84

Orthopaedics and Traumatology

4 119951.84-86 {No 2). (0 Urban &Vogel. Munich

Comment

to the contribution of T. M. Stoll, E. W. Morscher: Anterior Interbody Fusion Using the Cervical Spine Locking Plate

by Michael Blauth, Harald Tscherne, Unfallchirurgische Klinik der Medizinischen Hochschute Hannover

Prefacing remark: We can not report on personal ex- periences with the new system of the C. S. Locking Plate as described here by Stoll and Morscher. How- ever, between 1972 and 1993 we have performed an anterior, intervertebral fusion with plates in the treat- ment of unstable, mostly posttraumatic lesions in the lower cervical spine in 585 patients, using mostly at first the third tubular plate and later the conventional H-plate of AO [8]. The anterior fusion has proved to be the standard technique for treatment of these cer- vical lesions and has replaced in all but few indica- tions the posterior stabilization. The advantages of the anterior approach has been convincingly docu- mented by Stoll and Morscher.

Our comment concentrates mainly on three facts which seem to be important to us when comparing the method described by Stoll and Morscher with the more commonly used method, also preferred by us. The Cervical Spine Locking Plate has an angular sta- bility of screws whereas the conventional system does not have such a stability.

The Problem o f Stability o f Anterior Intervertebral Fusion

The authors state that an internal fixation without an- gular stability between screws and plate fails to pro- vide a sufficient stability. How can they proof this point?

The idea of angular stability was first advanced for the application of implants for the posterior vertebral fusion and realized by Magerl [5] through his fixator externe. An implant inserted posteriorly, however, serves a different purpose in the absence of an ante- rior support as an implant used in an anterior fusion in combination with a corticocancellous bone graft which resists compression. Moreover, the principle of angular stability can not easily be transferred from posterior to anterior methods as, at least theoretical- ly, the bone graft will not be exposed to the axial compression which is essential for consolidation.

The results of the biomechanical studies of Ulrich [9] have been cited frequently as proof of the insufficient

stability of a simple intervertebral fusion in the pre- sence of posterior ligament instability. Under these conditons the H-plate is inferior to the hook-plate of Magerl, however, one should be careful when trans- ferring these data to the in vivo situation.

Especially in the cervical spine the posterior muscles play an important role as tension bands for the stabil- ity; this fact could not be considered in the cited stud- ies of Ulrich. Stoll and Morscher are criticising the insufficient stability of a plate fixation which has no angular stability: however, they fail to show improved results when using a locking plate.

Many clinical observations [l, 2, 4, 6, 7] made in very different lesions speak in favor of a sufficient stability of purely anterior fixations which have no angular stability.

We also followed up 57 of 89 patients who had been operated on through a anterior approach between 1973 and 1982 for mostly discoligamentous or osteo- ligamentous instability of the lower cervical spine. Seventeen of those 89 patients had died in the mean- time, 2 could not be located, 13 were not available for a follow up for various reasons, unrelated to their injuries. The average follow up time was 10 7/12 years (!). The average postoperative loss of correc- tion in the lateral plane was 0.5 °. Sagital displace- ments were too small to be measured. Only in one in- stance of a clearly poorly performed fusion was an impaction of the vertebral body of 7 ° and an anterior displacement noticed during the early postoperative course.

These encouraging results were achieved although many patients presented with a posterior instability and the operative technique was new to 13 different surgeons.

The Problem o f Loosening and Anchorage o f Screws

A strong argument by Stoll and Morscher for the locking plate is the fact that the screws are unable to back out thanks to their locking device.

Comment c)~" Orthop Traumatol. 4 ( L 9951. S4-86 ( N~ 21 0 . ~

How important was the problem of screw migration among our patients? We noticed in 2 out of 57 pa- tients an early postoperative backing out of screws. However, they were caused by wrong placement of the screws into the disc space. In two further patients implant removal was performed in other institutions at a later time for supposedly loosened screws. Screw heads sitting obliquely in the plate are projected above the plate in lateral radiographs. Therefore often they only appear as being loose. We did not ob- serve any injury to the oesophagus.

The danger of screw loosening can only be avoided through optimal anchorage of the screws. It may be possible that a screw is not perfectly placed at once especially when it is inserted into a partially fractured vertebral body. The conventional H-plate, however, allows at any time to redirect a screw and to place it differently, i.e. into the corners of the vertebral body. This is an important disadvantage of the new locking system, as acknowledged also by the authors: the di- rection of insertion of the screws is exactly predeter- mined and can not be changed. We have repeatedly observed as for example in incomplete burst frac- tures, that part of the vertebral body can be used for anchorage of the screws thus avoiding the inclusion of another level in the fusion while at the same time de- creasing the extent of fusion. This, however, requires the possibility to alter the angle between screws and plates to a certain extent! Moreover a bone graft can be subjected to a certain compression when using a conventional H-plate which allows eccentric drilling.

Risk of Injury to the Spinal Cord by Screws

When using the conventional plates the purchase of ,,ordinary" screws is increased by placing them into the posterior cortex of the vertebral body. In regard to the concern postulated by the authors of a risk of iatrogenic injury to the spinal cord through drilling we did not find any injury to the dura or spinal cord. The theoretic risk of injury is much reduced by using an oscillating drill bit. The perforat ion of the posteri- or wall of the vertebral body can be detected at once. The image intensifier serves only to control the direc- tion of drilling. Tapping is not necessary, even with conventional screws, contrary to the authors claim. If plasma sprayed titanium plates are being used and the spongiosa is strong, one can omit in the conven- tional technique the anchorage of screws in the pos- terior cortex.

A further technical point to be made concerns the use of the four Hohmann retractors which will retract the soft tissues from the cervical spine. After many years of their use we prefer recently the spreaders developed by Casper [3]. If properly placed they al- low a much improved and constant exposure. They also avoid damage to the vertebral bodies as it can occur during hammering in of the retractors. Only one assistant is required which is of great advantage when using a freely draped intensifier. If the Hoh- mann retractors as said in the text of Stoll and Mor- scher ,,are placed into the anterior lateral walls of the vertebral body" then it is often necessary to remove a few of the retractors to be able to insert the plate onto the body from anterior. Contrary, when using the Casper spreader at this point of the operation a satisfactory exposure is maintained.

We believe that Advantages of the new plate for use in severely unstable multilevel lesions exist. Further experiences will show whether this technique will avoid an additional posterior intervention.In our pa- tients we were never obliged to use a combined stabi- lization after trauma.

In conclusion we are of the opinion that at least for most recent, mono- or bilevel lesions a fusion with a conventional H-plate, preferable from titanium, is fully sufficient. Although not allowing a stable seat- ing of screws in the plate, the conventional plates may be of greater advantage under certain circum- stances. Finally the much higher cost of the new locking system, the need for special instruments and the obviously not negligible number of errors should be taken in consideration when using the Cervical Spinal Locking System.

References

1. Aebi, M., K. Zuber, D. Marchesi: Treatment of cervical spine injuries with anterior plating. Indi- cations, techniques, and results. Spine 16 (1991), 38-45.

2. B6hler, J., T. Gaudernak: Anterior plate stabilisation for fracture-dislocations of the lower cervical spine. J. Trauma 20 (1980), 203-205.

3. Caspar, W.: Anterior cervical fusion and interbody stabilization with trapezial osteosynthetic technique. Aesculap Sci. Inf. 2 (1985), 1-36.

Resp~mse 8 6 Orthop. Traumaml 4 (1995 !. 86-88 (No. 2)

4. Illgner, A., N. Haas. M. Blauth, H. Tscherne: Die operative Behandlung von Verletzungen der Halswir- belsfiule. Unfallchirurg 92 (1989), 363-372.

5. Magerl, F. P.: Stabilization of the lower thoracic and lumbar spine with external skeletal fixation. Clin. Orthop. 189 (1984), 125-141.

6. Ripa, D. R., M. G. Kowall, P. R. Meyer, Jr., J. J. Rusin: Series of ninety-two traumatic cervical spine injuries stabilized with anterior ASIF plate fusion technique. Spine 16 (1991), 46-55.

7. Tscherne, H., G. Hiebler, G. Muhr: Zur operativen Behandlung von Frakturen und Luxationen der Hals- wirbels~iule. Hefte Unfallheilkd. 108 (1971), 142-144.

8. Tscherne, H., A. Illgner: Die ventrale interkorporelle Spondylodese der Halswirbels~ule. Operat. Orthop. Traumatol. 3 (1991), 147-157.

9. Ulrich, C., R. Kalff, L. Claes, O. W/3rsd0rfer, H.-J. Wilke: The relevance of torsional stability to anterior and posterior cervical spine fixation procedures - an experimental study. In: Louis, R., A. Weidner (eds.): Cervical spine II. Springer, Wien - New York (1989), 272-276.

Address all correspondence to: Priv.-Doz. Dr. Michael Blauth Unfallchirurgische Klinik Medizinische Hochschule D-30623 Hannover Germany

Response to the comments by M. Blauth and H. Tscherne

The goals of the development of the titanium Cervi- cal Spine Locking Plates have been to render the anterior plating of the cervical spine easier, less pro- blematic and safer. This system should obviate the need for a posterior stabilization. There is no doubt that these goals have been reached. We admit that surgeons having the same amount of experience as the commentators may not need a System with such advantages.

The foremost condition to reach our goals has been to find a safe approach which avoids not only the pe- netration of the posterior wall of the vertebral body during drilling but also the screw's encroachment of the spinal canal. A bicortical fixation has been de-

manded for adequate fixation when using the con- ventional plate systems. Naito et al. [2] have insisted on this point. Using the Caspar system they had ob- served a screw loosening in eight of 106 patients (7.5%). Six of the loosened screws had no purchase in the posterior wall. In order to achieve a primary and secondary stability of the monocort ical screw anchorage a design and a material had to be found which ensured this requirement. The typical charac- teristics of the CSLP system meets this demand by an intrinsic stability by convergent screw locking and a specific design of the titanium screws.

Response to the specific points: