Vertebral Disc Degeneration
The vertebral disc is the component of the spine structure that holds the vertebrae solidly together, while at the same time, enabling certain mobility to the spine. It also serves as a type of shock absorber similar to a blown-up automobile tire that insures a stable contact between the road and the wheel and blunts, somewhat, the shaking caused by the unevenness of the road surface. This disc function is made possible by its special structure, again similar to the tire, consisting of a strong, fibrous outer ring that surrounds a core which, in the case of the disc, is a liquid jelly-like substance. As we age, the disc, along with our other tissues, loses some of its liquid content, its elasticity is reduced and becomes “deflated".
One episode in the process might be the herniation of the disc (“defect") which occurs when a piece of the cartilage is pushed into the spinal canal, taking space away from the nerve passing through there and putting pressure on it. This, typically, results in complaints of pain, numbness and weakness of the lower extremities. We may also describe this phenomenon as space reducing, which can be remediated surgically by freeing the nerves.
This loss of elasticity (degeneration), however, may have other consequences, as well, if the neighboring vertebrae's capacity to control mobility has also been damaged. This condition is called instability and characterized, typically, by localized pain exacerbated at weight bearing. The surgical treatment in these cases is stabilization.
It has been a long ago basic tenet of orthopedic surgery to terminate pain inducing abnormal movements, as shown by its treatment of fractures. There was a time when the degenerated, painful major joints (i.e., hips, knees) were made rigid. The pain did, indeed, cease. Movement, however, became severely restricted. Thus, orthopedics introduced and has been using for several decades by now with great success, joint prostheses in the treatment of degeneration of the major joints.
It followed logically, that for spinal surgery the solution to instability would be the removal of the degenerated disc and the rigidifying of the vertebrae involved. This process is being widely used today as the “gold standard" with good results, however, we continue to seek new developments in spine treatment, not only for ceasing the pain, but for restoring the spine's function, which brings disc prostheses ever more to the fore.
Cervical Disc Prostheses
We first used the disc prosthesis on the cervical segment, the area under the least amount of gravitational stress. Presently, we perform the most numbers of such procedures in Hungary and, following our initial trepidation, after 10 years' of experience, we are now able to indicate it more widely to our patients. This is our method of choice in the surgical treatment of cervical herniated discs that are well circumscribed and without contraindications. This procedure provides an answer to the problems of space narrowing (herniated disc) and instability even in view of the fact that by preserving function, the weight bearing effect on the neighboring discs following surgery is milder.
A wide variety of prostheses, developed on the basis of differing biomechanical values and produced by diverse manufacturers, are available to us. Since clinical results, our own experiences and comparisons reported in international literature find no significant differences among them, the surgeon may choose according to his/her preference.
Lumbar Disc Prosthesis
Since the lumbar spine is under much more gravitational pressure, the lumbar disc prosthesis is slower at making inroads in the market place with pro and contra arguments formulated regarding surgical techniques, biomechanics and indicators. Moreover, vis á vis the cervical spine, there is not only serious competition in this area but several alternative surgical methods for maintaining spine mobility have also appeared. The disc prosthesis, however, has been in use the longest, has provided the most extensive experience and, we can clearly say, that this method has found its place in spinal surgery.
Development of prostheses for the lumbar spine, follow two paths. One, is for the treatment of less advanced degeneration, the “nucleus prosthesis", wherein only the core of the disc is replaced, resulting in an anatomic reconstruction almost. The other, is for the more serious degeneration requiring complete or total disc prostheses, wherein, in addition to the core, the fibrous ring is also removed and new joint components are implanted.
In addition, it is important to remember, that the path of the prosthesis implant diverges from the conventional fixation in that while the latter is, generally, performed in the posterior approach, the prosthesis is implanted from the abdomen, in the anterior approach. This might sound frightening or, perhaps, even illogical, especially considering that, relative to the vertical axis of the body, the spine is located to the back. With this approach, however, we can avoid the locomotor and the stabilizing spine muscles as well as the spinal canal and its nerves thus contributing to future good functional results. This also allows for a smaller suture that has an added esthetic benefit. Our own early experiences, as well as data reported in international publications, show that Patients who had undergone this treatment, needed a shorter period of rehabilitation enabling them to return to work earlier than their counterparts who had undergone fixation.
A variety of lumbar spine prostheses are available now on the market as well as through us. Clinical results show, that no significant differences are found between them. These procedures, as others, also have their limitations. In order to get optimal results, strict considerations must be followed and there must be a good indication for it. Currently we are very cautious about this procedure only undertaking it under very limited indications.