23 Nov 2008 07:29 pm

Writer’s Block Nov 23

Nov. 23, 2008

Twisting and the Herniated Disc

This is Part 4 in the series I’m tentatively calling: “What Causes Herniated Discs?”


In the previous installment, we discussed the various stress factors that contribute to disc failure and then examined one of those risk factors. This time I’d like to take a look at another one of those factors, but before we do that here’s a little something to refresh your memory:

“In physics, stress is classified according to type such as tensile strength (stretching the object), torsional strength (twisting the object), shear strength (lateral tearing of the object), and compressive strength (load bearing ability).

Of course, the normal intervertebral disc is designed to withstand all of these stress factors, but the two that appear to have the most impact on herniation are twisting and compressive loading.”

In that last article, we specifically looked at the effect of compressive loading and the role it plays in producing herniated discs. This time we’re going to examine torsional stress — sometimes referred to as axial torque — or what we laymen would simply call twisting.

And, for the sake of simplicity, we’re going to confine ourselves to answering the following three questions:

  1. What does twisting do to the walls of the disc?
  2. Can axial torque result in a herniated disc?
  3. What activities present the greatest risk for disc failure?

Let’s start with number one…

What Does Twisting Do to the Walls of the Disc?

You will recall that the outer portion of the disc is called the annulus and is a series of concentric rings of fibrous connective tissue that surrounds the nucleus much like a ring of forts built one inside the other.

Also, you may remember that the basic hypothesis is that as the disc dries out (either because of age, inactivity or both) the tough fibrous rings of the annulus start to break down and cracks begin to form. This process of deterioration is often referred to as degenerative disc disease.

So the question that is of most interest to us is what happens to this crumbling fortress when a twisting force is applied? McGill gives us a clue based on his observations…

“While we have not performed a lot of research on the effect of twisting on the discs, it appears that repeated twisting causes the annulus to slowly delaminate. This is evidenced by the tracking of the nucleus into the annulus in all directions. While we do not yet know the relationship between number of cycles and loads, we do know that added torsion reduces the compressive strength of the joint (Aultman et al., 2004).” [1]

That seems reasonable. If you take a sheet of rotten plywood and start flexing it, it would not be unexpected for the various layers to begin to separate (or delaminate as McGill puts it). So what about our next question:

Can Axial Torque Produce a Herniated Disc?

I believe that technically a disc can be considered herniated the moment the nucleus begins its initial break through the walls of the annulus even though it may not be to the point of causing a bulge and may be years away from actually extruding into the spinal canal.

So perhaps a better question might be, could a sudden twisting force cause a disc on the verge of rupturing to finally fail?

McGill gives us an interesting example that he and his colleagues observed during one of their research efforts:

“Our most recent work on disc herniation uncovered the dependency of the location of the herniating bulge on the axis of motion (Aultman et al., 2005). For example, in 20 motion segments, we flexed them repeatedly about an axis that was 30 degrees rotated from the pure flexion axis (mostly flexion with some lateral bend). One specimen simply failed abruptly and was removed…” [1]

When flexing disc segments about an axis of rotation, that is, ones that were twisted, one of them failed (herniated) immediately. The rest just took a little longer.

Again, this result comes as no surprise. It’s what you would expect to happen if you started applying torque to a weak disc. Perhaps a good analogy would be wringing the water out of a dishrag. Twisting the rag (disc) causes the liquid (nucleus) to seek a way out.

What is important to note is that this has been observed under laboratory conditions. It is not just the result of speculation.

So, since we know for a fact that twisting is a potentially harmful movement…

What activities present the greatest risk for disc failure?


The type of activity, which would apply a twisting force to the spine similar to what McGill is describing, would include such things as a golf swing, bowling, swinging a tennis racket, pitching a baseball, certain high velocity thrust-type spinal manipulations or any other forceful rotational movement.

None of the above activities are inherently dangerous or harmful to a healthy spine, but their cumulative effect needs to be recognized as a possible contributing factor to disc degeneration.

Discs that are not in perfect condition are no doubt going to be delaminating and accumulating additional cracks and tears each time one of these movements is performed.

And this says nothing about the impact of these activities on discs that are already in a weakened state and may be on the verge of catastrophic failure. Just teeing off on the ninth hole or bowling that last set may be all it takes.


That’s as far as we’re going to go today, next time we’ll take a look at what I call “the Catch-22” of rehabilitation, because there seems to be at least one hidden pitfall that we need to be aware of.

Till then,


1. McGill, S. Low Back Disorders, Evidence-Based Prevention and Rehabilitation, 2nd Edition. (p. 44-47) Human Kinetics (2007)

2. Tampier C, Drake JD, Callaghan JP, McGill SM. Progressive disc herniation: an investigation of the mechanism using radiologic, histochemical, and microscopic dissection techniques on a porcine model. Spine. 2007 Dec 1;32(25):2869-74.

3. Drake JD, Aultman CD, McGill SM, Callaghan JP. The influence of static axial torque in combined loading on intervertebral joint failure mechanics using a porcine model. Clin Biomech (Bristol, Avon). 2005 Dec;20(10):1038-45.

4. Aultman CD, Drake JD, Callaghan JP, McGill SM. The effect of static torsion on the compressive strength of the spine: an in vitro analysis using a porcine spine model. Spine. 2004 Aug 1;29(15):E304-9.


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