Relevant Surgical Anatomy of the Lateral and Anterior Lumbar Spine

Introduction

Anterior and lateral approaches to the lumbar spine are performed with increasing frequency and for a wider range of indications. To avoid complications and maximize patient outcomes, a clear understanding of the anatomy encountered during these approaches is necessary. Here we consider the bony, vascular, and neural anatomy most pertinent to the anterior and lateral transpsoas approaches.

Bony and Ligamentous Anatomy

The lumbar spine consists of five kidney-shaped vertebral bodies bordered by the thoracic spine above and the sacrum below ( Fig. 4.1 ). These five vertebral bodies typically have a combined lordosis of 20 to 45 degrees. Each vertebral body consists of a central depression surrounded by an apophyseal ring. The intervertebral disk sits in this depression between adjacent vertebral bodies. The pedicles, lamina, and spinous process form the boundaries of the spinal canal and compose the posterior elements. Facet joints link the superior and inferior articulating processes of adjacent vertebral bodies posteriorly. These posterior elements are not visualized during anterior or lateral approaches to the spine ( Fig. 4.2 ).

Anterior approaches allow direct visualization of the anterior lumbar spine and typically allow intervention at the L4-5 and the L5-S1 disk spaces. The lateral approaches typically utilize dilators and minimal access retractor systems with fluoroscopic visualization of the exact position of the retractor. Direct visualization and neural monitoring form a critical component of safe access to the lumbar spine. Not all levels of the lumbar spine (e.g., L5-S1) can be accessed via a lateral transpsoas approach. Careful preoperative evaluation with a lateral x-ray demonstrating the position of the iliac crests in relation to the vertebral bodies will help determine the lowest accessible disk space. Superior disk space access may be limited by the ribcage or diaphragm, although modifications of the approach may still allow access.

The ligaments most commonly encountered in anterior and lateral approaches to the lumbar spine are the anterior longitudinal ligament and the posterior longitudinal ligament. The anterior longitudinal ligament spans the entire spine and increases in width along the rostral-caudal axis. This multi-layered ligament is encountered early in the anterior approach and must be divided to access the disk space. In lateral approaches it provides a protective layer between the disk space and the large vessels located immediately anteriorly that are not directly visualized. This ligament also provides an anterior tension band preventing hyperextension when left in situ; however, with care, release of this ligament can allow for greater correction of sagittal deformity. Although the posterior longitudinal ligament does not contribute to stability to the extent of the anterior longitudinal ligament, it prevents herniation of nucleus pulposus centrally into the spinal canal. This ligament is not disrupted in either the anterior or lateral approaches but defines a plane posteriorly between the disk space and the spinal canal. The contralateral ligament is routinely released during lateral approaches and care must be taken during left-sided approaches to prevent the interbody graft from injuring vessels on the contralateral side. This risk is increased in patients with deformity, especially with axial rotation, as the vessels may lie outside their usual location.

Musculature of the Lumbar Spine

In anterior approaches the spinal musculature is not violated. Instead, the muscle layers divided are those of the abdominal wall. Closure of these layers and the fascia is important to prevent the development of true abdominal wall hernias. These are to be distinguished from abdominal wall pseudo hernias that are caused by abdominal wall weakness secondary to a neural injury (e.g., subcostal nerve). Lateral approaches also spare the paraspinous muscles but do require passing through the psoas muscle. The psoas muscle originates from the transverse processes and lateral vertebral bodies of L1-5 and along with the iliacus muscle inserts into the femur after passing under the posterior inguinal ligament ( Fig. 4.3 ).

Vascular Anatomy

Large Vessels of the Retroperitoneum

Anterior to the vertebral bodies, in close proximity within the retroperitoneal space, lie the aorta, its terminal branches, and the inferior vena cava (IVC). The close anatomic relationship of these large vessels to the lumbar spine places them at greater risk during lateral and anterior approaches than in posterior approaches. Cadaveric and radiologic studies have sought to describe more thoroughly these anatomic relationships in an attempt to define a safe working corridor as injuries to these vessels can cause serious morbidity and even death.

True anterior retroperitoneal lumbar spine approaches typically involve a paramedian or low abdominal incision to gain access to the retroperitoneum.

Access to the L5-S1 disk space is through a working corridor developed between the common iliac arteries and veins after sacrificing the median sacral artery. At L4-5, a left-sided approach is usually preferred, which involves retraction of the aorta to gain access to the mid-line disk space.

The aorta descends along the ventromedial spine only 2.1 cm from the center of the intervertebral disk. This large vessel begins at the fourth thoracic vertebra and continues to the fourth lumbar vertebra where it divides into the two common iliac arteries. The common iliac veins join ventral to the fifth lumbar vertebra forming the IVC. This vessel parallels the path of the aorta traveling along the right anterior aspect of the lumbar vertebrae, with a mean of 1.4 cm between the vessel and the center of the intervertebral disk. The IVC migrates posteriorly and laterally with caudal progression from L1 to L5. The iliolumbar vein crosses from the IVC at the level of the L5 vertebral body crossing the psoas muscle. In approaches that require dissection at L4-5, this vein is usually ligated and divided to the left of the left common iliac vein ( Fig. 4.4 ).

Analysis of the location of the IVC in magnetic resonance images of 48 individuals demonstrated that in 70% the position of this vessel at the L4-5 disk level would place it at risk during a right-sided lateral approach. Additionally, the right common iliac vein can lie draped across the anterolateral corner of the disk space, precluding safe entry at this point. During anterior lumbar interbody fusion, the disk space associated with the highest risk of vascular complication is the L4-5 disk space with reported vascular injury rates of 2% to 15%. At this level the left iliac artery is at risk since it must be mobilized for adequate exposure of the disk space. The iliac veins are also susceptible to injury at this level as they are mobilized. The left iliac vein and iliocaval junction lie in close proximity to the center of the disk space at L5-S1 and are thus at risk for injury in the anterior approach to this level. Anatomic variations in the relative positions of the aorta and the IVC have been described ( Fig. 4.5 ). The aorta typically lies ventral to the IVC and slightly to the left. Owing to this variation in the course of the aorta, IVC, their relative positions, and the location of the bifurcation, many authors advocate preoperative imaging to thoroughly define the vascular anatomy of each patient.