The Disc Replacement Book
By Jim Rider – Motion a Natural State
When it comes to your spine it’s all about the motion. Your spine provides strength and stability to your body as we stand, walk and function. If you take a look at the spine you should first consider the spine of an animal, say a horse, the spine is mostly horizontal, the vertebrae are across the bottom separated by soft discs and there are ligaments and muscles supporting it from above, much like a suspension bridge. This structure is fluid in its motion yet very strong. The discs have almost no weight bearing function, this is done by the ligaments and muscles. A brilliant design!
We refer to the three sections of the spine as the Cervical or Neck area, the thoracic, this is the area where the ribs form a cage like structure which also provides stability, and the Lumbar, an especially mobile area that allows us to bend and twist. The Spinal Cord runs through the spine and is protected by the vertebrae.
When we humans stand up straight the spine takes on a completely different challenge. Now the spine is mostly vertical, the discs become weight bearing and the muscles loose much of their ability to support the structure. Soon the body can no longer heal the damage this causes.
Let’s take a look at the vertebra and the discs. No two vertebra are the same, yet they do share common functions and structure. Each has a larger body structure, where the disc is attached and structures that surround the spinal cord and then extend outward to provide attachment of the ligaments and muscles. These outward structures also come together from one vertebrae above and one below to form what is called a facet joint.
The discs are between the vertebrae and because they are soft allow for motion between the vertebra. By compressing into wedge shapes each disc provides a bit of a curve and when you combine several of these you can cause a flex or spring like motion.
Now let’s talk more about this motion!
Preserving Natural Motion is of course the primary reason why we are moving towards disc replacement versus fusion surgery which eliminates motion. The Concept of motion in a spine is important to understand. What motion are we referring to? It is in fact the flexing forward and extending backwards that should be discussed first.
This motion affects the joints where the vertebrae come together called the facet joints. These joints are under great pressure when bending occurs and this bending is typically studied as a rotation of one vertebrae on the other. We can also measure the range of this motion and how the disc resists or allows this motion.
Often this rotation is thought of as having a “center” of rotation. This would presume that no shifting or sliding of this rotation point occurs, this is not in fact true. As the soft disc compresses, allowing for the spinal curve to increase by becoming a wedge shape, the vertebra are seen to naturally shift slightly back and forth.
Because of this shifting the concept of a rigid ball and socket rotation, like a hip or Knee, must be abandoned for a more natural compression type of motion.
The natural healthy human disc will also allow early rotation but begin to gradually resist and flexion increases. Since the bending varies and is spread out over many discs, some may be mechanical disc replacements and some will be natural discs, it is important that any implants replicate the resistance of the natural discs and allow for motion variations to create a natural variable balance between all discs in the curve as needed.
See the studies…
In reality the control of motion in all directions, including twisting or axial rotation and flexion extension is done by muscles as well as the disc and must be controlled prior to reaching a point where the facet joint or spinal processes are forced to limit the motion. Forcing the facet joints to do the work is a major cause of back pain in an unhealthy spine.
What about shock absorption?
Yes the natural spinal disc will absorb some shock forces, but most of this is done by the flex of the spinal curve. Shock absorption in a disc replacement implant is important, and safer that a hard implant, but the compression is mostly off center and allows for the wedging effect which promotes a natural flex of the curve.
Disc Disease and Pain
As explained the human spine, now vertical, has many parts, but he poor little discs are doing a job they are not designed for, flexing and supporting weight.
As these discs are put under increased stress they begin to lose their strength. The disc walls get weak and can have trouble holding up against the pressure of all the weight. This causes the inner soft disc material to push out and herniate. The disc can also dry out and shrink losing some of its ability to support the weight and causing loss of disc height. This loss of height and structural integrity is called degenerative disc disease.
Not really a disease but the result of years of wear and tear. This condition is present in every one of us and is progressive with age. This degeneration causes back pain and other symptoms in many forms.
Next Chapter – The Evolution of Disc Replacement > >
The Disc Replacement Book – A Back in Motion
By Jim Rider