Spinal Disc Mechanics: Understanding the relationship between rotation and compression

Have you ever stopped to think about the mechanics of how the vertebral disc in your spine works? Probably not, but if you look at the structure and makeup of the disc, you can see that it is made of two basic types of soft tissue. An outer fibrous tissue and an inner gel like tissue. The gel-like material (the nucleus pulpous) is the foundation of its shock absorption ability. The outer fibrous ring will be the focus of this blog entry.

The vertebral disc is very interesting as a structure. Its primary function is shock absorption, and for the spacing of the nerve roots that exit from the spinal cord. However when the disc is asked to become a means for mobility, meaning if it receives regular repetitive forces of bending, extending and rotation, then the structure undergoes a fair amount of stress. That stress causes the fibrous outer ring to develop radial and longitudinal fissuring (or cracks). Those fissures are usually not painful on the inner rim of the annulus fibrous because the only nerve innervation is on the outer rim. That is why our discs may begin to degenerate before we would even notice pain.

It is well know to the biomechanics community that the rolling, gliding and compression are physiological stresses that joints experience with normal movement. In fact these stresses are necessary for the maintenance of articular cartilage. Where there is an imbalance of rolling, gliding, and compression, joints begin to show the effects of wear and tear, marking the onset of Degenerative Joint Disease (DJD). For example, to much compression may occur with excessive running and jumping activities, which can then lead to DJD. On the other hand, not enough stress to the joint, as with prolonged bed rest or cast immobilization can also lead to DJD.

So, back to the annulus fibrous, it is made of college fibers and they are arranged in 10-20 concentric, circumference rings called lamellae (see Figure 2.3). The orientation of fibers alternates with respect to the vertical and is always the same and it measures about 65 degrees. During spinal rotation the oblique fibers of the annular fibrosis, running counter to the direction of the movement, are stretched while the intermediate fibers with the opposite orientation are relaxed. This tension reaches a maximum in the central fibers of the annulus, which are the most oblique. The nucleus (the inner gel like material) is therefore strongly compressed and the internal pressure rises in proportion to the angle of rotation. In short rotation is compression in terms of the spinal disc. This explains why flexion with axial rotation tends to tear the annulus and drive the nucleus (the inner gel like material) backward through the tears/fissures in the annulus. Thereby leading to disc herniation, prolapses and broken disc fragments in the spinal canal.

It is best to avoid joint compress techniques and exercise, as they can too easily aggravate a joint condition. Techniques that use indirect compression in the joint should also be used with caution, particularly in cases where compression tests are symptomatic. If you have a disc condition or think you might have one and are a looking for manual treatments and exercise programs that avoid compression then consult a knowledgeable physical therapist.