Feb 12, 2009
Compression Loading and Herniated Discs
Welcome back. Last time we took a look at general spinal anatomy with a particular emphasis on just how the disc is made. In a nutshell we saw that...
The intervertebral disc is basically made up of two parts and is often compared to a jelly donut. This donut-like structure is porous much like a sponge and (when healthy) is filled with fluid.
The center of this disc contains a jelly-like sack called the nucleus that -- along with the fluid in the disc itself -- acts like a hydraulic shock absorber.
The outer portion of the donut 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.
We ended by asking the question how can a disc fail? I believe the answer to that question has to do with:
- Disc degeneration
- Hydraulic pressure
The Root Cause of Disc Degeneration
The first part of the equation lies in the fact that the disc does not have a blood flow. It obtains its moisture and nutrients by a pumping action as the vertebrae above and below lift, flex and bend in all directions. Without this pumping movement of the vertebrae, the disk will not be able to replenish its moisture content and will dry out. It will literally starve to death.
As those once tough fibrous rings lose their moisture and dry out they begin to crack and delaminate just like an old rotten piece of plywood. Allow this degeneration to continue unchecked for year upon year and it becomes quite easy to see how a soft sack of jelly could break through that once formidable fortress.
Medical professionals have named this condition degenerative disc disease.
The Effect of Hydraulic Pressure
The second part of the answer lies in simple physics.
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. For now, let's just focus on compressive loading.
"Under spine compression the nucleus pressurizes, applying hydraulic forces to the end plates vertically and to the inner annulus laterally. This causes the annulus collagen fibers to bulge outward and become tensed."
- McGill, page 44 
If you've ever experienced a leaky basement, you've observed first hand the power of hydraulic pressure. Given the slightest crack, water under pressure can penetrate even the thick concrete walls of your basement.
When compressive force is applied to the disc, McGill noted that pressure is applied to the nucleus. This pressure pushes the nucleus in the opposite direction of the applied force. (In physics, this is known as "cause and effect.")
For example, if the pressure is applied to the front of the disc -- such as in the act of sitting or bending forward -- the nucleus will be squeezed towards the rear. If the fortress walls of the annulus are weakened, this hydrostatic pressure will begin to force the jelly-like nucleus through those walls.
Repeatedly apply this rearward pressure on the nucleus and it will eventually work its way through the walls of the fortress until it breaks completely through into the spinal canal.
This is not a sudden process. McGill has demonstrated that this requires a great deal of pressure and many thousands of repetitive cycles of forward bending.
"While no herniations were produced with 260 N (Newtons) of compressive load and up to 85,000 flexion cycles, herniations were produced with 867 N of load and 22,000 to 28,000 cycles, and with 1472 N and only 5000 to 9500 cycles (Callaghan and McGill, 2001)." 
McGill's research involved mechanically flexing spinal segments taken from swine (since they closely match the human spine) until those segments failed. While they did this, they tracked the migration of the nucleus as it pushed its way through the walls of the annulus.
It is probably not feasible to test for the effects of constant static pressure since it would most likely take months if not years before a single test specimen would fail. But I think we can safely speculate that constant hydrostatic pressure (hours spent in a sitting / bent forward posture) would most likely produce similar results.
Do You Know Where Your Nucleus Is?
If you're spending hours in a classroom, hours riding in cars, hours a day working at a desk and hours sitting on the couch at home, your discs are probably not getting pumped enough to adequately replenish the moisture lost during all that downtime.
Add to that the constant static pressure of the bent forward posture that sitting entails -- squeezing the nucleus in one direction -- and you don't have to be Einstein to figure out the end result. The nucleus is going to seek the path of least resistance.
Table of Contents for this series:
- What Causes Herniated Discs?
- The First Step in Repairing Herniated Discs
- Compression Loading and Herniated Discs
- Twisting and the Herniated Disc
- My Philosophy of Rehabilitation
About the Author
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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.
Last Updated: Feb 12, 2009