Justification of instrumentation in degenerative disease
by K.- P. Schulitz
Orthopaedic Department of Heinrich-Heine University, Düsseldorf, Germany
Since 30 years, I have been performing fusions on the vertebral spine, partly with good, excellent, but partly also with disappointing results. Many surgeons have this experience. I still belong to the grand-parents generation of the orthopaedic surgery and I have seen many different developments. Many methods have been digged out newly and had been dressed in new clothes to suit again, methods which have been provided with much approval and which have been thought to give good results but which have been abandoned after a certain period. Revolutionary innovation is minimal invasive surgery and implantation of techniques that lead to significant progress in surgical practice. I think we have all seen the development of minimal invasive surgery of the disc herniation: After chemonucleolysis had been in fashion, percutaneous discectomy was then introduced and then Onik's method was considered to be the major break-through with his cutting device and its suction capacity. Percutaneous laser disc-decompression was then invented. New methods are obliged to give better results. This has been thought or it had to be the fact at the time of invention, otherwise, the method would not have a justification to be invented. Chemonucleolysis was discredited, APLD soon was obsolete, we are concentrating our efforts on laser discectomy, the latest results being published around 78 % which we know from open surgery, i.e. that despite of the latest developments in this high-tech procedure, the results have not improved accordingly.
Spinal Fusion - Indication and Results
What about instrumented fusion which has been considered as another milestone of the last two decades? I have got the impression that the triumphal procession of pedicle screw fixation has continued until recently. In the United States in the period between 1979 and 1990 the operation rate for non-instrumented and instrumented spinal fusion in adults rose from 13 to 26 per 100,000 adult inhabitants, a 100 % increase (Taylor, 1994 (Fig. XX)). Another inpatient profile shows that only within one year, from 1991 to 1992 the fusion rate increased up to 30 %. Associated with that is a rise of the market volume of spinal implants. In Europe implants are used especially in Germany. It is supposed that in the United States about 59,000 spinal fusion had been carried out only in 1987 for the treatment of patients with low back disability and as costs per procedure including aftercare were estimated at 40,000 US-$ (Kostuik, 1992), the costs amount to an astonishing 2.4 billion US-$ per year. But these rates are essentially higher today. Astonishing is also that the surgery rates are higher in Eastern USA than elsewhere.
And the question I would like to be answered is: what is the reason for this explosive increase in the use of fixation devices for spinal fusion operations? In the United States like in other countries, the incidence of sickness benefit for low back pain has increased considerably in the recent decades. The increase of fusion could be still understandable if there had been a linear correlation between disease and fusion, but the number of spine operations grew supraproportionally. It had been noticed, too, that the number of spinal surgical procedures performed has been closely linked to the per capita supply of neurologic and orthopaedic surgeons (Cherkin, 1994) and there had been indeed a significant increase in the number of medical graduates world-wide. But what is astonishing is that distinct spine diseases are not only operated more and more, but that these diseases are also fused and instrumented additionnally more and more often. This might have something to do with the surgeons' new approach to see problems, as fusions and especially instrumentations. At least, I think this has nothing to do with over-enthousiasm among surgeons to try something new, as the instrumented fusion since the beginning in 1970 steadily going up is now an established method.
I thought that the increase of instrumented fusion might be based on significantly better results so that once I reviewed all relevant papers on the subject of spinal fusion presented at meetings of the International Society for the Study of the Lumbar Spine (ISSLS) and the Congress of Lumbar Spine Fusion. But I found that with and without internal fixation devices in the treatment of degenerative diseases, fusion rate and clinical results derived no benefits from instrumentation. I think that from the fusion rates between 78 and 94 % respectively 74 and 99 % you cannot derive any advantage for instrumentation, and the same is valid for the clinical results.
The idea of instrumented fusion was based on the assumption that the resultant rigidity of the spinal segment would lead to a more rapid and stronger spinal fusion. This opinion has been shared by many surgeons and is supported by some experimental studies on spinal fusion in animals while others expressed reservations about the influence of these devices in enhancing fusion. At least these results are not reflected clinically, as you can verify if you go through literature. One must of course admit that you cannot really compare the results without instruments of one author with those with instruments of the other, because the healing of the fusion is based on different criteria and is difficult to be proved exactly. So I looked for retrospective and prospective studies comparing fusions with and without instrumentation, but no clear-cut conclusion could be drawn, as the statements even in prospective studies had been different. It is striking from this list that especially the degenerative spondylolistheses did not only profit from fusion, but a little bit more and statistically significant from instrumentation, as you can gather from the investigation of Bridwell. This was also the statement of a meta-analysis, which had been performed by Mardjetko (1994). As you know, the whole question of the use of pedicle screws in the USA had been reviewed by the Federal Drug Administration (FDA). In July 1994 a meeting was held between the FDA and the FDA Orthopaedic Advisory Panel from which a recommendation emerged that the classification of pedicle screws should be upranged from class III to class II, but also only for use in the treatment of degenerative spondylolisthesis and fractures. It was decided that the use of these devices in the treatment of degenerative disc disease other than degenerative spondylolisthesis has to be considered as questionable. This decision was largely based on informations obtained from the cited study of Mardjetko (1994). Co-determinant was also a historical cohort study of pedicle screw fixation in thoracic lumbar and sacral spinal fusions of Yuan (1994) on 3,000 patients in which the degenerative spondylolisthesis showed a higher fusion rate in the instrumentation group. The advantage of pedicle screw fixation is theoretically the increase of rigidity, and practically the possibility of correction of malposition and reduction of the number of fusion levels.
Advantage of pedicle screw fixation
But, summa summarum, it can be stated that at present in literature there is no clear advantage in the use of instrumentation in the treatment of degenerative diseases of the lumbar spine except in rare circumstances, even if our feeling and our personal follow-ups seem to be contradictory. The fact that tens of thousands of patients treated with pedicle screws resulted in solid fusion alone should not influence us in the application of the pedicle screw if we could also obtain the same result without it. We must consider that in degenerative disc diseases many painful segments are not unstable but on the contrary, they show an increased rigidity so that when the preconditions for spinal fusion are set, such as a good tissue bed and a good osteoconductive potency of graft material, the results of non-instrumented fusions for most degenerative diseases seem to be essentially the same or at least not essentially worse than those achieved with the addition of pedicle screws.
Indications for pedicle screw fixation
Apart from a questionable amelioration of fusion rate, the definite advantage of instrumentation is the possibility of correction of malposition, from which tumours and fractures can profit in so far as the pedicle screw system has a three dimensional adjustability. So the indication for instrumentation is mostly seen for tumours, fractures and scoliosis. In the treatment of tumours and fractures, the pedicle screw system enables us to reduce the displaced vertebra and to stabilize the unstable spine and avoid neurological damage, remembering its limitation when resections of vertebral bodies are to be carried out or in the management of kyphotic deformities. In the management of degenerative disc disease, besides evident gross instabilities, too, situations may occur where in relation to malposition the use of pedicle screws certainly is beneficial or even essential. This may be the case in rotational, translational or retrolisthetic instabilities, e.g. the adult idiopathic lumbar rotational scoliosis or the lateral stenosis leading to an irritation of neural tissue which cannot be removed by decompression alone and which requires absolutely a realignment. Similarly, in the management of some cases of high grade isthmic spondylolisthesis, pedicle screw fixation with reduction of the deformity may find a place. Only in this context one should take an instrumented fusion into consideration, whilst in other cases one should make a critical application of instrumentation.
Disadvantage of pedicle screw fixation
Generally, we have to proceed on the assumption that essentially increased results are not due to more and more sophisticated procedures as e.g. instrumentation, but that they are almost with any exception due to indication. A good surgical technique is of course important, but, as always in medicine, indication - may it be with or without instruments - is the key of success for good results in fusion. Concerning fractures and tumours, we dispose of sufficiently defined concepts of stability so that it is not difficult to find an indication for instrumentation. However, in degenerative diseases, the pedicle screw fixation system is being over-estimated, in as far as there is no marked instability and no malposition with neurological consequences, and if we do not plan to reduce the number of fused segment with the aid of a screw fixation system. We must get rid of the imagination that much always helps much. We surely have to make further experiences in as far as indication for pedicle screw application is concerned. We should apply an optimal, but not a maximal therapy, as each overtreatment bears the danger of an increase of unnecessary risks. As for some diseases, we have certainly made great progress in the areas of pedicle screw fixation, but we also had to accept the disadvantages of spinal instrumentation. Anyhow, by experience and the so-called "CAOS-systems" lesions of pedicle and nerve root will be more seldom. But there are certain disadvantages which result from the implant itself, such as loosening, stress concentration in the adjacent segment, stress shielding, implant failure a.s.o.
In conclusion, the use of pedicle screw fixation must be considered as a mile-stone in developments in spinal surgery in the past decade because of its benefits in the field of correction of spinal deformities due to scoliosis, following fractures, in the treatment of spinal tumours and in some cases of spondylolistheses. Their widest use, however, has been in the treatment of degenerative disc diseases for which its continued use must be brought into question. Due to the high standard already obtained in spinal surgery and a constant development of potent spinal instruments the risk incurs that we are gradually going beyond ethical, moral and judicial boundaries, as with the help of modern anaesthesia and modern effectiveness of prae- and intraoperative blood saving a.s.o. we are placed in a position where we are able to create the optimal environment for the mechanical realisation of all spinal problems. We should in future remember the basic principles of primum non nocere in the treatment of patients with spinal disorders. What should become most important is that we should make every effort to deal with the causes of pain and the indications for therapy - so that weighing the pros and the cons involved in spinal technology, we can perhaps reduce the use of internal fixation devices in spinal surgery.
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