Introduction
The typical lumbar spine consists of five vertebrae that are connected in series and permit motion between each segment. Each lumbar vertebra is an anatomically complex structure that consists of multiple distinct subunits. Adjacent vertebrae are connected through the disk space anteriorly and the paired zygapophyseal (facet) joints posteriorly. Further stability is provided by a variety of supporting ligaments. The lumbar spinal canal houses the conus medullaris rostrally, along with the emerging cauda equina, with each lumbar nerve root extending caudally and exiting the canal through its neural foramen directly below the same-numbered pedicle. Understanding the anatomic relationships between these neural structures and the neighboring vertebral bone, disk, and ligament is key to performing effective and safe posterior interbody fusion.
Illustrated views of a lumbar vertebra are provided in Figs. 3.1 and 3.2 . The most ventral part of each vertebra is the vertebral body, a cylindrically shaped unit that serves to support axial loads. The vertebral bodies become progressively larger in a cranial –o-caudal direction. In the lumbar spine, where the bodies are largest, the average vertebral body height is 27 mm and is similar among all lumbar levels. In the axial plane, the anterior-posterior length is greater than the transverse width, and the bodies are longer and wider at either endplate than at their cranial-caudal midpoint. The transverse width and mid-sagittal length of the vertebral bodies increase progressively from L1 (29 mm wide and 40 mm long at the cranial-caudal midpoint) to L5 (32 mm wide and 46 mm long). The endplate is composed of cortical bone and is slightly concave. Its central portion is thinnest and porous, whereas the outer portion (the apophyseal ring) is thicker and stronger.
The pedicles are oriented primarily in an anterior-to-posterior direction and connect the vertebral body to the dorsal elements. Each pedicle is angled medially in the axial plane from posterior to anterior, and this angle increases progressively from L1 (average medial angulation of 11 degrees) to L5 (30 degrees). The transverse pedicle width also increases progressively from L1 (8.7 mm average width) to L5 (18 mm). The sagittal pedicle height displays an opposite relationship, decreasing slightly from L1 (15.4 mm) to L5 (14 mm). With the exception of L5, which has especially wide pedicles, the lumbar pedicles are taller than they are wide, and it is therefore the transverse width of the pedicle that limits its instrumentation.
The pedicle is connected to the dorsal vertebral elements at the junction of the superior articulating process (SAP) and the pars interarticularis (“pars”). The pars connects the SAP and pedicle to the lamina and the inferior articulating process (IAP). The lamina is a sheet-like subunit that forms the dorsal roof of the spinal canal. In the sagittal plane, it slopes posteriorly from superior to inferior; in the axial plane, it is angled posteriorly from lateral to medial, with an apex at the midline. When viewed in the coronal plane, the lamina is tall and narrow at the superior lumbar levels and becomes shorter and wider as it goes down to the lower lumbar levels. Between the SAP and IAP, the lamina is contiguous with the pars interarticularis, which forms the narrowest point along the lateral edge of the dorsal vertebra. The spinous process is oriented in the midline sagittal plane and projects dorsally from the lamina with downward angulation, lying slightly below its corresponding vertebral body and overlying the subjacent interlaminar space. The spinous process is the most dorsal part of the vertebra and the first bone encountered during posterior midline surgical exposure. The paired transverse processes originate from the junction of the pedicle with the SAP and project laterally.
The zygapophyseal (facet) joints are paired synovial joints that allow for articulation of the posterior portion of the vertebrae. Each facet joint consists of the IAP from the rostral vertebra (e.g., L4) and the SAP of the caudal vertebra (e.g., L5). Each of the apposed articular surfaces consists of smooth cortical bone covered with a layer of hyaline cartilage. The joint space contains synovial fluid and is enclosed posteriorly by a fibrous capsule. The facet joints in the lumbar spine are angled anteriorly (i.e., anterior-superior to posterior-inferior) in the sagittal plane, and medially (i.e., posterior-lateral to anterior-medial) in the axial plane. This orientation allows significant flexion/extension and moderate lateral bending, but minimal axial rotation. The facet joint angle in the axial plane (with respect to midline) decreases progressively at each level from rostral to caudal, such that the upper lumbar facet joints are oriented more in the sagittal plane and the lower facets are more coronally oriented. The articular surface is curved so that the posterior portion of the joint is more sagittally oriented and the most anterior portion is more coronally oriented, which makes the SAP articular surface concave, and the IAP surface convex. A clear understanding of facetal anatomy is mandatory to optimize bone drilling, especially during open and minimally invasive transforaminal lumbar interbody fusion (TLIF) surgeries.
The lumbar spine contains several ligaments that interconnect and stabilize the vertebrae: anterior and posterior longitudinal ligaments (ALL and PLL), supraspinous and interspinous ligaments, as well as the ligamentum flavum. The ALL runs vertically along the anterior edge of the spinal column and provides resistance to extension. The PLL runs vertically along the posterior aspect of the vertebral bodies (i.e., the ventral border of the spinal canal) and provides resistance to flexion. The PLL is narrowest behind the vertebral bodies and widens as it crosses each disk space. The ligamentum flavum (‘yellow ligament,’ named so owing to its color) is a discontinuous ligament that bridges the interlaminar space and forms part of the dorsal border of the spinal canal. The ligamentum flavum has its origin on the superior dorsal edge of the caudal lamina and inserts onto the inferior ventral edge of the superior lamina. It provides resistance to flexion at each level. The ligamentum flavum is surgically relevant because it is often hypertrophied in the degenerative spine, in which case it can cause compression of the central canal and lateral recess, and removal of this compressive ligament is key to an effective decompressive surgery. During laminectomy, the ligamentum protects the dura from violation during exposure and bone removal. Because of its discontinuity, the upper half of the lamina has no ligamentum ventrally between the bone and dura, a crucial anatomic landmark in tubular surgical procedures. The surgeon must also be aware that in patients who have undergone previous operations, the ligamentum flavum may be absent at a given level, a point of caution in reexploratory surgeries where inadvertent dural tears may occur. The lumbar interspinous ligament is discontinuous and spans the interval between spinous processes in the sagittal plane, whereas the supraspinous ligament is a continuous structure that runs in the midline along the dorsal edge of the spinous process; both provide resistance to flexion. In lumbar surgical procedures, it is important to preserve the interspinous ligaments wherever possible, to avoid unnecessary iatrogenic instability.
The intervertebral disk allows for transmission of axial loads between vertebral bodies while permitting motion at each segment. The disk consists of three main components: the annulus fibrosis, the outer ring composed of type I collagen, and fibrocartilage arranged in concentric lamellae; the nucleus pulposis, an amorphous inner core composed of water, type II collagen, and proteoglycans; and the cartilaginous endplates, which are composed of hyaline cartilage lining the bony endplates. Mean disk height increases progressively from L1-2 (8 mm) to a maximum at L4-5 (11 mm) before decreasing slightly at L5-S1, but there is significant variation among individuals and disk height is a dynamic property that varies with loading conditions. Significant loss of height can be found with degeneration of the disk. The disk is clinically and surgically relevant because degeneration and herniation can narrow the spinal canal, lateral recesses, and foramina and lead to symptomatic compression of neural elements (such as neurogenic claudication, radiculopathy, or cauda equina syndrome). Removal of ectopic disk material is therefore a principal component of many surgical interventions. There are 23 disks in the typical spine, one at each level from C2-3 through L5-S1, and these disk spaces are relevant to interbody fusion, as they serve as the site of arthrodesis. In this setting, it is important to perform a thorough diskectomy including removal of the cartilaginous endplates, to allow for sufficient exposure of the bony endplate and placement of ample bone graft to create optimal conditions for fusion.
The sacrum deserves brief mention because it articulates with the lumbar spine and is often instrumented in the setting of lumbar fusion. The sacrum is composed of five fused vertebrae that are arranged in a kyphotic shape and are tilted anteriorly in the sagittal plane. The rostral laminae are fused, with no interlaminar space, and the median sacral crest represents the fused former spinous processes. The posterior neuroforamina are arranged in paired vertical rows on each side and are the sites of exit of the dorsal rami from the spinal canal. S1 has a superior endplate and SAPs that are similar to those of the lumbar vertebrae, which allow it to articulate with L5 via the intervertebral disk and facet joints. S1 varies from the lumbar vertebrae in that the body and pedicles are flanked on each side by large alae. The S1 pedicle lies between the SAP and the S1 foramen. The S1 pedicles are unique from those of the lumbar vertebrae in that they are taller (21 mm), lack a lateral cortex (given that the pedicle is continuous with the ala), and allow for a shorter cortex-to-cortex screw trajectory. This means that S1 pedicle screws tend to be shorter and have less cortical bone surrounding them, making them more susceptible to pullout or toggling. Strategies for optimizing pullout strength given these limitations include bicortical purchase through the ventral S1 cortex, or tricortical purchase by directing the screw to the apex of the sacral promontory. S1 pedicle screws are at a further disadvantage when at the caudal end of a long construct given the long moment arm applied above the L5-S1 level. Iliac screws or additional points of sacral fixation may be helpful in this scenario.
The lumbar spinal canal has a triangular shape when viewed in the axial plane. It has a flat anterior edge formed by the posterior wall of the vertebral body and the PLL. The posterior edges of the canal meet at an apex in the midline, and are formed by the lamina and facet on each side, and the underlying ligamentum flavum. The canal’s transverse width is greater than its anterior-posterior height. The height remains relatively constant among levels in the lumbar spine (17 mm), whereas the width increases progressively from L1 (22 mm) to L5 (26 mm). The epidural space within the canal contains fat and a venous plexus that is most prominent ventrally. The venous plexus must often be coagulated in order to access the disk space and to retract the thecal sac and nerve root medially.
The neural foramen serves as the exit site for the nerve root and is frequently the site of symptomatic compression from degenerative pathology. When viewed in the sagittal plan, the foramen exhibits a keyhole shape, with a wider and circular upper portion and a narrower lower portion ( Fig. 3.3 ). The upper portion is bordered anteriorly by the vertebral body and superiorly by the pedicle of the same numbered vertebra. The inferior portion of the foramen is bordered anteriorly by the disk and inferiorly by the pedicle of the subjacent vertebra. The foramen is bordered dorsally by the ventral aspect of the facet joint (primarily the SAP, which lies anterior to the IAP) and its underlying ligamentum flavum.
