General Overview of the Spine

The spine is a complex structure of vertically stacked bones separated by joints and cartilage/collagen complexes known as discs. These anatomic elements are connected by ligaments and muscles. They are supplied by blood vessels and nerves. These structures works together to protect the spinal cord and nerves, allow motion, support the head, upper extremities, and internal organs, as well as to facilitate ambulation along with the lower extremities.

A knowledge of spinal anatomy is the foundation for understanding how the spine functions. Utilizing this foundation, we will explore the biomechanics of the spine in normal and abnormal conditions. This will give you an appreciation for the parts of the spine that can be injured and thus painful. Hopefully, this will lead to rational treatment options.

Lumbar Spine

Bones (Vertebrae)
      There are five lumbar vertebrae in the spine. Each vertebrae consists of a large body in the front (anterior - floor of the spinal canal) connected to two pedicles on each side (walls of the spinal canal). The pedicles join the superior and inferior articular processes, which contribute to the facet joints. They also join the lamina, which form the roof of the spinal canal.
     The vertebrae can be divided into an anterior and posterior column. The anterior column consists of the vertebral body. It gives the spine its bulk and height. The body is larger in front than in the rear, which makes the characteristic (lordotic) curve of the lumbar spine. The vertebral bodies withstand compressive loads applied to the spine including body weight and the forces imparted by contraction of the back muscles.
     The posterior column structures consist of the pedicles, articular processes, spinous processes, transverse processes, laminae, accessory processes,and mamillary processes. They help regulate the passive and active forces acting on the spine thereby controlling movement. The articular processes are the components of the facet joints, which resist forward sliding and rotation (twisting) of the vertebral bodies. The spinous, transverse, accessory, and mamillary processes provide areas for muscle attachment and act as levers for the action of the attached muscles. The laminae form the roof of he spinal canal and help protect the nerves. They also transmit forces from the spinous processes and the inferior articular processes to the pedicles. The pedicles connect the posterior column to the anterior column transferring forces between the columns.

Joints
      The space between two vertebral bodies is composed of an intervertebral disc. The articulation of the superior articular process of one vertebra and the inferior articular process of the vertebra above connect the posterior column. These joints are called facet joints (formal name zygapophyseal joints). The combination of the disc anteriorly and the two facet joints posteriorly is referred to as the three joint complex. The motion segment consists of the vertebra above, the three joint complex, and the vertebra below.

Intervertebral Disc
      The intervertebral disc is composed of two structures - a central nucleus pulposus and an outer annulus fibrosus. The nucleus consists of mostly type II collagen (cartilage like) and large protein macromolecules called proteoglycans. Proteoglycans absorb water into the disc and are extremely important to the mechanical properties of the disc. As we age, the disc loses proteoglycans and therefore water content. The alteration (drying) of the disc changes its biomechanical properties and makes the disc susceptible to injury.

The annulus fibrosus consists of tough, thick collagen fibers. The fibers are arranged in concentric layers (laminated) of alternating bands. The fibers are oriented parallel in each layer. However, each alternating layers' collagen is oriented obliquely (~120`) to each other. This oblique orientation allows the annulus to resist forces in both vertical and horizontal directions. Vertical tension resists bending and distraction (flexion and extension). Horizontal tension resists rotation and sliding (twisting). All layers of the annulus have vertical components; therefore, the disc can withstand forward and backward bending well. However, only 50% of the horizontal fibers are engaged during a rotational movement; therefore the disc is weak during a twisting motion. The disc is protected during rotation by the posterior facet joints.

The vertebral endplates are special cartilage structures that surround the top and bottom of each vertebra and are in direct contact with the disc. They are important to the nutrition of the disc because they allow the passage of nutrients and water into the disc. If these structures are injured, it can lead to deterioration of the disc and altered disc function.

Facet Joints
    The facet joints are typical synovial joints (like the knee) that have cartilage at the end of the bones, and are surrounded by tough ligamentous capsule. They also have a meniscus and a synovial membrane that can secrete fluid and become inflamed. Because of their shape and orientation, they can resist forward motion (shear) between the vertebral bodies and rotation through the disc. They permit sliding motion in a vertical direction during flexion and extension.

Ligaments
      In general, the ligaments do not significantly contribute to spinal stability. Rather, they tend to separate the compartments of muscle and bone around the spine. An exception is the iliolumbar ligament that connects the transverse process of L5 to the ilium. This protects the L5/S1 segment from rotational, forward sliding, and lateral bending forces.

Muscles
      The muscles acting on the lumbar spine can be divided into the back muscles (erector spinae), hip flexors, and muscles acting indirectly on the spine (abdominal, gluteal) through the thoracolumbar fascia. Muscles are rarely the source of chronic low back pain. However, because the same nerves that supply structures that can cause chronic pain innervate them, they can be secondarily involved in a painful symptom complex.

The lumbar back muscles (erector spinae) cover the posterior portion of the vertebrae. Their arrangement is multiple and complex but systematic. A detailed anatomic description is not warranted here; however, a discussion of their function is important. Because of their orientation, these muscles have very little ability to extend the lumbar spine. Their primary role is to control forward bending of the vertebrae. The lumbar spine muscles also generate tremendous compression forces on the vertebral bodies and discs, which can be important in loading of the lumbar spine.

The hip flexor muscles (psoas major and psoas minor) originate from the anterior (front) of the spine and connect to the femur (thigh) bone. They act to flex the hip joint and have little ability to flex the spine. However, they exert significant compression loads on the vertebrae and discs.

The muscles acting indirectly on the spine do so through the thoracolumbar fascia. This structure is a thick, fibrous tissue that surrounds the back muscles and connects the spine to the abdominal muscles and the gluteal muscles. The abdominal muscles support or brace the spine and help determine (along with pelvic tilt) the amount of curve or lordosis in the lumbar spine. The gluteal muscles through their attachment to the thoracolumbar fascia, are the main extensors of the spine important in lifting.

Nerves
      The lumbar spine has an extensive and complicated nerve supply. Any structure that receives a nerve supply is a potential source of pain if a pain producing pathology stimulates it. The nerve roots exit the spinal canal and join to form the major nerves of the leg. They supply the leg with the sensory and motor fibers that produce the sharp, shooting, electric like pain referred to as sciatica.

Smaller branches of the nerve root divide to become the dorsal (front) and ventral (back) rami nerve groups. The dorsal rami nerves supply the vertebral body, disc, posterior longitudinal ligament, and dura. The ventral rami nerves supply the facet joints, posterior back muscles, and ligaments. Painful stimuli of these structures typically produce poorly defined low back,buttock, hip, or thigh pain. There is extensive overlap of these nerve groups and their referred pain patterns, which make the precise diagnosis of low back pain difficult.

Blood Supply
      The blood supply of the lumbar spine is beyond the scope of this discussion; however, a few major points are useful. The bones and muscles have excellent blood flow and therefore have good healing potential. The intervertebral disc has a very limited blood supply and limited healing potential. This is the reason a tear or degeneration of the disc often leads to chronic pain. The disc receives nutrition primarily from two arterial sources. Small arteries supply the periphery of the annulus and small capillaries beneath the vertebral end plates. The majority of the disc is the largest avascular structure in the adult human body. Disc nutrition is facilitated by diffusion of fluids across the vertebral end plates into the proteoglycan matrix of the nucleus and enhanced by repeated compression of the disc by activity of daily living and muscle action.

Biomechanics
      Movement of the lumbar spine is described in six planes: flexion, extension, compression, rotation, side bending, and distraction.

Flexion and Extension
      Flexion and extension of the spine combine forward sliding and rotation of the vertebrae. The facet joints and the annulus of the disc resist the forward sliding. Rotation is resisted by the annulus of the disc, capsules of the facet joints, and the action of the back muscles as well as the passive tension generated by the thoracolumbar fascia. Extension is resisted by the facet joints and secondarily by the annulus of the disc.

Compression
      Compression of the spine is due to body weight and loads applied to the spine. Body weight is a minor compressive load. The major compressive load on the spine is produced by the back muscles. As a person bends forward, the body weight plus an external load must be balanced by the force generated by the back muscles. The external force is calculated by multiplying the load times the perpendicular distance of the load from the spine. The greater the distance from the spine, the larger the load. Since the back muscles act close to the spine, they must exert large forces to balance the load. The force generated by the back muscles act to compress the spine structures.

Most of the compressive loads (~80%) are born by the anterior column (disc and vertebral body). The disc is a hydrostatic system. The nucleus acts as a confined fluid within the annulus. It converts compressive (axial) loads into tension on the annular fibers and the vertebral end plates. The fluid properties of the nucleus are secondary to the ability of the proteoglycans to imbibe water.

Rotation
      Rotation of the spine is accomplished by the contraction of the abdominal muscles acting through the thorax and the thoracolumbar fascia. There are no primary muscles responsible for lumbar rotation. The facet joints and the collagen fibers of the annulus resist this rotation. In rotation, only 50% of the collagen fibers are in tension at any time, which renders the annulus susceptible to injury.

Side bending
      Bending is a combination of lateral flexion and rotation through the annulus and facet joints. It has not been thoroughly studied.

Distraction
      Pure distraction rarely occurs and is usually a combination of tension and compression on the spinal joints depending on the direction of applied force. Spinal traction as a treatment acts to unload the spine and is an example of a distraction force.

Mechanical Injuries

Flexion
      The spine is resistant to injury if the force is only in pure flexion. The combination of the facet joints and disc are intrinsically stable in this plane. However, the spinal muscles can be injured during forceful flexion since they are important in controlling this motion. Fortunately, this does not lead to chronic pain.

Extension
      Extension is stopped by impaction of the facet joints and eventually the inferior articular process against the lamina. This can result in a cartilage injury of the facet joint, disruption of the facet capsule, facet joint or pars interarticularis fracture.

Flexion and rotation
      The spine is very susceptible to injury in this loading combination. Flexion prestresses the anular fibers. As the spine rotates, compression occurs on the facet joint surfaces of the joint opposite the rotation. Distraction occurs on the facet joint on the same side of the rotation. The center of rotation of the motion segment shifts from the back of the disc to the facet joint in compression. The disc shifts sideways and shear forces on the anular fibers are significant. Since the anular fibers are weak in this direction, they can tear. If the rotation continues, the facet joints can sustain cartilage injury, fracture, and capsular tears while the anulus can tear in several different ways. Any of these injuries can be a source of pain.

Compression
      Compression injuries occur by two main mechanisms; axial loading by gravity or by muscle action. Gravitational injuries result from a fall onto the buttocks while muscular injuries result from severe exertion during pulling or lifting. A serious consequence of the injury is a fracture of the vertebral end plate. Since the end plate is critical to disc nutrition, an injury can change the biochemical and metabolic state of the disc. If the end plate heals, the disc may suffer no malice. However, if the end plate does not heal, the nucleus can undergo harmful changes. The nucleus loses its proteoglycans and thus its water-binding capacity. The hydrostatic properties of the nucleus are compromised. Instead of sharing the load between the nucleus and the anulus, more load is transferred to the anulus. The anular fibers then fail. In addition to anular tears, the layers of the anular separate (delaminate). The disc may collapse or it may maintain its height with progressive anular tearing. If the anulus is significantly weakened, there may be a rupture of the disc whereby the nuclear material migrates into the anulus or into the spinal canal causing nerve root compression.