We have now seen a wealth of studies demonstrating that spinal manipulation is effective, but it is quite another matter to fully understand how and why. The search for an explanation has absorbed the attention of chiropractors since D.D. Palmer founded the profession in 1895. The history of chiropractic, like all healing arts, is largely one in which empirical process has preceded theoretical formulation. In other words, from the earliest days practitioners have applied new manual treatment methods on an intuitive, empirical basis, noted that some are more effective than others, and theorized on the basis of these findings as to the underlying mechanisms.

When certain methods have demonstrated their effectiveness over a period of time, they, along with the theories used to explain them, become part of what we might call the "chiropractic corpus," the body of tradition, evidence, and practice which is the contribution of the chiropractic school of knowledge to the healing arts as a whole.

Not until the late Twentieth Century was this accumulated body of chiropractic knowledge sufficiently grounded in scientific research to allow wide recognition across professional boundaries. Fortunately, that point has now been reached. It therefore seems timely to review the nature of the chiropractic diagnostic and therapeutic model, so that it can be well understood by the public and other health professionals.

Part of this review is an examination of chiropractic theory past and present. It is important to sift out ideas which may have been state of the art in 1910 or 1950, but which are no longer fully tenable. Chief among these is the idea that the chiropractic adjustment works primarily by physically moving a vertebra that is out of place back into place.

The Bone-Out-of-Place Theory
The early chiropractors assumed that their adjustments worked by moving misaligned vertebrae back into line, thereby relieving pressure caused when those bones impinged directly on spinal nerves. The standard explanation given to patients was the analogy of stepping on a garden hose if you step on the hose the water canât get through, and then if you lift your foot off the hose, the free flow of water is restored. Similarly, the explanation went, the chiropractic adjustment removes the pressure of bone on nerve, thus allowing free flow of nerve impulses.

Based on the information available in the early years, such a theory was plausible. Chiropractors were able to feel interruptions in the symmetry of the spinal column with their well-trained hands, and in many cases could verify this on x-ray (discovered in 1895, the same year as chiropractic). They would then adjust the vertebra with manual pressure, attempting to move it back into line. More often than not, patients reported significant functional improvements and healing effects.

But there are problems with the theory. This can most simply and directly be illustrated by noting the fact that, after an adjustment resulting in dramatic relief from headaches or sciatica, an x-ray will rarely show any discernible change in alignment. (Such comparative x-rays are now considered inappropriate, because of the unnecessary radiation exposure). Long-term positive health changes have not been definitively shown to correlate with symmetrical alignment of spinal bones on any consistent basis.

Though much excellent work has been done by chiropractors whose understanding of their healing art was based on the bone-out-of-place theory, the theory has not stood the test of time. This does not mean that chiropractic is invalid, only that this late nineteenth century explanation has been overtaken by later developments.

While misalignments may play a role in the interpretation of spinal subluxations, they are no longer believed to play the central role. But if the old explanation of misaligned bones pressing on nerves is inadequate, what new theory has replaced it? To answer this question, we need to move beyond the essentially two-dimensional viewpoint of the misalignment theory, and include motion as an added dimension.

The Intervertebral Motion Theory
In the 1930s, Belgian chiropractor Henri Gillet developed a theory of intervertebral motion and fixation, in which he asserted that it was loss of normal spinal joint movement, rather than misalignment, that was the underlying explanation for the vertebral subluxation. He agreed with the bone-out-of-place adherents that the interplay between the skeletal system and the nervous system was crucial, but parted ways with them regarding the causal process underlying the abnormal nerve signaling. Rather than attributing the subluxationâs effects to direct pressure of misaligned bone on nerve, Gillet theorized that loss of proper joint dynamics was the underlying issue.

Later work by medical researchers Schmorl and Junghans, and many more who followed, verified the complex role of the "vertebral motor unit," consisting of bones, muscles, ligaments, blood vessels, and nerves. This model is now widely accepted.

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