How does the disc move?
“Clinically, disc herniation is most commonly observed posteriorly or posterior-laterally”
Very rarely does the disc herniated straight backwards (posteriorly). There is a very strong ligament at the back side of the spine that prevents a posterior herniation. The material in the disc will typically make a trough in the disc and go sideways from that point.
“Specifically, when the spine is loaded asymmetrically, the nucleus tracks along a radial fissure formed in the contralateral corner of the disc suggesting that annular delamination is load/direction dependent.”
When the spine is loaded asymmetrically is similar to bending sideways. Picture the spine as a bunch of books loaded on top of each other with big bars of wet soap in between. When you bend to the right, you would bring the right sided edges of the books closer together (i.e. they would be compressed) and the left side of the books would be gaped open (i.e. they would decompress). If there is a wet bar of soap in the middle, it would move away from the compressed side. This is what the above sentence is saying in a lot of words. There is more to it than this, such as forming a trough, which is similar to the annular fissure.
The disc is composed of two separate parts: the annular rings and the nucleus. The annular rings are a cartilaginous protector of the nucleus. It’s similar to putting Play Do in a sealed freezer bag. When you push on one end of the freezer bag, the play do goes in the opposite direction. This holds true for most discs. There is a study (I have trouble remembering the name, but its by a Japanese author…I’ll try to find it and write a post on it) that demonstrates that as the discs age and dehydrate, they may not always move away from the compressed side, but this is a different story for a different day. Just know that for the most part, most discs operate like this on most days.
“Callaghan and McGill (2001) determined that posterior disc herniations are consistently created with repetitive flexion under modest static compressive forces”
McGill…Oh great guru of spines…has done much research regarding the biomechanics of the spine. Stu, as he likes to be called, is actually very approachable and responsive to e-mails, which is surprising considering how high profile he actually is in the rehab world. Anywho…what they found was that a disc herniation is predictable with specific forces. If one repeatedly bends forward while standing and does this bending from the spine (NO-NO) instead of from the hips.
“Their data suggests that disc herniations are an injury that result from cumulative bending trauma and can initiate after only 5870 cycles of flexion/extension while under a compressive load of only 867N.”
It’s funny that they use the term “only 5870 cycles”. That’s a whole heck of a lot of flexion/extension cycles. This is more of the case of the straw that broke the camel’s back more so than one cycle of flexion/extension. The number that the authors gave for force is equivalent to about 200 pounds. Now take your upper body weight plus any external weight and this is the number of flexion cycles that it would take to cause a herniation.
“Bending the motion segments about an axis oriented 30 (degrees) to the left of the sagittal plane flexion axis resulted in the focused nucleus tracking toward the posterior right side of the disc in 15 of the 16 trials”
What this means is that in a majority of people, bending towards one direction will cause a movement of the disc material in the opposite direction. The are treatment strategies that are based on this exact theory.
“Discovering that the side that the nucleus tracks is dependent upon the direction of bending motion is of use in understanding injury mechanics”
This means that we could reverse engineer the injury if we have a picture of the disc. It also means that we could reverse the injury if the disc is still intact by moving in the opposite direction of the problematic motion. These are theories of course, but this type of theory is used in treatment.
NOTE: This article uses a porcine disc model, which is commonly used in the research to mimic the motion of the human disc.
Aultman CD, Scannell J, McGill SM. The direction of progressive herniation in porcine spine motion segments is influenced by the orientation of the bending axis. Clinical Biomechanics. 2005;20:126-129.