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Normal Functional Spine

The spine can be broken down into two major components:

  • The musculoskeletal portion (bones, joints, discs, ligaments, muscles)
  • The neurologic portion (spinal cord, nerve roots)

On a basic level the purpose of the spine is to allow for motion while protecting the delicate neurologic structures within. This allows us to move about to find food for survival and to reproduce to maintain the species. Of course most of us are concerned with more modern activities such as earning a living and enjoying a high quality of life, but even these activities relate to locomotion for survival of the individual and the species.

Enough about philosophy. Let’s first consider the musculoskeletal component. When looking at a spine the most obvious thing we all see are the bones. With the exception of the two uppermost vertebrae (C1 and C2) there is a remarkable consistency of shape and design from head to pelvis, the vertebrae larger from top to bottom consistent with the load they are required to bear. Generally speaking, a spool-like or cylindrical vertebral body in the front, and posterior structures behind that house the spinal cord and nerve roots, and serve as points of attachment for muscles and ligaments. The discs are in front of the nerves (anterior) between the vertebral bodies. Facet joints are behind the cord and nerves in the back (posterior), two per level. These spinal components are present throughout the animal kingdom, common to all vertebrates including mammals, even in giraffes. We generally all have 7 cervical (neck), 12 thoracic (chest, mid-back), and 5 lumbar (low back) vertebrae, and the sacrum (posterior portion of the pelvis). Spinal motion occurs between vertebrae between discs anteriorly and facet joints posteriorly within limits defined by ligaments that join the vertebrae together. This structure of bones, joints and ligaments allows for motion while protecting the contained neural elements. Most motion occurs in the cervical (neck) and lumbar (low back) areas. The thoracic (mid-back) area has limited motion because of the attached rib cage. Motion of the spine is facilitated by the attached muscles ranging from large familiar ones (trapezius, latissimus) to smaller less known but equally important ones (erector spinae, multifidus, etc.). To function properly there is a generally accepted shape to the spine – vertically straight from head to pelvis as viewed from front or back, and a gentle S-shaped curve as viewed from the side. These side curves are known as lordosis (concave posteriorly) in the cervical and lumbar/sacral regions, and kyphosis (convex posteriorly) in the thoracic area. Generally speaking, deviations from this arrangement beyond normal individual variation can cause pain, dysfunction and disability. This is of critical importance when we consider surgery on the spine. Maintaining or restoring coronal (viewed from the front/back) and sagittal (viewed from the side) balance is essential to pain-free normal physiologic motion and function.

Many pathologic conditions occur that affect these bony, ligamentous, muscular, and disc structures that can cause pain, dysfunction and disability. These range from trauma to degeneration, infection, tumor, congenital abnormalities, and idiopathic (we don’t know the cause) or metabolic conditions. Keep in mind that from a physiology viewpoint, calcium is of paramount importance to normal nerve and muscle function and your endocrine system considers the bony skeleton as nothing more than a storage depot for excess calcium ready at a moment’s need to be leached into the bloodstream to serve this metabolic function. So reduced calcium intake or absorption can result in weakening of the bones as they are depleted of calcium to keep this element within a very narrow, highly regulated range for normal nerve and muscle function.

The neurologic component of the spine is comprised of the spinal cord, nerve roots, and the structures surrounding and protecting them (dura, cerebrospinal fluid – CSF) for the most part. These are very delicate structures housed in the vertebrae posteriorly, behind the vertebral bodies and discs, in front of the facets and posterior muscles and bone (i.e. the lamina, which covers the cord and nerves from behind). The dura is a long tube (think empty hot dog casing) that extends head to pelvis. It is filled with CSF (contains proteins, glucose and other nutrients but for all practical purposes looks and feels like pure water). Inside the dura and floating in the CSF is the spinal cord which extends from the head or upper cervical spine, to just below the ribs (a truly elegant design to provide for ultimate protection of the neurologic structures). Most people are unaware that it ends there, not extending to the pelvis or the lowest extent of the spine. The cord gives rise to paired nerve roots at each spinal level. In the neck (cervical) area, these supply function to the arms. In the chest (thoracic) area they are for intercostal nerves that follow the ribs. In the low back (lumbar) area the nerve roots supply the pelvis and legs, but since the cord ends higher in the lumbar spine, the nerve roots traveling to the legs form a ponytail (or cauda equina) at the end of the cord that continues down the spinal canal, still within the CSF inside the dura, a pair of roots exiting at each level).

It is for this reason that spinal cord injuries are less common in the lumbar spine – the spinal cord doesn’t travel that far down the spine. When trauma or disease affects the vertebrae or musculoligamentous spinal structures, damage can occur to the adjacent and contained spinal cord and nerve roots, causing serious pain, dysfunction and disability.

Hopefully this overview of the anatomy will allow for a greater understanding of the various pathologic conditions that would cause patients to seek treatment for pain or loss of function. In fact, with knowledge of normal anatomy, and an understanding of the disease process, the treatment or surgical solution is typically intuitive and easily understood.