13: Cervical Pedicle Screw Fixation

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Procedure 13 Cervical Pedicle Screw Fixation

Kuniyoshi Abumi, Manabu Ito, Yoshihiro Hojo

Overview

image Despite increasing acceptance of the use of pedicle screws in the lumbar and thoracic spine, screw insertion into the cervical pedicle has been considered by spine surgeons to be too risky for the neurovascular structures, except at C2 and C7. Leconte first reported C2 pedicle screw insertion for osteosynthesis of the C2 hangman’s fracture. In the late 1980s, Goel and Laheri started to use C2 pedicle screws for atlantoaxial plate fixation in combination with C1 lateral mass screws. However, there had been no reports of pedicle screw fixation from C3 to C6 until the 1994 report by Abumi and colleagues of pedicle screw fixation for traumatic lesions of the lower cervical spine. Biomechanical studies revealed the superior stabilizing effect of pedicle screw fixation compared with other internal fixation procedures used with the cervical spine, including lateral mass screw fixation. In their recent experimental study, Johnston and colleagues revealed the superior pullout strength of a cervical pedicle screw versus a lateral mass screw after repetitive loading. Dunlop and colleagues demonstrated, by an in-vitro biomechanical study, that cervical pedicle screw/rod constructs support a greater axial load than lateral mass screw/rod constructs.

image The pedicle screw fixation procedure allows rigid fixation that provides the high correction capability needed to restore physiologic sagittal alignment of the cervical spine, as well as sufficient correction of malalignment in the occipitoatolantoaxial region. In addition, the pedicle screw fixation procedure, which does not require use of the lamina for stabilization, is quite valuable in patients who undergo one-stage posterior cervical decompression and stabilization, and in patients who undergo posterior reconstruction after previous posterior cervical decompression. On the other hand, the risks of neurovascular complications caused by inadequate screw placement into the cervical pedicle cannot be completely obviated. Thorough knowledge of local anatomy, sufficient preoperative radiologic examinations, and the application of established surgical techniques are essential for this procedure.

Indications

image Almost all the pathologic conditions requiring posterior stabilization of the occipitocervical junction, cervical spine, or cervicothoracic junction.

image Most middle and lower cervical injuries with posterior disruption, or anterior and posterior disruption without a severely disrupted anterior column, can be managed by posterior surgery using cervical pedicle screw fixation alone.

image Cervical spinal instability caused by nontraumatic lesions, including metastatic tumor, rheumatoid arthritis, destructive spondyloarthropathy, cervical intervention for posterior decompression of the spinal cord or nerve root, and so forth, are well managed by this procedure.

image Cervical kyphosis caused by many causes, including postlaminectomy and posttraumatic kyphosis, cervical spondylotic myelopathy associated with kyphosis, and so forth, can be corrected sufficiently by this procedure.

image A degenerative cervical spine with segmental instability requiring posterior decompression also can be managed by simultaneous decompression and stabilization using pedicle screw fixation.

image This procedure is beneficial for stabilization of the unstable motion segment caused by extensive decompression of the nerve root or spinal cord, affecting stability of the facet joint.

image Salvage of pseudarthrosis of anterior fusion.

image Fusion-level elongation for adjacent segment degeneration after anterior or posterior fusion surgery.

Indications Pitfalls

• Infectious disorders at the posterior portion of the cervical spine are contraindications for pedicle screw fixation.

• Pedicles destroyed by injuries, tumors, rheumatoid arthritis

• Marked osteoporosis

• Extremely small pedicles

• Pedicles of the vertebra associated with major anomalies of the vertebral artery, and so forth, are inadequate and risky for screw insertion (Figures 13-1 and 13-2).

• Figure 13-1 shows an abnormal condition of the cervical pedicle. Pedicles destroyed by injuries, tumors, or marked osteoporosis; extremely small pedicles; pedicles of the vertebrae associated with major anomalies of the vertebral artery, and so forth, are inadequate and risky for screw insertion. Figure 13-1, A shows a fracture of the pedicle in a patient with a lateral mass fracture. Figure 13-2, B shows a pedicle of an extremely small size in a patient with rheumatoid arthritis.

• Figure 13-2 shows a small size of the pedicle of the axis. Figure 13-2, A shows that the diameter of right side of the pedicle of the axis is too small for screw insertion by high-riding vertebral artery bends into the lateral mass of the axis (arrow). Figure 13-2, B shows extremely small pedicles of the axis for screw insertion.

• Patients sometimes have extreme unilateral dominance of the vertebral artery. In this condition, the dominant-side foramen transversarium enlarges, and the ipsilateral side of the pedicle decreases in size (Figure 13-3). Retrogression of the pedicle is found on the side of the dominant vertebral artery. Figure 13-3, A shows that patients sometimes show extreme right-left dominance of the vertebral artery. Figure 13-3, B shows that the dominant side of the foramen transversarium enlarges and that the ipsilateral side of the pedicle decreases in size (white arrow).

• For the patient with unilateral obstruction of the vertebral artery by injury, tumor, congenital anomaly, and so forth, screw insertion on the preserved artery side must be conducted with great care, or screw insertion should only be performed on the obstructed side.

image image

FIGURE 13-1, A-B

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FIGURE 13-2, A-B

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FIGURE 13-3, A-B

Examination/Imaging

image Preoperative oblique-projection plain radiograph films are valuable for evaluation of the pedicle size. In an oblique projection, the contralateral pedicle is seen as an oval projected onto the vertebral body, showing the outer and inner diameter of the pedicle. If the projection shows no inner diameter, the pedicle does not have a medullary canal.

image Computerized tomography (CT) evaluations (adjusted to the bone windows) are essential to assess the pedicle morphometry and to determine pedicle size, which allows surgeons to choose the appropriate pedicle screw diameter, length, direction in the coronal plane, and screw insertion point. Reconstructive CT in the oblique plane provides useful information about the size of the neural foramen.

image Preoperative evaluation of the morphology of the vertebral artery is important in preventing serious complications involving the artery.

• Duan and colleagues demonstrated, in an imaging anatomic study, that the incidence of abnormal course of the vertebral artery is high at the craniovertebral junction.
• The incidence of ischemic brain complications caused by unilateral obstruction of the vertebral artery is low.
♦ However, if the dominant vertebral artery is injured, serious neurologic complications can occur.
• CT and magnetic resonance imaging (MRI) provide information regarding right-left dominance and anatomic variations of the vertebral artery. Magnetic resonance angiography (MRA) must be conducted for patients with evidence of the abnormalities or in whom these abnormalities are suspected.

Surgical Anatomy

image According to previous studies by Panjabi and colleagues and by Karaikovic and colleagues, the pedicle of the cervical spine in a normal population has a sufficient diameter to allow insertion of a screw with a diameter of 3.5 mm or more.

• These authors defined this quantitatively by measuring the cadaveric cervical spine. Both the outer width and outer height were largest at C2 and smallest at C3, with subsequent increasing size if progressing to C7.
• Pedicles in some patients have a diameter that is too small to allow screw insertion.

image According to an anatomic study by Reinhold and colleagues, the average overall angle between the sagittal plane and the longitudinal pedicle axis was 46 degrees, varying from 30 degrees to 62 degrees. The smallest angle was at C7, the largest at C4. Their results were similar to previous studies by Karaikovic and colleagues.

image Pedicle screw insertion into a vertebra with an extremely large angle between the pedicle axis and the sagittal plane may be possible but puts the vertebral artery and the spinal cord at risk. Figure 13-4 shows an extremely large angle between the pedicle axis and the sagittal plane. The left side of the foramen transversarium is enlarged toward the vertebral body (Figure 13-4, A, open arrow), and the angle between the pedicle axis (black line) and the sagittal plane is extremely large because of deformation of the foramen. In a case of C6 spondylolysis, the angle between the pedicle axis (Figure 13-4, B, black line) and the sagittal plane is extremely large. Screw insertion into the left side of the pedicle is too risky for the vertebral artery and the spinal cord.

image Karaikovic and colleagues defined the inner morphology of the cervical pedicles. They revealed that the thinnest pedicle cortex was always the lateral cortex, and some pedicles had no medullary canal (i.e., where solid cortical bone is expected: 0.9% for C2, 2.8% for C3 and C4, and 3.8% for C5 pedicles).

image The vertebral artery sometimes bends into the vertebral body forming the loop, and screw insertion into the ipsilateral side of the pedicle may put the artery at risk. Figure 13-5 shows loop formation of the vertebral artery.

image MRA shows the medial loop of the left vertebral artery (Figure 13-5, A, arrow).

image CT (Figure 13-5, B) and MRI (Figure 13-5, C) images show that the vertebral artery bends into the vertebral body, forming the loop (arrows). Screw insertion into the left side of the pedicle is too risky for the artery.

image image

FIGURE 13-4, A-B

image image image

FIGURE 13-5, A-C

Positioning

image The authors prefer to stand at the head of the patient, to ensure symmetric insertion of the right and left screws, while the assistant for a right-handed surgeon usually stands on the left side of the patient.

image The C-arm display is placed on the left side of the patient near the patient’s pelvis for easy viewing by the surgeon. The authors’ preferred operation room setup for posterior cervical spinal procedures is shown in Figure 13-6.

image The patient is placed prone on a Relton-Hall frame, using a horseshoe-type headrest or a Mayfield headholder.

image The shoulders are pulled caudally using heavy bandage for intraoperative lateral fluoroscopic imaging of the lower cervical spine.

image

FIGURE 13-6

Portals/Exposures

image A skin incision is made, usually longer than that required for a standard spinous process wiring. The cephalad adjacent lamina of the most cephalad-fixed vertebra should entirely be exposed, taking care to protect the surrounding facet joint capsule. The paravertebral muscles are dissected laterally to expose the lateral margins of the articular masses for exact mediolateral determination of the screw insertion point.

Procedure

Step 1: Manual Screw Placement

image The cranial margin of the C2 lamina is the craniocaudal landmark for the screw insertion point for C2. To confirm the screw insertion points in C2, a small spatula can be inserted into the spinal canal along the cranial margin of the C2 lamina to the medial surface of the C2 pedicle. Figure 13-7 shows the pedicle screw insertion point for C2. The cranial margin of the C2 lamina (white broken line) is the landmark for the screw insertion point for C2 (asterisk). To confirm the screw insertion points in C2, a small spatula can be inserted into the spinal canal along the margin cortex of the C2 pars interarticularis to the medial surface of the C2 pedicle. The black broken arrow indicates the screw direction toward the C2 pedicle.

image The angle for the C2 pedicle should be 15 to 25 degrees medial to the midline in the transverse plane.

image The screw insertion points for the C3 to C7 pedicles are slightly lateral to the center of the articular mass and close to the inferior margin of the inferior articular process of the cranially adjacent vertebra. However, the shape and size of the lateral mass are variable in each vertebra and in each patient.

image The lateral margin of the articular mass of the cervical spine has a notch approximately at the level of the pedicle. The pedicles are located approximately below the lateral vertebral notch at C2, at the notch at C3 through C6, and at or slightly above the notch at C7. Figure 13-8 shows the pedicle screw insertion points for C3 to C7. Three-dimensional CT reconstruction shows the screw insertion points for C3 to C7. The lateral margin of the articular mass of the cervical spine has a notch approximately at the level of the pedicle (white arrow). The pedicles are located approximately below the lateral vertebral notch at C2, at C3 to C6, and at or slightly above the notch at C7. Screw insertion points (black crosses) are 2 to 4 mm medial to the notch. The white asterisk shows the C2 pedicle screw insertion point.

• The screw insertion points for the C3 to C7 pedicles are slightly laterally to the center of the articular mass and close to the inferior margin of the inferior articular process of the cranially adjacent vertebra. Craniocaudal orientation of the screw insertion point can be confirmed by a lateral image intensifier.
• The anatomic direction of the pedicle axis in the transverse plane varies from a minimum for the C7 pedicle to a maximum for the C5 pedicle.
• Pedicle screw insertion with a large angle relative to the sagittal plane can be difficult. Because of short length of cervical pedicle axis, however, the screw can be inserted at a smaller angle than the angle of the anatomic axis.
• The authors usually insert screws at an angle of 25 degrees to 45 degrees relative to the sagittal plane for the pedicles from C3 to C7.
• By making the funnel-shaped hole bigger and deeper with a curette or high-speed burr, the surgeon can see the medial cortex of the posterior portion of the pedicle and the pedicle cavity directly in most cases. This funnel-shaped resection of the outer portion of the articular mass toward the entrance of the pedicle cavity allows more freedom and potential angulation for positioning the screw. Figure 13-9 shows the starting point and direction of the cervical pedicle screw. The authors usually create a funnel-shaped hole at the screw insertion point using a high-speed burr. The two dashed black lines indicate the anatomic axis of the pedicle. The semicircular shaded area denotes the excised outer portion of the articular mass. Through funnel-shaped resection of the articular mass toward the entrance of the pedicle cavity using a high speed burr, the starting point of the screw approaches the entrance of the pedicle cavity. Consequently, the surgeon obtains more freedom with the screw insertion angle. The triangular area between the two black lines indicates the possible screw insertion direction.
• In addition, the surgeon can see the pedicle cavity directly, in many cases, by enlarging the insertion hole with a curette or high-speed burr.
• After creating the insertion hole, a small pedicle probe, tap, and screws are inserted into the pedicle with the help of a lateral image intensifier, to confirm the direction and insertion depth. The authors recommend confirming the proper creation of the screw insertion path after probing and tapping, using a pedicle sounder. Screw insertion is regulated using a C-arm. The two dashed white lines indicate the cranial and caudal margins of the pedicle (Figure 13-10). The pedicle probe, tap, and screws must be advanced between the two lines. Figure 13-10, A shows the making of a funnel-shaped hole. Figure 13-10, B shows the probing of the pedicle, and Figure 13-10, C shows the tapping of the pedicle. Screw insertion is shown in Figure 13-10, D.
• Yukawa and colleagues demonstrated that the use of oblique projection fluoroscopy imaging increased the rate of proper screw insertion.

image The thinnest pedicle cortex is always the lateral cortex. Therefore the surgeon should keep this in mind while probing and tapping the pedicle and while placing the screws.

image The medial pedicle cortex must be used as a safe guide for the insertion of the screw into the vertebral body through the pedicle isthmus.

image Use of a drill bit cannot be recommended to penetrate the cortex of the lateral mass or to make a hole for screw advancement. However, the neurocentral junction in the cervical spine, which is near the base of the pedicle in the vertebral body, is sometimes hard to pass with the pedicle probe, because it is in the thoracic and lumbar spine. In such cases, the junction can be perforated with a Kirschner wire or with small-sized high-speed diamond burr to make the path for the pedicle probe into the vertebral body.

image The intended angle of screw insertion in the sagittal plane is parallel to the upper end plate for the pedicles of C5 to C7, and it is slightly in the cephalad direction for C2 to C4 (based on angulation in the sagittal plane). The C2 screw usually is perpendicular to the anterior surface of the vertebral body.

image

FIGURE 13-7

image

FIGURE 13-8

image

FIGURE 13-9

image image image image

FIGURE 13-10, A-D

Computer-Assisted Screw Placement

image Modern technology for computer navigation systems has been developing in the field of cervical spine surgery.

image Kim and colleagues and Ludwig and colleagues conducted a study to compare current computer-assisted technology with the original fluoroscopy-based technique in the laboratory. They concluded that the use of a computer-assisted image guidance system did not enhance the safety or accuracy of placing pedicle screws. However, their computer-assisted system (StealthStation; Medtronic Sofamor-Danek USA, Memphis, Tenn.) only navigated a screw guide tube at the insertion point of the bone surface and did not guide the actual tip of the pedicle probe, tap, or screw within the pedicle.

image Kotani and colleagues developed a new computer-assisted guidance system for cervical pedicle screw placement. The system facilitates obtaining real-time, three-dimensional instrument/screw tip information at each step (probing, tapping, and screw insertion into the vertebra) and is useful in improving the safety and accuracy of pedicle screw placement in the cervical spine.

image More recently, several authors have reported that intraoperative three-dimensional fluoroscopy-based navigation improved the safety and accuracy of the pedicle screw insertion in the cervical spine.

image Rajasekaran and colleagues emphasized that the advantages conferred by cervical pedicle screws into adults also can be extended to the pediatric population.

image Further modifications and developments in technology will increase safety and refine surgical techniques.

Step 1 Instrumentation/Implantation

• Screw diameters vary from 3.5 to 4.5 mm for cervical pedicle screw fixation. However, a screw with a proper diameter must be chosen to obtain sufficient bite of the screw thread in the pedicle cortex. The length of the screw is usually 20 to 24 mm for C3 to C7. A screw length of 24 mm or more is sometimes required to penetrate the anterior cortex of the vertebral body to increase the C2 screw stability. A constrained type of locking mechanism is essential for connecting the screws and plates/rods, to obtain the rigid stabilizing effect of this procedure. Regarding the longitudinal connectors of screws, a rod rather than a plate is recommended for multisegmental fixation.

Step 2

image Before plate or rod application, a posterior decompression by laminoplasty or laminectomy is recommended for a patient with a narrow spinal canal, to avoid possible neurologic deterioration caused when the vertebral alignment is changed after longitudinal connection of the screws.

image The lateral masses and laminae must be decorticated, and bone chips obtained from spinous processes and laminae must be placed.

image In the final stage of instrumentation, inserted screws are connected with a plate or rod.

• Simple plate fixation is preferred for one- or two-segment fixation. Monosegmental fixation is shown in Figure 13-11; the patient has cervical spondylotic myelopathy associated with segmental instability at C3-4. Radiograph films in the extension and flexion positions show segmental instability at C3-4 (see Figure 13-11, A and B, respectively). MRI images and a CT scan (see Figure 13-11, C to E) show a narrowed spinal canal and spinal cord compression. Figure 13-11, in parts F and G, shows postoperative images of C3-4 single-level pedicle screw/plate fixation. Axial CT scans (see Figure 13-11, G and I) show the location of inserted screws.

image The alignment of the screw head in the coronal plane may be off in multilevel fixation. Therefore a rod with a screw rather than plate is recommended for multilevel fixation across three segments.

image image image image image image image image image

FIGURE 13-11, A-I

Step 3

• The pedicle screw procedure is a strong tool for correction of the deformities in the cervical spine.

• Plates and rods are contoured in the sagittal plane, with the expected correction of kyphotic deformity.

• Correction of the kyphosis is performed by tightening the nuts or by rotating the rods using rodholders (Figure 13-12). A preoperative myelogram (see Figure 13-12, A) shows postlaminectomy kyphosis. MRI images (see Figure 13-12, B and C) demonstrate dumbbell-type recurrent spinal cord tumor (schwannoma). A postmyelogram CT (see Figure 13-12, D) shows that the tumor is expanding from intracanal-extradural to the outside of the spinal canal. The patient sustained progressive spinal cord compression symptoms. The patient underwent simultaneous posterior decompression by tumor extirpation and correction of kyphosis (see Figure 13-12, E and F). Consequently, the posterior part of the cervical spine can be shortened.

• Cases of cervical scoliosis requiring surgical correction are uncommon. If necessary, however, the deformity is correctable using this procedure by applying distraction force on the concave side (Figure 13-13). Preoperative images (see Figure 13-13, A and B) show 31 degrees of scoliosis after laminoplasty. The patient suffered from severe radiculopathy of the left C5 and C6 caused by C4-5 and C5-6 foraminal stenosis. Preoperative CT scans (see Figure 13-13, C to F) show proliferated degenerative changes of the left lateral mass. In Figure 13-13, G and H, postoperative images show sufficient correction of scoliosis. Postoperative CT images (see Figure 13-13, G to K) show proper screw placement in each vertebra.

image image image image image image

FIGURE 13-12, A-F

image image image image image image image image image image image

FIGURE 13-13, A-K

Step 3 Pearls

• The neural foramina in patients with degenerative disorders are sometimes stenotic preoperatively. There is the risk of iatrogenic nerve root lesion because of foraminal stenosis caused by reduction of anterior translation or correction of kyphosis.

• Reconstructive CT performed in the oblique plane provides useful information about the size of the neural foramen.

• The use of a washer under the plate/rod for the cranial vertebral screws is helpful in situations where excessive reduction would occur during screw tightening.

• During the correction of kyphosis, the surgeon must also avoid applying excessive compression force at the spinal segment having neural foraminal stenosis because of degenerative changes. A prophylactic foraminotomy is recommended for the segments with marked stenosis of the neural foramen.

Step 3 Pitfalls

• Surgeons must be careful to avoid excessive shortening of the spine because of the potential for developing nerve root lesions by iatrogenic foraminal stenosis.

Postoperative Care and Expected Outcomes

image Complications directly attributable to screw insertion

• There have been several published reports in English referring to results of cervical pedicle screw fixation. In total, 306 patients underwent cervical reconstructive surgery in those three reports.
♦ Two cases of vertebral artery injury and five cases of nerve root lesion directly attributable to pedicle screw insertion were described in the three reports.
♦ There were no cases of spinal cord injury caused by pedicle screw insertion in any of the reports.
• Onishi and colleagues reported a case of a late complication of cerebral infarction caused by a laterally perforated cervical pedicle screw.
• Based on the experience of Abumi and associates in 571 cases of cervical pedicle screw fixation from 1990 to 2009, the rate of neurovascular complication was relatively low.
♦ In the authors’ series, there were five patients with neurovascular complications directly attributable to screw insertion into the cervical pedicles: two with vertebral artery injury and three with radiculopathy by screw insertion.
♦ Intraoperative injury of the vertebral artery during the tapping of a fractured pedicle occurred in one patient with an injury at C6-7. Bleeding stopped immediately by packing bone wax into the insertion hole.
♦ Obstruction of the vertebral artery by a laterally dislodged screw occurred in one patient who underwent occipitoatlantoaxial fixation.

• No further ischemic neurologic complications of the brain were observed in these two cases.
♦ Radiculopathy caused by an inserted pedicle screw was identified in three patients.

• A C6 nerve root lesion caused by superiorly perforated C6 screw threads resolved during the course of the follow-up without screw removal.
• Two C5 nerve root lesions associated with muscle weakness was caused by an inferiorly perforated C4 screw, and recovered to normal strength upon screw removal.

image Complications not directly attributable to screw insertion

• Based on Abumi and colleague’s published results of 227 cases of cervical pedicle screw fixation, late neurologic deficits after posterior correction surgery occurred in 2.6% of cases.
♦ The neurologic deficits were associated with correction of cervical kyphosis by posterior instrumentation.
♦ The neurologic deficits occurred in 20% of patients who underwent correction of cervical kyphosis.
♦ Possible causes of this complication are iatrogenic foraminal stenosis and/or tethering of the nerve because of posterior shift of the spinal cord after correction of kyphosis.
♦ As the correction angle increases, the incidence of this complication was higher.

• Prophylactic foraminotomy at C4-5 is recommended when foraminal stenosis is present on preoperative CT images.
• Excessive correction, exceeding 9.7 degrees per segment in the middle cervical spine, should not be conducted to prevent this complication.
• When this complication occurs, regardless of revision, decompression, or observation, the majority of patients will improve on their own over time.

Summary

image Pedicle screw fixation is a strong procedure for reconstruction of the cervical spine in various types of disorders. Furthermore, a screw inserted into the cervical pedicle screw can be a strong anchor for reconstruction of the craniocervical junction and cervicothoracic spine. Surgeons must keep in mind that there must be limitations of cervical pedicle screw placement based on anatomic variations of the pedicle and vertebral artery. Complications associated with cervical pedicle screw fixation cannot be completely obviated; however, they can be minimized by sufficient preoperative imaging studies of the pedicles, thorough knowledge of local anatomy, and strict control of screw placement during surgery.

image Cervical pedicle screw fixation may become a more definitive tool for reconstruction of the cervical spine in the future with progress in supplemental technology, including computer navigation systems, monitoring procedures of the nerve, and so forth.

Evidence

Abumi K, Ito M, Kotani Y. Complications of cervical pedicle screw placement. Semin Spine Surg. 2002;14:112-124.

Abumi K, Ito H, Taneichi H, et al. Transpedicular screw fixation for traumatic lesions of the middle and lower cervical spine. Description of the techniques and preliminary report. J Spinal Disorder. 1994;7:19-28.

Abumi K, Kaneda K, Shono Y, et al. One-stage posterior decompression and reconstruction of the cervical spine by using pedicle screw fixation systems. J Neurosurg. 1999;90(Suppl 1):19-26.

Abumi K, Shono Y, Ito M, et al. Complication of pedicle screw fixation in reconstructive surgery of the cervical spine. Spine. 2000;25:962-969.

Abumi K, Shono Y, Taneichi T, et al. Correction of cervical kyphosis using pedicle screw fixation systems. Spine. 1999;24:2389-2396.

Abumi K, Takada T, Shono Y, et al. Posterior occipitocervical reconstruction using cervical pedicle screws and plate-rod systems. Spine. 1999;24:1425-1434.

Duan S, Lv S, Ye F, Lin Q. Imaging anatomy and variation of vertebral artery and bone structure at craniocervical junction. Eur Spine J. 2009;18:1102-1108.

Dunlap BJ, Karaikovic EE, Park HS, Sokolowski MJ, Zhang LQ. Load-sharing properties of cervical pedicle screw-rod constructs versus lateral mass screw-rod constructs. Eur Spine J. 2010;19:803-808.

Goel A, Leheri V. Plate and screw fixation for atlanto-axial subluxation. Acta Neurochir (Wien). 1994;129:47-53.

Hojo Y, Ito M, Abumi K, et al. A late neurological complication following posterior correction surgery of severe cervical kyphosis. Eur Spine J. 2010;20:890-898.

Ishikawa Y, Kanemura T, Yoshida G, et al. Clinical accuracy of three-dimensional fluoroscopy-based computer-assisted cervical pedicle screw placement: a retrospective comparative study of conventional versus computer-assisted cervical pedicle screw placement. J Neurosurg Spine. 2010;13:606-611.

Johnston TL, Karaikovic EE, Lautenschlager EP, et al. Cervical pedicle screws vs. lateral mass screws: uniplanar fatigue analysis and residual pullout strengths. Spine J. 2006;6:667-672.

Karaikovic EE, Daubs MD, Madsen RW, et al. Morphologic characteristics of human cervical pedicles. Spine. 1997;22:493-550.

Karaikovic EE, Kunakornsawat S, Daubs MD, et al. Surgical anatomy of the cervical pedicles: landmarks for posterior cervical pedicle entrance localization. J Spinal Disord. 2000;13:63-72.

Karaikovic EE, Yngsakmongkol W, Gaines RW. Accuracy of cervical pedicle screw placement using the funnel technique. Spine. 2001;26:2456-2462.

Kast E, Mohr K, Richter HP, Borm W. Complication of transpedicular screw fixation in the cervical spine. Eur Spine J. 2005;15:327-334.

Kim HS, Heller JG, Hudgins PA, et al. The accuracy of computed tomography in assessing cervical pedicle screw placement. Spine. 2003;28:2441-2446.

Kotani Y, Abumi K, Ito M. Improved accuracy of computer-assisted cervical pedicle screw insertion. J Neurosurg. 1993;99(Suppl 3):257-263.

Kothe R, Ruther W, Schneider E, et al. Biomechanical analysis of transpedicular screw fixation in the subaxial cervical spine. Spine. 2004;29:1969-1975.

Leconte P. Fracture et luxation des deux premieres vertebres cervicales. In: Judet R, editor. Luxation Congénitale de la Hanche. Fractures du Cou-de-pied Rachis Cervical. Actualités de Chirurgie Orthopédique de l’Hôpital Raymond-Poincaré, vol 3. Paris: Masson et Cie; 1964:147-166.

Ludwig SC, Kowalski JM, Edwards CC2nd, et al. Comparative accuracy of two insertion techniques. Spine. 2000;25:2675-2681.

Oda I, Abumi K, Sell LC, et al. Biomechanical evaluation of five different occipito-atlanto-axial fixation techniques. Spine. 1999;24:2377-2382.

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