are contiguous with the anterior longitudinal ligament. During anterior and lateral approaches, the crura may need to be mobilized for access to the T12-L1 vertebral bodies and adjacent disk spaces. This is typically performed by leaving a tendinous cuff of about 1 cm from the vertebral body. Repair of the diaphragmatic crura during closure must be meticulous, robust, and anatomic. Failure to repair both the diaphragm, as well as the abdominal musculature fascia superficially, can result in a postoperative diaphragmatic or incisional abdominal hernia, respectively.


Complications associated with lateral approaches depend in part on the laterality of the approach. Left-sided approaches are often preferred in the upper lumbar spine as they avoid the liver and the mobilization of the vena cava. Vena cava lacerations or perforations are notoriously morbid, owing to their relatively thin-walled structures and extremely high current. Right-sided approaches, however, may be necessary for certain pathology such as accessing the convexity of a dextroscoliotic thoracolumbar curve. If extension above the thoracolumbar junction is anticipated, some authors advocate for a right-sided approach, given its avoidance of the artery of Adamkiewicz.


For extension into the lumbar spine, psoas muscle retraction and/or mobilization may be necessary. The complications associated with psoas violation or aggressive retraction are well documented, and often include L4 root injuries, psoas and/or quadriceps weakness, and thigh numbness. Complications are less likely to occur at the cranial regions of the psoas’ origin (L1-3) than in the thicker, caudal regions (L3-5); however, they can occur throughout. At the thoracolumbar junction and the superior lumbar spine, the psoas may be elevated subperiosteally, as opposed to using a trans-psoas approach; this may reduce chance of injury to the lumbar roots.


The mid-to-lower thoracic spine is considered a watershed area of the spinal cord and thus may be vulnerable to ischemia in the event of excessive manipulation, misplaced implants or instruments, or overly aggressive reduction maneuvers. Injury to the artery of Adamkiewicz, the largest anterior segmental medullary artery that supplies the inferior third of the spinal cord, is reported with anterior or lateral approaches at the thoracolumbar junction. Classically, the artery arises from a left posterior intercostal artery at a level between the 9th to 12th thoracic vertebra; however, in up to 25% to 30% of people, it arises on the right side. It can also originate as low as L2 and branch from the aorta proper or from other lumbar vessels. Injury to the artery of Adamkiewicz classically results in anterior spinal artery syndrome, marked by distal and disproportionate motor versus sensory loss, coupled with urinary and fecal incontinence.


Other intraoperative complications associated with accessing the thoracolumbar junction include injury to the sympathetic plexus, which can occur when exposing and/or ligating the intercostal vessels. Conus medullaris syndrome has also been reported with this approach, as has hepatic ischemia resulting from aggressive deformity correction and resultant tethering of the vascular celiac trunk by the diaphragmatic crura.


Pulmonary complications are relatively common in the postoperative setting, especially following procedures in which ipsilateral lung deflation is performed. Such complications include lung contusions, pulmonary effusions, atelectasis, persistent pneumothoraces, and pneumonia. Failure to perform a meticulous closure or intraoperative pleural injury can result in a postoperative pneumothorax. Postoperative chest tubes are often used to decrease related morbidities. Intraoperative vascular injury and failure to achieve adequate hemostasis—especially when ligation of segmental vessels is required—are associated with postoperative hemothoraces, requiring chest-tube decompression and often surgical reexploration.


Outcomes


Surgical outcomes of fusions performed at the thoracolumbar junction depend largely on the inciting pathology, patient comorbidities, and the nature and extent of the procedure. Overall, the thoracolumbar junction is a region vulnerable to both implant failure and bony nonunion, owing, in part, to its kyphotic inflection and the lever-arm generated by the rigid thoracic spine.


In a notable retrospective analysis of 96 patients with adult idiopathic scoliosis patients undergoing primary instrumented fusion demonstrated a 17% overall pseudoarthrosis rate, almost 60% of which occurred between T9 and L1. Additional risk factors identified for thoracolumbar pseudarthrosis include advanced age (>55 years), longer fusion constructions (12 or more vertebral levels), and preoperative thoracolumbar kyphotic curves of 20 or greater degrees. The same study team also identified osteoarthritis of the hip as well as use of the thoracoabdominal approach (versus the paramedian approach) as additional pseudarthrosis risk factors.


Despite the elevated risk of nonunion, several case series have demonstrated that effective kyphotic deformity correction can be achieved using thoracolumbar osteotomies and instrumentation junctional fusions. One study demonstrated an average 46-degree improvement in overall sagittal alignment, as well as improvements in lumbar lordosis and sagittal imbalance, utilizing posterior-only approaches. In patients with ankylosing spondylitis (AS), the thoracolumbar junction (especially T11-12 and T12-L1) is the single most common region for focal kyphotic deformity and pre interventional pseudarthrosis in an otherwise autofused spine. Historically, these patients with complex and rigid deformity have required combined anterior or extensive anterior alone procedures to correct their imbalance; however, improved instrumentation and osteotomy techniques are allowing surgeons to achieve excellent outcomes using posterior approaches alone. In a series of 30 consecutive adult patients with AS, Chang et al. reported an average of 38-degrees of junctional kyphosis correction without radiographic evidence of postoperative pseudarthrosis at an average of 4.7 years.


For shorter segmental fusions, including single- or double-level procedures, lateral interbody fusion has been demonstrated to be an effective and safe technique. A retrospective analysis of 22 lateral interbody fusions (14 patients) performed at the thoracolumbar junction (T11-12 and T12-L1) demonstrated bony union in all but one patient who died soon thereafter from metastatic cancer. The majority of such cases were augmented with posterior instrumented fixation. Albeit a retrospective and clinically disparate series, there were relatively few complications, allowing the authors to conclude that lateral interbody techniques can be effective in the thoracolumbar junction.


The thoracolumbar junction is disproportionately vulnerable to posttraumatic kyphotic deformity The case detailed in Fig. 22.4 demonstrates the occurrence and treatment of such sequelae. Several studies have explored the potential use of minimally invasive surgical (MIS) transforaminal interbody fusion techniques between T10 and L2 with somewhat mixed results. In a series of 15 patients with posttraumatic junctional deformities undergoing single-level MIS-TLIF procedures in an attempt to correct an average of 19.1 degrees of focal kyphosis, there was no significant improvement in kyphosis at an average follow-up of 26 months. Despite this less than ideal outcome, the authors reported no significant complications and union was achieved in 13 of 15 patients.


Nov 11, 2019 | Posted by in NEUROSURGERY | Comments Off on Interbody Fusion Strategies in Thoracic and Sacral Overlap Diseases
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