Lateral Transpsoas and Anteropsoas Interbody Fusion Indications and Techniques

Indications and Contraindications

The goals of LTIF and LAIF are to access the lumbar disc space safely, release the lateral annular attachments, remove disc material, and place a structural graft. The objectives are increased interbody height, restoration of collapse or deformity, and stabilization of interbody motion. The current indications include interbody access to the lumbar spine for stenosis (specifically foraminal), spondylolisthesis (grade I or II), degenerative scoliotic deformity, and disk disease from L1 to L5 (S1 for LAIF). ^, Relative contraindications include severe osteoporosis, active infection, history of serious retroperitoneal infection or disease, as well as prior retroperitoneal dissection or injury. Additional contraindications for LAIF include severe central canal stenosis and high grade spondylolisthesis. These procedures can be performed at L1-2, but depending on the anatomy may require either removal of, or significant maneuvering around, the descending twelfth rib ( Fig. 157.1 ). The ability to perform LTIF at the L4-5 level depends on the anatomy of the iliac crest, and has a higher chance of nerve root injury at this level. ^, LTIF at L5-S1 is generally contraindicated due to obstruction by the iliac wing (see Fig. 157.1 ). ^, In contrast, LAIF has been demonstrated as a feasible approach to the L5-S1 disc space, with its main limitation based on the level of bifurcation of the common iliac veins which determines their need for retraction and thus increased risk for injury. ^, For many conditions, these approaches can be combined with anterior longitudinal ligament sectioning, staged posterior decompression, and posterior–lateral fusion if necessary to achieve surgical goals.

Clinical Results

Both LTIF and LAIF have been shown in the literature to adequately and successfully decompress neural elements. Oliveira et al. first demonstrated how XLIF could be used to indirectly decompress the neural elements, and found an average increase in disk height, foraminal height, cross-sectional foraminal area, and central spinal canal diameter of 42%, 13%, 25%, and 33%, respectively, when compared to posterior techniques. Later series showed an even greater average increase in cross-sectional foraminal area, mean anterior disk height, and posterior disk height of 35%, 58%, and 70%, respectively. Lang et al. reviewed the literature and found that XLIF achieved similar clinical and radiographic results compared to TLIF and ALIF for foraminal stenosis. However, the data were inconsistent with regard to the effect on central canal and lateral recess stenosis. Similar results of neural decompression and radiographic increases in canal decompression were noted with the LAIF approach. Additional studies on LAIF have demonstrated significant increase in the spinal canal area. ^,

These approaches have been shown to be especially beneficial in treating certain complex scoliotic deformities, as they provide excellent coronal and sagittal corrective ability given the opportunity for using larger interbody grafts ( Fig. 157.2 ). LTIF has demonstrated improvements in mean segmental lordosis (from −5.3° pre- to −8.2° postoperatively), yet with no effect on the global lumbar lordosis. Similarly, local segmental coronal plane changes were achieved (from 21° pre- to 10° postoperatively), without an effect on the global coronal plane. Phillips et al. evaluated adult degenerative scoliosis treated with XLIF and found statistically significant improvements in average ODI and VAS scores for back and leg pain, as well as SF-36 physical and mental components. Kotwal et al. published a prospective series of MIS-LIF degenerative scoliosis, with 2-year follow-up showing average improvement in VAS of 53%, ODI of 43%, and SF-12 of 41%. A review of XLIF for degenerative spinal deformity concluded that the procedure is a promising alternative for coronal deformity correction, but not for lumbar lordosis or sagittal balance. LAIF had been shown in degenerative lumbar kyphoscoliosis to significantly improve balance parameters, axial back pain, and to significantly decrease Cobb angles from 16° to 9.3° on average or about 2.5° per level. ^,

LTIF and LAIF can also both be performed with a minimally invasive approach. Youssef et al. report an average improvement in VAS and ODI scores of 77% and 56%, respectively, with MIS-LTIF. Ahmadian et al. also found significant improvements in ODI, VAS pain, and SF-36 general health measures. In 2017, Jin et al. compared MIS-LAIF to MIS-DLIF and found no significant difference in perioperative parameters and early clinical outcomes. However, they reported LAIF possibly having less approach-related perioperative morbidities.

MIS-LTIF procedures may also be utilized for lumbar total disc replacement (TDR). Pimenta et al. reported patients treated with the Extreme Lateral Total Disc Replacement (XL-TDR, NuVasive) system had VAS back pain decreased from 8.5 pre- to 2.5 postoperatively on average and ODI improved from an average of 54% pre- to 21% postoperatively. Moreover, Malham and Parker reported significant improvements in back and leg pain, ODI, and SF-36 physical and mental component scores using this system. Also, as part of the XL TDR clinical trial, Tohmeh and Smith showed clinically significant improvements in pain and function with few complications. Artificial discs placed in the lateral position have not yet been evaluated or approved by the FDA in the United States. Additionally, no published reports of placing artificial discs from an LAIF approach have been published to date.

One advantage of these procedures over traditional posterior only approaches are that they may be combined with other operations depending on the patient’s needs. For instance, LTIF can be combined with other minimally invasive techniques, such as transaxial lumbar interbody fusion. Heo et al. utilized a combined endoscopic disc resection, followed by LAIF to achieve decompression without the need for repositioning for a posterior laminectomy.

Surgical Anatomy

As in all surgical disciplines, a thorough understanding of the surgical anatomy involved in these approaches is crucial for maximizing patient benefit and complication avoidance. For LTIF, the most critical anatomy is the distribution of the lumbar plexus within the psoas muscle, because the approach inevitably requires the surgeon to traverse the psoas muscle, which places the lumbar plexus at risk of injury. For LAIF, the most critical anatomy is the great vessels, specifically at what level the common iliac vessels branch. Much of the anatomy for these two approaches is similar and thus will be described in this section together.

The lateral abdominal wall is formed by three layers: the outer and inner oblique, and transversus abdominis muscles, with fibers running in at right angles to each other. Their attachments include the iliac crest, ribs, serratus anterior, latissimus dorsi, and abdominal rectus sheath. Beneath these layers is the pre-peritoneum, a potential space filled with pre-peritoneal fat. For the LAIF procedure, this is an important layer to identify early, as this is the plane that is to be followed into the retroperitoneal space and excellent fascial reapproximation and closure is vital to avoiding the formation of a postoperative hernia. The paired psoas major muscles form a major part of the posterior abdominal wall. They are long, thick, and fusiform shaped, and lie lateral to the lumbar vertebrae. They arise from the roots of the lumbar transverse processes and pass inferolaterally, deep to the inguinal ligament, to reach and insert onto the lesser trochanter of the femur. Another major component of the posterior abdominal wall is the paired quadratus lumborum muscles that form a thick muscular sheet in the posterior abdominal wall alongside the lumbar vertebral column. They lie posterior and lateral to the origin of the psoas muscle.

The lumbar plexus is embedded mainly in the posterior portion of the psoas, anterior to the lumbar transverse processes ( Fig. 157.3 ). It is composed of the ventral rami of the L1 through L4 roots. Major cutaneous branches include (1) the ilioinguinal and iliohypogastric nerves (L1), which supply the skin of the suprapubic and inguinal regions; (2) the genitofemoral nerve (L1 and L2), which supplies the cremaster muscle and the skin over femoral triangle; and (3) the lateral femoral cutaneous nerve (L2 and L3), which supplies the skin on the anterolateral surface of the thigh. The genitofemoral nerve pierces the anterior surface of the psoas muscle and runs inferiorly, deep to the psoas fascia. The two major motor branches of lumbar plexus are the obturator nerve (L2-L4), which emerges from the lower part of the medial border of psoas muscle and supplies the adductor muscles, and the femoral nerve (L2-L4), which emerges from the lower part of the lateral border of psoas muscle and supplies the hip flexors and knee extensors.

In 2003, Moro et al. defined the relationship between the psoas major muscle and the lumbar plexus using cadaveric dissection. Excluding the genitofemoral nerve, the roots and critical branches of the lumbar plexus were found to be overlapping with the dorsal half of the vertebral column above L4-5 in a lateral projection view. The genitofemoral nerve, however, traverses through the psoas to emerge on the ventral surface between the rostral third of the L3 and the L4 vertebral bodies. When the genitofemoral nerve is taken into account, only the ventral half of the vertebral column above L2-3 is free of the lumbar plexus. The safest corridor, then, for the MIS-LTIF approach is the ventral half of the vertebral body above L2-3. Damage to the genitofemoral nerve usually causes only a transient sensory disturbance to the ipsilateral scrotum and medial thigh, which rarely becomes a serious problem. The ventral half of the vertebral column above L4-5 is therefore considered safe if transient genitofemoral nerve dysfunction is acceptable.

Not all levels are equally accessed by these approaches. The LAIF has the distinct advantage of direct visualization and increased avoidance of the neuromuscular structures that are at risk in the transpsoas LTIF approach. Accessing the lateral vertebral body via the LTIF approach at or below L5 carries significant risk of damaging critical structures such as the L4 and L5 nerve roots, femoral nerve, and/or obturator nerve. Thus, although there is considerably more space between the psoas major muscle and the quadrates lumborum muscle at L5-S1 compared to at L4-5 and above, L5 and below are generally not amenable to LTIF. This is primarily because of interference by the iliac crest with the approach at L5 and caudal. Benglis et al. also found an obvious dorsal to ventral migration of the lumbar contribution to the lumbosacral plexus within the psoas muscle from L2 to L5 in their cadaver studies, which would make the LTIF approach more dangerous at lower levels. During an LAIF approach, multiple significant structures are potentially encountered including the lumbar plexus, iliohypogastric and ilioinguinal nerve after passing through internal oblique, genitofemoral nerve on the anterior aspect of the psoas muscle, ureter, segmental vessels, and great vessels.

Using the lateral transpsoas approach in cadaveric dissection to identify the structures at risk with transpsoas K-wire and dilator placement, Banagan et al. found a serious potential anatomic problem: The nerve roots and the genitofemoral nerve were at risk in all their dissections in which the transpsoas approach is re-created. K-wire placement caused damage to the nerve root in 25% of cases at L4-5. It was also found that the lumbar plexus was placed under tension after sequential dilator placement even when no direct injury occurred during insertion. In addition to the lumbar plexus, the sympathetic chain was identified in the anterior third of the psoas over the disc spaces of L1 to L4, putting it at risk of potential damage with the transpsoas approach. Davis et al. found the femoral nerve is consistently at risk as it crosses the L4-5 interspace and can be compressed against the L5 transverse process when retractors are used during the procedures. In contrast, an LAIF cadaver study has demonstrated the feasibility of MIS-LAIF from L2 to S1.

Using a morphometric analysis of the ventral lumbar nerve roots and large vessels with the vertebral end plate, from hundreds of MRI studies, Regev et al. found the overlap of either nerve roots or large vessels with vertebral body end plates gradually increased from L1 to L5. At the L4-5 level, the overlap can reach up to 87%, resulting in a very narrow corridor for the LTIF approach. Scoliosis was found to further decrease the potential safe corridor for LTIF. In similar studies, Molinares cautioned approaching the L4-S1 levels with an LAIF approach due to the variations in iliac vessel bifurcation, but overall reported the technical feasibility from L2-S1. These anatomic and morphometric studies show the importance for neuromonitoring or direct visualization when establishing safe passage through the psoas muscle during LTIF procedures. Yet, without traversing the caudal aspect of the psoas, and the addition of direct intra-op visualization, the majority of current literature does not support the use of neuromonitoring during LAIF.

Surgical Technique

Open Lateral Transpsoas Interbody Fusion

Access to the lateral lumbar spine using an open approach allows for several potential surgical dissection planes. In the lateral extracavitary approach popularized by Larson et al., the lumbar spine is accessed with the plane of dissection posterior to the quadrates lumborum and anterolateral to the erector spinae muscles. The erector spinae muscle group is elevated and retracted medially to expose the lateral elements of the spine. In 2002, Wolfla et al. described a retroperitoneal L2 to L5 lumbar interbody fusion using a true lateral trajectory to treat symptomatic nonunion. The access to the lumbar spine was provided by retraction of the psoas and quadrates lumborum muscles posteriorly. In their series of 15 patients with painful pseudarthrosis from one or more (average 2.1) previous posterior lumbar operations, 87% had significant improvement after open LTIF, and a 90% radiographic fusion rate was reported. In 1997, Mayer first described the LAIF technique with an approach similar to ALIF. In 2012, Silvestre et al. performed the first true LAIF via a mini-open anterior retroperitoneal lumbar interbody fusion using the “Sliding Window Technique” of utilizing the same skin incision for up to three consecutive disc spaces.

In the open approach, the patient is placed in the lateral decubitus position in a plane perpendicular to the floor to facilitate obtaining lateral radiographs ( Fig. 157.4 ). Generally, a left-sided approach is chosen for preferential retraction of the descending aorta as opposed to the inferior vena cava, the main reason being the difficulty of repairing venous structures compared to arteries in case of vascular injury. In addition, for upper lumbar levels, the liver may prevent right-sided exposure. The ipsilateral lower extremity is preferably flexed at the hip to reduce tension on the psoas muscle. However, caution should be taken in not positioning the patient with too much side bend, as this may increase the risk of L4 nerve root traction injury due to tension on the psoas muscle and decreased perfusion to the nerve.

A standard left flank retroperitoneal exposure is performed based on the required level of exposure. With LAIF the incision is generally 2 finger breaths anterior to the iliac crest. Access can reliably be provided from L2 to L5 in LTIF, hindered superiorly by the crura of the diaphragm and inferiorly by the ileum. Conversely, access with the LAIF can be as wide as L1-S1, hindered mainly by the 12th rib, variant iliac bone, or iliac vessel anatomy. After skin incision, the external oblique muscle and fascia are exposed and divided along its fibers. The underlying internal oblique and transverse abdominis muscles are then transected. After the deep fascia of the transverse abdominis is opened, the retroperitoneal space is entered. Blunt finger dissection is used to strip the retroperitoneal contents anteriorly away from the quadratus lumborum and psoas muscles ( Fig. 157.5 ), exposing the anterior spinal column and the great vessels. After this step, and until the disc space is encountered, these two approaches diverge greatly. In the LTIF, with or without neuromonitoring, the psoas is divided and retracted along with the quadratus lumborum dorsally, exposing the lumbar vertebrae. The ureter, genitofemoral nerve, and sympathetic chain are identified and protected. During an LAIF, the psoas muscle is retracted laterally and the anterolateral attachments are dissected with great care for any neurovascular structures, as well as the ureters. The great vessels are retracted medially. The posterior border of the ALL is identified to serve as an important landmark later for the placement of the interbody fusion material. To perform interbody fusion, all disc spaces to be fitted with instrumentation are opened from the lateral border of the ALL to the base of the transverse process; the discs are then removed using angled curettes and rongeurs with the aid of self-retaining retractors. During this step in the LTIF, the assistant must retract the psoas muscle out of the way manually. Retraction of the psoas should be from ventral to dorsal to protect the traversing nerve roots in the psoas muscles.

After the disc space is prepared, an implant (e.g., a polyetherketone, or PEEK, cage) is chosen to ensure solid engagement to the bone surface with maximal height restoration. Lateral radiography is used to confirm a true lateral trajectory in LTIF, and then the tang retractor is placed into the disc space laterally. In the LAIF, radiography is also utilized to confirm an oblique trajectory.

One of the benefits of the open lateral lumbar approach is the possibility of performing a vertebrectomy for extensive lumbar vertebral infection or resection of neoplasm. In addition, by placing an angled retractor into the disc space of L5-S1 at a 35- to 45-degree angle from the true lateral plane, it is possible to place a cage into the L5-S1 disc space using the same incision and thus avoid the iliac crest. However, this is generally more difficult to perform compared to ALIF and LAIF at L5-S1.