Pre-psoas (Oblique) Lateral Interbody Fusion

Introduction

Lumbar interbody fusion (LIF) is a well-accepted treatment for spinal pathologies. Traditionally, LIF is performed using posterior approaches. These open strategies require broad dissection of the paraspinal musculature and nerve root retraction to access the disk space for interbody fusion. A more refined minimally invasive method, transforaminal lumbar interbody fusion (TLIF), approaches the disk space through a tube via a more posterior oblique trajectory. Access to the disk space is gained through the Kambin’s triangle. This posterior variant approach preserves the contralateral musculoligamentous elements and avoids excessive manipulation of the dural sac. Therefore, many of the complications associated with traditional open posterior approaches are mitigated. TLIF remains an important interbody fusion strategy. Nonetheless, the technique relies on a small surgical window—a particularly limiting factor when attempting more extensive diskectomy, graft placement, and fusion.

Anterior lumbar interbody fusion (ALIF) affords the most impressive fusion area of any surgical technique. The improvements in arthrodesis can be attributed to superb visualization of the disk space. This allows for robust removal of the disk and placement of the graft with maximal biological footprint. Other theoretic benefits of ALIF include avoidance of the spinal canal and the potential for more impressive degrees of correction via sectioning of the anterior longitudinal ligament (ALL). Nonetheless, ALIF is not without significant risks. ALIF requires entry in the abdominal cavity, which may be intraperitoneal or retroperitoneal, and careful dissection of important vascular structures. This puts the patient at risk for potentially devastating hollow-viscous and large-vessel injuries (including the iliac and great vessels). Other important considerations include ureteral injury, sexual dysfunction, and the need for a secondary access surgeon.

Various strategies have been proposed to improve on ALIF. These include laparoscopic, endoscopic, and mini-open approaches. Although strong proponents of these respective methods remain, many now consider laparoscopic strategies as historical stepping-stones. Laparoscopic methods, however, have laid the foundation for the more modern lateral lumbar interbody fusion strategies (LLIF). These include a number of synonyms, namely the direct lateral (DLIF) and extreme lateral (XLIF) approaches.

The LLIF technique allows for lateral exposure of the disk space through a retroperitoneal route. The traditional approach is a transpsoas approach, which allows for clearance of the disk tissue in the anterior column. This approach affords access to the disk space, avoiding the spinal canal posteriorly, and abdominal cavity and large vessels anteriorly. Other benefits of LLIF include no need for a secondary access surgeon, reduced incidence of ileus, preservation of the ALL, lack of bony resections, reduced operative time, reduced hospital stay, and reduced analgesic requirements. These features have slowly made the LLIF an acceptable method for lumbar interbody fusion. Nonetheless, certain issues remain if the transpsoas LLIF is to be embraced as a gold-standard interbody fusion strategy. Chief among them include injury to the lumbar plexus with neurologic injury and access to the lumbosacral junction. In a broader sense, it remains to be seen whether minimally invasive (MIS) techniques such as this can, as a whole, accomplish comparable metrics in global coronal and sagittal alignment.

In this chapter, we discuss an alternative lateral interbody approach, the pre-psoas (oblique) lateral interbody fusion (OLIF) technique. We review anatomic considerations of the approach including the rationale for the shift from a transpsoas to a pre-psoas trajectory and limitations of accessing the L5-S1 disk space via the lateral approach. Our discussion offers a step-by-step description of the technique including patient positioning, preoperative planning, operative procedure, and closure. We discuss common complications and management strategies. In our discussion, we review the available literature for preliminary outcome data and discuss future endeavors moving forward. Finally, we introduce relatively new techniques, such as lordotic cages, used to improve global alignment—a primary limiting factor in MIS correction.

Limitations and Anatomic Considerations

A thorough anatomic understanding is a vital tool in the spinal surgeon’s armamentarium. This is particularly important given the shift toward minimally invasive methods. Surgeons once trained in posterior approaches must now familiarize themselves with the novel anatomic landscape of OLIF.

Landmarks

A comprehensive review must not overlook key anatomic landmarks. The intercrestal line is the theoretic plane between respective superior iliac crests. The landmark classically represents the L4-5 interspace. Surgeons must be weary of variations based on abdominal circumference, body mass index, age, gender, and degree of flexion. Furthermore, the intercrestal line denotes the lower limit of the minimally invasive transpsoas lateral approach as the iliac crest hinders lateral L5-S1 disk space entry. An oblique anterior pre-psoas approach (OLIF) allows surgeons access to the lumbosacral junction avoiding the iliac crest and this could be a distinct advantage of OLIF over LLIF.

Superficial Musculature

The abdominal musculature consists of the external oblique, internal oblique, and transverse abdominis from superficial to deep. It is important to note the direction of the respective muscle fibers. In particular, the external oblique travels diagonally from the superior-lateral to the inferior-medial direction. This plane is followed during superficial dissection in order to improve cosmesis with closure. Another important consideration is the inferior lumbar triangle, also known as Petit’s triangle ( Fig. 12.1 ). This anatomic space is bordered by the iliac crest inferiorly, latissimus dorsi posteriorly, and external abdominal oblique anteriorly. The area could be targeted during the initial dissection in order to gain access to the retroperitoneal space.

Deep Musculature

The psoas muscle lies posterior to the retroperitoneal space. It traverses on the lateral surface of vertebral bodies and intervertebral spaces and originates from the anterior surface of the transverse processes from T12 to L5 and eventually inserts inferiorly on the lesser trochanter of the femur. Embedded within the muscle fibers of the psoas run key components of the lumbosacral plexus. Careful attention must be given in order to avoid damage to these neuronal structures. Another important deep muscle is the quadratus lumborum. This spinal stabilizer attaches to the transverse processes, 12th rib, and iliac crest. Fig. 12.2 illustrates the musculature of the lumbar spine.

Lumbosacral Plexus

The lumbosacral plexus arises from the ventral rami of T12-S3. The major terminal branches of the lumbar plexus include the iliohypogastric (L1), ilioinguinal (L1), genitofemoral (L1-2), lateral femoral cutaneous (L2-3), femoral (L2-4), and oburator (L2-4) nerves ( Fig. 12.3 ). These branches have mixed motor and sensory innervation with the exception of the lateral femoral cutaneous nerve, a purely sensory nerve. The plexus originates posterior and medial to the psoas muscle and descends in an anterior and lateral direction, traversing through the fibers of the psoas muscle.

Benglis et al. measured the ratio of the plexus location relative to the disk length on three cadavers. The plexus migrated in a progressively ventral direction with a ratio of 0 at L1-2 and 0.28 at L4-5. Moro et al. detailed the distribution of the lumbar plexus based on six cadaveric specimens. From L1-5, vertebral bodies were divided into four zones from anterior to posterior. All parts of the lumbar plexus were found in zone IV and dorsally at levels L2-3 and above. Excluding the genitofemoral nerve, the plexus was found posterior to zone II from L4-5 and above ( Fig. 12.4 ). Uribe et al. dissected 20 lumbar segments and divided vertebral bodies into 4 similar zones to identify anatomic “safe zones” during the lateral retroperitoneal transpsoas approach. These safe zones were found at the midpoint of zone III and posterior from L1-4 and at the midpoint of the vertebral body at the L4-5 disk space.

Positioning

The patient is placed in the lateral decubitus position (LDP). A right LDP (with left side facing up) is preferred as it avoids the inferior vena cava and right common iliac vein that lies slightly right and anterior to the vertebral body, especially at L4-5. We prefer to access the left side always with the OLIF approach. The iliac crest is placed slightly below the kidney rest table to maximize the opening between the ribs and iliac crest when the kidney rest is elevated. With the OLIF procedure, unlike the transpsoas procedure, the iliac crest is not as much of a problem because the incision is anterior and avoids the iliac crest. We continue to use short-latency somatosensory evoked potentials (SSEP) with both real time and triggered electromyography (EMG), although again with the pre-psoas approach it may not be needed as the lumbar plexus is not transgressed. Generous padding, axillary rolls, and tape are used to maintain the patient’s position, prevent stretching or compression injuries, and minimize skin abrasion. Neutral positioning of the head is particularly important in avoiding brachial plexus disturbances.

Fluoroscopic images are next obtained with the C-arm perpendicular to the floor. Adjustments to obtain a true-lateral image are made by tilting the table rather than rotating the C-arm. This ensures that the surgical intervention remains perpendicular relative to the floor and maintains the surgeon’s sense of the spinal anatomy. A true-lateral image must clearly delineate super-imposed pedicles and a “flat” superior endplate ( Fig. 12.5 ). It is also possible to perform the procedure with intraoperative 3D image acquisition and navigation. Some systems allow one to navigate all instruments and implants, thus no need for intraoperative fluoroscopy.

Procedure

The true lateral image is obtained to confirm the relative anatomy and identify the appropriate disk space. Anatomic landmarks, including the iliac crest, help pinpoint the lumbar triangle (Petit’s triangle). The disk spaces are marked and an incision is marked two finger breadths anterior to the anterior margin of the disk. We tend to access the L4-5 and L3-4 with one incision, L2-3 and L1-2 through another, and the L5-S1 is accessed separately ( Fig. 12.6 ). The patient is prepared and draped in the usual manner. The incision is made splitting the fibers of the external and internal oblique muscle along their orientation. The surgeon then uses a single finger to push through the transversalis fascia toward the iliac crest and sweep the peritoneum off the inside of the iliac crest and over the psoas to gain entry to the retroperitoneal space.

The anatomy of retroperitoneal space is explored using tactile stimuli. Upon entering the retroperitoneal space, the quadratus lumborum can be palpated posterolaterally. Following the muscle medially, the transverse process of the respective vertebrae and overlying psoas can be recognized. The under surface of the rib and inner surface of the iliac crest can also be palpated to confirm entry into the retroperitoneal space. Fig. 12.7 is an illustration of the retroperitoneal space along with the relative anatomy.

A retractor is brought in to reflect the anterior contents of the abdomen and expose the anterior border of the psoas muscle ( Fig. 12.8 ). A sponge stick or peanut dissector may be used to dissect the soft tissue over the psoas to expose the anterior border of the psoas. Once the relevant anatomy is appreciated, a blunt dilator is guided down to the disk just anterior to the psoas muscle ( Fig. 12.9A ). A lateral fluoroscopic image is obtained to confirm the appropriate position of the first dilator anterior to the psoas ( Fig. 12.9B ). The ideal position for the dilator is the anterior third of the disk space just behind the ALL. A more anterior approach reduces potential injury to the lumbosacral plexus and its branches housed within the psoas muscle. Once a satisfactory position is confirmed, triggered-EMG (t-EMG) is used to stimulate the dilator and confirm safe distance from neural elements. The guidewire is then advanced through the initial dilator and into the disk space. Fluoroscopic images are obtained to confirm the final location of the guidewire. Dilators are placed with sequentially larger diameters to retract the surrounding soft tissue ( Fig. 12.10A,B ).