Lateral Lumbar Interbody Fusion

Overview

Conditions such as spinal deformity, degenerative disk disease, adjacent segment disease, low-grade spondylolisthesis, spinal oncology, and traumatic deformity or instability are examples of conditions that may require instrumented spinal fusion. There are a variety of approaches to the spine, and these include anterior, posterior, and combined approaches. The choice of approach is largely dependent on the nature and location of the spinal pathology, surgeon preference and experience, and patient medical comorbidities. The lateral approach to the spine uses a retroperitoneal dissection to access the lateral aspect of the vertebral body or intervertebral disk. The minimally invasive lateral transpsoas approach for spinal fusion, also known as direct lateral interbody fusion (DLIF) or extreme lateral interbody fusion (XLIF), is designed to provide lateral access to the intervertebral disk and lateral vertebral body. This technique involves a retroperitoneal transpsoas dissection by splitting the fibers of the psoas muscle body to minimize the approach-related morbidity of an open lateral approach.

Pimenta first introduced the idea of a lateral approach to the anterior spine in 2001, and Ozgur later popularized the lateral transpsoas approach in what he called the “extreme lateral interbody fusion.” Although this technique has been expanded to include performing a corpectomy through a minimally invasive transpsoas approach, the focus of this chapter will be on lateral interbody fusion. The biomechanical advantages of using an interbody fusion to augment the anterior and middle column have been demonstrated and take advantage of the increased load sharing of the vertebral body compared with the posterior column. Furthermore, the use of the DLIF approach allows for indirect neural decompression without exposing the thecal sac or the nerve roots. Likewise, the transpsoas approach does not require mobilization of the great vessels, nor does it carry the risk of retrograde ejaculation associated with a transabdominal retroperitoneal approach for anterior lumbar interbody fusion (ALIF).

The goal of the lateral transpsoas approach is to deliver a large interbody graft, while minimizing blood loss, and to reduce approach-related morbidity associated with larger lateral approaches. One of the greatest risks of the lateral transpsoas approach is injury to the lumbar plexus and genitofemoral nerve during the approach and dissection through the psoas muscle. The risk of neural injury can be minimized with the use of multimodal neuromonitoring and appropriate radiographic guidance.

Indications

The lateral transpsoas approach can be used for any condition that requires access to the interbody space from T12–L1 through L4–L5 ( Fig. 47-1 ). This approach cannot be used at L5–S1 because of the location of the iliac crest, which obstructs direct lateral access. Likewise, the lumbar plexus courses more anteriorly at the more caudal levels of the lumbar spine, and the iliac vasculature courses more laterally at the more caudal levels; thus both are at great risk. Oftentimes, and particularly in men, the L4–L5 disk space is also not accessible because of the size of the iliac crest. In the setting of a lumbar scoliosis, the more caudal levels may be accessible only on the convexity of the curvature, because the approach angle is more rostral ( Fig. 47-2 ). Acosta and colleagues showed that a large interbody graft delivered through a lateral transpsoas approach can provide some degree of coronal correction and focal restoration of sagittal alignment. Although the lateral transpsoas approach can have many applications, the ideal candidate is typically a patient with focal coronal imbalance or disk degeneration who does not require direct neural decompression. For example, a patient with adjacent segment degeneration above a prior posterolateral fusion may benefit from a lateral transpsoas interbody fusion (LTIF), because the interbody can restore some disk height and can supplement extension of the posterior instrumented fusion ( Figs. 47-3 and 47-4 ). The posterior elements do not need to be disrupted, and the challenges of posterior revision surgery can be avoided; however, the patient must have favorable anatomy in terms of access to the intervertebral space and the working channel between the twelfth rib, and the iliac crest must be such that the procedure can be done safely and effectively.

Contraindications

There are a few technical/anatomic aspects that preclude the use of a lateral transpsoas approach in certain circumstances. For example, the lumbar plexus courses progressively more anteriorly at the more caudal levels. Thus despite the use of neuromonitoring, the risk of nerve damage at the level of L5–S1 is significant, and the lateral transpsoas approach should be avoided. Likewise, the iliac crest can often block direct lateral access to L5–S1.

Contraindications to the use of LTIF without posterior column support center on the biomechanical factors at a given level. Stand-alone LTIF should not be used at a level of high biomechanical stress, such as adjacent to a previous fusion or with a high-grade spondylolisthesis. In the setting of increased segmental stress, such as a pars fracture or at the apex of a kyphosis or scoliosis, posterior column support should be strongly considered. Posterior stabilization is often necessary to increase the stability of the construct, because lateral fixation has been not been shown to add construct stiffness compared with lateral interbody fusion alone. The lateral transpsoas interbody approach is also contraindicated in patients who have undergone prior retroperitoneal surgery or those with a retroperitoneal abscess. Preoperative imaging may reveal abnormal vascular anatomy or an abnormally large psoas muscle that prevents safe access to the lateral spine. Any patient who requires direct neural decompression is also a poor candidate for a lateral transpsoas approach, because the lateral interbody fusion provides only indirect decompression with restoration of disk height and ligamentotaxis. Although LTIF has been shown to improve focal coronal alignment, it has not been shown to provide meaningful global sagittal correction.

Preoperative Planning

Careful study of preoperative imaging is essential when planning LTIF. The patient’s anatomy must be closely evaluated to ensure that the disk space can be accessed safely and effectively. For example, a large psoas muscle, seen best on magnetic resonance imaging (MRI; Fig. 47-5 ) may prevent a transpsoas dissection. Likewise, the anatomic location of the aorta, inferior vena cava, and iliac vessels must be completely visualized to minimize the risk of vascular injury. When planning an approach to the upper lumbar spine, the eleventh or twelfth ribs may block direct access, thus necessitating an intercostal approach or a rib resection. The height of the iliac crest must also be taken into consideration, because it can block not only L5–S1 but sometimes L4–L5 also.

When performing an LTIF in the setting of lumbar scoliosis, the disk space can be accessed from either the concavity or the convexity of the curve. The advantages of approaching from the convexity include the fact that the lateral access of the spine is closer to the abdominal surface, thus minimizing the working depth through the tube. Likewise, the disk space is often widened on the convex side, making entering the disk space easier. Conversely, although the concavity is deeper, it allows the surgeon to reach multiple levels through a single incision. However, the lateral aspect of the disk space is often more collapsed on the concavity, making access to the disk space more difficult. The lumbar plexus also runs more anteriorly on the concavity, increasing the risk of nerve injury during the approach.

The approach to the T12–L1 and L1–L2 disk spaces is transdiaphragmatic. Thus it is important to plan for an intrathoracic exposure. Most often taking down the diaphragm does not necessitate the placement of a chest tube postoperatively, unless the pleura or lung parenchyma has been violated. It is important to close the diaphragm in layers completely, which can be done over a red rubber catheter, draining the intrathoracic space. After the final suture is placed, a Valsalva maneuver is performed, the red rubber catheter is removed, and the suture is tied down securely. Having support from colleagues in thoracic surgery is essential in the event of a complication.

Intraoperative use of fluoroscopy is essential when performing LTIF, and it is important to ensure that the appropriate radiology staff are available during the case. Intraoperative stereotactic navigation is an alternative to fluoroscopic guidance. The use of fluoroscopy provides significant radiation exposure; to minimize that exposure, stereotactic navigation can be used. However, the advantage of fluoroscopy is that it provides real-time anatomic assessment. Stereotactic navigation only provides a static image of the anatomy. As the diskectomy is performed, or in the setting of placing multiple interbodies, the navigation registration can become inaccurate. Likewise, the reference frame for stereotactic navigation must remain undisturbed throughout the case, or the navigation will become inaccurate.

Addressing the risks and benefits of LTIF with the patient before surgery is essential. The greatest risk is injury to the lumbar plexus. As many as 36% of patients will have ipsilateral iliopsoas weakness postoperatively, and the most commonly affected levels are L3–L4 and L4–L5. Moller and colleagues have also reported that 84% of those with subjective ipsilateral iliopsoas weakness improved completely by 6 months postoperatively. The etiology of such weakness is multifactorial in nature and includes dissection through the psoas muscle, edema, nerve stretch, and placement of the tubular retractors through the muscle down to the level of the lateral annulus. This risk can be minimized by docking the tubular retractor superficial to the psoas muscle and performing careful intramuscular dissection guided by neuromonitoring and direct visualization of the genitofemoral nerve.

Operative Technique

Positioning is a key component to performing a safe and successful LTIF. The patient is placed in the lateral decubitus position with the hip, not the waist, over the break in the operating table ( Fig. 47-6 ). A beanbag can be used to help maintain position. The lateral aspect of the bottom knee must be thoroughly padded to reduce the risk of peroneal nerve compression. Likewise, the top leg should be bent as much as possible to relax the psoas muscle to aid in dissection. A pillow should be placed between the patient’s legs, an axillary roll should be placed along the downside lateral chest wall, and all bony prominences should be fully padded to reduce the risk of additional injury. The bed should then be flexed to help open the lateral disk space on the side of approach. This also increases the working space between the twelfth rib and the iliac crest. Finally, the patient must be well secured to the bed. During the case, the fluoroscopy C-arm will remain in the neutral position in both the anterior-posterior (AP) and lateral planes, and thus the patient and bed can be manipulated to obtain true AP and lateral images. The patient must also be placed in a position on the bed such that the C-arm can freely pass beneath the table.