Musculoskeletal Anatomy, Neuroanatomy, and Biomechanics of the Lumbar Spine

“You will have to learn many tedious things which you will forget the moment you have passed your final examination, but in anatomy it is better to have learned and lost than never to have learned at all.”

–Authors

It is a convention observed by most authors of medical texts to start the book with a chapter devoted to the anatomy of the subject covered. In many instances, this is a form of brownian movement having very little purposive significance. Having skipped through many such essays with ill-concealed impatience, it was with considerable trepidation that we continued to follow this well-established precedent. The purpose of this introductory chapter is to remind the reader of anatomic terminology and to correlate the gross anatomic features of the lumbar vertebrae with normal biomechanics and pathologic changes of clinical significance. Remember, the key to understanding disease and completing exacting surgical techniques is an intimate knowledge of anatomy.

Functional Musculoskeletal Anatomy

There are five lumbar vertebrae and the sacrum making up the lumbar spine. We can consider each vertebra as having three functional components: the vertebral bodies, designed to bear weight; the neural arches, designed to protect the neural elements; and the bony processes (spinous and transverse), designed as outriggers to increase the efficiency of muscle action.

The vertebral bodies are connected together by the intervertebral discs, and the neural arches are joined by the facet (zygapophyseal) joints (Fig. 1-1). The discal surface of an adult vertebral body demonstrates on its periphery a ring of cortical bone. This ring, the epiphysial ring, acts as a growth zone in the young and in the adult as an anchoring ring for the attachment of the fibers of the annulus. The hyaline cartilage plate lies within the confines of this ring (Fig. 1-2). The size of the vertebral body increases from L1 to L5, which is indicative of the increasing loads that each lower lumbar vertebral level has to absorb.

FIGURE 1.1 The components of a lumbar vertebra: the body, the pedicle, the superior and inferior facets, the transverse and spinous processes, and the intervertebral foramen and its relationship to the intervertebral disc and the posterior joint.

FIGURE 1-2 The epiphysial ring is wider anteriorly and surrounds the hyaline cartilaginous plate.

The neural arch is composed of two pedicles and two laminae (Fig. 1-1). The pedicles are anchored to the cephalad half of the vertebral body and form a protective cover for the cauda equina contents of the lumbar spinal canal. The ligamentum flavum (yellow ligament) fills in the interlaminar space at each level.

The outriggers for muscle attachment are the transverse processes and spinous process.

The Intervertebral Disc

The intervertebral discs (Fig. 1-3) are complicated structures, both anatomically and physiologically. Anatomically, they are constructed in a manner similar to that of a car tire, with a fibrous outer casing, the annulus, containing a gelatinous inner tube, the nucleus pulposus. The fibers of the annulus can be divided into three main groups: the outermost fibers attaching between the vertebral bodies and the undersurface of the epiphysial ring; the middle fibers passing from the epiphysial ring on one vertebral body to the epiphysial ring of the vertebral body below; and theinnermost fibers passing from one cartilage endplate to the other. The anterior fibers are strengthened by the powerful anterior longitudinal ligament. The posterior longitudinal ligament affords
only weak reinforcement, especially at L4-5 and L5–S1, where it is a midline, narrow, unimportant structure attached to the annulus. The anterior and middle fibers of the annulus are most numerous anteriorly and laterally but are deficient posteriorly, where most of the fibers are attached to the cartilage plate (Fig. 1-3).

FIGURE 1-3 The annulus fibrosus is composed of concentric fibrous rings that surround the nucleus pulposus (A). The nucleus pulposus abuts against the hyaline cartilage plate (B). The outermost annulus fibers are most numerous anteriorly and are attached to the vertebral body immediately deep to the epiphysial ring. C: The epiphysial fibers run from one epiphysial ring to the other. The cartilaginous fibers run from one cartilage plate to the other cartilage plate. These comprise 90% of the annulus fibers posteriorly. The anterior fibers of the annulus are strongly reinforced by the powerful anterior longitudinal ligament, but the posterior longitudinal ligament only gives weak reinforcement to the posterior fibers of the annulus.

With the onset of degenerative changes in the disc, abnormal movements occur between adjacent vertebral bodies. These abnormal movements apply a considerable traction strain on the outermost fibers of the annulus, resulting in the development of a spur of bone, the so-called traction spur (Macnab spur) (6). Because the outermost fibers attach to the vertebral body beneath the epiphysial ring, this spur develops about 1 mm away from the discal border of the vertebral body and projects horizontally. This differs in its radiologic morphology from the common claw-type osteophyte, which develops at the edge of the vertebral body and curves over the outer fibers of the intervertebral disc (Fig. 1-4). The clinical significance of a traction spur lies in the fact that it indicates the presence of a vertebral segment in the early stage of instability.

FIGURE 1-4 The traction spur projects horizontally from the vertebral body about 1 mm away from the discal border. It is indicative of segmental instability. The common claw spondylophyte, on the other hand, extends from the rim of the vertebral body and curves as it grows around the bulging intervertebral disc. It is associated with disc degeneration. It does not represent the radiologic manifestation of osteoarthritis.

The first stage of a disc rupture would appear to be detachment of a segment of the hyaline cartilage plate. The integrity of the confining ring of the annulus is then disrupted. Nuclear material can escape between the vertebral body and the displaced portion of the cartilage plate. On occasion, as a result of a compression force, a whole segment of the annulus may be displaced posteriorly, carrying with it the nucleus pulposus and displaced portion of the hyaline plate (Fig. 1-5A). This pathology is more common in younger patients (Fig. 1-5B).

FIGURE 1-5 A: The first morphologic change to occur in a disc rupture is a separation of a segment of the cartilage plate from the adjacent vertebral body. Fissures run through the annulus on each side of the detached portion of the cartilage. When a vertical compression force is then applied, the detached portion of the cartilage plate is displaced posteriorly, and the nucleus exudes through the torn fibers of the annulus. B: Computed tomography (CT) of young patient with end-plate fracture (arrow) and herniated nucleus pulposus.

The fibers of the annulus are firmly attached to the vertebral bodies and arranged in lamellae, with the fibers of one layer running at an angle to those of the deeper layer (Fig. 1-6). This anatomic arrangement permits the annulus to limit vertebral movements. This important function is reinforced by the investing vertebral ligaments.

FIGURE 1-6 A: The annulus is a laminated structure with the fibrous lamellae running obliquely. This disposition of the fibers permits resistance of torsional strains. B: The nucleus pulposus is constrained by the fibers of the annulus. When a vertical load is applied to the vertebral column, the force is dissipated radially by the gelatinous nucleus pulposus. Distortion and disruption of the nucleus pulposus are resisted by the annulus.

Because the nucleus pulposus is gelatinous, the load of axial compression is distributed not only vertically but also radially throughout the nucleus (5,8). This radial distribution of the vertical load (tangential loading of the disc) is absorbed by the fibers of the annulus and can be compared with the hoops around a barrel (Fig. 1-7).

FIGURE 1-7 Hoop stress. This diagram shows how the load of water in a barrel is resisted by the hoops around the barrel. When too great a load is applied, the hoops will break. The annulus functions in a manner similar to that of the hoops around a water barrel.

Weight is transmitted to the nucleus through the hyaline cartilage plate. The hyaline cartilage is ideally suited to this function because it is avascular. If weight were transmitted through a vascularized structure, such as bone, the local pressure would shut off blood supply, and progressive areas of bone would die. This phenomenon is seen when the cartilage plate presents congenital defects and the nucleus is in direct contact with the spongiosa of bone. The pressure occludes the blood supply, a small zone of bone dies, and the nucleus progressively intrudes into the vertebral body. This phenomenon was first described by Schmorl and Junghanns (9), and the resulting lesion bears the name Schmorl’s node (Fig. 1-8).

FIGURE 1-8 A Schmorl’s node (L2–3) (arrow), likely of no clinical significance. Have you ever seen a herniated nucleus pulposus at the same disc space as a Schmorl’s node?

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