The Lateral Transpsoas Approach versus ALIF: Do the Risks of Lateral Interbody Fusion Outweigh the Benefits Compared to Anterior Lumbar Interbody Fusion?

5 The Lateral Transpsoas Approach versus ALIF: Do the Risks of Lateral Interbody Fusion Outweigh the Benefits Compared to Anterior Lumbar Interbody Fusion?

MIS: Steven M. Spitz, Hasan R. Syed, and Jean-Marc Voyadzis
Open: David L. Scott and Peter G. Whang

5.1 Introduction

Lumbar interbody fusion has been successfully used for decades to manage degenerative, neoplastic, developmental, and traumatic conditions of the lumbar spine. Potential advantages of interbody fusion over posterolateral arthrodesis include the ability to place a large graft under compressive forces to enhance fusion, restoration of disc height, foraminal distraction and indirect decompression, and deformity correction.1,2 Lumbar interbody fusion utilizing a posterior approach (posterior lumbar interbody fusion [PLIF]) was first described in 1952 by Cloward. This technique proved effective in achieving interbody fusion but required extensive nerve root and thecal sac retraction to allow for adequate disc excision for placement of the interbody graft.³ In an attempt to minimize the risk of nerve root retraction and injury, Harms and Jeszensky popularized a technique that utilized a transforaminal corridor (transforaminal lumbar interbody fusion [TLIF]) for interbody graft placement. This approach provides greater lateral access with enhanced visualization of the intervertebral disc.⁴ These traditional posterior procedures have proven to be relatively safe and effective in achieving spinal fusion with favorable clinical outcomes; however, they require significant soft-tissue dissection, muscle retraction, and the removal of osteoligamentous structures, all of which can adversely affect patient recovery.^3,5

The anterior transabdominal approach for interbody fusion (anterior lumbar interbody fusion [ALIF]), first described in 1932 by Capener for the treatment of spondylolisthesis and later modified to a retroperitoneal approach in 1960 by Harmon, provides wide anterior access to the lumbar disc space without disruption of the posterior spinal elements, with the advantage of placing a larger interbody graft than afforded by the posterior approaches.^6,7 Mayer in 1997 modified the retroperitoneal approach utilizing a smaller incision combined with a muscle-splitting exposure.⁸ There remains, however, an increased approach-related risk of vascular and visceral injury, along with sympathetic dysfunction in males.^9,10,11,12 Recently, technological advancements in spinal instrumentation along with an improved understanding of surgical anatomy have led to minimally invasive techniques to mitigate muscle damage and blood loss and to reduce patient recovery time incurred by traditional open spinal fusions. The lateral transpsoas approach to the lumbar spine (lateral lumbar interbody fusion [LLIF]) was first introduced in 2006 by Pimenta and colleagues as the extreme lateral interbody fusion (XLIF).¹³ Unlike ALIF, this approach does not require mobilization of the bowel, great vessels, or autonomic plexus. It involves the use of tubular retractors through a small incision that allows for a wide discectomy while maintaining the competence of the anterior and posterior longitudinal ligaments, and as such represents a true minimally disruptive technique. Only a few clinical studies directly comparing LLIF and ALIF have been published ( Table 5.1).

5.2 Indications of Lateral Lumbar Interbody Fusion

LLIF is generally indicated for the treatment of degenerative conditions of the spine, including degenerative disc disease, spondylolisthesis, degenerative scoliosis, mechanical instability, and facet arthropathy involving all lumbar levels with the exception of L5–S1. The ability to place a large interbody graft makes it ideal for the restoration of disc height and indirect decompression of severe foraminal stenosis and moderate canal stenosis in the setting of degenerative disc disease with radiculopathy 14,15 LLIF has also been used successfully for the treatment of Grade 1 to 2 spondylolisthesis ( Table 5.2). Lateral positioning, disc space preparation, and height expansion can allow for reduction, particularly in cases of overt instability. Multilevel LLIF has also been shown to be a powerful procedure for the correction of coronal deformities ( Table 5.3).^16,17 Other indications include nonunion, discitis and osteomyelitis, and trauma.^2,13,16 Instrumentation options following LLIF include lateral plating, pedicle or transfacet screws, or the use of an interspinous device.^{18,19,20,21,22} Recent biomechanical studies have shown that lateral plating is not as robust when compared to posterior fixation in resisting flexion and extension forces.^18,23 Posterior fixation typically requires prone repositioning, although posterior percutaneous screw insertion has been described in the lateral decubitus position.²² Several studies have shown that stand-alone LLIF, particularly with larger grafts, is safe and effective owing to the minimal disruption of osteoligamentous structures afforded by this approach ( Table 5.4).^15,24

Relative contraindications to LLIF include previous retroperitoneal surgery or fibrosis, vascular abnormalities, Grade 3 and above spondylolisthesis, severe disc space collapse with osteophyte formation, and L5–S1 pathology due to the anatomic constraints of the iliac crest. Patients with concomitant severe canal stenosis require a decompressive laminectomy because indirect decompression will be ineffective. This can be performed through a tubular retractor in the prone position after LLIF. Alternatively, a posterior approach for fusion with decompression should be considered. A standard preoperative anteroposterior (AP) X-ray is essential when considering an L4–L5 approach, given the presence of a “high-riding” crest at the L4 pedicle or above may complicate the approach. A “rising psoas sign” on axial magnetic imaging at L4–L5 manifested by a more ventral and lateral position of the psoas muscle with respect to the vertebral body may portend difficulty during the initial docking phase due to the proximity of the lumbar plexus.²⁵

5.3 Advantages of Lateral Lumbar Interbody Fusion

The LLIF technique utilizes a lateral retroperitoneal approach to access the psoas major muscle that is subsequently traversed to expose the lateral intervertebral disc space. This approach, as with the ALIF, provides a wide working corridor for intervertebral disc excision and end plate preparation, leading to a larger surface area available for fusion. Furthermore, this approach, as with ALIF, allows for the insertion of a larger interbody graft that engages the cortical ring apophysis, which may lessen the risk of subsidence and hence maintain disc height, indirect foraminal decompression, and/or deformity correction.20,26 The lateral transpsoas approach, in contradistinction to ALIF, does not require an approach surgeon and minimizes dissection or manipulation of the great vessels, viscera, and sympathetic chain, thus potentially mitigating the attendant risks associated with injury to these structures. In addition, LLIF maintains the structural integrity of the anterior and posterior longitudinal ligaments, reducing the potential for iatrogenic destabilization.^15,18 LLIF can be utilized to treat all lumbar levels with the exception of L5–S1.

The use of tubular retractors with LLIF reduces incision size and tissue dissection, thus minimizing blood loss, infection rates, and postoperative pain. This has translated into shortening hospital stays and faster return to work.^{2,5,16,24,27,28}

5.4 Indications for Anterior Lumbar Interbody Fusion

Despite ALIF’s long-standing use and popularity, evidence-based indications continue to evolve.29,30 Improved instrumentation, better graft materials, and a reduction in associated complications have expanded its clinical application. ALIF, in comparison to traditional posterior approaches, is associated with reduced operative times, results in less blood loss, and yields comparable outcomes. The anterior approach provides direct, unobstructed, and unmatched access to the disc space, allowing complete removal of disc material, meticulous preparation of the vertebral end plates, and the insertion of a large graft or implant. The technique may prove particularly useful for patients who have one or more risk factors for poor bone healing. The ALIF technique permits restoration of disc space height, correction of sagittal and coronal balance, and the reduction of spondylolisthesis. Posterior elements, including facet joints and pars, must be carefully scrutinized preoperatively to determine the need for ancillary stabilization.

Operative indications are often subjective (severe back pain), and surgery should be restricted to patients who have failed a reasonable trial of nonoperative care. Clinical research supports the use of ALIF for the following: grade I or II isthmic or degenerative spondylolisthesis (72–94% success); degenerative disc disease with disabling central or discogenic low back pain (71– 100% success); degenerative lumbar scoliosis; pseudoarthrosis after posterior procedures; recurrent disc herniation; and the revision or retrieval of failed XLIF grafts. Contraindications to ALIF include osteoporosis severe enough to compromise graft stability, a Grade 3 or higher spondylolisthesis, prior retroperitoneal surgery, severe peripheral vascular disease, active disc space infection, an infrarenal aortic aneurysm, an anomalous genitourinary system with only a single ureter, obesity, and men still desiring to father children. All patients should undergo a preoperative bone density testing and males with a history of sexual dysfunction referred for evaluation. Although posterior surgical approaches and techniques are generally indicated for patients with severe nerve compression, ALIF, either alone or with supplemental foraminal decompression, may be sufficient to improve modest radicular symptoms.

5.5 Advantages of Anterior Lumbar Interbody Fusion

The anterior approach to the spine has become popular, in part due to its relative ease and its potential for application to all lumbar segments. Degenerative disc disease most commonly affects the L4–L5 and L5–S1 disc levels. Lateral approaches are unable to provide access to the L5–S1 disc space due to the constraints of the bony pelvis. Surgery at the L4–L5 level may also prove impossible or difficult if the patient has relatively high-riding iliac crests. Access to more rostral levels (e.g., L2–L3) may be limited by the rib cage. Although orthopaedic and neurological surgeons who are uncomfortable with the anterior approach should rely on the assistance of a general or vascular access surgeon, well-trained spine surgeons can achieve safe and comparable outcomes.31

The anterior column of the spine bears 80% of the forces associated with axial physiological loads. ALIF reconstructs the anterior load-bearing column of the spine, places the graft under compression, thereby increasing the likelihood of fusion, and improves sagittal and coronal alignment. The ideal position for fused lumbar segments is in lordosis, and ALIF restores this through resection of the anterior longitudinal ligament, through discectomy and annular release, by allowing the insertion of large lordotic grafts and by retaining the posterior tension band. Lateral interbody techniques also preserve the posterior tension band but fail to provide an anterior release and often cannot accommodate comparably sized grafts or implants. Obtaining autograft, particularly from the iliac crests, is much easier with the supine positioning of ALIF and may not require a separate incision. Both anterior and lateral approaches leave the paravertebral muscles and facet joints untouched, and the spinal canal inviolate, and may reduce the potential for iatrogenic adjacent level disease.³²

The anterior approach provides an unobstructed view of the entire disc space, permitting complete removal of the disc and a thorough anterior decompression. Lateral interbody techniques, in contrast, provide a more limited view of the disc space and nociceptive disc remnants may be retained. Disc space elevation during ALIF graft or cage placement can increase spinal volume and neural foramen cross-sectional area by 20 and 30%, respectively. Additional anterior techniques such as microscopic anterior foraminal decompression have been developed to supplement this indirect nerve decompression.

Although resection of the anterior longitudinal ligament and the anterior annulus fibrosus during ALIF has been determined to be destabilizing, particularly in extension and axial rotation, the relevance of this biomechanical finding to fusion success and clinical outcome is unclear. Anterior fusion with structural grafting, threaded cages, or stand-alone anterior cages is often augmented with posterior pedicle screws (open or percutaneous), low-profile anterior plates, translaminar facet screws, and spinous process anchors. The posterior procedures significantly increase both operative time and costs but the increased stability appears to improve outcomes, particularly for patients with preoperative spondylolistheses or instability. Newer anterior devices constructed of PEEK (polyetheretherketone) with integrated locking screws offer more secure stand-alone fixation.

5.6 Case Illustration

A 50-year-old woman developed worsening low back and right leg pain that failed to respond to conservative measures, including physical therapy and epidural injections over the course of 1 year. Imaging studies demonstrate a Grade 1 spondylolisthesis at L4–L5 with a broad disc protrusion causing significant foraminal stenosis without significant canal stenosis ( Fig. 5.1). Flexion and extension radiographs showed 5 mm of anterolisthesis on flexion without reduction on extension.

5.7 Surgical Technique (Lateral Lumbar Interbody Fusion)

After starting intravenous lines and the administration of preoperative antibiotics, general endotracheal anesthesia is obtained. A Foley can be placed at the surgeon’s discretion. The side of the approach should be chosen based on a careful assessment of preoperative X-ray and magnetic resonance imaging (MRI). The patient is placed in a true lateral decubitus position with the table break at the midpoint of the iliac crest and greater trochanter. All pressure points are padded, and the leg is flexed to relax the psoas muscle. The patient’s chest and hip are secured to the table with elastic tape. The table is then flexed in such a way as to increase the distance between the iliac crest and the rib cage. Once in position, a proper fluoroscopic AP view is obtained demonstrating parallel end plates, with the spinous processes being equidistant to the pedicles. Proper lateral imaging is also confirmed, eliminating parallax of the end plates and pedicles with adjustments of the table. A Kirschner wire (K-wire) is used to identify the intervertebral disc of interest at its midpoint with fluoroscopy. A mark is made on the flank overlying the center of the affected disc space. The patient is then draped after antiseptic is applied and the skin is infiltrated with a local anesthetic.

A 3- to 4-cm transverse incision is made along Langer’s line. The incision is then carried down to the aponeurosis of the external oblique. Access to the retroperitoneal space is performed by blunt dissection in parallel to the orientation of the muscle fibers of the external oblique, followed by the internal oblique, and finally the transversus abdominis. Blunt dissection is of paramount importance so as not to injure the superficial sensory nerves (subcostal, iliohypogastric, ilioinguinal, and lateral femoral cutaneous) and motor fibers of the abdominal wall musculature.1,33 At this point, the retroperitoneal fat will be in view, and blunt dissection is continued using sweeping movements to mobilize the peritoneum and its contents anteriorly to visualize the psoas major muscle with the aid of hand-held retractors. Once the psoas major is in view, the surgeon’s index finger is then used to guide the first dilator through the abdominal musculature to the surface of the psoas major overlying the center of the disc space. A lateral fluoroscopic image is obtained to ensure that the first dilator is coaxial with the center of the disc space of interest.

Given the intrapsoas location of the lumbar contribution to the lumbosacral plexus, it is extremely important to have real-time electrophysiologic monitoring to aid in safe passage through the psoas muscle.^1,3,3,34,35 Each dilator is equipped with an isolated electrode at its distal tip. Prior to traversing the psoas muscle, a stimulation clip is applied to the proximal end of the dilator for intraoperative continuous electromyographic (EMG) monitoring. With this system, safe passage is most likely if the EMG threshold is greater than 8 mA, thus demonstrating that the nerve is a safe distance from the dilator. The dilator is passed through the psoas muscle and docked on the center of the disc space. At this point, a K-wire is advanced through the dilator to the middle of the disc space. Successively larger dilators are then placed over the K-wire, followed by the retractor system, which in turn is affixed to the table-mounted flexible arm. All these steps are performed with stimulation and EMG. Lateral fluoroscopy is obtained to confirm placement over the center of the index disc space. A blunt-tip nerve probe is then used to confirm the location of lumbar nerves outside the confines of the retractor system, and the working channel is secured to the disc space or vertebral body and expanded. If the dilation was effective, only scant muscular fibers are identified and easily swept away exposing the disc space. A discectomy is performed in standard fashion using a combination of pituitary rongeurs, end plate shavers, rasps, and curettes. A Cobb elevator is used to release the contralateral annulus with AP fluoroscopy to allow for distraction and disc height expansion. All cartilaginous material is meticulously removed from the end plates. A series of trials are then inserted to determine the appropriate dimensions of the spacer. The graft is filled with bone graft extenders and is then tapped into place as confirmed with AP and lateral fluoroscopy. The wound is copiously irrigated and hemostasis deemed appropriate with bipolar electrocautery and the application of commercially available hemostatic agents. The retractor is slowly removed and any bleeding cauterized under direct visualization to insure meticulous hemostasis. The fascia of the external oblique is closed when possible, and the wound is closed in layers with absorbable sutures.

Fig. 5.1 Imaging studies of a 50-year-old woman with low back and right leg pain that failed to improve with conservative measures. (a) Standing lateral X-ray and (b) sagittal and (c) axial MRI showing a Grade 1 L4–L5 spondylolisthesis and a broad disc protrusion causing significant foraminal stenosis without canal stenosis.

5.8 Surgical Technique (Anterior Lumbar Interbody Fusion)

The patient should undergo a thorough bowel clean-out the night prior to surgery. Prophylactic intravenous antibiotics are infused approximately 1 hour before the planned incision. The patient is positioned supine on the cystoscopy table with legs abducted (French position), allowing direct caudal access to the lower abdomen. Both arms are positioned at 90° to the plane of the operating table to permit unfettered lateral access to the abdomen by the surgeon and by the fluoroscopic unit. Vulnerable sites, including elbows and heels, are carefully padded to reduce the incidence of peripheral nerve injury. Additional lumbar lordosis, if needed, can be obtained either by reverse flexing of the table or by positioning a lumbar bump. Endotracheal intubation is performed, general anesthesia provided, and a Foley catheter placed. Ureteral stents may be advisable for women or for individuals with complex abdominal anatomy. Oxygenation probes and pulse oximeters are placed on toes and neuromonitoring considered. Patients should be typed and cross-matched. The abdomen is prepped, with inclusion of the anterior iliac crest if bone grafting is planned.

A left paramedian vertical incision is marked below the umbilicus with fluoroscopic guidance. The incision is carried through the subcutaneous tissues using electrocautery to expose the anterior rectus sheath. The sheath is divided longitudinally in the direction of its fibers with preservation of a tissue cuff to permit tight closure. The rectus muscle is retracted medially and the posterior rectus sheath/transversalis fascia divided as required. The ureters are identified and protected. Great care is also taken to prevent damage to the iliohypogastric and ilioinguinal nerves between the layers of the internal and transverse abdominal muscles.

The peritoneum and its contents are retracted medially by blunt dissection to expose the iliopsoas muscles and the anterior longitudinal ligament. Even small rents in the peritoneum are immediately repaired to reduce the possibility of herniation. A circular frame (e.g., Synthes SynFrame) is attached to the operating table and the abdominal contents gently retracted behind padded blades. The left common iliac artery and vein are traced to their bifurcations, and the iliolumbar vein (for L4– L5), middle sacral vessels (for L5–S1), and segmental vessels (for proximal exposure) are ligated and divided as necessary. The great vessels, including the aorta, inferior vena cava, and iliac, are mobilized to the right with hand-held retractors. Pressure on the vessels is ideally released at least hourly.

The appropriate disc level is confirmed by AP and lateral fluoroscopy. The annulus fibrosus is vertically incised at its midline and the two halves retracted laterally. The disc is incised and then completely removed. End plates are curetted, removing cartilaginous remnants while avoiding deep penetration. The disc space is distracted and the cage/graft size determined by an intraoperative assessment of annular tension with temporary trials.

Tricortical structural graft or cancellous graft for packing for the interior of titanium or PEEK spacers is harvested from the anterior iliac crest. For ALIF at the L4–L5 level, a separate incision is not needed. The graft or cage is impacted under lateral fluoroscopic guidance. The interbody can now be further stabilized, if indicated, with either anterior screws or a plate. The final construct is evaluated critically on both AP and lateral fluoroscopic views and the wound thoroughly lavaged. A suction drain is placed retroperitoneally, and the fascia, subcutaneous tissue, and skin closed meticulously.

Although this patient’s spondylolisthesis appears stable on flexion–extension views ( Fig. 5.1), she would still likely benefit from posterior stabilization. The degree of foraminal compromise, the radiological improvement seen with the ALIF, and the patient’s tolerance of the anterior procedure would be important variables in determining whether the patient undergoes a subsequent foraminotomy/open fusion or a percutaneous posterior procedure. In addition to the usual postoperative neural checks, lower extremity pulses, particularly the dorsalis pedis artery, must be carefully evaluated.