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What do pigs and humans have in common?

“The majority of in vitro research has examined repeated axial loading with the spine in a neutral position from which observed herniations are extremely rare.”

 

This means that loading much weight onto your shoulders doesn’t appear to affect the disc negatively, aside from compressing it. Picture the people doing strongman, powerlifting, Olympic weightlifting or Crossfit. All of these sports are safe regardless of how much weight is being used, as long as technique remains good, while under the weight.

 

“The most consistent development of disc herniation with repeated loading conditions was achieved by Gordon et al. In vitro human lumbar motion segments were flexed from a neutral posture to 7 degrees of flexion with a small axial twist motion. All 14 of the motion segments examined failed with herniations of the Intervertebral disc (either nuclear protrusion or extrusion) with an average of 40,000 loading cycles to failure. It appears that load, motion, degenerative condition, and repetition require further investigation as prerequisites to disc herniation.”

 

Stu is one of the great gurus of back pain. He states in his papers that he does not endorse a specific number of flexion cycles to create a herniation. This is individual for each person. Also of note is that the above experiment is not done on a live person, but on a cadaver. This means that there is little compensatory motion that can occur, which may occur in real life. For example, there is one paper (don’t have it currently, but I will find it for later) that postulates that the posterior longitudinal ligament (a strong ligament on the back of the spine) may be a protective mechanism for back pain, which would then work to prevent a disc herniation by absorbing some of the flexion load. It’s just an idea though and is no more right or wrong than the number of loading cycles found in the above quote.

 

“The cervical spines of 26 porcine specimens were obtained immediately following death. Pig cervical spines have been shown to be the section closest to human lumbar spines for anatomical and biomechanical characteristics.”

 

The authors make is sound so humane that they waited until the pigs died, but then went on to say that the mean age was 6 months. They died for science. What is most important though is that this study was performed on pig spines! The results can be correlated to humans, but again this will not be precise because the subjects aren’t real live humans.

 

“The remains of any soft tissue and discs were dissected from the cranial and caudal endplates.”

 

The muscles were removed. The muscles, tendons and ligaments provide active and passive support to the joint. Without this support, we are only looking at how the spine joint moves in a vacuum. This again makes it hard to take the results of this study and apply them to humans. We can though take the idea of the study and generalize it to another spine.

“Herniation occurred with modest levels of compression and flexion/extension movements but with a high number of motion cycles. Specimens tested in the lowest compressive force group had nuclei that were intact after 86,400 flexion cycles…All herniations that were created during testing occurred in the posterior or posterior-lateral areas of the annulus.”

 

The first thing to take from this is that the spinal segment is strong. It can withstand over 80,000 cycles of flexion/extension, without resting, and some were able to withstand the force without significant anatomical changes. All herniations were posterior or posterior lateral. This is consistent with what we see in the clinic. Very rarely is there an anterior herniation, but in real life there is also a very strong ligament on the anterior portion of the spine, which would impede a herniation in this direction.

 

“…highly repetitive flexion/extension motions and modest flexion/extension moments, even with relatively low magnitude compression joint forces, consistently resulted in Intervertebral disc herniations. Larger axial compressive force resulted in more frequent and more severe disc injuries…there is no doubt that disc herniation is a cumulative process that can result with modest forces if sufficient flexion/extension cycles are applied.”

 

This is a mouthful. Let’s start by saying that if you spend a lot of time in a flexed (slouched posture position), this may lead to a posterior disc herniation. It’s kind of like the straw that broke the camel’s back. It may not happen the first time, but the more often one spends in flexion the more that the nucleus (the pudding substance inside the disc) will travel towards the border of the disc (annulus). This article doesn’t state what happens to the disc when we rest and stop spending time in a flexed position. For instance, what is not stated is that if we flexion for an entire day, but then move in the opposite direction (extension), do we then counteract the effects of flexion? This article doesn’t say this, but one would have to infer if we could create a herniation that we can reduce a herniation with movement. More to come in future posts.

 

“While there may be a tendency to identify an event that ‘caused’ an intervertebral disc herniation, this work together with our other experiments have led us to form the opinion that this is only a culminating event and that the real cause had already occurred.”

 

This quote says it best and I will leave it at that.

 

Thanks for reading. If you would like to learn more about a topic, feel free to ask a question on here or at my Facebook page @movementthinker. I love reading research and if I can read something that may help you specifically then it is more functional than just reading stuff that I enjoy.

 

Callaghan JP, McGill SM. Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compression force. Clinical Biomechanics. 2001;16:28-37.

 

link to article

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Categories: Physical therapy

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