Neurosurg. Rev. 15 (1992) 275-283
Osteosynthesis of injuries and rheumatic or congenital instabilities of the upper cervical spine using double-threaded screws Peter KniJringer D e p a r t m e n t o f Neurosurgery o f the University o f Ulm, District H o s p i t a l Gtinzburg, Fed. Rep. o f G e r m a n y
Abstract The surgical treatment of instabilities of the upper cervical spine, independent of their cause, requires the use of differentiated procedures if functional anatomy is to be largely restored. Successful procedures have been the diagonal screw fixation of the axis from the anterolateral aspect in case of acute fractures of the odontoid process, transpedicular screw osteosynthesis of C2 in hangman's fractures with bony instability, and the transarticular screw fixation of C 1/2 with interarcual fusion for atlantoaxial instabilities. Compared with conventional screws, the use of doublethreaded screws which are almost totally imbedded in the vertebral body has the advantage of eliminating local irritation, reducing the risk of surgery, and simplifying the operative procedure itself. Keywords: Double-threaded screws, osteosynthesis, upper cervical spine.
1 Introduction The objective of any surgical treatment of traumatic, congenital, or acquired instabilities of the upper cervical spine is the reconstitution of the original, anatomical conditions. W i t h regard to the affected organs - nerves, vessels, and spine - , the following aspects must be considered. Reposition and stability are required to relieve the spinal cord, nerve roots and vertebral arteries and protect them from continuous microtrauma. Decompression is designed to improve any acute neurological impairments, to procure stable conditions in order to prevent chronic disorders like progressive cervical myelopathy, constant nerve r o o t irritation, and recurring vertebrobasilar insufficiency, 9 1992 by Walter de Gruyter & Co. Berlin - New York
One aim in spinal mobility is that osteosynthesis produces no or only the least possible functional loss. This aim is o f p a r a m o u n t i m p o r t a n c e for the upper cervical spine, which requires the o p t i m u m of mobility for head motion. The following account will delineate some aspects o f the treatment o f o d o n t o i d fractures, h a n g m a n ' s fractures, and atlantoaxial instabilities o f various causes using double-threaded screws (Table I). Table I. Indications for osteosynthesis with doublethreaded screws in instabilities of the upper cervical column Compression-screw osteosynthesis of the odontoid process Acute fracture of the odontoid process Transpedicular screw fixation C 2 Hangman's fracture (with dominant bony instability) Transarticular screw fixation C 1/2 Jefferson fracture Combined odontoid and Jefferson fracture Odontoid pseudarthrosis Rotatory subluxation C t Post traumatic incongruency C 1/2 Rheumatic arthritis Os odontoideum Atlas-assimilation and instability
2 Material and methods 2.1 Functional principle of the double-threaded screw The double-threaded screw has two different, self cutting threads which are independently arranged
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along a slim, threadless centerpiece. The threads of the front portion are not only steeper than those in the back, they are also further apart (Figure 1). When the screw is introduced, the front thread enters the distal bone fragment after it has passed the proximal bone fragment and the fracture line, and the back thread enters the proximal bone fragment. Due to its steeper angle and the greater distance between threads, the entry velocity of the front thread is greater than that of the back thread. This causes the distal fragment to be moved towards the proximal fragment in the course of screw fixation, so that both fragments are ultimately pressed together. Since the front thread has a diameter of 3.2 m m (with a core diameter of 1.8 ram), and the back thread has a diameter of 5.0 m m (with a core diameter of 3.7 mm), both threads can cut without interfering with each other.
in a halofixateur or a minerva cast, screw fixation of the axis, without placing undue strain on the patient, leads to quicker and more stable healing results after about six weeks of wearing a removable neck brace [11]. In the hands of a practiced surgeon, therefore, screw fixation of acute odontoid fractures presents a true alternative to conservative treatment and is to be seen the method of choice among the other surgical procedures. The use of double-threaded screws and the development of a corresponding application set resolved some of the disadvantages inherent in the original procedure. The heads of the generally employed small fragment spongy bone screws almost invariably irritate the segment C 2/3, leading to painful inhibition of ventral flexion combined with posterior headaches and osteophytic reactions at C 2/3 [15, 16, 17]. The double-threaded screws, which are almost completely imbedded in the axis, do not produce mechanical irritation of the segment C 2/3. With regard to the diameters and lengths of the threaded portions, the dimensions of the double-threaded screws are exactly adapted to the anatomic conditions of the fracture types encountered at the axis. The cannulated preburr for the back thread increases the diameter of the burr hole from 1.8 m m to 3.7 ram, so that it can be easily found when the screw is introduced. This and the fact that it is no longer necessary to prethread the burr hole serve to simplify the procedure.
Surgical procedure [19, 21]: Preoperative diagnos-
Figure 1. Double-threaded screw with two differently styled threaded portions. The threads of the front portion are not only steeper than those of the back portion, they are also further apart.
2.20dontoid screw fixation The osteosynthetic repair of odontoid fractures with a pair of screws introduced from the anterolateral aspect has the advantage over the dorsal method [4, 8] that the atlantoaxial joint is maintained or reconstituted [2, 3, 15]. Compared with the transoral procedure [6, 7] there is less intraand postoperative burden on the patient, and the risk of infection is reduced. Whereas conservative treatment necessitates long periods of constraint
tics include X-rays (transoral a.p. and lateral), computed tomography of C 1/2, and possibly Xray tomography [12]. Repositioning is performed with an extension frame modified according to U~cmACH (4 spikes), and the result is maintained by appropriate positioning in extension. The patient is orally intubated for the operation. It is imperative that the tubus is situated to the left of the midline and that the mouth is adequately opened by placing a bandage roll between the teeth. If this is not done, the odontoid process will be covered by the tube or the teeth during a.p. radiography, and cannot be aimed of accurately during predrilling. The 4 - 6 cm long skin incision is drawn at a slight angle within the tension lines of the right side of the neck at the level of the cricoid cartilage (C 5/6). This is important, because only this permits an accurate procedure in the fascial spaces, by which damage to the Nn. laryngeus superior and hypoglossus can be avoided. This also determines the direction for the Neurosurg. Rev. 15 (1992)
Kn/Sringer, Osteosynthesis of the upper cervical spine using double-threaded screws
Figure 2a--d. Schematic presentation of odontoid screw fixation with double-threaded screws. (a) The repositioned fracture is bridged by a 1.8 mm drill wire with a short chisel tip, which enters the lower anterior axis surface through the intervertebral disk C 2/3 and just barely perforates the cortical bone of the odontoid tip. (b) Guided by the implanted drill wires, the cannulated preborer with fixed depth of entry provides the core diameter for the posterior self-cutting threads. (c) A
borer to be introduced later. After vertically cleaving the platysma, the upper border of the omohyoid muscle is prepared and the cervical spine is bluntly probed behind the neck muscles approximately at C 4/5. From here, we proceed by sight (headlamp) retropharyngealty up to C 2/3. Once C 2 is reached, two image converters are positioned and the metal retractors are exchanged for X-ray negative ones [13, 14]. Under simultaneous imaging from both views, the fracture is bridged by 2 drill wires, which are introduced at the anterior base of the axis through the intervertebral disk of C 2/3 and which just barely perforate the dorsal cortical bone near the tip of the odontoid. Neurosurg, Rev. 15 (1992)
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fitting double-threaded screw is implanted with a screwholder and a screwdriver. (d) When precisely positioned, the anterior thread is located in the cortical bone of the odontoid tip, and the posterior thread is in the cortical bone of the anterior base, so that the hard vertebral structures are fully utilized for anchoring, and the surrounding parts are not irritated by cutting screw portions.
Using a fixed-depth cannulated drill, a hole is prebored along the situated drill wires for the broader, self cutting, posterior thread of the screw. After one drill wire is removed, the length of the screw is determined with a depth gauge. At this stage, both the second drilt wire, which is stilJ in position, and the extension stabilize the fragments, so that the respective burr holes at the axis body and odontoid are not disaligned and can be found when crossing the fracture line. A fitting screw is then inserted with a screwholder and a screwdriver (Figure 2c) and turned until it bridges the fracture line. The same procedure is then repeated in the region of the second drill wire. The implanted
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Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws if the upper vertebral joints C 2 can be repositioned and the fracture does not reach too deeply into the anterior, ventral cortical bone of the axis body. In these fractures, the lower thread prohibits a ventrocaudal dislocation of the odontoid. An anti-glide plate, as used in BOHLER'Sprocedure, is not required. Postoperatively, immobilization is achieved for 6 - 8 weeks by means of a removable neck brace with chin support. This is required for safe healing of the fracture and is supposed to support the scarry stabilization of the overdistended ligaments C 1/2.
2.3 Transpedicular screw fixation C 2 Hangman's fractures are generally treated conservatively by repositioning and a 6 - 1 2 week's course of immobilization in a halofixateur. An indication for primary intercorporal fusion C 2/3 with instrumentation is usually assumed in case there is considerable damage to intervertebral disk C 2/3 with a corresponding dislocation (procedure according to Cloward or Smith-Robinson with plate osteosynthesis). This is also the case for fractures which achieved bone stability after conservative therapy, but with a remaining discotigamental instability of C 2/3. These cases require only intercorporal fusion without metal implants.
Figure 3a--d. Type II odontoid fracture according to ANDERSON.(a) Postaccident image of dorsally dislocated odontoid and atlas. (b) and (e) Status after reposition and stabilization by double-threaded screws. Both screws are located in the odontoid, the hard vertebral structures are used for anchoring. The segment C 2/3 is unaffected. (d) Postoperative axial CT at the level of the odontoid showing the spatial relationship of the screws fixed in the odontoid. screw is now the stabilizing agent, so that the burr hole in corpus and odontoid remains in alignment. After the extension weights are removed, the screws are alternately tightened. The fragments are drawn closer together and subjected to physiological pressure. In order to achieve a rotationsproof osteosynthesis, it is always necessary to implant 2 screws. If the positioning was exact, the front thread will rest in the cortical bone of the odontoid tip, while the back thread will be anchored in the cortical bone of the anterior, lower base of the axis [17, 21] (Figures 2 a - d and 3 a - d ) . This procedure can also be used to treat fractures extending from the upper back to the lower front,
I f the instability is primarily of bony nature, as is the case in most injuries, transpedicular screw fixation [10] can provide stability without loss of function, so that this method can be considered an alternative to conservative therapy. Stabilization with exact repositioning is particularly important when the injury, either due to high-grade instability or a fracture in the region of the transverse foramina, affects the vertebral arteries and thus produces vertebrobasilar symptoms, which are often misinterpreted as contusional cerebral damage [20]. Transpedicular screw fixation provides much better repositioning and stability than external immobilization, which additionally poses considerable nursing and intensive care problems in a restless patient with lapses of consciousness [26].
Surgical procedure: The procedure can either be performed with the patient in a supine position with the extension in place or fixed in a sitting position with a sharp mounting support. Repositioning is achieved and maintainted by positioning the patient appropriately. The course of the skin incision is in the medial line vertically from the occipital region to C 7. The spinal processes of the Neurosurg. Rev. 15 (1992)
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b
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C
Figure 4a--e. Transpedicular C 2 screw osteosynthesis of a Hangman's fracture. (a) Operative site at C 1/2 (C 1 = arcus posterior atlantis; PS C 2 = processus spinosus C 2; D = Dura). The star marks the point of entry, the arrow indicates the direction of drilling. (b) and (e) Schematic presentation of the screw position between
the transverse foramen and the inner pedicular corticalis. The screw is located medial to the artery. At this point, it has its smallest diameter of 2.0 mm. The screw does not affect the segment C 1/2 and its threaded parts are anchored in the anterior and posterior cortical bone of the axis and the lamina.
vertebrae are accessed via the septum nuchae, and the neck muscles of C 2 up to about C 4 are detached. In order to avoid injury of the dura and the spinal cord, the internal contours of the vertebral arch are bilaterally visualized by preparing subperiostally up to the corpus (Figure 4a). A surgical microscope can be useful at this stage. Microscopic viewing usually reveals fracture lines in the arch and corpus region.
Once both screws are tightened alternatingly and the Redon-drains are inserted, the wound is closed in layers. Immobilization of the cervical spine by means of a removable neck brace with chin support is recommended for 4 - 6 weeks. Figure 5a - c shows an example of a clinical case.
The fracture is bilaterally bridged with a 1.8 m m drill wire. The starting point is the caudal region of the transition of the lamina into the joint structure of the 2nd cervical vertebra. The direction converges slightly to the midline (about 15~ and ascends cranially, approximately parallel to the upper edge of the arch. The drill wires and, later on, the screws are medial to the vertebral artery (Fig. 4b, c). The core diameter for the back thread of the screw is produced by preboring with a cannulated drill along the positioned drill wires. As in odontoid fixation, one drill wire remains in place to stabilize the fracture site, while the length of the screw is measured via depth gauge under lateral imaging control. The measurement can also be performed by the tightened depth arrest, which is to be placed at the entry site in the lamina. After the drill wire is removed, the length of the screw can be determined by measuring the distance between the tip of the drill wire and the depth arrest. This method requires that the drill wire has perforated the anterior cortical bone for about 1 - 2 mm. After a fitting screw is inserted, the other pedicle is screwed in the analogous fashion. Neurosurg. Rev. 15 (1992)
Transpedicular screw fixation is accompanied by the risk of injury to the vertebral arteries, the dura, and the spinal cord. This risk can be decisively reduced if the following points are observed. Besides native X-ray imaging with lateral and transoral a.p. views, possibly enhanced by X-ray tomography, one should require computed tomography of C 1 - C 3 in bone window adjustment and 2 m m layers, whose sections should be parallel to the line of the upper arch of C 2. This not only provides more precise visualization of the respective fracture types, it also reveals anomalous vertebral arteries, recognized by alterations in the transverse formania. As a rule, preoperative angiography can be waived. The exact subperiostal view of the medial borders of the arches and the localization of the vertebral artery by intraoperative doppler sonography [18] permits the relatively safe preboring and screw implantation between these two landmarks. A further reduction of the risk of injury is achieved by using double-threaded screws instead of the cortical or small fragment spongy bone screws [26] in use to date. The preboring for the implantation of the double-threaded screws is done with a 1.8 m m drill wire with a chisel tip, whereas the other
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Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws
Figure 5a--e. X-ray image of a transpedicular screw osteosynthesis of Hangman's fracture with doublethreaded screws. (a) Postaccident status. (b) and (e)
Status after transpedicular screw fixation with doublethreaded screws.
two screw types require a 2.0 or 2.5 m m spiral drill. While the thread does not need to be precut for an osteosynthesis with double-threaded screws, this is necessary for the other two screw types. The risk of injury is thus reduced not only by the smaller diameter of the drill wire and its smooth surface when compared to a spiral drill, but also by the fact that a threadcutter, which probably represents the greatest intraoperative risk, can be dispensed with. The double-threaded screw also has a distinct spatial advantage after implantation. The site with the greatest injury potential is in the vertebral arch between the vertebral artery and the internal corticalis (dura and spinal cord). At this site, the diameter of the implanted doublethreaded screw is 2.0 ram, that of the cortical screw 3.5 ram, and that of the small fragment spongy bone screw 2.4 m m (in the unthreaded portion) or 4.0 m m (in the threaded portion). Since the small fragment spongy bone screw has too long a threaded portion for this indication, one can assume that at least part of this will come to lie in the danger zone, so that this screw type has the least suitable dimensions. Even if the transverse foramen is opened, the chisel tip and the smooth drill wire are more likely to only displace the vertebral artery, whereas the spiral drill and particularly the threadcutter are more apt to cause injury to the vessel.
After the screws are tightened, the vertebral arteries should be doppler-sonographically examined for patency. In case of stenosis or occlusion by compression, an immediate correction of the screw position can reinstate the patency of an undamaged vessel. This can prevent serious circulation problems. This is particularly true when the error was made on one side only. Exactly positioned screws retain the ventral corticalis below the upper joint surfaces of the axis with the front thread, and the lamina corticalis at the transition into the lower joint structure with the back thread. If the angle is too steep, the joints of C 1/2 may be affected. If this should happen, then the metal must be removed after the injury has healed and stability is assured.
2.4 Transarticular screw fixation C 1/2 Atlantoaxial instabilities, as they occur in odontoid pseudoarthroses, os odontoideum, rheumatic affection of the odontoid or o f the ligamentum transversum atlantis, as well as Jefferson fractures not stabilized by conservative therapy, are an indication for transarticular screw fixation C 1/2 [9, 22]. Compared with the lateral atlantoaxial screw arthrodesis [5, 25], which needs to be performed bilaterally, this procedure has the advantage of a Neurosurg. Rev. 15 (1992)
Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws single access, which is furthermore generally known and technically uncomplicated. In contrast to the other methods like fusion according to GALLIE [8], BROOKS [4], and compression clamp spondylodesis [23, 24], this procedure provides the secm'ity that a reposition once performed will be maintained. The disadvantage of a higher level of technical difficulty is offset by its advantages. Surgical procedure: This surgery is best performed in supine position with extension in place rather than seated with sharp mounted support, because the steep drill direction required is much more difficult to achieve while the patient is sitting. The route of access is basically the same as that for transpedicular C 2 screw fication, only that in this case the arcus posterior atlantis needs to be visualized and that, due to the steeper drilling angle, the skin incision needs to be extended to C 7/Th 1, and it is recommended to detach the muscles up to around C 5. After subperiostal visualization of the arch C 2 at its medial aspect and the massae laterales of the atlas, one Kirschner wire each is drilled into the massae laterales a little above the lower joint surface. This protects the roots of C 2. The joints of C 1/2 can be inspected by lifting the wires and the cartilage can be removed, if necessary. The point of entry of the transarticular drill wire is again in the caudal region of the transition of the lamina into the joint structure. The direction is sagittal and obliquely upward, and is achieved by aiming at the arcus anterior atlantis under lateral imaging control. The wires perforate the upper joint surfaces C 2 between the dorsal and
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middle third, enter the lower joint surfaces C 1 and should just barely perforate the massae laterales ventrally. Screw implantation is performed as described for transpedicular screw fixation C 2, and the same safety measures and advantages of osteosynthesis with double-threaded screws apply here as welt. For transarficular screw fixation C 1/2, the burr holes and screws are located above the foramen transversarium, thus, the risk of injury to the vertebral arteries is reduced (Figure 6a-c). In order to achieve a permanent fusion, an additional interarcual fusion C 1/2 is performed using an autologous corticospongeous bone graft and wire cerclage, providing an osteosynthesis stable to all sides which does not require an additional orthetic device. For Jefferson fractures or fractures of the arch in C 1 and C 2, the cartilage needs to be removed from the joint surfaces C 1/2. A spongy bone interposition between the joints is subsequently performed, because an interarcual spondylodesis is not possible in these cases. Older Jefferson fractures, in which the massae laterales have shifted outward and can no longer be completely repositioned, require that the screws are not entirely sagittal, but diverge slightly outward, so that they do not miss the massae laterales medially. This direction increases the risk of vascular damage, so that intraoperative doppler-sonography should not be waived in these particular cases. After screw fixation of a Jefferson fracture, the head should be immobilized with a neck brace with chin support for 6 - 8 weeks.
.,:. ......... sot
a
b
Figure 6a--e. Schematic presentation of transarticular screw fixation C 1/2. (a) The drill wire enters the caudal region of the arch C 2 at the transition of the lamina into the joint mass, penetrates the joint C 1/2, which is fixed by this manipulation, and barely perforates the anterior corticalis of the massa lateralis. The cannulated Neurosurg. Rev. 15 (1992)
preborer provides the core diameter for the posterior thread. (b) and (e) The screw direction is sagittal and more inclined than for transpedicular C 2 screw fixation. The screw is behind the vertebral artery, and is anchored in the cortical bone of lamina C 2 and the massa lateralis atlantis.
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Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws wires. In the rare cases of a posterior atlas dislocation, repositioning can be achieved by axial traction and exerting force along the drill wires into the massae laterales by ventral pressure. This position is maintained by the transarticular drill wires, one wire again securing this position while the appropriate screw is implanted into the other side. Good repositioning is absolutely important, because only this guarantees that the spinal canal again obtains its necessary width and that the spinal cord is completely free from pressure (Figure 7 a - d). 3 Discussion
Figure 7a--d. Atlantoaxial instability due to PCP with debilitating motional dysfunction and incipient cervical myelopathy. (a) Lateral X-ray image in middle position. (b) Axial CT of odontoid and atlas. As in the X-ray, there is a considerably increased atlanto-odontoid distance of 10 mm with high-grade stenosis of the spinal lumen. (c) and (d) Status after exact reposition and stabilization by transarticular screw fixation and interarcuai fusion C 1/2 with corticospongeous bone graft and wire cerclage. Immediate postoperative remission of instability and improvement of the medullary symptoms.
If adequate repositioning was not achieved by extension and positioning immediately before the operation, this can be completed intraoperatively before the drill wires are introduced. A ventrally dislocated atlas is reverted to its correct position by bridling the arcus posterior and drawing backward, and this position is secured with the drill
Among the surgical treatment methods of recent type II and III odontoid fractures as classified by ANDERSON [1], osteosynthesis with doublethreaded screws constitutes the procedure of choice. Odontoid screw fixation preserves or reestablishes proper head motion. Since doublethreaded screws are almost completely imbedded into the axis, the irritation of C 2/3 movement regularly induced by the heads of small fragment spongy bone screws or cortical bone screws is avoided. Double-threaded screws are exactly adapted to the anatomy of the atlas and to the fracture types encountered at this site. The operative procedure is simplified by the instruments, which include two X-ray negative retractors. Like odontoid screw fixation, transpedicular C 2 screw fixation of hangman's fractures is a procedure associated with no functional loss of vertebral mobility, so that his method fulfills the demands for an optimal osteosynthesis. Due to the technique of 3-point fixation, transarticular C 1/2 screw fixation combined with interarcual fusion in the treatment of atlantoaxial instabilities of various causes provides the advantage of an osteosynthesis with stability in all directions, in which the result of reposition can be securely maintained. The advantage of the double-threaded screws in the latter two surgical procedures does not only consist in their simpler implantation when compared with the 3.5 and 4.0 mm screws used until now, but also in the reduction of operative risk.
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References
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odontoid process of the axis. J Bone Jt Surg 56 A (1974) 1663-- 1674 B6HLER J: Schraubenosteosynthese von Frakturen des Dens axis. Unfallheilkunde 84 (1981) 221-223 B6HLERJ: Anterior stabilization for acute fractures and non-unions of the dens. J Bone Jt Surg 64 A (1982) I8--27 BROOKSAL, FB JENKINS:Atlanto-axial arthrodesis by the wedge compression method. J Bone Jt Surg 60 A (1978) 279-284 Du TOlT G: Lateral atlanto-axial arthrodesis, a screw fixation technique. South African J Surg 14 (1976) 9 - 1 2 ESTRIDGE MN, RA SMITH: Transoral fusion of odontoid fracture. J Neurosurg 27 (1967) 462-465 FANG HSY, GB ONG: Direkt anterior approach to the upper cervical spine. J Bone Jt Surg 44 A (1962) 1588-1604 GALLIEWE: Fractures and dislocations of the cervical spine. Am J Surg 46 (1939) 495-499 GROB D, F MAGERL, P SEEMAN~:Operative atlantoaxiale Stabilisierung. In: HOHMANND, A K~GELGEN, K LmBIG(Eds) Neuroorthopfidie 4. SpringerVerlag Berlin (1988) 137-140 JUDET R: Colloque sur tes fractures et luxations r6centes du rachis cervical. Rev Chir orthop 55 (1969) 71 KNORINGERP: Zur Behandlung frischer Frakturen des Dens axis durch Kompressionsschraubenosteosynthese. Neurochirurgia 27 (1984) 6 8 - 7 2 KNORINGERP: Diagnostischer Wert der Computertomographie bei spinalen Verletzungen. Unfallchirurg 88 (1985) 6 3 - 7 4 KNORINGER P: R6ntgennegative Retraktoren bei Osteosynthesen an der Wirbels/iulen. Unfallchirurg 88 (1985) 373-376 KN6RINGERP: X-ray translucent retractors in spine surgery. J Neurosurg 64 (1986) 520-521 KN6RINGER P: Osteosynthese der Densfraktur durch Zweigewindekompressionsschrauben. Hefte zur Unfallheilkunde, Springer-Verlag Berlin 181 (1986) 9 3 - 9 7 KORINGERP: Double-threaded compression screws for osteosynthesis of acute fractures of the odontoid process. In: VOTHD, P GLEES(eds): Diseases in the cranio-cervical junction. Walter de Gruyter, Berl i n - N e w York (1987) 127-136
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[17] KNORINGERP: Operative Versorgung der Densfraktur von ventral durch Doppelgewindeschrauben. In: HOHMANND, B KUGELGEN, K LIEBIG(Hrsg): Neuroorthop/idie 4. Springer-Verlag, Heidelberg-New York (1988) 199-206 [18] KNORINGER P: Intraoperative dopplersonography during surgery of the cervical spine. In: WALTERW, M BRANDT,M BROCK,M KLINGER(eds): Advances in Neurosurgery Springer-Verlag, Berlin 16 (1988) 7 3 - 79 [19] KNORINGERP: Osteosynthese der Densfraktur mit Doppelgewindeschrauben. Videofilm (1988) Ulrich HC Ulm und Ortho-Video [20] KN6RINGERP: Neurochirurgische Indikationen bei Wirbels/iulenverletzungen. In: Hauptverband der Gewerblichen Berufsgenossenschaften e.V. Bonn (Hrsg): Diagnostik und Therapie yon Wirbels/iulenverletzungen. Heft 68 (1989) 5 1 - 6 2 [21] KN6RINGERP: Die Osteosynthese der Densfraktur mit Doppelgewindeschrauben. Operat Orthop u Traumatol (1989) 178-192 [22] MAGERL F, PS SEEMANN:Stable posterior fusion of the atlas and axis by transarticular screw fixation. In: KEHR P, A WEIONER (eds): Cervical spine I, Springer-Verlag, Wien (1987) 322- 327 [23] ROOSENK, A TRAUSCH~L,W GROTE: Posterior atlanto-axial fusion: A new clamp for laminar osteosynthesis. Arch Orthop trauma Surg 100 (1982) 27-31 [24] ROOSENK, A TRAUSCI-IEL,R KALFF,W GROTE:Die posteriore dynamische Kompressionsspondylodese bei atlantoaxialer Instabilit/it. In: HOHMANND, B KOGELGEN, K LIEBIG (Hrsg): Neuroorthop/idie 4. Springer-Verlag Berlin (1988) 192-198 [25] SIMMONSEH, G DE TOIT: Lateral atlanto-axial arthrodesis. Orthop Clin North Am 9 (1978) t 101 [26] TSCI-mRNEH, G MUHR, R O v DEN WINKEL: Frakturen und Luxationen der HWS - operative Behandlung und Ergebnisse. In: BURRI C, A ROTER (Hrsg): Verletzungen der Wirbels/iule. Hefte zur Unfallheilkunde. Springer-Verlag, Berlin 149 (1980) 89 - 94 Submitted March 29, 1989. Accepted June 3, 1992. Dr. med. Peter Kn6ringer Neurochirurg und Neuroradiologe Neurochirurgische Abteilung der Universit/it Ulm im Bezirkskrankenhaus Gfinzburg Ludwig-Heilmeyer-Str. 2 W-8870 Giinzburg Fed. Rep. of Germany
Osteosynthesis of injuries and rheumatic or congenital instabilities of the upper cervical spine using double-threaded screws Peter KniJringer D e p a r t m e n t o f Neurosurgery o f the University o f Ulm, District H o s p i t a l Gtinzburg, Fed. Rep. o f G e r m a n y
Abstract The surgical treatment of instabilities of the upper cervical spine, independent of their cause, requires the use of differentiated procedures if functional anatomy is to be largely restored. Successful procedures have been the diagonal screw fixation of the axis from the anterolateral aspect in case of acute fractures of the odontoid process, transpedicular screw osteosynthesis of C2 in hangman's fractures with bony instability, and the transarticular screw fixation of C 1/2 with interarcual fusion for atlantoaxial instabilities. Compared with conventional screws, the use of doublethreaded screws which are almost totally imbedded in the vertebral body has the advantage of eliminating local irritation, reducing the risk of surgery, and simplifying the operative procedure itself. Keywords: Double-threaded screws, osteosynthesis, upper cervical spine.
1 Introduction The objective of any surgical treatment of traumatic, congenital, or acquired instabilities of the upper cervical spine is the reconstitution of the original, anatomical conditions. W i t h regard to the affected organs - nerves, vessels, and spine - , the following aspects must be considered. Reposition and stability are required to relieve the spinal cord, nerve roots and vertebral arteries and protect them from continuous microtrauma. Decompression is designed to improve any acute neurological impairments, to procure stable conditions in order to prevent chronic disorders like progressive cervical myelopathy, constant nerve r o o t irritation, and recurring vertebrobasilar insufficiency, 9 1992 by Walter de Gruyter & Co. Berlin - New York
One aim in spinal mobility is that osteosynthesis produces no or only the least possible functional loss. This aim is o f p a r a m o u n t i m p o r t a n c e for the upper cervical spine, which requires the o p t i m u m of mobility for head motion. The following account will delineate some aspects o f the treatment o f o d o n t o i d fractures, h a n g m a n ' s fractures, and atlantoaxial instabilities o f various causes using double-threaded screws (Table I). Table I. Indications for osteosynthesis with doublethreaded screws in instabilities of the upper cervical column Compression-screw osteosynthesis of the odontoid process Acute fracture of the odontoid process Transpedicular screw fixation C 2 Hangman's fracture (with dominant bony instability) Transarticular screw fixation C 1/2 Jefferson fracture Combined odontoid and Jefferson fracture Odontoid pseudarthrosis Rotatory subluxation C t Post traumatic incongruency C 1/2 Rheumatic arthritis Os odontoideum Atlas-assimilation and instability
2 Material and methods 2.1 Functional principle of the double-threaded screw The double-threaded screw has two different, self cutting threads which are independently arranged
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along a slim, threadless centerpiece. The threads of the front portion are not only steeper than those in the back, they are also further apart (Figure 1). When the screw is introduced, the front thread enters the distal bone fragment after it has passed the proximal bone fragment and the fracture line, and the back thread enters the proximal bone fragment. Due to its steeper angle and the greater distance between threads, the entry velocity of the front thread is greater than that of the back thread. This causes the distal fragment to be moved towards the proximal fragment in the course of screw fixation, so that both fragments are ultimately pressed together. Since the front thread has a diameter of 3.2 m m (with a core diameter of 1.8 ram), and the back thread has a diameter of 5.0 m m (with a core diameter of 3.7 mm), both threads can cut without interfering with each other.
in a halofixateur or a minerva cast, screw fixation of the axis, without placing undue strain on the patient, leads to quicker and more stable healing results after about six weeks of wearing a removable neck brace [11]. In the hands of a practiced surgeon, therefore, screw fixation of acute odontoid fractures presents a true alternative to conservative treatment and is to be seen the method of choice among the other surgical procedures. The use of double-threaded screws and the development of a corresponding application set resolved some of the disadvantages inherent in the original procedure. The heads of the generally employed small fragment spongy bone screws almost invariably irritate the segment C 2/3, leading to painful inhibition of ventral flexion combined with posterior headaches and osteophytic reactions at C 2/3 [15, 16, 17]. The double-threaded screws, which are almost completely imbedded in the axis, do not produce mechanical irritation of the segment C 2/3. With regard to the diameters and lengths of the threaded portions, the dimensions of the double-threaded screws are exactly adapted to the anatomic conditions of the fracture types encountered at the axis. The cannulated preburr for the back thread increases the diameter of the burr hole from 1.8 m m to 3.7 ram, so that it can be easily found when the screw is introduced. This and the fact that it is no longer necessary to prethread the burr hole serve to simplify the procedure.
Surgical procedure [19, 21]: Preoperative diagnos-
Figure 1. Double-threaded screw with two differently styled threaded portions. The threads of the front portion are not only steeper than those of the back portion, they are also further apart.
2.20dontoid screw fixation The osteosynthetic repair of odontoid fractures with a pair of screws introduced from the anterolateral aspect has the advantage over the dorsal method [4, 8] that the atlantoaxial joint is maintained or reconstituted [2, 3, 15]. Compared with the transoral procedure [6, 7] there is less intraand postoperative burden on the patient, and the risk of infection is reduced. Whereas conservative treatment necessitates long periods of constraint
tics include X-rays (transoral a.p. and lateral), computed tomography of C 1/2, and possibly Xray tomography [12]. Repositioning is performed with an extension frame modified according to U~cmACH (4 spikes), and the result is maintained by appropriate positioning in extension. The patient is orally intubated for the operation. It is imperative that the tubus is situated to the left of the midline and that the mouth is adequately opened by placing a bandage roll between the teeth. If this is not done, the odontoid process will be covered by the tube or the teeth during a.p. radiography, and cannot be aimed of accurately during predrilling. The 4 - 6 cm long skin incision is drawn at a slight angle within the tension lines of the right side of the neck at the level of the cricoid cartilage (C 5/6). This is important, because only this permits an accurate procedure in the fascial spaces, by which damage to the Nn. laryngeus superior and hypoglossus can be avoided. This also determines the direction for the Neurosurg. Rev. 15 (1992)
Kn/Sringer, Osteosynthesis of the upper cervical spine using double-threaded screws
Figure 2a--d. Schematic presentation of odontoid screw fixation with double-threaded screws. (a) The repositioned fracture is bridged by a 1.8 mm drill wire with a short chisel tip, which enters the lower anterior axis surface through the intervertebral disk C 2/3 and just barely perforates the cortical bone of the odontoid tip. (b) Guided by the implanted drill wires, the cannulated preborer with fixed depth of entry provides the core diameter for the posterior self-cutting threads. (c) A
borer to be introduced later. After vertically cleaving the platysma, the upper border of the omohyoid muscle is prepared and the cervical spine is bluntly probed behind the neck muscles approximately at C 4/5. From here, we proceed by sight (headlamp) retropharyngealty up to C 2/3. Once C 2 is reached, two image converters are positioned and the metal retractors are exchanged for X-ray negative ones [13, 14]. Under simultaneous imaging from both views, the fracture is bridged by 2 drill wires, which are introduced at the anterior base of the axis through the intervertebral disk of C 2/3 and which just barely perforate the dorsal cortical bone near the tip of the odontoid. Neurosurg, Rev. 15 (1992)
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fitting double-threaded screw is implanted with a screwholder and a screwdriver. (d) When precisely positioned, the anterior thread is located in the cortical bone of the odontoid tip, and the posterior thread is in the cortical bone of the anterior base, so that the hard vertebral structures are fully utilized for anchoring, and the surrounding parts are not irritated by cutting screw portions.
Using a fixed-depth cannulated drill, a hole is prebored along the situated drill wires for the broader, self cutting, posterior thread of the screw. After one drill wire is removed, the length of the screw is determined with a depth gauge. At this stage, both the second drilt wire, which is stilJ in position, and the extension stabilize the fragments, so that the respective burr holes at the axis body and odontoid are not disaligned and can be found when crossing the fracture line. A fitting screw is then inserted with a screwholder and a screwdriver (Figure 2c) and turned until it bridges the fracture line. The same procedure is then repeated in the region of the second drill wire. The implanted
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Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws if the upper vertebral joints C 2 can be repositioned and the fracture does not reach too deeply into the anterior, ventral cortical bone of the axis body. In these fractures, the lower thread prohibits a ventrocaudal dislocation of the odontoid. An anti-glide plate, as used in BOHLER'Sprocedure, is not required. Postoperatively, immobilization is achieved for 6 - 8 weeks by means of a removable neck brace with chin support. This is required for safe healing of the fracture and is supposed to support the scarry stabilization of the overdistended ligaments C 1/2.
2.3 Transpedicular screw fixation C 2 Hangman's fractures are generally treated conservatively by repositioning and a 6 - 1 2 week's course of immobilization in a halofixateur. An indication for primary intercorporal fusion C 2/3 with instrumentation is usually assumed in case there is considerable damage to intervertebral disk C 2/3 with a corresponding dislocation (procedure according to Cloward or Smith-Robinson with plate osteosynthesis). This is also the case for fractures which achieved bone stability after conservative therapy, but with a remaining discotigamental instability of C 2/3. These cases require only intercorporal fusion without metal implants.
Figure 3a--d. Type II odontoid fracture according to ANDERSON.(a) Postaccident image of dorsally dislocated odontoid and atlas. (b) and (e) Status after reposition and stabilization by double-threaded screws. Both screws are located in the odontoid, the hard vertebral structures are used for anchoring. The segment C 2/3 is unaffected. (d) Postoperative axial CT at the level of the odontoid showing the spatial relationship of the screws fixed in the odontoid. screw is now the stabilizing agent, so that the burr hole in corpus and odontoid remains in alignment. After the extension weights are removed, the screws are alternately tightened. The fragments are drawn closer together and subjected to physiological pressure. In order to achieve a rotationsproof osteosynthesis, it is always necessary to implant 2 screws. If the positioning was exact, the front thread will rest in the cortical bone of the odontoid tip, while the back thread will be anchored in the cortical bone of the anterior, lower base of the axis [17, 21] (Figures 2 a - d and 3 a - d ) . This procedure can also be used to treat fractures extending from the upper back to the lower front,
I f the instability is primarily of bony nature, as is the case in most injuries, transpedicular screw fixation [10] can provide stability without loss of function, so that this method can be considered an alternative to conservative therapy. Stabilization with exact repositioning is particularly important when the injury, either due to high-grade instability or a fracture in the region of the transverse foramina, affects the vertebral arteries and thus produces vertebrobasilar symptoms, which are often misinterpreted as contusional cerebral damage [20]. Transpedicular screw fixation provides much better repositioning and stability than external immobilization, which additionally poses considerable nursing and intensive care problems in a restless patient with lapses of consciousness [26].
Surgical procedure: The procedure can either be performed with the patient in a supine position with the extension in place or fixed in a sitting position with a sharp mounting support. Repositioning is achieved and maintainted by positioning the patient appropriately. The course of the skin incision is in the medial line vertically from the occipital region to C 7. The spinal processes of the Neurosurg. Rev. 15 (1992)
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b
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C
Figure 4a--e. Transpedicular C 2 screw osteosynthesis of a Hangman's fracture. (a) Operative site at C 1/2 (C 1 = arcus posterior atlantis; PS C 2 = processus spinosus C 2; D = Dura). The star marks the point of entry, the arrow indicates the direction of drilling. (b) and (e) Schematic presentation of the screw position between
the transverse foramen and the inner pedicular corticalis. The screw is located medial to the artery. At this point, it has its smallest diameter of 2.0 mm. The screw does not affect the segment C 1/2 and its threaded parts are anchored in the anterior and posterior cortical bone of the axis and the lamina.
vertebrae are accessed via the septum nuchae, and the neck muscles of C 2 up to about C 4 are detached. In order to avoid injury of the dura and the spinal cord, the internal contours of the vertebral arch are bilaterally visualized by preparing subperiostally up to the corpus (Figure 4a). A surgical microscope can be useful at this stage. Microscopic viewing usually reveals fracture lines in the arch and corpus region.
Once both screws are tightened alternatingly and the Redon-drains are inserted, the wound is closed in layers. Immobilization of the cervical spine by means of a removable neck brace with chin support is recommended for 4 - 6 weeks. Figure 5a - c shows an example of a clinical case.
The fracture is bilaterally bridged with a 1.8 m m drill wire. The starting point is the caudal region of the transition of the lamina into the joint structure of the 2nd cervical vertebra. The direction converges slightly to the midline (about 15~ and ascends cranially, approximately parallel to the upper edge of the arch. The drill wires and, later on, the screws are medial to the vertebral artery (Fig. 4b, c). The core diameter for the back thread of the screw is produced by preboring with a cannulated drill along the positioned drill wires. As in odontoid fixation, one drill wire remains in place to stabilize the fracture site, while the length of the screw is measured via depth gauge under lateral imaging control. The measurement can also be performed by the tightened depth arrest, which is to be placed at the entry site in the lamina. After the drill wire is removed, the length of the screw can be determined by measuring the distance between the tip of the drill wire and the depth arrest. This method requires that the drill wire has perforated the anterior cortical bone for about 1 - 2 mm. After a fitting screw is inserted, the other pedicle is screwed in the analogous fashion. Neurosurg. Rev. 15 (1992)
Transpedicular screw fixation is accompanied by the risk of injury to the vertebral arteries, the dura, and the spinal cord. This risk can be decisively reduced if the following points are observed. Besides native X-ray imaging with lateral and transoral a.p. views, possibly enhanced by X-ray tomography, one should require computed tomography of C 1 - C 3 in bone window adjustment and 2 m m layers, whose sections should be parallel to the line of the upper arch of C 2. This not only provides more precise visualization of the respective fracture types, it also reveals anomalous vertebral arteries, recognized by alterations in the transverse formania. As a rule, preoperative angiography can be waived. The exact subperiostal view of the medial borders of the arches and the localization of the vertebral artery by intraoperative doppler sonography [18] permits the relatively safe preboring and screw implantation between these two landmarks. A further reduction of the risk of injury is achieved by using double-threaded screws instead of the cortical or small fragment spongy bone screws [26] in use to date. The preboring for the implantation of the double-threaded screws is done with a 1.8 m m drill wire with a chisel tip, whereas the other
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Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws
Figure 5a--e. X-ray image of a transpedicular screw osteosynthesis of Hangman's fracture with doublethreaded screws. (a) Postaccident status. (b) and (e)
Status after transpedicular screw fixation with doublethreaded screws.
two screw types require a 2.0 or 2.5 m m spiral drill. While the thread does not need to be precut for an osteosynthesis with double-threaded screws, this is necessary for the other two screw types. The risk of injury is thus reduced not only by the smaller diameter of the drill wire and its smooth surface when compared to a spiral drill, but also by the fact that a threadcutter, which probably represents the greatest intraoperative risk, can be dispensed with. The double-threaded screw also has a distinct spatial advantage after implantation. The site with the greatest injury potential is in the vertebral arch between the vertebral artery and the internal corticalis (dura and spinal cord). At this site, the diameter of the implanted doublethreaded screw is 2.0 ram, that of the cortical screw 3.5 ram, and that of the small fragment spongy bone screw 2.4 m m (in the unthreaded portion) or 4.0 m m (in the threaded portion). Since the small fragment spongy bone screw has too long a threaded portion for this indication, one can assume that at least part of this will come to lie in the danger zone, so that this screw type has the least suitable dimensions. Even if the transverse foramen is opened, the chisel tip and the smooth drill wire are more likely to only displace the vertebral artery, whereas the spiral drill and particularly the threadcutter are more apt to cause injury to the vessel.
After the screws are tightened, the vertebral arteries should be doppler-sonographically examined for patency. In case of stenosis or occlusion by compression, an immediate correction of the screw position can reinstate the patency of an undamaged vessel. This can prevent serious circulation problems. This is particularly true when the error was made on one side only. Exactly positioned screws retain the ventral corticalis below the upper joint surfaces of the axis with the front thread, and the lamina corticalis at the transition into the lower joint structure with the back thread. If the angle is too steep, the joints of C 1/2 may be affected. If this should happen, then the metal must be removed after the injury has healed and stability is assured.
2.4 Transarticular screw fixation C 1/2 Atlantoaxial instabilities, as they occur in odontoid pseudoarthroses, os odontoideum, rheumatic affection of the odontoid or o f the ligamentum transversum atlantis, as well as Jefferson fractures not stabilized by conservative therapy, are an indication for transarticular screw fixation C 1/2 [9, 22]. Compared with the lateral atlantoaxial screw arthrodesis [5, 25], which needs to be performed bilaterally, this procedure has the advantage of a Neurosurg. Rev. 15 (1992)
Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws single access, which is furthermore generally known and technically uncomplicated. In contrast to the other methods like fusion according to GALLIE [8], BROOKS [4], and compression clamp spondylodesis [23, 24], this procedure provides the secm'ity that a reposition once performed will be maintained. The disadvantage of a higher level of technical difficulty is offset by its advantages. Surgical procedure: This surgery is best performed in supine position with extension in place rather than seated with sharp mounted support, because the steep drill direction required is much more difficult to achieve while the patient is sitting. The route of access is basically the same as that for transpedicular C 2 screw fication, only that in this case the arcus posterior atlantis needs to be visualized and that, due to the steeper drilling angle, the skin incision needs to be extended to C 7/Th 1, and it is recommended to detach the muscles up to around C 5. After subperiostal visualization of the arch C 2 at its medial aspect and the massae laterales of the atlas, one Kirschner wire each is drilled into the massae laterales a little above the lower joint surface. This protects the roots of C 2. The joints of C 1/2 can be inspected by lifting the wires and the cartilage can be removed, if necessary. The point of entry of the transarticular drill wire is again in the caudal region of the transition of the lamina into the joint structure. The direction is sagittal and obliquely upward, and is achieved by aiming at the arcus anterior atlantis under lateral imaging control. The wires perforate the upper joint surfaces C 2 between the dorsal and
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middle third, enter the lower joint surfaces C 1 and should just barely perforate the massae laterales ventrally. Screw implantation is performed as described for transpedicular screw fixation C 2, and the same safety measures and advantages of osteosynthesis with double-threaded screws apply here as welt. For transarficular screw fixation C 1/2, the burr holes and screws are located above the foramen transversarium, thus, the risk of injury to the vertebral arteries is reduced (Figure 6a-c). In order to achieve a permanent fusion, an additional interarcual fusion C 1/2 is performed using an autologous corticospongeous bone graft and wire cerclage, providing an osteosynthesis stable to all sides which does not require an additional orthetic device. For Jefferson fractures or fractures of the arch in C 1 and C 2, the cartilage needs to be removed from the joint surfaces C 1/2. A spongy bone interposition between the joints is subsequently performed, because an interarcual spondylodesis is not possible in these cases. Older Jefferson fractures, in which the massae laterales have shifted outward and can no longer be completely repositioned, require that the screws are not entirely sagittal, but diverge slightly outward, so that they do not miss the massae laterales medially. This direction increases the risk of vascular damage, so that intraoperative doppler-sonography should not be waived in these particular cases. After screw fixation of a Jefferson fracture, the head should be immobilized with a neck brace with chin support for 6 - 8 weeks.
.,:. ......... sot
a
b
Figure 6a--e. Schematic presentation of transarticular screw fixation C 1/2. (a) The drill wire enters the caudal region of the arch C 2 at the transition of the lamina into the joint mass, penetrates the joint C 1/2, which is fixed by this manipulation, and barely perforates the anterior corticalis of the massa lateralis. The cannulated Neurosurg. Rev. 15 (1992)
preborer provides the core diameter for the posterior thread. (b) and (e) The screw direction is sagittal and more inclined than for transpedicular C 2 screw fixation. The screw is behind the vertebral artery, and is anchored in the cortical bone of lamina C 2 and the massa lateralis atlantis.
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Kn6ringer, Osteosynthesis of the upper cervical spine using double-threaded screws wires. In the rare cases of a posterior atlas dislocation, repositioning can be achieved by axial traction and exerting force along the drill wires into the massae laterales by ventral pressure. This position is maintained by the transarticular drill wires, one wire again securing this position while the appropriate screw is implanted into the other side. Good repositioning is absolutely important, because only this guarantees that the spinal canal again obtains its necessary width and that the spinal cord is completely free from pressure (Figure 7 a - d). 3 Discussion
Figure 7a--d. Atlantoaxial instability due to PCP with debilitating motional dysfunction and incipient cervical myelopathy. (a) Lateral X-ray image in middle position. (b) Axial CT of odontoid and atlas. As in the X-ray, there is a considerably increased atlanto-odontoid distance of 10 mm with high-grade stenosis of the spinal lumen. (c) and (d) Status after exact reposition and stabilization by transarticular screw fixation and interarcuai fusion C 1/2 with corticospongeous bone graft and wire cerclage. Immediate postoperative remission of instability and improvement of the medullary symptoms.
If adequate repositioning was not achieved by extension and positioning immediately before the operation, this can be completed intraoperatively before the drill wires are introduced. A ventrally dislocated atlas is reverted to its correct position by bridling the arcus posterior and drawing backward, and this position is secured with the drill
Among the surgical treatment methods of recent type II and III odontoid fractures as classified by ANDERSON [1], osteosynthesis with doublethreaded screws constitutes the procedure of choice. Odontoid screw fixation preserves or reestablishes proper head motion. Since doublethreaded screws are almost completely imbedded into the axis, the irritation of C 2/3 movement regularly induced by the heads of small fragment spongy bone screws or cortical bone screws is avoided. Double-threaded screws are exactly adapted to the anatomy of the atlas and to the fracture types encountered at this site. The operative procedure is simplified by the instruments, which include two X-ray negative retractors. Like odontoid screw fixation, transpedicular C 2 screw fixation of hangman's fractures is a procedure associated with no functional loss of vertebral mobility, so that his method fulfills the demands for an optimal osteosynthesis. Due to the technique of 3-point fixation, transarticular C 1/2 screw fixation combined with interarcual fusion in the treatment of atlantoaxial instabilities of various causes provides the advantage of an osteosynthesis with stability in all directions, in which the result of reposition can be securely maintained. The advantage of the double-threaded screws in the latter two surgical procedures does not only consist in their simpler implantation when compared with the 3.5 and 4.0 mm screws used until now, but also in the reduction of operative risk.
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[17] KNORINGERP: Operative Versorgung der Densfraktur von ventral durch Doppelgewindeschrauben. In: HOHMANND, B KUGELGEN, K LIEBIG(Hrsg): Neuroorthop/idie 4. Springer-Verlag, Heidelberg-New York (1988) 199-206 [18] KNORINGER P: Intraoperative dopplersonography during surgery of the cervical spine. In: WALTERW, M BRANDT,M BROCK,M KLINGER(eds): Advances in Neurosurgery Springer-Verlag, Berlin 16 (1988) 7 3 - 79 [19] KNORINGERP: Osteosynthese der Densfraktur mit Doppelgewindeschrauben. Videofilm (1988) Ulrich HC Ulm und Ortho-Video [20] KN6RINGERP: Neurochirurgische Indikationen bei Wirbels/iulenverletzungen. In: Hauptverband der Gewerblichen Berufsgenossenschaften e.V. Bonn (Hrsg): Diagnostik und Therapie yon Wirbels/iulenverletzungen. Heft 68 (1989) 5 1 - 6 2 [21] KN6RINGERP: Die Osteosynthese der Densfraktur mit Doppelgewindeschrauben. Operat Orthop u Traumatol (1989) 178-192 [22] MAGERL F, PS SEEMANN:Stable posterior fusion of the atlas and axis by transarticular screw fixation. In: KEHR P, A WEIONER (eds): Cervical spine I, Springer-Verlag, Wien (1987) 322- 327 [23] ROOSENK, A TRAUSCH~L,W GROTE: Posterior atlanto-axial fusion: A new clamp for laminar osteosynthesis. Arch Orthop trauma Surg 100 (1982) 27-31 [24] ROOSENK, A TRAUSCI-IEL,R KALFF,W GROTE:Die posteriore dynamische Kompressionsspondylodese bei atlantoaxialer Instabilit/it. In: HOHMANND, B KOGELGEN, K LIEBIG (Hrsg): Neuroorthop/idie 4. Springer-Verlag Berlin (1988) 192-198 [25] SIMMONSEH, G DE TOIT: Lateral atlanto-axial arthrodesis. Orthop Clin North Am 9 (1978) t 101 [26] TSCI-mRNEH, G MUHR, R O v DEN WINKEL: Frakturen und Luxationen der HWS - operative Behandlung und Ergebnisse. In: BURRI C, A ROTER (Hrsg): Verletzungen der Wirbels/iule. Hefte zur Unfallheilkunde. Springer-Verlag, Berlin 149 (1980) 89 - 94 Submitted March 29, 1989. Accepted June 3, 1992. Dr. med. Peter Kn6ringer Neurochirurg und Neuroradiologe Neurochirurgische Abteilung der Universit/it Ulm im Bezirkskrankenhaus Gfinzburg Ludwig-Heilmeyer-Str. 2 W-8870 Giinzburg Fed. Rep. of Germany
