Dynamic Stabilization with Flexible Rods

The principal behind dynamic stabilization of the spine is to provide internal support and protection to the area of the spine affected by wear and tear changes without having to rigidly fuse or stiffen the segments of the spine involved.

The spine is meant to move and the advantages in terms of function of preserving that movement are clear. However, there is a rather less immediately apparent advantage, but one which is perhaps even more important. Fusing one part of the spine results in stresses being transferred to other parts of the spine, particularly the neighbouring segments. This can potentially lead to problems affecting these levels in future years, a condition known as adjacent level disease. Dynamic stabilization techniques aim to avoid this problem by providing support for the spine where it is needed but still allowing continuing movement. This minimizes the transfer of stress to other parts of the spine.
There are various techniques of dynamic stabilization. Perhaps the most minimally invasive method is the Wallis® stabilization technique, but this cannot be undertaken in certain circumstances and would be inappropriate in some others. Dynamic stabilization using flexible rods offers an excellent alternative which may be suitable in these situations.

Dynamic Stabilization with Flexible Rods

Rationale

There are two problems that cause back pain with moderate wear and tear in the lower back. Abnormal loads through the disc and facet joints can be painful. The affected segment also becomes a little loose and soft causing abnormal movements to occur.

The disc is often compared with a car tyre. Like a good, well inflated tyre the normal disc is firm but compliant, able to absorb stresses and allow some degree of movement. Degeneration causes the disc to become soft, like a deflated tyre. Bumps and stresses are not well absorbed, jarring the back, and the disc becomes rather 'wobbly' or unstable. This instability is experienced as the back 'going', often with quite trivial movements, and can cause further damage to the disc, with tears in the outer wall of the disc developing, much like splits in the wall of a flat tyre.

Flexible rods help to transfer loads and stresses away from the discs and facet joints. While being flexible the rods do impart stability to the affected part of the spine, eliminating the loose abnormal movements that can occur.

Advantages

1. Screws fixed into the vertebrae, called pedicle screws, provide a very strong and robust attachment for the rods. This allows a large amount of load to be relieved from the disc and facet joints. Because of this the technique can be used where there are somewhat more advanced wear and tear changes.
2. Interspinous stabilization devices such as the Wallis® system provide good stability in bending and extending movements of the spine but are less effective in controlling side bending and twisting movements. The use of screws and flexible rods can help to stabilize the spine in all these movements.
3. There are certain situations where the bony anchorage points for the Wallis® system may be absent, inadequate, or need to be removed as part of the treatment of a spinal condition. This is particularly the case at the lowest level of the lumbar spine (L5/S1) where the spinous process of the sacrum is often too small to allow Wallis® stabilization. In these situations the use of flexible rods still allows stabilization, preserving movement in the spine.

Possible Drawbacks

1. The surgery may potentially cause some damage to the important muscles of the back, and is a little more invasive than the Wallis® Stabilization technique. However, special minimally invasive approaches and retractor systems can keep any muscle damage to a minimum. The use of screws does have some specific risks, such as the very small possiblity of injuring a nerve root.
2. Movement will lead to loads being placed on the screws. Unlike a spinal fusion, where the fusion bone takes over the load bearing role from the screws after a few months, these loads will have to be borne by the screws for many years. As a result there is a small risk that the screws may eventually loosen or break and this may require further spinal surgery.

Flexible Rod Systems

There are two flexible rod devices which can be used, the CD Horizon® Agile™ system, by Medtronic, and the Dynesys system. Both systems share a number of similarities. The Dynesys system uses a tensioning polyethylene cord, which will control bending movements. This is combined with a flexible spacer that can bend with movement but helps to off-load the disc and facet joints. The CD Horizon® Agile ™system is a more recent refinement in which the flexible spacers and the tensioning cord (in this case a braided cable) are built into titanium rods. Different sizes of spacers can be selected with slightly different degrees of flexibility. One advantage of this system is that it can be combined with normal rods, allowing a fusion to be undertaken at one level and a dynamic stabilization at the next
(see Hybrid Techniques).

How are the Screws and Rods adapted to an individual?

Screw sizes are chosen to give the best fit and fix within a particular vertebra. A wide range of diameters and lengths are available. With the CD Horizon® Agile™ system the rods with the built in spacers also come in a range of sizes and the off-loading distraction adjustment can be undertaken very precisely as the rods are secured to the screws. In the Dynesys system the surgeon will gauge the amount of distraction force required and, as this is done, measure precisely the length of spacer required.

 

 

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