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