Cervical Pedicle Screws: Conventional Versus Computer-Assisted Placement of Cannulated Screws

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1 Cervical Pedicle Screws: Conventional Versus Computer-Assisted Placement of Cannulated Screws SPINE Volume 30, Number 20, pp , Lippincott Williams & Wilkins, Inc. Marcus Richter, MD, PhD,* Balkan Cakir, MD, and René Schmidt, MD Study Design. Prospective clinical study with postoperative radiologic control of pedicle screw placement in the cervical spine. Objectives. To evaluate whether cervical pedicle screws can be placed safely in a conventional technique when using cannulated screws and separate stab incisions. Also, to evaluate if accuracy and safety of pedicle screw placement can be improved using a computerassisted surgery (CAS) system (VectorVision ; BrainLAB AG, Heimstetten, Germany). Summary of Background Data. Pedicle screws are rarely used in the cervical spine compared to the use in lumbar and thoracic spine. The main reason is probably the potential risk of iatrogenic damage to the spinal cord, nerve roots, or vertebral artery caused by screw misplacement as well as the more demanding technique of pedicle screw placement in the cervical spine. Methods. A total of 52 consecutive patients with posterior cervical or cervicothoracic instrumentations using pedicle screws were evaluated prospectively. For the first 20 patients, 93 pedicle screws were implanted using the conventional technique with the image intensifier in the lateral view, and for the next 32 patients (167 screws), a CAS system was additionally used. For registration of the vertebra, surface-matching algorithms were used. For evaluation of screw placement, postoperative computerized tomography with multiplanar reconstructions in the screw axis was performed for each screw. Results. No implant-related complications were observed. No neurologic or vascular complications were found related to pedicle screws. The rate of pedicle perforations was 8.6% (8 screws) in the conventional group and 3.0% (5 screws) in the CAS group, and in all cases, less than 2-mm displacement. None of the screws with pedicle perforation had to be revised as a result of nonsufficient biomechanical stability or compression of neural/vascular structures. Conclusions. Transpedicular screws in the cervical spine and cervicothoracic junction can be applied safely and with high accuracy in a conventional technique. Cannulated screws and the use of separate stab incisions from C3 C6 with a trocar system allow for reduced screw misplacement rates. The CAS system leads to significantly reduced screw misplacement rates. Therefore, because of the potential risk of injury to the vertebral artery From the *Spine Center, St. Josefs Hospital, Wiesbaden, and Department of Orthopaedics, University of Ulm, Ulm, Germany. Acknowledgment date: July 14, First revision date: November 5, Acceptance date: November 17, The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication. No funds were received in support of this work. One or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript: e.g., honoraria, gifts, consultancies, royalties. Address correspondence and reprint requests to Marcus Richter, MD, PhD, Spine Center, St. Josefs-Hospital, Solmsstrasse 15, Wiesbaden, Germany; mrichter@joho.de and neural elements, the use of a CAS system seems to be beneficial, especially for pedicle instrumentation C3 C6. Key words: computer-assisted surgery, cervical spine, posterior instrumentation, pedicle screws, in vivo. Spine 2005;30: The use of pedicle screw-based spinal instrumentation systems in the lumbar and thoracic spine has increased tremendously during the last decade because of their superior biomechanical properties and reduction possibilities. 1 In the lumbar and thoracic spine, conventional screw insertion techniques have been associated with higher screw misplacement rates in cadaver studies compared to instrumentation with the assistance of a computer-assisted surgery (CAS) system (VectorVision ; BrainLAB AG, Heimstetten, Germany) Pedicle screws in the cervical spine and the cervicothoracic junction have become increasingly popular, and promising clinical results have been published. 1,12,13 Because of improved biomechanical stability of pedicle screws as compared to lateral mass screws, pedicle screws allow for shorter instrumentations with improved reposition capacities. 14,15 Nevertheless, because of the small size of the cervical pedicles, there are potential risks of iatrogenic damage to neural or vascular structures In vitro studies have shown that cervical pedicle screws can be safely applied using a CAS system. 14,21,22 Therefore, we started using a CAS system intraoperatively for posterior instrumentation of the cervical and cervicothoracic spine after using the conventional technique of cervical pedicle screw placement without CAS system in the years before. The purpose of this study was to evaluate the feasibility and accuracy of pedicle screw placement in the cervical spine and cervicothoracic junction using the conventional as well as the computerassisted technique. Because the cervical pedicles from C3 C6 have an average angulation of (Figure 1), application of the screws over separate stab incisions reduces the size of the approach and, therefore, the morbidity of the procedure. To reduce the risk of screw misplacement and make screw application over the stab incisions easier, an implant system with cannulated cervical and thoracic pedicle screws that can be applied over a blunt K-wire was recently developed. Materials and Methods From August 2000 to October 2003, the senior author (M.R.) performed posterior instrumentation of the occipito-cervical, cervical, and cervicothoracic spine with pedicle screws in 20 patients using the conventional technique with the image inten- 2280

2 Cervical Pedicle Screws Richter et al 2281 Figure 1. Anatomy of cervical pedicles C3 C6 and position of pedicle screw in all 3 planes. sifier in the lateral plane and, in 32 patients, an additional computer navigation system (Vector Vision ) was used. A total of 260 pedicle screws were placed, including 228 screws cervical and 32 screws high thoracic. There were 93 pedicle screws placed in the conventional group and 167 screws in the CAS group. The data for mean age, follow-up, blood loss, operating time, average of instrumented segments, and indications are shown in Table 1, in total and separately for both groups. A spinal screw and rod system (neon occipito-cervical system, Ulrich Medizintechnik, Ulm, Germany) was used for the instrumentations. This modular implant system was recently developed with special regard for the use of cervical and highthoracic pedicle screws, as well as the combination with computer navigation. Therefore, cannulated pedicle screws were developed, allowing application of pedicle screws over separate stab incisions, using a trocar system. Additional preoperative imaging of the patients for the placement of the pedicle screws was performed using a helicoidal computerized tomography (CT) scan (Somatom Plus 4, Siemens, Germany) with a 1-mm, nonoverlapping slice thickness for the patients of the conventional group and according to the special BrainLAB protocol (Heimstetten, Germany) for the computer navigation group. For the conventional group, before surgery, multiplanar CT reconstructions of each pedicle intended for screw placement were performed, and the pedicle angle was measured and noted in a protocol. Table 1. Data for Groups Total (range) Conventional (range) Navigated (range) No. patients Mean age (yrs) 56.4 (29 76) 54.5 (29 69) 58.4 (30 76) No. screws Operating time (mins) 145 (60 240) ( ) 135 (60 180) Blood loss (ml) 238 (50 800) 239 ( ) 237 (50 800) Follow-up (mos) 22.8 (6 37) 29.6 (22 37) 18.6 (7 22) No. instrumented 3.3 (1 7) 3.8 (1 7) 3.0 (1 7) segments Indications No. cervical spondylotic myelopathy No. fractures No. metastases No. implant failures No. Corrections of kyphotic deformities No. rheumatoid instabilities No. iatrogenic instabilities Operative Technique. All operations were performed with the patient under general endotracheal anesthesia with muscle relaxation. In supine position, preparing and insertion of Mayfield clamp approximately 2 cm above the porus acusticus externus was performed. Positioning of the head in a mold without clamp fixation may also be possible but is not recommended because the possibilities of reduction are limited with the head mold. The patients were then turned into prone position on a gel filled mattress; support of thorax and pelvis was performed with foam pillows, and stabilization of both arms was achieved by adhesive tape and continuous pull of the arms with a pulley system with 2 4-kg weights. Under imaging control, a closed reduction, if necessary, was performed. After a standard midline posterior approach, the reference clamp was fixed to the spinous process of the vertebra intended for instrumentation in the navigation group, and the CAS system is positioned over the feet of the patient once the posterior aspect of the spine has been approached. To have a good line of sight, the camera is positioned over the legs of the patient. The surgeon stands at the head of the patient for inserting pedicle screws. Every vertebra that was instrumented was separately registered. Registration was performed using the surface-matching algorithm, in which a predicted accuracy of 1.0 mm was accepted. If the predicted accuracy was 1.0 mm, we repeated the registration procedure until we reached an accuracy of 1.0 mm. This process was necessary in approximately 10% of cases. After registration of the vertebra, verification was performed to ensure that the virtual reality of the CAS system corresponded to the surgical reality. If verification was accurate, the navigated instrumentation could be started, otherwise the entire registration procedure was repeated. For pedicle screw instrumentation, a drill guide, which was navigated in the CAS group to prepare the pedicle screw holes, was used (Figure 2). The advantage of the drill compared to a pedicle awl is the better feeling of bony resistance as a result of reduced friction of the rotating drill and the reduced anteroposterior forces on the vertebra. To avoid bending and torsion problems caused by the small rigidity of the 2.6 or 3.5-mm drill bit, we tracked the drill guide rather than the drill itself (Figure 6). With this technique, the CAS system can only visualize the trajectory of the drill guide and not the drill itself. To avoid perforation of the anterior vertebral body wall, in addition to Figure 2. Modular drill guide for pedicle preparation with 2.6-mm trocar insert for insertion over separate stab incisions and 2.6-mm insert, as well as 3.5-mm insert for the open technique.

3 2282 Spine Volume 30 Number Figure 3. Screenshot during preoperative planning with multiplanar reconstructions and measurement of the angle of each pedicle intended for instrumentation. the CAS system, we used an image intensifier in the lateral view to control the position of the drill in the vertebral body. After registration and verification of the proper vertebral level, pedicle screws were prepared with a 2.6-mm drill bit for 4.0-mm screws and a 3.5-mm bit for the 5.0-mm screws in the CAS group. After the correct position of the pedicle hole was confirmed with a pedicle probe, we inserted a blunt 1.5-mm K-wire into the vertebral body, and the cannulated pedicle screws were implanted over the k-wire. The K-wire reduces the risk of screw malpositioning. Moreover, it also avoids a lateral breakout of Figure 4. A, Intraoperative picture showing navigated pedicle preparation over separate stab incision with the navigated drill guide and trocar insert. B, Determination of the pedicle angle for the conventional technique with angle measurement device is shown as well. the screws from the pedicle, which could endanger the vertebral artery. There is a high risk of lateral breakout of the pedicle screws from C3 C6 because of the high inclination angle of the cervical pedicle in this region of approximately 45 and the strong posterior cervical muscles, forcing the screwdriver handle to the midline during screw insertion when using an open technique. Therefore, we used separate stab incisions for pedicle screw implantation from C3 C6, over which we inserted the navigated trocar system. The navigation system was used to find the ideal position for the stab incision in the CAS group (Figure 5), and the image intensifier was used in the conventional group. The entrance point in C3 and C4 was chosen at the upper margin of the pedicle with a descending direction, so that the drill would not perforate the upper endplate of the vertebra. Because of the lower pedicle inclination angle ( 30 ) in C2 and from C7 down to high thoracic vertebrae, we preferred the open technique in this region. In the conventional group, before surgery, the pedicle angle of each instrumented pedicle was measured using multiplanar CT reconstructions. Intraoperatively, the correct pedicle angle was secured using an angle measurement device (Figure 4), and anatomic landmarks determined the correct entrance point. Screw Evaluation. Screw position was evaluated after surgery using CT with multiplanar reconstructions in each screw axis (Figure 3). Screw position of the pedicle screws was divided into 3 groups: group 1, correct screw placement without pedicle perforation or with pedicle perforation 1.0 mm (depth of the thread); group 2, pedicle perforation 1.0 mm without the need for screw revision; and group 3, pedicle perforation 1.0 mm with the need for screw revision because of irritation or injury of roots or the spinal cord, or because of reduced biomechanical stability.

4 Cervical Pedicle Screws Richter et al 2283 Figure 5. Screenshot during computer navigation showing determination of the stab incision for the trocar insert. Spinal Instrumentation. A modular, constrained, titanium alloy screw and rod system (neon occipito-cervical system) was used for all instrumentations in this article. The system consists of 4.5-mm rods and polyaxial connectors. The screw size depended on the cervical region to be instrumented. There were 4.0-mm cannulated, self-tapping screws for C2 C7 pedicle instrumentation and 5.0-mm cannulated, self-tapping screws for high thoracic pedicle instrumentation. A special modular drill guide with adjustable depth stop, capable for different CAS systems, was used. For pedicle screw instrumentations, 3 dif- Figure 6. Screenshot during computer navigation showing preparation of the pedicle with the navigated drill guide and the trocar insert using a separate stab incision.

5 2284 Spine Volume 30 Number Figure 7. Clinical case of a 70- year-old male. Patient had unstable C4/6 fracture with ankylosing spondylitis, additional epidural hematoma C6 Th5. Navigated posterior fixation C3/Th1 with pedicle screws C3, C4, C7, Th1, and fusion, as well as removal of the epidural hematoma were performed without additional anterior fixation. There was no need for an orthosis postoperatively. All screws were placed correctly without pedicle perforation as a result of CT evaluation. At 6 months postoperatively, there no loss of correction or implant failure, and fusion could be documented. A, preoperative MRI T2. B, preoperaive MRI T1. C, 6-month postoperative radiograph, anteroposterior view. D, 6-month postoperative radiograph, lateral view. E, postoperative CT lateral reconstruction to show the Th1 screws. ferent adapters are available: 2.6-mm trocar insert for cervical pedicle screw instrumentation over separate stab incisions, especially C3 C6; 2.6-mm insert for open cervical pedicle screw instrumentation, especially C2 and C7; and 3.5-mm insert for open thoracic pedicle screw instrumentation (Figure 2). CAS System. The CAS system consists of 2 calibrated infrared cameras, a high-end workstation with a touch screen monitor, and universal instrument adapters that are attached to trackable probes. The instrument tracking is based on passive marker technology (i.e., infrared-light emitting diodes that are positioned around the cameras). There is a direct line of sight between the cameras and localization sensors attached to the surgical instruments, which are wireless and do not require any batteries or electrical power, and are, thus, free to move without power cords. The surgeon can use any instrument after a quick calibration, which usually takes less than 1 minute, with the instrument calibration matrix. During the operation, the instrument can be visualized in a 3-dimensional (D) surface model and in 2-D multiplanar reconstructed images. Navigation is also possible with a precalibrated pointing device. Before navigation, the patient s anatomy must be matched intraoperatively to the previously acquired CT data set. The software used offers a variety of patient registration methods, such as paired point, fluoro to CT, and surface matching. Throughout this study, surface matching was used. Surface Matching. During the necessary preregistration of the anteroposterior direction, 1 starting point and the head direction must be acquired using the pointer. Following prereg- Figure 8. Axial CT corresponding to clinical case in Figure 7. A, Pedicle screws C4 (metal window). B, Pedicle screws C4 (bone window) with better visualization of the bony structures, especially the transverse foramen. C, Pedicle screws Th1 (metal window). D, Pedicle screws Th1 (bone window).

6 Cervical Pedicle Screws Richter et al 2285 istration, a variety of surface points are acquired on the actual patient, which is matched to the CT bone structure surface model using a refined iterative closest point algorithm. Generally, up to 20 points are acquired to match the actual spine to the 3-D CT image. In this study, for every vertebra that was instrumented, a separate surface matching and registration was performed. Results Mean operating time was minutes (range ) in the conventional group compared to 135 (range ) in the CAS group. No postoperative neurologic deterioration was observed. A deep wound infection occurred in 1 patient with ankylosing spondylitis caused by a cervical fracture C5/6, which was successfully treated by one-time surgical debridement without implant removal. No other intraoperative or postoperative complications occurred. All pedicle screws could be implanted using the CAS system or conventional technique as planned before surgery. Of 93 pedicle screws, 85 (91.4%) were graded in group 1, 8 (8.6%) in group 2, and no screw was graded in group 3 in the conventional group. In the CAS group, 162 of 167 pedicle screws (97.0%) were graded in group 1, 5 (3.0%) in group 2, and no screw was graded in group 3. No complication caused by pedicle perforation occurred in any of the groups, especially injury of the vertebral artery, and none of the screws with pedicle perforation had to be revised. A case report of a patient out of the navigated group with postoperative screw placement control is shown in Figures 7 and 8. Discussion Although pedicle instrumentation is very common in the lumbar and thoracic spine, it is still uncommon in the cervical spine. The small dimensions of the cervical pedicle, and proximity of vascular and neural structures may explain this. The posterior screw technique in the cervical spine mainly used (i.e., lateral mass screw fixation) results in a biomechanical stability below the one achieved with pedicle screws. 16,23 Nevertheless, lateral mass screws are useful in cases of low instability with the need for short range stabilization (e.g., single-level ligamentous instability). Whether one uses pedicle screws or lateral mass screws in these cases, it must be decided on an individual basis. For this decision, the surgeons must consider their experience and preference. Conversely, the potential complications of both methods, such as lateral mass fractures (6% to 7%) or injury, or irritation of the posterior nerve root (4% to 10%), depending on the technique used 24 with lateral mass screws, and the potential risk of iatrogenic damage to neural or vascular structures when using cervical pedicle screws, must be weighed against each other. Indeed, for longer fusions with the need for high biomechanical stability (e.g., in the correction of kyphotic deformities), the higher stability of pedicle screws can enhance fusion rate, shorten the amount of segments to be fused, ensure better reposition capacities, and ease rehabilitation because of the less restrictive postoperative immobilization. Therefore, they are more preferable. Because of these facts, the interest of spine surgeons in cervical and high thoracic pedicle screws is increasing rapidly. 1,12,13 Although excellent results with very low screw misplacement rates were published by Abumi et al, 20 showing a 7% misplacement rate out of 669 cervical pedicle screws when using a conventional screw insertion technique without a CAS system, these data may not be applicable to other spine surgeons with less experience in cervical pedicle screws. In 1993, CAS systems were developed for the installation of pedicle screws in the lumbar spine 6,25,26 based on reports of misplacement rates between 5% and 40% using conventional techniques In vitro studies showed that the misplacement rates of pedicle screws can be significantly reduced when using CAS systems. 5,9,34,35 In vivo studies confirmed these results. 7,8,10,11,36 Because in vitro studies have shown that cervical pedicle screws can be safely applied using a CAS system, 14,21,22 we started using a CAS system intraoperatively for pedicle screws in the cervical and cervicothoracic spine. We decided to use a drill to prepare the pedicles because of the reduced friction of a rotating drill compared to a pedicle awl. Because 2.6-mm drills bend significantly during drilling, resulting in reduced accuracy, we designed a drill guide suitable for the CAS system. The results of our in vitro study 14 show that this technique is suitable for cervical pedicle instrumentation. In this study, we had a perforation rate of 8% without harm to vascular or neural structures. The 2 minor pedicle perforations in our in vitro study in pedicles with a width less than 4.0 mm indicate possible anatomic restrictions. The low pedicle perforation rate in the CAS group in the present study, without any screw-related complication, also highlights this. In addition, the low pedicle perforation rate in the conventional group in the present study of 8.6%, without any screw related complication, shows that cervical pedicles can also be applied safely without the CAS system when using the described technique with cannulated screws and application over separate stab incisions from C3 C6. A preoperative plan and determination of the pedicle angles are mandatory for cervical pedicle screw placement without computer navigation. The entrance point can be defined well with anatomic landmarks, and the pedicle can be identified in the lateral image intensifier view, but it is very difficult for the surgeon to define the exact angle of the drill guide. This is especially the case if the angle is too low, then there is a high risk of injury to the vertebral artery. Therefore, intraoperatively, the use of an angle measurement device is always mandatory. As shown in this study, the use of a CAS system reduces the risk of screw misplacement compared to conventional screw placement and should, therefore, be used if available.

7 2286 Spine Volume 30 Number Pedicle widths of C3 averages 4.9 mm in males and 4.5 mm in females, with the minimum reported width being 3.0 mm. C4 averages 4.7 mm in males and 4.6 mm in females, with the minimum reported width of 3.1 mm. 19 The pedicle width in C5 and C6 is slightly higher. These anatomic data show that some pedicles may not be suitable for pedicle screws. Therefore, pedicle width should be measured before surgery using CT. Although the risk associated with pedicle screws in C3 C6 is obviously higher compared to C7 and high thoracic pedicle screws, the use of a CAS system for C7 and high thoracic pedicle screws may also be beneficial because the quality of the intraoperative image intensifier picture in this region is usually rather poor because of the shoulders of the patient. Although a CAS system is a useful and fascinating tool for spine surgery, anyone using such a system should be aware of possible errors. Because the system can crash at any time during the operation because of hardware, software, or human failure, every surgeon using the system should be experienced in the operation without the use of a CAS system. Furthermore, the surgeon should never rely on the virtual information from the CAS system without his own verification. The surgeon must ensure that the correct vertebrae are instrumented via image intensifier control or other techniques. Although registration of the instrumented vertebrae was possible in all cases, it can be difficult, especially in C3 and C4, because these vertebrae have very small spinous processes, and in many patients the posterior surface of the vertebrae, which is used for the surface matching, is quite similar. Therefore, it is possible to achieve an acceptable registration on C3 with the surface data of C4 or vice versa. This problem underlines the mandatory need for the surgeon to verify that he/she instruments the correct vertebra. Another error may result from the reference clamp, which is attached to the vertebra on which the surgeon will be working. In the middle of the cervical spine, the small dimensions of the spinous processes makes it difficult to achieve a stable fixation of the reference clamp, especially because most of the reference clamps available for the different CAS systems were initially designed for the lumbar and thoracic spine. Therefore, adapted reference clamps or other fixation techniques for the reference marker star should be developed for the cervical spine anatomy. Conclusions Transpedicular screws in the cervical spine and cervicothoracic junction can be applied safely and with high accuracy in a conventional technique. Cannulated screws and the use of separate stab incisions from C3 C6 with a trocar system allow for reduced screw misplacement rates. Furthermore, the CAS system leads to significantly reduced screw misplacement rates. Therefore, because of the potential risk of injury to the vertebral artery, the use of a CAS system seems to be beneficial, especially for pedicle instrumentation C3 C6. Nevertheless, it is of outmost importance that the surgeon using a CAS system is experienced, and must be able to perform the surgery without the CAS system as well. Key Points Transpedicular cannulated screws in the cervical spine and cervicothoracic junction can be applied safely in a conventional technique. The described percutaneous assisted technique for pedicle screws C3 C6 reduces the morbidity and screw misplacement rate. The use of a CAS system leads to significantly reduced screw misplacement rates. References 1. Ludwig SC, Kramer DL, Vaccaro AR, et al. Transpedicle screw fixation of the cervical spine. Clin Orthop Relat Res 1999;359: Laine T, Makitalo K, Schlenzka D, et al. Accuracy of pedicle screw insertion: A prospective CT study in 30 low back patients. 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