It is well known that the human spine has coupled motion when it rotates. Now, what do I mean by that?

When the human spine rotates it also bends sideways at the same time, and that phenomenon is called coupled motion. What happens in the normal spine is that, when we bend our lumbar spine to the right it also rotates counter clockwise, causing it to stay in the right position i.e the spinous processes pointing to the concavity of the lumbar spine. Interestingly, the opposite occurs in the thoracic spine; here, the spinous processes end up pointing to the convexity of the thoracic spine.

What happens in Thoracic Idiopathic Scoliosis?

When observing x-rays of the thoracic spine affected by idiopathic scoliosis, one can see the spinous processes pointing to the concavity of the spine vs convexity in a normal spine. What causes this abnormal behavior in the spine?

To understand why this happens, we had to dig through layers of dust laden papers and finally found a paper from 1903 with the possible answer. Dr. Robert W. Lovett from the Anatomical Laboratory in Harvard Medical School, managed to solve the puzzle through his simple but ingenious experiments.

“With the aid of Prof. I. N. Hollis, of Harvard University, two general laws governing flexible rods were formulated as follows :

(A) Although a straight flexible rod (e. g., a quadrilateral rod of rubber or lead) may be bent in one plane without twisting, if such a rod is already bent in one plane it cannot be bent in another plane without twisting.

(B) Although a straight flexible rod may be twisted without acquiring a side bend, a flexible rod already bent in one plane cannot be twisted without acquiring a side bend.”

Dr Lovett repeated the above experiments [22] and found the same results; he then studied the spine using those principles and the results were consistent. In his third experiment, he took away the facet joints in the spine (so as to eliminate the occurrence of coupled motion) and studied the behavior of that spine in the same manner; again, the findings were the same as the first two experiments’. What Dr. Lovett proved in all his experiments was that if the spine was lordotic the spine rotated in the opposite direction; and if the spine was kyphotic the spine rotated in the same direction as the side bend.

With the above findings, we have a better understanding of the behavior of the spine affected by thoracic idiopathic scoliosis.

22. Lovett AW. A contribution to the study of the mechanics of the spine. Am J Anat 1903 2:457-462